Apparatus and method for encoding or decoding a multi-channel signal

Abstract

Embodiments provide an apparatus for encoding a multi-channel signal having at least three channels. The apparatus comprises an iteration processor, a channel encoder and an output interface. The iteration processor is configured to calculate, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using a multi-channel processing operation to derive first multi-channel parameters for the selected pair and to derive first processed channels. Further, the iteration processor is configured to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive second multi-channel parameters and second processed channels. The channel encoder is configured to encode channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels. The output interface is configured to generate an encoded multi-channel signal having the encoded channels and the first and the second multi-channel parameters.

Description

(19) *EP004675616A2* (11) EP 4 675 616 A2 (12) EUROPEAN PATENT APPLICATION (43) Date of publication: 07.01.2026 Bulletin 2026 / 02 (21) Application number: 25211436.8 (22) Date of filing: 08.03.2016 (51) International Patent Classification (IPC): G10L 19 / 008 (2013.01) (52) Cooperative Patent Classification (CPC): G10L 19 / 008 (84) Designated Contracting States: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR (30) Priority: 09.03.2015 EP 15158234 17.06.2015 EP 15172492 (62) Document number(s) of the earlier application(s) in accordance with Art. 76 EPC: 19157636.2 / 3 506 259 16709344.2 / 3 268 959 (71) Applicant: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. 80686 München (DE) (72) Inventors: • DICK, Sascha 91058 Erlangen (DE) • SCHUH, Florian 91058 Erlangen (DE) • RETTELBACH, Nikolaus 91058 Erlangen (DE) • SCHWEGLER, Tobias 90419 Nürnberg (DE) • FÜG, Richard 91058 Erlangen (DE) • HILPERT, Johannes 91058 Erlangen (DE) • NEUSINGER, Matthias 91189 Rohr (DE) (74) Representative: Schlenker, Julian et al Schoppe, Zimmermann, Stöckeler Zinkler, Schenk & Partner mbB Patentanwälte Radlkoferstraße 2 81373 München (DE) Remarks: •This application was filed on 27‑10‑2025 as a divisional application to the application mentioned under INID code 62. •Claims filed after the date of filing of the application / after the date of receipt of the divisional application (Rule 68(4) EPC) (54) APPARATUS AND METHOD FOR ENCODING OR DECODING A MULTI‑CHANNEL SIGNAL (57) Embodiments provide an apparatus for encod- ing amulti-channel signal having at least three channels. The apparatus comprises an iteration processor, a chan- nel encoder and an output interface. The iteration pro- cessor is configured to calculate, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using amulti-channel processing operation to derive first multi-channel parameters for the selected pair and to derive first processed channels. Further, the iteration processor is configured to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive second multi-channel parameters and second pro- cessed channels. The channel encoder is configured toencodechannels resulting froman iterationprocessing performed by the iteration processor to obtain encoded channels. The output interface is configured to generate an encoded multi-channel signal having the encoded channels and the first and the second multi-channel parameters. EP 4 67 5 61 6 A 2 Processed by Luminess, 75001 PARIS (FR) Description

[0001] Thepresent invention relates toaudiocoding / decodingand, inparticular, to audiocodingexploiting inter-channel signal dependencies.

[0002] Audio coding is the domain of compression that deals with exploiting redundancy and irrelevancy in audio signals. In MPEG USAC [ISO / IEC 23003‑3:2012 - Information technology - MPEG audio technologies Part 3: Unified speechandaudio coding], joint stereo codingof two channels is performedusing complexprediction,MPS2‑1‑2or unified stereo with band-limited or full-band residual signals. MPEG surround [ISO / IEC 23003‑1:2007 - Information technology - MPEG audio technologies Part 1: MPEGSurround] hierarchically combines OTTand TTT boxes for joint coding of multi- channelaudiowithorwithout transmissionof residual signals.MPEG-HQuadChannelElementshierarchically applyMPS 2‑1‑2 stereo boxes followed by complex prediction / MS stereo boxes building a fixed 4x4 remixing tree. AC4 [ETSI TS 103 190 V1.1.1 (2014‑04) - Digital Audio Compression (AC‑4) Standard] introduces new 3‑, 4‑ and 5‑ channel elements that allow for remixing transmitted channels via a transmitted mix matrix and subsequent joint stereo coding information. Further, prior publicationssuggest touseorthogonal transforms likeKarhunen-LoeveTransform(KLT) for enhancedmulti- channel audio coding [Yang, Dai and Ai, Hongmei and Kyriakakis, Chris and Kuo, C.‑C. Jay, 2001: Adaptive Karhunen- Loeve Transform for Enhanced Multichannel Audio Coding, http: / / ict.usc.edu / pubs / Adaptive%20Karhunen-Loeve% 20Transform%20for%20Enhanced %20Multichannel%20Audio%20Coding.pdf].

[0003] In the 3Daudio context, loudspeaker channels are distributed in several height layers, resulting in horizontal and vertical channel pairs. Joint coding of only two channels as defined in USAC is not sufficient to consider the spatial and perceptual relations between channels. MPEG Surround is applied in an additional pre‑ / postprocessing step, residual signals are transmitted individually without the possibility of joint stereo coding, e.g. to exploit dependencies between left and right vertical residual signals. InAC‑4dedicatedN-channel elements are introduced that allow for efficient encoding of joint coding parameters, but fail for generic speaker setups with more channels as proposed for new immersive playback scenarios (7.1+4, 22.2). MPEG-HQuad Channel element is also restricted to only 4 channels and cannot be dynamically applied to arbitrary channels but only a preconfigured and fixed number of channels.

[0004] It is an object of the present invention to provide an improved encoding / decoding concept.

[0005] This object is achieved by an apparatus for encoding a multi-channel signal having at least three channels according to claim 1, an apparatus for decoding an encoded multi-channel signal having encoded channels and at least first and second multi-channel parameters according to claim 12, a method for encoding a multi-channel signal having at least three channels according to claim 21, a method for decoding an encoded multi-channel signal having encoded channels and at least first and second channel multi-channel parameters according to claim 22, or a computer program according to claim 23.

[0006] Embodiments provide an apparatus for encoding a multi-channel signal having at least three channels. The apparatus comprises an iteration processor, a channel encoder and an output interface. The iteration processor is configured to calculate, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using a multi-channel processing operation to derive first multi-channel parameters for the selected pair and to derive first processed channels. Further, the iteration processor is configured to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive second multi-channel parameters and second processed channels. The channel encoder is configured to encode channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels. The output interface is configured to generate an encodedmulti-channel signal having the encoded channels and the first and the second multi-channel parameters.

[0007] Further embodiments provide an apparatus for decoding an encoded multi-channel signal, the encoded multi- channel signal having encoded channels and at least first and second multi-channel parameters. The apparatus comprises a channel decoder and a multi-channel processor. The channel decoder is configured to decode the encoded channels to obtain decoded channels. The multi-channel processor is configured to perform a multi-channel processing using a second pair of the decoded channels identified by the second multi-channel parameters and using the second multi-channel parameters toobtain processedchannelsand toperforma furthermulti-channel processingusingafirst pair of channels identified by the first multi-channel parameters and using the first multi-channel parameters, wherein the first pair of channels comprises at least one processed channel.

[0008] In contrast to commonmulti-channel encoding concepts which use a fixed signal path (e.g., stereo coding tree), embodiments of the present invention use a dynamic signal path which is adapted to characteristics of the at least three input channelsof themulti-channel input signal. Indetail, the iterationprocessor102canbeadapted tobuild thesignal path (e.g, stereo tree), in the first iteration step, based on an inter-channel correlation value between each pair of the at least three channels CH1 to CH3, for selecting, in the first iteration step, a pair having the highest value or a value above a threshold, and, in the second iteration step, based on inter-channel correlation values between each pair of the at least three channels and corresponding previously processed channels, for selecting, in the second iteration step, a pair having 2 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 the highest value or a value above a threshold.

[0009] Further embodiments provide a method for encoding a multi-channel signal having at least three channels. The method comprises: - calculating, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and processing the selected pair using a multichannel processing operation to derive first multichannel parameters for the selected pair and to derive first processed channels; - performing the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive second multichannel parameters and second processed channels; - encoding channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels; and - generating an encoded multi-channel signal having the encoded channels and the first and the second multichannel parameters.

[0010] Further embodiments provide amethod for decodinganencodedmulti-channel signal having encoded channels and at least first and second multichannel parameters. The method comprises: - decoding the encoded channels to obtain decoded channels; and - performing a multichannel processing using a second pair of the decoded channels identified by the second multichannel parameters and using the second multichannel parameters to obtain processed channels, and performing a further multichannel processing using a first pair of channels identified by the first multichannel parameters and using the first multichannel parameters, wherein the first pair of channels comprises at least one processed channel.

[0011] Embodiments of the present invention are described herein making reference to the appended drawings. Fig. 1 shows a schematic block diagram of an apparatus for encoding a multi-channel signal having at least three channels, according to an embodiment; Fig. 2 shows a schematic block diagram of an apparatus for encoding a multi-channel signal having at least three channels, according to an embodiment; Fig. 3 shows a schematic block diagram of a stereo box, according to an embodiment; Fig. 4 shows a schematic block diagram of an apparatus for decoding an encoded multi-channel signal having en- coded channels and at least first and second multi-channel parameters, according to an embodiment; Fig. 5 shows a flowchart of a method for encoding a multi-channel signal having at least three channels, according to an embodiment; and Fig. 6 shows a flowchart of a method for decoding an encoded multi-channel signal having encoded channels and at least first and second multi-channel parameters, according to an embodiment.

[0012] Equal or equivalent elements or elements with equal or equivalent functionality are denoted in the following description by equal or equivalent reference numerals.

[0013] In the following description, a plurality of details are set forth to provide a more thorough explanation of embodiments of the present invention. However, it will be apparent to those skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form rather than in detail in order to avoid obscuring embodiments of the present invention. In addition, featuresof thedifferentembodimentsdescribedhereinaftermaybecombinedwitheachother, unlessspecifically noted otherwise.

[0014] Fig. 1 shows a schematic block diagram of an apparatus (encoder) 100 for encoding a multi-channel signal 101 having at least three channels CH1 to CH3. The apparatus 100 comprises an iteration processor 102, a channel encoder 104 and an output interface 106.

[0015] The iteration processor 102 is configured to calculate, in a first iteration step, inter-channel correlation values between each pair of the at least three channels CH1 toCH3 for selecting, in the first iteration step, a pair having a highest valueor havingavalueabovea threshold, and for processing the selectedpair usingamulti-channel processingoperation 3 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 to derivefirstmulti-channel parametersMCH_PAR1 for theselectedpair and toderivefirst processedchannelsP1andP2. Further, the iteration processor 102 is configured to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels P1 or P2 to derive second multi-channel parameters MCH_PAR2 and second processed channels P3 and P4.

[0016] For example, as indicated in Fig. 1, the iteration processor 102 may calculate in the first iteration step an inter- channel correlation value between a first pair of the at least three channels CH1 to CH3, the first pair consisting of a first channel CH1 and a second channel CH2, an inter-channel correlation value between a second pair of the at least three channels CH1 to CH3, the second pair consisting of the second channel CH2 and a third channel CH3, and an inter- channel correlation value betweena third pair of the at least three channelsCH1 toCH3, the third pair consisting of the first channel CH1 and the third channel CH3.

[0017] In Fig. 1 it is assumed that in the first iteration step the third pair consisting of the first channel CH1 and the third channelCH3comprises thehighest inter-channel correlation value, such that the iterationprocessor102selects in thefirst iteration step the third pair having the highest inter-channel correlation value andprocesses the selected pair, i.e., the third pair, using amulti-channel processing operation to derive firstmulti-channel parametersMCH_PAR1 for the selected pair and to derive first processed channels P1 and P2.

[0018] Further, the iteration processor 102 can be configured to calculate, in the second iteration step, inter-channel correlation valuesbetweeneachpair of theat least three channelsCH1 toCH3and theprocessedchannelsP1andP2, for selecting, in the second iteration step, a pair having a highest inter-channel correlation value or having a value above a threshold. Thereby, the iteration processor 102 can be configured to not select the selected pair of the first iteration step in the second iteration step (or in any further iteration step).

[0019] Referring to the example shown in Fig. 1, the iteration processor 102 may further calculate an inter-channel correlation value between a fourth pair of channels consisting of the first channel CH1 and the first processed channel P1, an inter-channel correlation value between a fifth pair consisting of the first channel CH1 and the second processed channel P2, an inter-channel correlation value between a sixth pair consisting of the second channel CH2 and the first processed channel P1, an inter-channel correlation value between a seventh pair consisting of the second channel CH2 and the second processed channel P2, an inter-channel correlation value between an eighth pair consisting of the third channel CH3 and the first processed channel P1, an inter-correlation value between a ninth pair consisting of the third channel CH3 and the second processed channel P2, and an inter-channel correlation value between a tenth pair consisting of the first processed channel P1 and the second processed channel P2.

[0020] InFig. 1, it is assumed that in the second iteration step the sixthpair consistingof thesecondchannelCH2and the first processed channel P1 comprises the highest inter-channel correlation value, such that the iteration processor 102 selects in the second iteration step the sixth pair and processes the selected pair, i.e., the sixth pair, using amulti-channel processing operation to derive second multi-channel parameters MCH_PAR2 for the selected pair and to derive second processed channels P3 and P4.

[0021] The iteration processor 102 can be configured to only select a pair when the level difference of the pair is smaller thana threshold, the thresholdbeingsmaller than40dB,25dB,12dBor smaller than6dB.Thereby, the thresholdsof 25or 40 dB correspond to rotation angles of 3 or 0.5 degree.

[0022] The iteration processor 102 can be configured to calculate normalized integer correlation values, wherein the iteration processor 102 can be configured to select a pair, when the integer correlation value is greater than e.g. 0.2 or preferably 0.3.

[0023] Further, the iteration processor 102may provide the channels resulting from the multichannel processing to the channel encoder104.For example, referring toFig. 1, the iterationprocessor 102mayprovide the thirdprocessedchannel P3and the fourth processed channelP4 resulting from themultichannel processing performed in the second iteration step and thesecondprocessedchannelP2 resulting from themultichannel processingperformed in the first iteration step to the channel encoder 104. Thereby, the iteration processor 102 may only provide those processed channels to the channel encoder 104 which are not (further) processed in a subsequent iteration step. As shown in Fig. 1, the first processed channel P1 is not provided to the channel encoder 104 since it is further processed in the second iteration step.

[0024] The channel encoder 104 can be configured to encode the channels P2 to P4 resulting from the iteration processing (or multichannel processing) performed by the iteration processor 102 to obtain encoded channels E1 to E3.

[0025] For example, the channel encoder 104 can be configured to usemono encoders (ormono boxes, ormono tools) 120_1 to 120_3 for encoding the channels P2 to P4 resulting from the iteration processing (or multichannel processing). Themono boxesmay be configured to encode the channels such that less bits are required for encoding a channel having less energy (or a smaller amplitude) than for encoding a channel having more energy (or a higher amplitude). The mono boxes 120_1 to 120_3 can be, for example, transformation based audio encoders. Further, the channel encoder 104 can be configured to use stereo encoders (e.g., parametric stereo encoders, or lossy stereo encoders) for encoding the channels P2 to P4 resulting from the iteration processing (or multichannel processing).

[0026] The output interface 106 can be configured to generate and encoded multi-channel signal 107 having the encoded channels E1 to E3 and the first and the second multi-channel parameters MCH_PAR1 and MCH_PAR2. 4 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0027] For example, the output interface 106 can be configured to generate the encoded multi-channel signal 107 as a serial signal or serial bit stream, and so that the second multi-channel parameters MCH_PAR2 are in the encoded signal 107 before the first multi-channel parameters MCH_PAR1. Thus, a decoder, an embodiment of which will be described later with respect to Fig. 4, will receive the second multi-channel parameters MCH_PAR2 before the first multi-channel parameters MCH-PAR1.

[0028] In Fig. 1 the iteration processor 102 exemplarily performs two multi-channel processing operations, a multi- channel processing operation in the first iteration step and a multi-channel processing operation in the second iteration step. Naturally, the iteration processor 102 also can perform further multi-channel processing operations in subsequent iteration steps. Thereby, the iteration processor 102 can be configured to perform iteration steps until an iteration termination criterion is reached. The iteration termination criterion can be that a maximum number of iteration steps is equal to or higher than a total number of channels of the multi-channel signal 101 by two, or wherein the iteration termination criterion is, when the inter-channel correlation values do not have a value greater than the threshold, the threshold preferably being greater than 0.2 or the threshold preferably being 0.3. In further embodiments, the iteration termination criterion can be that amaximumnumber of iteration steps is equal to or higher than a total number of channels of the multi-channel signal 101, or wherein the iteration termination criterion is, when the inter-channel correlation values do not have a value greater than the threshold, the threshold preferably being greater than 0.2 or the threshold preferably being 0.3.

[0029] For illustration purposes themulti-channel processing operationsperformedby the iteration processor 102 in the first iteration step and the second iteration step are exemplarily illustrated in Fig. 1 by processing boxes 110 and 112. The processing boxes 110 and 112 can be implemented in hardware or software. The processing boxes 110 and 112 can be stereo boxes, for example.

[0030] Thereby, inter-channel signal dependency can be exploited by hierarchically applying known joint stereo coding tools. In contrast to previous MPEG approaches, the signal pairs to be processed are not predetermined by a fixed signal path (e.g., stereo coding tree) but can be changed dynamically to adapt to input signal characteristics. The inputs of the actual stereo box can be (1) unprocessed channels, such as the channels CH1 to CH3, (2) outputs of a preceding stereo box, such as the processed signals P1 toP4, or (3) a combination of anunprocessed channel and an output of a preceding stereo box.

[0031] The processing inside the stereo box 110 and 112 can either be prediction based (like complex prediction box in USAC) or KLT / PCA based (the input channels are rotated (e.g., via a 2x2 rotation matrix) in the encoder to maximize energy compaction, i.e., concentrate signal energy into one channel, in the decoder the rotated signals will be retransformed to the original input signal directions).

[0032] In a possible implementation of the encoder 100, (1) the encoder calculates an inter channel correlation between every channel pair and selects one suitable signal pair out of the input signals and applies the stereo tool to the selected channels; (2) the encoder recalculates the inter channel correlation between all channels (the unprocessed channels as well as the processed intermediate output channels) and selects one suitable signal pair out of the input signals and applies the stereo tool to the selected channels; and (3) the encoder repeats step (2) until all inter channel correlation is below a threshold or if a maximum number of transformations is applied.

[0033] As already mentioned, the signal pairs to be processed by the encoder 100, or more precisely the iteration processor 102, are not predetermined by a fixed signal path (e.g., stereo coding tree) but can be changed dynamically to adapt to input signal characteristics. Thereby, the encoder 100 (or the iteration processor 102) can be configured to construct the stereo tree in dependence on the at least three channels CH1 toCH3 of themulti-channel (input) signal 101. Inotherwords, theencoder100 (or the iterationprocessor102) canbeconfigured tobuild thestereo treebasedonan inter- channel correlation (e.g., by calculating, in the first iteration step, inter-channel correlation values betweeneachpair of the at least threechannelsCH1 toCH3, for selecting, in thefirst iterationstep, apair having thehighest valueoravalueabovea threshold, and by calculating, in a second iteration step, inter-channel correlation values between each pair of the at least three channels and previously processed channels, for selecting, in the second iteration step, a pair having the highest value or a value above a threshold). According to a one step approach, a correlationmatrix may be calculated for possibly each iteration containing the correlations of all, in previous iterations possibly processed, channels.

[0034] As indicated above, the iteration processor 102 can be configured to derive first multi-channel parameters MCH_PAR1 for the selected pair in the first iteration step and to derive secondmulti-channel parameters MCH_PAR2 for the selected pair in the second iteration step. The firstmulti-channel parametersMCH_PAR1maycomprise a first channel pair identification (or index) identifying (or signaling) the pair of channels selected in the first iteration step, wherein the secondmulti-channel parametersMCH_PAR2may comprise a second channel pair identification (or index) identifying (or signaling) the pair of channels selected in the second iteration step.

[0035] In the following, an efficient indexing of input signals is described. For example, channel pairs can be efficiently signaled using a unique index for each pair, dependent on the total number of channels. For example, the indexing of pairs for six channels can be as shown in the following table: 5 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 0 1 2 3 4 5 0 0 1 2 3 4 1 5 6 7 8 2 9 10 11 3 12 13 4 14 5

[0036] For example, in the above table the index 5 may signal the pair consisting of the first channel and the second channel. Similarly, the index 6 may signal the pair consisting of the first channel and the third channel.

[0037] The total number of possible channel pair indices for n channels can be calculated to:

[0038] Hence, the number of bits needed for signaling one channel pair amount to:

[0039] Further, the encoder 100may use a channel mask. Themultichannel tool’s configurationmay contain a channel mask indicating for which channels the tool is active. Thus, LFEs (LFE = low frequency effects / enhancement channels) can be removed from the channel pair indexing, allowing for a more efficient encoding. E.g. for a 11.1 setup, this reduces the number of channel pair indices from 12*11 / 2=66 to 11*10 / 2 = 55, allowing signaling with 6 instead of 7 bit. This mechanism can also be used to exclude channels intended to be mono objects (e.g. multiple language tracks). On decoding of the channel mask (channelMask), a channel map (channelMap) can be generated to allow re-mapping of channel pair indices to decoder channels.

[0040] Moreover, the iteration processor 102 can be configured to derive, for a first frame, a plurality of selected pair indications, wherein the output interface 106 can be configured to include, into the multi-channel signal 107, for a second frame, following the first frame, a keep indicator, indicating that the second frame has the same plurality of selected pair indications as the first frame.

[0041] The keep indicator or the keep tree flag can be used to signal that no new tree is transmitted, but the last stereo tree shall be used. This can be used to avoid multiple transmission of the same stereo tree configuration if the channel correlation properties stay stationary for a longer time.

[0042] Fig. 2 shows a schematic block diagram of a stereo box 110, 112. The stereo box 110, 112 comprises inputs for a first input signal I1 and a second input signal I2, and outputs for a first output signal O1 and a second output signal O2. As indicated inFig. 2, dependenciesof theoutput signalsO1andO2 from the input signals I1 and I2 canbedescribedby the s- parameters S1 to S4.

[0043] The iteration processor 102 can use (or comprise) stereo boxes 110,112 in order to perform the multi-channel processing operations on the input channels and / or processed channels in order to derive (further) processed channels. For example, the iteration processor 102 can be configured to use generic, prediction based or KLT (Karhunen-Loève- Transformation) based rotation stereo boxes 110,112.

[0044] A generic encoder (or encoder-side stereo box) can be configured to encode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation:

[0045] A generic decoder (or decoder-side stereo box) can be configured to decode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation: 6 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0046] A prediction based encoder (or encoder-side stereo box) can be configured to encode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation wherein p is the prediction coefficient.

[0047] A prediction based decoder (or decoder-side stereo box) can be configured to decode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation:

[0048] AKLT based rotation encoder (or encoder-side stereo box) can be configured to encode the input signals I1 to I2 to obtain the output signals O1 and O2 based on the equation:

[0049] AKLT based rotation decoder (or decoder-side stereo box) can be configured to decode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation (inverse rotation):

[0050] In the following, a calculation of the rotation angle α for the KLT based rotation is described.

[0051] The rotation angle α for the KLT based rotation can be defined as: with cxy being the entries of a non-normalized correlation matrix, wherein c11, c22 are the channel energies.

[0052] This can be implemented using the atan2 function to allow for differentiation between negative correlations in the numerator and negative energy difference in the denominator:

[0053] Further, the iteration processor 102 can be configured to calculate an inter-channel correlation using a frame of each channel comprising a plurality of bands so that a single inter-channel correlation value for the plurality of bands is obtained, wherein the iteration processor 102 can be configured to perform the multi-channel processing for each of the plurality of bands so that the first or the secondmulti-channel parameters are obtained from each of the plurality of bands.

[0054] Thereby, the iteration processor 102 can be configured to calculate stereo parameters in the multi-channel processing,wherein the iterationprocessor 102 canbe configured to only performastereoprocessing in bands, inwhich a stereo parameter is higher than a quantized-to-zero threshold defined by a stereo quantizer (e.g., KLT based rotation encoder). The stereo parameters can be, for example, MS On / Off or rotation angles or prediction coefficients).

[0055] For example, the iteration processor 102 can be configured to calculate rotation angles in the multi-channel processing,wherein the iteration processor 102 canbe configured to only performa rotation processing in bands, inwhich a rotation angle is higher thanaquantized-to-zero threshold definedbya rotation angle quantizer (e.g., KLTbased rotation encoder).

[0056] Thus, the encoder 100 (or output interface 106) can be configured to transmit the transformation / rotation information either as one parameter for the complete spectrum (full band box) or as multiple frequency dependent parameters for parts of the spectrum. 7 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0057] The encoder 100 can be configured to generate the bit stream 107 based on the following tables: 8 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 9 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 10 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 11 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 12 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 Table 6 - Value of usacExtElementType usacExtElementType Value ID_EXT_ELE_FILL 0 ID_EXT_ELE_MPEGS 1 ID_EXT_ELE_SAOC 2 ID_EXT_ELE_AUDIOPREROLL 3 ID_EXT_ELE_UNI_DRC 4 ID_EXT_ELE_OBJ_METADATA 5 ID_EXT_ELE_SAOC_3D 6 ID_EXT_ELE_HOA 7 ID_EXT_ELE_FMT_CNVRTR 8 ID_EXT_ELE_MCC 9 or 10 / * reserved for ISO use * / 10‑127 / * reserved for use outside of ISO scope * / 128 and higher NOTE: Application-specific usacExtElementType values are mandated to be in the space reserved for use outside of ISO scope. These are skipped by a decoder as a minimum of structure is required by the decoder to skip these extensions. 13 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 Table 7 - Interpretation of data blocks for extension payload decoding usacExtElementType The concatenated usacExtElementSegmentData represents: ID_EXT_ELE_FILL Series of fill_byte ID_EXT_ELE_MPEGS SpatialFrame() ID_EXT_ELE_SAOC SaocFrame() ID_EXT_ELE_AUDIOPREROLL AudioPreRoll() ID_EXT_ELE_UNI_DRC uniDrcGain() as defined in ISO / IEC 23003‑4 ID_EXT_ELE_OBJ_METADATA object_metadata() ID_EXT_ELE_SAOC_3D Saoc3DFrame() ID_EXT_ELE_HOA HOAFrame() ID_EXT_ELE_FMT_CNVRTR FormatConverterFrame() ID_EXT_ELE_MCC MultichannelCodingFrame() unknown unknown data. The data block shall be discarded.

[0058] Fig. 3 shows a schematic block diagram of an iteration processor 102, according to an embodiment. In the embodiment shown in Fig. 3, the multichannel signal 101 is a 5.1 channel signal having six channels: a left channel L, a right channel R, a left surround channel Ls, a right surround channel Rs, a center channel C and a low frequency effects channel LFE.

[0059] As indicated in Fig. 3, the LFE channel is not processed by the iteration processor 102. This might be the case since the inter-channel correlation values between the LFEchannel and eachof the other five channels L,R, Ls,Rs, andC are to small, or since the channel mask indicates not to process the LFE channel, which will be assumed in the following.

[0060] In a first iteration step, the iteration processor 102 calculates the inter-channel correlation values between each pair of the five channels L, R, Ls, Rs, andC, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold. InFig. 3 it is assumed that the left channel L and the right channel R have the highest value, such that the iteration processor 102 processes the left channel L and the right channelRusing a stereo box (or stereo tool) 110, which performs the multi-channel operation processing operation, to derive first and second processed channels P1 and P2.

[0061] In a second iteration step, the iteration processor 102 calculates inter-channel correlation values between each pair of the five channels L, R, Ls, Rs, and C and the processed channels P1 and P2, for selecting, in the second iteration step, a pair havingahighest value or havinga value abovea threshold. InFig. 3 it is assumed that the left surround channel Ls and the right surround channel Rs have the highest value, such that the iteration processor 102 processes the left surround channel Ls and the right surround channel Rs using the stereo box (or stereo tool) 112, to derive third and fourth processed channels P3 and P4.

[0062] In a third iteration step, the iteration processor 102 calculates inter-channel correlation values between eachpair of the five channels L, R, Ls, Rs, andC and the processed channels P1 to P4, for selecting, in the third iteration step, a pair having a highest value or having a value above a threshold. In Fig. 3 it is assumed that the first processed channel P1 and the third processed channel P3have thehighest value, such that the iteration processor 102processes the first processed channel P1and the third processed channel P3using the stereo box (or stereo tool) 114, to derive fifth and sixth processed channels P5 and P6.

[0063] In a fourth iteration step, the iteration processor 102 calculates inter-channel correlation values between each pair of the fivechannels L,R, Ls,Rs, andCand theprocessedchannelsP1 toP6, for selecting, in the fourth iteration step, a pair having a highest value or having a value above a threshold. In Fig. 3 it is assumed that the fifth processed channel P5 and the center channel C have the highest value, such that the iteration processor 102 processes the fifth processed channel P5 and the center channel C using the stereo box (or stereo tool) 115, to derive seventh and eighth processed channels P7 and P8.

[0064] The stereo boxes 110 to 116 can be MS stereo boxes, i.e. mid / side stereophony boxes configured to provide a mid-channelandaside-channel. Themid-channel canbe thesumof the input channelsof thestereobox,wherein theside- channel can be the difference between the input channels of the stereo box. Further, the stereo boxes 110 and 116 can be rotation boxes or stereo prediction boxes.

[0065] In Fig. 3, the first processed channel P1, the third processed channel P3 and the fifth processed channel P5 can be mid-channels, wherein the second processed channel P2, the fourth processed channel P4 and the sixth processed channel P6 can be side-channels. 14 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0066] Further, as indicated in Fig. 3, the iteration processor 102 can be configured to perform the calculating, the selecting and the processing in the second iteration step and, if applicable, in any further iteration step using the input channels L, R, Ls, Rs, and C and (only) the mid-channels P1, P3 and P5 of the processed channels. In other words, the iteration processor 102 can be configured to not use the side-channels P1, P3 and P5 of the processed channels in the calculating, the selecting and the processing in the second iteration step and, if applicable, in any further iteration step.

[0067] Fig. 4 shows a schematic block diagram of an apparatus (decoder) 200 for decoding an encoded multi-channel signal 107 having encoded channels E1 to E3 and at least first and second multi-channel parameters MCH_PAR1 and MCH_PAR2. The apparatus 200 comprises a channel decoder 202 and a multi-channel processor 204.

[0068] The channel decoder 202 is configured to decode the encoded channels E1 to E3 to obtain decoded channels in D1 to D3.

[0069] For example, the channel decoder 202 can comprise at least three mono decoders (or mono boxes, or mono tools) 206_1 to 206_3,wherein eachof themono decoders 206_1 to 206_3 canbe configured to decodeoneof the at least threeencodedchannelsE1 toE3, to obtain the respective decodedchannelE1 toE3. Themonodecoders 206_1 to 206_3 can be, for example, transformation based audio decoders.

[0070] Themulti-channel processor 204 is configured for performing amulti-channel processing using a second pair of thedecodedchannels identifiedby thesecondmulti-channel parametersMCH_PAR2andusing the secondmulti-channel parametersMCH_PAR2 to obtain processed channels, and for performing a furthermulti-channel processing using a first pair of channels identified by the first multi-channel parameters MCH_PAR1 and using the first multi-channel parameters MCH_PAR1, where the first pair of channels comprises at least one processed channel.

[0071] As indicated in Fig. 4 by way of example, the second multi-channel parameters MCH_PAR2 may indicate (or signal) that thesecondpair of decodedchannels consistsof thefirst decodedchannelD1and theseconddecodedchannel D2. Thus, the multi-channel processor 204 performs a multi-channel processing using the second pair of the decoded channels consisting of the first decoded channel D1 and the second decoded channel D2 (identified by the second multi- channel parameters MCH_PAR2) and using the second multi-channel parameters MCH_PAR2, to obtain processed channelsP1* andP2*. The firstmulti-channel parametersMCH_PAR1may indicate that the first pair of decoded channels consists of the first processed channel P1* and the third decoded channel D3. Thus, the multi-channel processor 204 performs the further multi-channel processing using this first pair of decoded channels consisting of the first processed channelP1* and the third decoded channelD3 (identifiedby the firstmulti-channel parametersMCH_PAR1) andusing the first multi-channel parameters MCH_PAR1, to obtain processed channels P3* and P4*.

[0072] Further, themulti-channel processor 204may provide the third processed channel P3* as first channel CH1, the fourth processed channel P4* as third channel CH3 and the second processed channel P2* as second channel CH2.

[0073] Assuming that the decoder 200 shown in Fig. 4 receives the encodedmulti-channel signal 107 from the encoder 100 shown in Fig. 1, the first decoded channel D1 of the decoder 200may be equivalent to the third processed channel P3 of theencoder100,wherein the seconddecodedchannelD2of thedecoder200maybeequivalent to the fourth processed channel P4 of the encoder 100, and wherein the third decoded channel D3 of the decoder 200 may be equivalent to the second processed channel P2 of the encoder 100. Further, the first processed channel P1* of the decoder 200 may be equivalent to the first processed channel P1 of the encoder 100.

[0074] Further, the encoded multi-channel signal 107 can be a serial signal, wherein the second multichannel parameters MCH_PAR2 are received, at the decoder 200, before the first multichannel parameters MCH_PAR1. In that case, the multichannel processor 204 can be configured to process the decoded channels in an order, in which the multichannel parameters MCH_PAR1 and MCH_PAR2 are received by the decoder. In the example shown in Fig. 4, the decoder receives thesecondmultichannel parametersMCH_PAR2before thefirstmultichannel parametersMCH_PAR1, and thus performs themultichannel processing using the second pair of the decoded channels (consisting of the first and seconddecodedchannelsD1andD2) identifiedby thesecondmultichannel parameterMCH_PAR2beforeperforming the multichannel processing using the first pair of the decoded channels (consisting of the first processed channel P1* and the third decoded channel D3) identified by the first multichannel parameter MCH_PAR1.

[0075] In Fig. 4, the multichannel processor 204 exemplarily performs two multi-channel processing operations. For illustration purposes, themulti-channel processing operations performed bymultichannel processor 204 are illustrated in Fig. 4 by processing boxes 208and210. The processing boxes 208and210 canbe implemented in hardware or software. The processing boxes 208 and 210 can be, for example, stereo boxes, as discussed above with reference to the encoder 100, suchasgeneric decoders (or decoder-side stereo boxes), prediction baseddecoders (or decoder-side stereo boxes) or KLT based rotation decoders (or decoder-side stereo boxes).

[0076] For example, the encoder 100canuseKLTbased rotationencoders (or encoder-side stereoboxes). In that case, the encoder 100may derive the first and secondmultichannel parametersMCH_PAR1andMCH_PAR2 such that the first and secondmultichannel parametersMCH_PAR1 andMCH_PAR2 comprise rotation angles. The rotation angles can be differentially encoded. Therefore, themultichannel processor 204 of the decoder 200 can comprise a differential decoder for differentially decoding the differentially encoded rotation angles.

[0077] The apparatus 200 may further comprise an input interface 212 configured to receive and process the encoded 15 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 multi-channel signal 107, to provide the encoded channels E1 to E3 to the channel decoder 202 and the first and second multi-channel parameters MCH_PAR1 and MCH_PAR2 to the multi-channel processor 204.

[0078] As already mentioned, a keep indicator (or keep tree flag) may be used to signal that no new tree is transmitted, but the last stereo tree shall be used. This can be used to avoidmultiple transmission of the same stereo tree configuration if the channel correlation properties stay stationary for a longer time.

[0079] Therefore, when the encoded multi-channel signal 107 comprises, for a first frame, the first or the second multichannel parameters MCH_PAR1 and MCH_PAR2 and, for a second frame, following the first frame, the keep indicator, the multichannel processor 204 can be configured to perform the multichannel processing or the further multichannel processing in the second frame to the same second pair or the same first pair of channels as used in the first frame.

[0080] Themultichannel processing and the furthermultichannel processingmay comprise a stereo processing using a stereo parameter, wherein for individual scale factor bands or groups of scale factor bands of the decoded channels D1 to D3, a first stereo parameter is included in the first multichannel parameter MCH_PAR1 and a second stereo parameter is included in the second multichannel parameter MCH_PAR2. Thereby, the first stereo parameter and the second stereo parameter can be of the same type, such as rotation angles or prediction coefficients. Naturally, the first stereo parameter and the second stereo parameter can be of different types. For example, the first stereo parameter can bea rotation angle, wherein the second stereo parameter can be a prediction coefficient, or vice versa.

[0081] Further, the first or the second multichannel parameters MCH_PAR1 and MCH_PAR2 can comprise a multi- channel processing mask indicating which scale factor bands are multichannel processed and which scale factor bands are not multichannel processed. Thereby, the multichannel processor 204 can be configured to not perform the multi- channel processing in the scale factor bands indicated by the multichannel processing mask.

[0082] Thefirst and the secondmultichannel parametersMCH_PAR1andMCH_PAR2mayeach includeachannel pair identification (or index), wherein the multichannel processor 204 can be configured to decode the channel pair identifications (or indexes) using a predefined decoding rule or a decoding rule indicated in the encoded multi-channel signal.

[0083] For example, channel pairs can be efficiently signaled using a unique index for each pair, dependent on the total number of channels, as described above with reference to the encoder 100.

[0084] Further, the decoding rule can be a Huffman decoding rule, wherein the multichannel processor 204 can be configured to perform a Huffman decoding of the channel pair identifications.

[0085] The encoded multi-channel signal 107 may further comprise a multichannel processing allowance indicator indicating only a sub-group of the decoded channels, for which the multichannel processing is allowed and indicating at least onedecoded channel forwhich themultichannel processing is not allowed. Thereby, themultichannel processor 204 can be configured for not performing any multichannel processing for the at least one decoded channel, for which the multichannel processing is not allowed as indicated by the multichannel processing allowance indicator.

[0086] For example, when the multichannel signal is a 5.1 channel signal, the multichannel processing allowance indicatormay indicate that themultichannel processing is only allowed for the 5 channels, i.e. right R, left L, right surround Rs, left surround LS and center C, wherein the multichannel processing is not allowed for the LFE channel.

[0087] For the decoding process (decoding of channel pair indices) the following c-code may be used. Thereby, for all channel pairs, the number of channels with active KLT processing (nChannels) as well as the number of channel pairs (numPairs) of the current frame is needed. 16 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0088] For decoding the prediction coefficients for non-bandwise angles the following c-code can be used.

[0089] For decoding the prediction coefficients for non-bandwise KLT angles the following c-code can be used. 17 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0090] Toavoid floatingpoint differencesof trigonometric functionsondifferent platforms, the following lookup-tables for converting angle indices directly to sin / cos shall be used: 18 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0091] For decoding of multi-channel coding the following c-code can be used for the KLT rotation based approach.

[0092] For bandwise processing the following c-code can be used. 19 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0093] For an application of KLT rotation the following c-code can be used. 20 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55

[0094] Fig. 5 shows a flowchart of amethod 300 for encoding amulti-channel signal having at least three channels. The method300 comprises a step302of calculating, in a first iteration step, inter-channel correlation valuesbetweeneachpair of the at least three channels, selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and processing the selected pair using a multichannel processing operation to derive first multichannel parameters for the selected pair and to derive first processed channels; a step 304 of performing the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive second multichannel parameters and second processed channels; a step 306 of encoding channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels; and a step 308 of generating an encoded multi-channel signal having the encoded channels and the first and the second multichannel parameters.

[0095] Fig. 6showsaflowchart of amethod400 fordecodinganencodedmulti-channel signal havingencodedchannels and at least first and second multichannel parameters. The method 400 comprises a step 402 of decoding the encoded channels to obtain decoded channels; and a step 404 of performing amultichannel processing using a second pair of the decoded channels identified by the second multichannel parameters and using the second multichannel parameters to obtainprocessedchannels, andperforminga furthermultichannel processingusingafirst pair of channels identifiedby the first multichannel parameters and using the first multichannel parameters, wherein the first pair of channels comprises at least one processed channel.

[0096] In the following, additional embodiments and aspects of the invention will be described which can be used individually or in combination with any of the features and functionalities and details described herein.

[0097] According to a first aspect, an apparatus 100 for encoding a multi-channel signal 101 having at least three channelsCH1:CH3, comprises: an iteration processor 102 for calculating, in a first iteration step, inter-channel correlation values between each pair of the at least three channels CH:CH3, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using a multichannel processing operation 110,112 to derive first multichannel parameters MCH_PAR1 for the selected pair and to derive first processed channels P1,P2, wherein the iteration processor 102 is configured to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels P1 to derive second multichannel parameters MCH_PAR2 and second processed channels P3,P4;a channel encoder for encoding channels P2:P4 resulting from an iteration processing performed by the iteration processor 104 to obtain encoded channels E1:E3; and an output interface 106 for generating an encodedmulti-channel signal 107 having the encoded channels E1:E3 and the first and the second multichannel parameters MCH_PAR1,MCH_PAR2.

[0098] According to a second aspect when referring back to the first aspect, the output interface 106 is configured to generate the encoded multi-channel signal 107 as a serial bitstream and so that the second multichannel parameters MCH_PAR2 are in the encoded signal before the first multichannel parameters MCH_PAR1.

[0099] According to a third aspect when referring back to any one of the first or second aspects, the iteration processor 102 is configured to perform stereo processing comprising at least one of a group including rotation processing using a rotation angle calculation from the selected pair and prediction processing.

[0100] According to a fourth aspect when referring back to any one of the first to third aspects, the first multichannel parametersMCH_PAR1 comprise a first identification of the channel in the selected pair for the first iteration step, and the 21 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 second multichannel parameters MCH_PAR2 comprise a second identification of the channels in a selected pair of the second iteration step.

[0101] According to a fifth aspectwhen referring back to any oneof the first to fourth aspects, the iteration processor 102 is configured to calculate an inter-channel correlation using a frameof each channel comprising aplurality of bands so that a single inter-channel correlation value for the plurality of bands is obtained, and the iteration processor 104 is configured to perform the multichannel processing for each of the plurality of bands so that the first or the second multichannel parameters MCH_PAR1,MCH_PAR2 are obtained for each of the plurality of bands.

[0102] According toasixthaspectwhen referringback toanyoneof thefirst to fifthaspects, the iterationprocessor102 is configured to derive, for a first frame, a plurality of selected pair indications, and the output interface 106 is configured to include, into themulti-channel signal 107, for a second frame, following the first frame, a keep indicator, indicating that the second frame has the same plurality of selected pair indications as the first frame.

[0103] According to a seventh aspect when referring back to any one of the first to sixth aspects, the iteration processor 102 is configured to only select a pair when the level difference of the pair is smaller than a threshold, the threshold being smaller than 40 dB, or 25 dB, or 12 dB, or smaller than 6 dB.

[0104] According to an eighth aspect when referring back to any one of the first to seventh aspects, the iteration processor 102 is configured to calculate normalized correlation values, and the iteration processor 102 is configured to select a pair, when the correlation value is greater than 0.2 and preferably 0.3.

[0105] According to a ninth aspect when referring back to any one of the first to eighth aspects, the iteration processor 102 is configured to calculate stereo parameters in the multichannel processing, and the iteration processor 102 is configured to only perform a stereo processing in bands, in which a stereo parameter is higher than a quantized-to-zero- threshold defined by a stereo parameter quantizer.

[0106] According to a tenth aspectwhen referringback toanyoneof the first to ninthaspects, the iterationprocessor 102 is configured to calculate rotation angles in the multichannel processing, and the iteration processor 102 is configured to only perform rotation processing in bands, in which a rotation angle is higher than a decoder-side dequantized-to-zero- threshold.

[0107] According to an eleventh aspect when referring back to any one of the first to tenth aspects, the iteration processor 102 is configured to not select the selected pair of the first iteration step in the second iteration step and, if applicable, in any further iteration steps.

[0108] According to a twelfth aspect when referring back to any one of the first to eleventh aspects, the iteration processor 102 is configured to perform iteration steps until an iteration termination criterion is reached, wherein the iteration termination criterion is that a maximum number of iteration steps is equal to or higher than a total number of channels CH1:CH3 of themulti-channel signal 101 by two, or wherein the iteration termination criterion is, when the inter- channel correlation values do not have a value greater than the threshold.

[0109] According to a thirteenth aspect when referring back to any one of the first to twelfth aspects, the iteration processor 102 is configured to process, in the first iteration step, the selected pair using themultichannel processing such that the processed channels P1,P2 are a mid-channel P1 and a side-channel P2; and the iteration processor 102 is configured to perform the calculating, the selecting and the processing in the second iteration step using only the mid- channel P1 of the processed channels P1,P2 as the at least one of the processed channels P1,P2 to derive the second multichannel parameters MCH_PAR2 and second processed channels P3,P4.

[0110] According to a fourteenth aspect when referring back to any one of the first to thirteenth aspects, the channel encoder comprises channel encoders 120_1:120_3 for encoding the channels P2:P4 resulting from the iteration processing, wherein the channel encoders are configured to encode the channels P2:P4 so that less bits are used for encoding a channel having less energy than for encoding a channel having more energy.

[0111] According to a fifteenth aspect, an apparatus 200 for decoding an encoded multi-channel signal 107 having encoded channels E1:E3 and at least first and second multichannel parameters MCH_PAR1,MCH_PAR2 comprises: a channel decoder 202 for decoding the encoded channels E1:E3 to obtain decoded channels D1:D3; and a multichannel processor 204 for performing amultichannel processing using a second pair of the decoded channels D1:D3 identified by the second multichannel parameters MCH_PAR2 and using the second multichannel parameters MCH_PAR2 to obtain processed channels P1*,P2*, and for performing a further multichannel processing using a first pair of channels D1:D3,P1*,P2* identified by the first multichannel parameters MCH_PAR1 and using the first multichannel parameters MCH_PAR1, wherein the first pair of channels comprises at least one processed channel P1*,P2*.

[0112] According to a sixteenth aspectwhen referring back to the fifteenth aspect, the encodedmulti-channel signal 107 comprises, for a first frame, the first and the secondmultichannel parametersMCH_PAR1,MCH_PAR2 and, for a second frame, following the first frame, a keep indicator, and the multichannel processor 204 is configured to perform the multichannel processing and the further multichannel processing in the second frame to the same second pair and the same first pair of channels as used in the first frame.

[0113] According to a seventeenth aspect when referring back to any one of the fifteenth to sixteenth aspects, the multichannel processing and the furthermultichannel processing comprise a stereo processing using a stereo parameter, 22 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 wherein for individual scale factor bands or groups of scale factor bands of the decoded channels D1:D3, a first stereo parameter is included in the first multichannel parameter MCH_PAR1 and a second stereo parameter is included in the second multichannel parameter MCH_PAR2.

[0114] According to aneighteenth aspectwhen referring back to any oneof the fifteenth to seventeenth aspects, the first or the second multichannel parameters MCH_PAR1,MCH_PAR2 comprise a multichannel processing mask indicating which scale factor bands are multichannel processed and which scale factor bands are not multichannel processed, and the multichannel processor 204 is configured to not perform the multichannel processing in the scale factor bands indicated by the multichannel processing mask.

[0115] According to a nineteenth aspect when referring back to any one of the fifteenth to eighteenth aspects, the first and the second multichannel parameters MCH_PAR1,MCH_PAR2 each include a channel pair identification, and the multichannel processor 204 is configured to decode the channel pair identifications using a predefined decoding rule or a decoding rule indicated in the encoded multi-channel signal.

[0116] According to a twentieth aspect when referring back to the nineteenth aspect, the decoding rule is a Huffman decoding rule and the multichannel processor 204 is configured to perform a Huffman decoding of the channel pair identifications.

[0117] According to a twenty-first aspect when referring back to any one of the fifteenth to twentieth aspects, the encodedmulti-channel signal 107comprisesamultichannel processing allowance indicator indicatingonly asub-groupof the decoded channels, for which the multichannel processing is allowed and indicating at least one decoded channel for which themultichannel processing is not allowed, and themultichannel processor 204 is configured for not performingany multichannel processing for the at least one decoded channel, for which the multichannel processing is not allowed as indicated by the multichannel processing allowance indicator.

[0118] According to a twenty-second aspect when referring back to any one of the fifteenth to twenty-first aspects, the first and second multichannel parameters MCH_PAR1,MCH_PAR2 comprise stereo parameters, and the stereo para- meters are differentially encoded, and the multichannel processor 204 comprises a differential decoder for differentially decoding the differentially encoded stereo parameters.

[0119] According to a twenty-third aspect when referring back to any one of the fifteenth to twenty-second aspects, the encoded multi-channel signal 107 is a serial signal, wherein the second multichannel parameters MCH_PAR2 are received, at the decoder 200, before the firstmultichannel parametersMCH_PAR1, and themultichannel processor 204 is configured to process the decoded channels D1:D3 in an order, in which the multichannel parameters MCH_PAR1,MCH_PAR2 are received by the decoder 200.

[0120] According to a twenty-fourth aspect, a method 300 for encoding a multi-channel signal having at least three channels comprises: calculating 302, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and processing the selected pair using amultichannel processing operation to derive firstmultichannel parameters for the selected pair and to derive first processed channels, performing 304 the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive secondmultichannel parameters and second processedchannels; encoding306channels resulting froman iterationprocessingperformedby the iterationprocessor to obtain encoded channels; and generating 308an encodedmulti-channel signal having the encoded channels and the first and the second multichannel parameters.

[0121] According to a twenty-fifth aspect, a method 400 of decoding an encoded multi-channel signal having encoded channels andat least first and secondmultichannel parameters, comprises: decoding 402 theencodedchannels to obtain decoded channels; and performing 404 amultichannel processing using a second pair of the decoded channels identified by thesecondmultichannel parametersandusing thesecondmultichannel parameters toobtainprocessedchannels, and performing a further multichannel processing using a first pair of channels identified by the first multichannel parameters and using the first multichannel parameters, wherein the first pair of channels comprises at least one processed channel.

[0122] A twenty-sixth aspect relates to a computer program for performing, when running on a computer or processor, themethod of encoding themulti-channel signal of the twenty-fourth aspect or themethod of decoding an encodedmulti- channel signal of the twenty-fifth aspect.

[0123] A twenty-seventh aspect relates to an apparatus, method or computer program of any of the first to twenty-sixth aspects, wherein multichannel processing means a joint stereo processing or a joint processing of more than two channels, and wherein a multichannel signal has two channels or more than two channels.

[0124] Although the present invention has been described in the context of block diagrams where the blocks represent actual or logical hardware components, the present invention can also be implemented by a computer-implemented method. In the latter case, theblocks represent correspondingmethodstepswhere thesestepsstand for the functionalities performed by corresponding logical or physical hardware blocks.

[0125] Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the correspondingmethod,where ablock or device corresponds to amethod step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a 23 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 corresponding block or itemor feature of a corresponding apparatus. Some or all of themethod stepsmay be executed by (or using) a hardware apparatus, like for example, amicroprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus.

[0126] The inventive transmitted or encoded signal can be stored on adigital storagemediumor can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet.

[0127] Depending on certain implementation requirements, embodiments of the invention can be implemented in hardwareor in software.The implementation canbeperformedusingadigital storagemedium, for exampleafloppydisc, a DVD, a Blu-Ray, a CD, a ROM, a PROM, and EPROM, an EEPROM or a FLASHmemory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable.

[0128] Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperatingwith a programmable computer system, such that one of themethods described herein is performed.

[0129] Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of themethods when the computer program product runs on a computer. The program code may, for example, be stored on a machine readable carrier.

[0130] Other embodiments comprise the computer program for performing oneof themethods described herein, stored on a machine readable carrier.

[0131] Inotherwords, anembodiment of the inventivemethod is, therefore, a computer programhavingaprogramcode for performing one of the methods described herein, when the computer program runs on a computer.

[0132] A further embodiment of the inventive method is, therefore, a data carrier (or a non-transitory storage medium such as a digital storagemedium, or a computer-readablemedium) comprising, recorded thereon, the computer program for performing one of themethods described herein. The data carrier, the digital storagemedium or the recordedmedium are typically tangible and / or non-transitory.

[0133] A further embodiment of the inventionmethod is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may, for example, be configured to be transferred via a data communication connection, for example, via the internet.

[0134] A further embodiment comprises a processingmeans, for example, a computer or a programmable logic device, configured to, or adapted to, perform one of the methods described herein.

[0135] A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.

[0136] A further embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of themethods described herein to a receiver. The receivermay, for example, beacomputer, amobile device, amemorydevice or the like. Theapparatusor systemmay, for example, comprise a file server for transferring the computer program to the receiver .

[0137] In someembodiments, aprogrammable logicdevice (for example, a fieldprogrammablegatearray)maybeused to perform some or all of the functionalities of themethods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus.

[0138] The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein. Claims 1. Apparatus (100) for encoding a multi-channel signal (101) having at least three channels (CH1:CH3), comprising: an iteration processor (102) for calculating, in a first iteration step, inter-channel correlation values between each pair of theat least three channels (CH:CH3), for selecting, in the first iteration step, a pair havingahighest valueor having a value abovea threshold, and for processing the selected pair using amultichannel processing operation (110,112) to derive first multichannel parameters (MCH_PAR1) for the selected pair and to derive a first pair of processed channels (P1,P2), wherein the iterationprocessor (102) is configured toperform thecalculating, theselectingand theprocessing ina second iteration step using unprocessed channels of the at least three channels (CH1:CH3) and at least one of the processed channels (P1,P2) to derive second multichannel parameters (MCH_PAR2) for the selected pair and a second pair of processed channels (P3,P4), wherein the iteration processor (102) is configured to provide 24 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 channels resulting from the iteration processing and channels of the at least three channels (CH1:CH3) not processed by the iteration processor to the channel encoder; a channel encoder for encoding channels (P2:P4) provided by by the iteration processor (104) to obtain encoded channels (E1:E3); and an output interface (106) for generating an encoded multi-channel signal (107) having the encoded channels (E1:E3) and the first and the second multichannel parameters (MCH_PAR1,MCH_PAR2), wherein the first multichannel parameters (MCH_PAR1) comprise a first identification of the channel in the selected pair for the first iteration step, andwherein the secondmultichannel parameters (MCH_PAR2) comprise a second identification of the channels in a selected pair of the second iteration step. 2. Apparatus (100) of claim 1, wherein the output interface (106) is configured to generate the encoded multi-channel signal (107) as a serial bitstreamandso that the secondmultichannel parameters (MCH_PAR2)are in theencodedsignal before the first multichannel parameters (MCH_PAR1), and / or wherein the iteration processor (102) is configured to perform stereo processing comprising at least one of a group including rotation processing using a rotation angle calculation from the selected pair and prediction processing. 3. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to calculate an inter-channel correlation using a frameof each channel comprising a plurality of bands so that a single inter-channel correlation value for the plurality of bands is obtained, and wherein the iteration processor (104) is configured to perform themultichannel processing for eachof theplurality of bands so that the first or the secondmultichannel parameters (MCH_PAR1,MCH_PAR2) are obtained for each of the plurality of bands. 4. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to derive, for a first frame, a plurality of selected pair indications, andwherein theoutput interface (106) is configured to include, into themulti-channel signal (107), for a second frame, following the first frame, a keep indicator, indicating that the second frame has the same plurality of selected pair indications as the first frame. 5. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to only select a pair when the level difference of the pair is smaller than a threshold, the threshold being smaller than 40 dB, or 25 dB, or 12 dB, or smaller than 6 dB, and / or wherein the iteration processor (102) is configured to calculate normalized correlation values, and wherein the iteration processor (102) is configured to select a pair, when the correlation value is greater than 0.2 and preferably 0.3. 6. Apparatus (100) of one of the preceding claims, wherein the iterationprocessor (102) is configured to calculate stereoparameters in themultichannel processing, and wherein the iteration processor (102) is configured to only perform a stereo processing in bands, in which a stereo parameter is higher than a quantized-to-zero-threshold defined by a stereo parameter quantizer, and / or wherein the iteration processor (102) is configured to calculate rotation angles in the multichannel processing, and wherein the iteration processor (102) is configured to only perform rotation processing in bands, in which a rotation angle is higher than a decoder-side dequantized-to-zero-threshold. 7. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to not select the selected pair of the first iteration step in the second iteration step and, if applicable, in any further iteration steps, 25 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 and / or wherein the iteration processor (102) is configured to perform iteration stepsuntil an iteration termination criterion is reached, wherein the iteration termination criterion is that a maximum number of iteration steps is equal to or higher than a total number of channels (CH1:CH3) of the multi-channel signal (101) by two, or wherein the iteration termination criterion is, when the inter-channel correlation values do not have a value greater than the threshold. 8. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to process, in the first iteration step, the selected pair using the multichannel processing such that the processed channels (P1,P2) are a mid-channel (P1) and a side-channel (P2), wherein the iteration processor (102) is configured to perform the calculating, the selecting and the processing in the second iteration step using only the mid-channel (P1) of the processed channels (P1,P2) as the at least one of the processed channels (P1,P2) to derive the second multichannel parameters (MCH_PAR2) and second processed channels (P3,P4), and / or wherein the channel encoder comprises channel encoders (120_1:120_3) for encoding the channels (P2:P4) resulting from the iteration processing, wherein the channel encoders are configured to encode the channels (P2:P4) so that less bits are used for encoding a channel having less energy than for encoding a channel having more energy. 9. Apparatus (200) for decoding an encoded multi-channel signal (107) having encoded channels (E1:E3) and at least first and second multichannel parameters (MCH_PAR1,MCH_PAR2), comprising: a channel decoder (202) for decoding the encoded channels (E1:E3) to obtain decoded channels (D1:D3); and a multichannel processor (204) for performing a multichannel processing using a second pair of the decoded channels (D1:D3) identified by the second multichannel parameters (MCH_PAR2) and using the second multichannel parameters (MCH_PAR2) to obtain a pair of processed channels (P1*,P2*), and for performing a furthermultichannel processingusingafirst pair of channels (D1:D3,P1*,P2*) identifiedby the firstmultichannel parameters (MCH_PAR1) and using the first multichannel parameters (MCH_PAR1), wherein the first pair of channels comprises at least one processed channel (P1*,P2*), wherein the first multi-channel parameters comprise a first channel pair identification identifying the pair of channels selected in the first iteration step, wherein the second multi-channel parameters comprise a second channel pair identification identifying the pair of channels selected in the second iteration step. 10. Apparatus (200) of claim 9, wherein the encoded multi-channel signal (107) comprises, for a first frame, the first and the second multichannel parameters (MCH_PAR1, MCH_PAR2) and, for a second frame, following the first frame, a keep indicator,wherein themultichannelprocessor (204) is configured toperform themultichannelprocessingand the further multichannel processing in the second frame to the same second pair and the same first pair of channels as used in the first frame, and / or wherein the multichannel processing and the further multichannel processing comprise a stereo processing using a stereo parameter, wherein for individual scale factor bands or groups of scale factor bands of the decoded channels (D1:D3), a first stereo parameter is included in the first multichannel parameter (MCH_PAR1) and a second stereo parameter is included in the second multichannel parameter (MCH_PAR2. 11. Apparatus (200) of one of claims 9 to 10, wherein the first or the second multichannel parameters (MCH_PAR1,MCH_PAR2) comprise a multichannel processing mask indicating which scale factor bands are multichannel processed and which scale factor bands are not multichannel processed, wherein the multichannel processor (204) is configured to not perform the multichannel processing in the scale factor bands indicated by the multichannel processing mask, and / or wherein the first and the secondmultichannel parameters (MCH_PAR1,MCH_PAR2)each includeachannel pair identification, and wherein the multichannel processor (204) is configured to decode the channel pair identifica- tions using a predefined decoding rule or a decoding rule indicated in the encoded multi-channel signal. 12. Apparatus (200) of one of claims 9 to 11, 26 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 wherein the encoded multi-channel signal (107) comprises a multichannel processing allowance indicator indicating only a sub-group of the decoded channels, for which the multichannel processing is allowed and indicating at least one decoded channel for which the multichannel processing is not allowed, wherein the multichannel processor (204) is configured for not performing any multichannel processing for the at least one decoded channel, for which the multichannel processing is not allowed as indicated by the multichannel processing allowance indicator, and / or wherein the first and second multichannel parameters (MCH_PAR1,MCH_PAR2) comprise stereo parameters, and wherein the stereo parameters are differentially encoded, and wherein the multichannel processor (204) comprises a differential decoder for differentially decoding the differentially encoded stereo parameters, and / or wherein the encoded multi-channel signal (107) is a serial signal, wherein the second multichannel parameters (MCH_PAR2)are received, at thedecoder (200), before thefirstmultichannel parameters (MCH_PAR1),wherein themultichannel processor (204) is configured to process the decoded channels (D1:D3) in an order, inwhich the multichannel parameters (MCH_PAR1,MCH_PAR2) are received by the decoder (200). 13. Method (300) for encoding a multi-channel signal having at least three channels, comprising: Calculating (302), in a first iteration step, inter-channel correlation values between each pair of the at least three channels, selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and processing the selected pair using a multichannel processing operation to derive first multichannel para- meters for the selected pair and to derive a first pair of processed channels, Performing (304) the calculating, the selecting and the processing in a second iteration step using unprocessed channels of the at least three channels (CH1:CH3) and at least one of the processed channels to derive second multichannel parameters and a second pair of processed channels; Encoding (306) channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels; and generating (308) an encoded multi-channel signal having the encoded channels and the first and the second multichannel parameters, wherein the first multichannel parameters comprise a first identification of the channel in the selected pair for the first iteration step, and wherein the second multichannel parameters (comprise a second identification of the channels in a selected pair of the second iteration step. 14. Method (400) of decoding an encoded multi-channel signal having encoded channels and at least first and second multichannel parameters, comprising: decoding (402) the encoded channels to obtain decoded channels; and performing (404) amultichannel processingusing a secondpair of thedecodedchannels identifiedby the second multichannel parameters and using the second multichannel parameters to obtain processed channels, and performing a further multichannel processing using a first pair of channels identified by the first multichannel parameters and using the first multichannel parameters, wherein the first pair of channels comprises at least one processed channel, wherein the first multi-channel parameters comprise a first channel pair identification identifying the pair of channels selected in the first iteration step, wherein the second multi-channel parameters comprise a second channel pair identification identifying the pair of channels selected in the second iteration step. 15. Computer program for performing, when running on a computer or processor, the method of encoding the multi- channel signal of claim 24 or the method of decoding an encoded multi-channel signal of claim 25. 27 EP 4 675 616 A2 5 10 15 20 25 30 35 40 45 50 55 28 EP 4 675 616 A2 29 EP 4 675 616 A2 30 EP 4 675 616 A2 31 EP 4 675 616 A2 32 EP 4 675 616 A2 33 EP 4 675 616 A2 34 EP 4 675 616 A2 REFERENCES CITED IN THE DESCRIPTION This list of references cited by the applicant is for the reader’s convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard. Non-patent literature cited in the description • YANG, DAI ;AI, HONGMEI ;KYRIAKAKIS, CHRIS ; KUO, C.‑C. Adaptive Karhunen-Loeve Transform for Enhanced Multichannel Audio Coding, January 2001, http: / / ict.usc.edu / pubs / Adaptive%20Karhu- nen-Loeve%20Transform%20for%20Enhanced % 20Multichannel%20Audio%20Coding.pdf

[0002] (19) *EP004675616A3* (11) EP 4 675 616 A3 (12) EUROPEAN PATENT APPLICATION (88) Date of publication A3: 04.03.2026 Bulletin 2026 / 10 (43) Date of publication A2: 07.01.2026 Bulletin 2026 / 02 (21) Application number: 25211436.8 (22) Date of filing: 08.03.2016 (51) International Patent Classification (IPC): G10L 19 / 008 (2013.01) (52) Cooperative Patent Classification (CPC): G10L 19 / 008 (84) Designated Contracting States: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR (30) Priority: 09.03.2015 EP 15158234 17.06.2015 EP 15172492 (62) Document number(s) of the earlier application(s) in accordance with Art. 76 EPC: 19157636.2 / 3 506 259 16709344.2 / 3 268 959 (71) Applicant: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. 80686 München (DE) (72) Inventors: • DICK, Sascha 91058 Erlangen (DE) • SCHUH, Florian 91058 Erlangen (DE) • RETTELBACH, Nikolaus 91058 Erlangen (DE) • SCHWEGLER, Tobias 90419 Nürnberg (DE) • FÜG, Richard 91058 Erlangen (DE) • HILPERT, Johannes 91058 Erlangen (DE) • NEUSINGER, Matthias 91189 Rohr (DE) (74) Representative: Schlenker, Julian et al Schoppe, Zimmermann, Stöckeler Zinkler, Schenk & Partner mbB Patentanwälte Radlkoferstraße 2 81373 München (DE) (54) APPARATUS AND METHOD FOR ENCODING OR DECODING A MULTI‑CHANNEL SIGNAL (57) Embodiments provide an apparatus for encod- ing amulti-channel signal having at least three channels. The apparatus comprises an iteration processor, a chan- nel encoder and an output interface. The iteration pro- cessor is configured to calculate, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using amulti-channel processing operation to derive first multi-channel parameters for the selected pair and to derive first processed channels. Further, the iteration processor is configured to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive second multi-channel parameters and second pro- cessed channels. The channel encoder is configured toencodechannels resulting froman iterationprocessing performed by the iteration processor to obtain encoded channels. The output interface is configured to generate an encoded multi-channel signal having the encoded channels and the first and the second multi-channel parameters. EP 4 67 5 61 6 A 3 Processed by Luminess, 75001 PARIS (FR) 2 EP 4 675 616 A3 5 10 15 20 25 30 35 40 45 50 55 3 EP 4 675 616 A3 5 10 15 20 25 30 35 40 45 50 55 用於編碼或解碼多通道訊號的裝置和方法 摘要 本發明提供一種用於編碼至少具有三個通道的多通道訊號的裝置。該裝置包括迭代處 理器、通道編碼器和輸出介面。迭代處理器被配置為在第一迭代步驟中計算至少三個 通道中每對通道之間的通道間相關值，以便在第一迭代步驟中選擇具有最高相關值或 相關值高於閾值的通道對，並使用多通道處理操作處理所選通道對，從而導出所選通 道對的第一多通道參數和第一處理通道。此外，迭代處理器也被配置為在第二迭代步 驟中使用至少一個已處理通道執行計算、選擇和處理操作，從而導出第二多通道參數 和第二處理通道。通道編碼器被配置為對迭代處理器執行迭代處理後所得到的通道進 行編碼，以獲得編碼後的通道。輸出介面配置為產生編碼多通道訊號，該訊號具有編 碼通道以及第一和第二多通道參數。 摘要

Claims

1. Apparatus (100) for encoding a multi-channel signal (101) having at least three channels (CH1:CH3), comprising: an iteration processor (102) for calculating, in a first iteration step, inter-channel correlation values between each pair of the at least three channels (CH:CH3), for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using a multichannel processing operation (110,112) to derive first multichannel parameters (MCH_PAR1) for the selected pair and to derive a first pair of processed channels (P1,P2), wherein the iteration processor (102) is configured to perform the calculating, the selecting and the processing in a second iteration step using unprocessed channels of the at least three channels (CH1:CH3) and at least one of the processed channels (P1,P2) to derive second multichannel parameters (MCH_PAR2) for the selected pair and a second pair of processed channels (P3,P4), wherein the iteration processor (102) is configured to provide channels resulting from the iteration processing and channels of the at least three channels (CH1:CH3) not processed by the iteration processor to the channel encoder; a channel encoder for encoding channels (P2:P4) provided by by the iteration processor (104) to obtain encoded channels (E1:E3); and an output interface (106) for generating an encoded multi-channel signal (107) having the encoded channels (E1:E3) and the first and the second multichannel parameters (MCH_PAR1,MCH_PAR2), wherein the first multichannel parameters (MCH_PAR1) comprise a first identification of the channel in the selected pair for the first iteration step, and wherein the second multichannel parameters (MCH_PAR2) comprise a second identification of the channels in a selected pair of the second iteration step.

2. Apparatus (100) of claim 1, wherein the output interface (106) is configured to generate the encoded multi-channel signal (107) as a serial bitstream and so that the second multichannel parameters (MCH_PAR2) are in the encoded signal before the first multichannel parameters (MCH_PAR1), and / or wherein the iteration processor (102) is configured to perform stereo processing comprising at least one of a group including rotation processing using a rotation angle calculation from the selected pair and prediction processing.

3. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to calculate an inter-channel correlation using a frame of each channel comprising a plurality of bands so that a single inter-channel correlation value for the plurality of bands is obtained, and wherein the iteration processor (104) is configured to perform the multichannel processing for each of the plurality of bands so that the first or the second multichannel parameters (MCH_PAR1,MCH_PAR2) are obtained for each of the plurality of bands.

4. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to derive, for a first frame, a plurality of selected pair indications, and wherein the output interface (106) is configured to include, into the multi-channel signal (107), for a second frame, following the first frame, a keep indicator, indicating that the second frame has the same plurality of selected pair indications as the first frame.

5. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to only select a pair when the level difference of the pair is smaller than a threshold, the threshold being smaller than 40 dB, or 25 dB, or 12 dB, or smaller than 6 dB, and / or wherein the iteration processor (102) is configured to calculate normalized correlation values, and wherein the iteration processor (102) is configured to select a pair, when the correlation value is greater than 0.2 and preferably 0.3.

6. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to calculate stereo parameters in the multichannel processing, and wherein the iteration processor (102) is configured to only perform a stereo processing in bands, in which a stereo parameter is higher than a quantized-to-zero-threshold defined by a stereo parameter quantizer, and / or wherein the iteration processor (102) is configured to calculate rotation angles in the multichannel processing, and wherein the iteration processor (102) is configured to only perform rotation processing in bands, in which a rotation angle is higher than a decoder-side dequantized-to-zero-threshold.

7. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to not select the selected pair of the first iteration step in the second iteration step and, if applicable, in any further iteration steps, and / or wherein the iteration processor (102) is configured to perform iteration steps until an iteration termination criterion is reached, wherein the iteration termination criterion is that a maximum number of iteration steps is equal to or higher than a total number of channels (CH1:CH3) of the multi-channel signal (101) by two, or wherein the iteration termination criterion is, when the inter-channel correlation values do not have a value greater than the threshold.

8. Apparatus (100) of one of the preceding claims, wherein the iteration processor (102) is configured to process, in the first iteration step, the selected pair using the multichannel processing such that the processed channels (P1,P2) are a mid-channel (P1) and a side-channel (P2), wherein the iteration processor (102) is configured to perform the calculating, the selecting and the processing in the second iteration step using only the mid-channel (P1) of the processed channels (P1,P2) as the at least one of the processed channels (P1,P2) to derive the second multichannel parameters (MCH_PAR2) and second processed channels (P3,P4), and / or wherein the channel encoder comprises channel encoders (120_1:120_3) for encoding the channels (P2:P4) resulting from the iteration processing, wherein the channel encoders are configured to encode the channels (P2:P4) so that less bits are used for encoding a channel having less energy than for encoding a channel having more energy.

9. Apparatus (200) for decoding an encoded multi-channel signal (107) having encoded channels (E1:E3) and at least first and second multichannel parameters (MCH_PAR1,MCH_PAR2), comprising: a channel decoder (202) for decoding the encoded channels (E1:E3) to obtain decoded channels (D1:D3); and a multichannel processor (204) for performing a multichannel processing using a second pair of the decoded channels (D1:D3) identified by the second multichannel parameters (MCH_PAR2) and using the second multichannel parameters (MCH_PAR2) to obtain a pair of processed channels (P1*,P2*), and for performing a further multichannel processing using a first pair of channels (D1:D3,P1*,P2*) identified by the first multichannel parameters (MCH_PAR1) and using the first multichannel parameters (MCH_PAR1), wherein the first pair of channels comprises at least one processed channel (P1*,P2*), wherein the first multi-channel parameters comprise a first channel pair identification identifying the pair of channels selected in the first iteration step, wherein the second multi-channel parameters comprise a second channel pair identification identifying the pair of channels selected in the second iteration step.

10. Apparatus (200) of claim 9, wherein the encoded multi-channel signal (107) comprises, for a first frame, the first and the second multichannel parameters (MCH_PAR1, MCH_PAR2) and, for a second frame, following the first frame, a keep indicator, wherein the multichannel processor (204) is configured to perform the multichannel processing and the further multichannel processing in the second frame to the same second pair and the same first pair of channels as used in the first frame, and / or wherein the multichannel processing and the further multichannel processing comprise a stereo processing using a stereo parameter, wherein for individual scale factor bands or groups of scale factor bands of the decoded channels (D1:D3), a first stereo parameter is included in the first multichannel parameter (MCH_PAR1) and a second stereo parameter is included in the second multichannel parameter (MCH_PAR2.

11. Apparatus (200) of one of claims 9 to 10, wherein the first or the second multichannel parameters (MCH_PAR1,MCH_PAR2) comprise a multichannel processing mask indicating which scale factor bands are multichannel processed and which scale factor bands are not multichannel processed, wherein the multichannel processor (204) is configured to not perform the multichannel processing in the scale factor bands indicated by the multichannel processing mask, and / or wherein the first and the second multichannel parameters (MCH_PAR1,MCH_PAR2) each include a channel pair identification, and wherein the multichannel processor (204) is configured to decode the channel pair identifications using a predefined decoding rule or a decoding rule indicated in the encoded multi-channel signal.

12. Apparatus (200) of one of claims 9 to 11, wherein the encoded multi-channel signal (107) comprises a multichannel processing allowance indicator indicating only a sub-group of the decoded channels, for which the multichannel processing is allowed and indicating at least one decoded channel for which the multichannel processing is not allowed, wherein the multichannel processor (204) is configured for not performing any multichannel processing for the at least one decoded channel, for which the multichannel processing is not allowed as indicated by the multichannel processing allowance indicator, and / or wherein the first and second multichannel parameters (MCH_PAR1,MCH_PAR2) comprise stereo parameters, and wherein the stereo parameters are differentially encoded, and wherein the multichannel processor (204) comprises a differential decoder for differentially decoding the differentially encoded stereo parameters, and / or wherein the encoded multi-channel signal (107) is a serial signal, wherein the second multichannel parameters (MCH_PAR2) are received, at the decoder (200), before the first multichannel parameters (MCH_PAR1), wherein the multichannel processor (204) is configured to process the decoded channels (D1:D3) in an order, in which the multichannel parameters (MCH_PAR1,MCH_PAR2) are received by the decoder (200).

13. Method (300) for encoding a multi-channel signal having at least three channels, comprising: Calculating (302), in a first iteration step, inter-channel correlation values between each pair of the at least three channels, selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and processing the selected pair using a multichannel processing operation to derive first multichannel parameters for the selected pair and to derive a first pair of processed channels, Performing (304) the calculating, the selecting and the processing in a second iteration step using unprocessed channels of the at least three channels (CH1:CH3) and at least one of the processed channels to derive second multichannel parameters and a second pair of processed channels; Encoding (306) channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels; and generating (308) an encoded multi-channel signal having the encoded channels and the first and the second multichannel parameters, wherein the first multichannel parameters comprise a first identification of the channel in the selected pair for the first iteration step, and wherein the second multichannel parameters (comprise a second identification of the channels in a selected pair of the second iteration step.

14. Method (400) of decoding an encoded multi-channel signal having encoded channels and at least first and second multichannel parameters, comprising: decoding (402) the encoded channels to obtain decoded channels; and performing (404) a multichannel processing using a second pair of the decoded channels identified by the second multichannel parameters and using the second multichannel parameters to obtain processed channels, and performing a further multichannel processing using a first pair of channels identified by the first multichannel parameters and using the first multichannel parameters, wherein the first pair of channels comprises at least one processed channel, wherein the first multi-channel parameters comprise a first channel pair identification identifying the pair of channels selected in the first iteration step, wherein the second multi-channel parameters comprise a second channel pair identification identifying the pair of channels selected in the second iteration step.

15. Computer program for performing, when running on a computer or processor, the method of encoding the multi-channel signal of claim 24 or the method of decoding an encoded multi-channel signal of claim 25.

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