Encoder with an inter-channel phase difference calculator device and method for operating such an encoder

Abstract

The present invention provides an encoder for generating an audio bitstream from a stereo audio signal, the encoder comprising: a downmixer configured to downmix the stereo audio signal to generate a mono audio signal; an inter-channel phase difference calculator device configured to calculate an inter-channel phase difference for each time segment of a plurality of consecutive time segments of the stereo audio signal; a bitstream generator configured to generate an audio bitstream such that the mono audio signal and inter-channel phase differences for a plurality of consecutive time segments are embedded in the audio bitstream; Equipped with The inter-channel phase difference calculator device comprises an inter-channel phase difference calculator configured to calculate an inter-channel phase difference according to a global inter-channel phase difference for the current time segment and according to an average band-specific inter-channel phase difference change for the current time segment.

Description

[Background technology]

[0001] The inter-channel phase difference (IPD) describes how two channels are aligned in terms of the phase of their respective signals. The inter-channel phase difference ranges from 0 (perfectly phase-aligned) to + / -π (perfectly out of phase). The more out of phase two channels are, the more problems can arise when creating a single downmix channel from them, as such phase shifts can have severe cancellation effects that significantly reduce the energy in the downmix. Therefore, to avoid these effects, it is desirable to estimate and compensate for the inter-channel phase difference of strongly out-of-phase signals. In audio coders that use parametric stereo techniques, i.e., transmit only one downmix and side information used to upmix the downmix back to a stereo representation, the inter-channel phase differences are usually part of the side information. They are estimated in the encoder (either wideband or in multiple smaller frequency bands) and then compensated to align the channels for a better downmix. In the decoder, they are finally reapplied as part of the upmix to restore the original phase shift between the channels.

[0002] However, for inter-channel phase difference compensation to have a positive effect on the final audio quality, the stability of the inter-channel phase difference used plays an important role: if the inter-channel phase difference varies significantly over time, this is likely to also lead to audible spatial variations in the output, which will be perceived negatively by the listener.

[0003] Stereo coding, which relies on a single downmix channel and a parametric representation of spatial cues, is a well-known method for efficient audio data compression of stereo signals. It is used in several established techniques, such as binaural cue coding [1][2] or parametric stereo coding [3][4].

[0004] For some types of signals, e.g., stereo speech signals recorded with a mid-side microphone setup, it can be shown that using only inter-channel loudness difference (ILD) and inter-channel coherence (IC) as stereo parameters is sufficient to achieve high-quality coding results [5, 6]. However, for other types of inputs, especially binauralized signals, it has been found that also considering inter-channel time difference (ITD) and inter-channel phase difference is very important for efficient coding of such signals [7].

[0005] Later, further techniques were developed to improve the use of inter-channel time difference and inter-channel phase difference in audio codecs, such as using full-band inter-channel time difference / inter-channel phase difference for low bit-rate scenarios [8], adaptively switching inter-channel phase difference compensation on and off [9], or using it in coders specifically tuned for speech

[10] .

[0006] For inputs with phase differences that are strongly frequency-dependent, such as binauralized inputs, using a large bandwidth to estimate inter-channel phase differences can result in large fluctuations in the estimate over short periods of time, depending on the spectral distribution of the signal within a given frame, even if the actual phase difference remains nearly constant. This effect can be minimized by estimating inter-channel phase differences in smaller bandwidths, where inter-channel phase differences tend to be much more stable over time. However, transmitting more inter-channel phase differences for smaller bandwidths also requires devoting more bits to inter-channel phase differences, which reduces the number of bits available for everything else. In addition, simultaneously compensating for many different narrowband inter-channel phase differences also risks introducing instability. Therefore, it is desirable to keep the number of transmitted inter-channel phase differences relatively low while also avoiding strong fluctuations in inter-channel phase differences for signals that are actually relatively stable over time. To achieve this, a stabilization mechanism based on more precise phase analysis is needed. Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved is to provide an improved encoder for stereo coding that relies on a single downmix channel and a parametric representation of spatial cues.

[0008] This problem is solved by an encoder for generating an audio bitstream from a stereo audio signal and a method for operating an encoder for generating an audio bitstream from a stereo audio signal according to the independent claims. [Means for solving the problem]

[0009] In a first aspect, the present invention provides an encoder for generating an audio bitstream from a stereo audio signal, the encoder comprising: a downmixer configured to downmix the stereo audio signal to generate a mono audio signal; an inter-channel phase difference calculator device configured to calculate an inter-channel phase difference for each time segment of a plurality of consecutive time segments of the stereo audio signal; a bitstream generator configured to generate an audio bitstream such that the mono audio signal and inter-channel phase differences for a plurality of consecutive time segments are embedded in the audio bitstream; Equipped with the inter-channel phase difference calculator device comprises a global inter-channel phase difference calculator configured to calculate a global inter-channel phase difference for each time segment of a plurality of consecutive time segments based on a frequency band of the stereo audio signal; the frequency band includes a plurality of sub-bands, and the inter-channel phase difference calculator device comprises a band-specific inter-channel phase difference calculator configured to calculate a band-specific inter-channel phase difference for each of a subset of the sub-bands for each time segment of the plurality of consecutive time segments; the inter-channel phase difference calculator device comprises a band-specific inter-channel phase difference change calculator configured to calculate a band-specific inter-channel phase difference change for each of the subset of subbands for each time segment of the plurality of consecutive time segments based on a band-specific inter-channel phase difference of a current time segment of the plurality of consecutive time segments and an inter-channel phase difference of at least one previous time segment of the plurality of consecutive time segments of a respective subband; the inter-channel phase difference calculator device comprises an average per-band inter-channel phase difference change calculator configured to calculate an average per-band inter-channel phase difference change for each time segment of a plurality of consecutive time segments based on the per-band inter-channel phase difference change of each of the subset of subbands; The inter-channel phase difference calculator device comprises an inter-channel phase difference calculator, the inter-channel phase difference calculator configured to calculate an inter-channel phase difference according to a global inter-channel phase difference for the current time segment and according to an average band-specific inter-channel phase difference change for the current time segment.

[0010] A downmixer is a device that can generate a mono audio signal from a stereo audio signal. The downmixer may comprise or be a processor. Both audio signals may be digital audio signals.

[0011] The term processor refers to an electronic device configured for a specific task. A processor may comprise hardware or a combination of hardware and software. Different processors may share hardware and / or software components.

[0012] The inter-channel phase difference calculator device is a device capable of calculating and outputting an inter-channel phase difference for each time segment of a plurality of time segments of stereo audio received by the inter-channel phase difference calculator device. The time segments may be frames of the digital stereo audio signal. The time segments may have a length between 10 ms and 1 s. The inter-channel phase difference calculator device may comprise a processor or may be a processor.

[0013] The bitstream generator is a device capable of generating a digital bitstream comprising the mono audio signal received from the downmixer and the associated inter-channel phase difference received from the inter-channel phase difference calculator device. The inter-channel bitstream generator may comprise or be a processor.

[0014] The global inter-channel phase difference calculator is a device capable of calculating a global inter-channel phase difference for each time segment of a plurality of consecutive time segments based on a frequency band of the stereo audio signal. The frequency band may be a wideband frequency band having a range of at least 40 Hz to 4 kHz, in particular at least 20 Hz to 8 kHz. The global inter-channel phase difference calculator may comprise or be a processor.

[0015] A frequency band comprises a number of sub-bands, the number of which may vary depending on the use case, and may be, for example, in the range of 4 to 16.

[0016] The band-specific inter-channel phase difference calculator is a device capable of calculating a band-specific inter-channel phase difference for each of a plurality of sub-bands for each time segment of a plurality of consecutive time segments. The band-specific inter-channel phase difference calculator may comprise or be a processor.

[0017] The band-specific inter-channel phase difference change calculator is a device capable of calculating a band-specific inter-channel phase difference change for each of the plurality of subbands for each time segment of the plurality of consecutive time segments based on a band-specific inter-channel phase difference of a current time segment of the plurality of consecutive time segments and a band-specific inter-channel phase difference of at least one previous time segment of the plurality of consecutive time segments of the respective subband. The band-specific inter-channel phase difference change calculator may comprise a processor or may be a processor.

[0018] The average per-band inter-channel phase difference change calculator is a device capable of calculating an average per-band inter-channel phase difference change for each time segment of a plurality of consecutive time segments based on the per-band inter-channel phase difference change for a respective time segment of each of a plurality of subbands. The average per-band inter-channel phase difference change calculator may comprise a processor or may be a processor.

[0019] The inter-channel phase difference calculator is a device that can receive the global inter-channel phase difference of the current time segment and the average band-specific inter-channel phase difference change of the current time segment, and calculate the inter-channel phase difference of the current time segment according to the global inter-channel phase difference of the current time segment and according to the average band-specific inter-channel phase difference change of the current time segment. The inter-channel phase difference calculator may comprise a processor or may be a processor.

[0020] The present invention minimizes undesired variations in inter-channel phase differences embedded in an audio bitstream by analyzing a global inter-channel phase difference derived from a larger frequency band and by analyzing band-specific inter-channel phase differences derived from smaller sub-bands within the larger frequency band. For each sub-band, a measure of band-specific inter-channel phase difference change is derived from the current band-specific inter-channel phase difference in the sub-band of the current time segment and one or more of the band-specific inter-channel phase differences from previous time segments in the same band.

[0021] The individual band-wise inter-channel phase difference changes for different subbands are then averaged to obtain an average band-wise inter-channel phase difference change, which is a stability measure for the complete frequency band. At the same time, a global inter-channel phase difference estimate for the complete frequency band is calculated. The inter-channel phase difference to be embedded in the audio bitstream for the current time segment is then calculated depending on the global inter-channel phase difference of the current time segment and depending on the average band-wise inter-channel phase difference change of the current time segment.

[0022] If the average band-wise inter-channel phase difference change is sufficiently small, indicating high stability in the frequency band, strong fluctuations in the inter-channel phase difference estimates are prevented, for example, by limiting the maximum change in the inter-channel phase difference from the previous time segment to the current time segment, or even by forcing the current inter-channel phase difference to the same value as the previous time segment. The stabilized inter-channel phase difference is then used at the decoder side to align the channels of the reconstructed stereo audio signal across a given frequency band.

[0023] Such a feature can avoid strong fluctuations in inter-channel phase difference. Furthermore, for each time segment, only one inter-channel phase difference value needs to be embedded in the audio bitstream.

[0024] In summary, the present invention provides an innovative method for using and transmitting inter-channel phase differences in an efficient manner while minimizing the effects of undesirable variations.

[0025] According to some embodiments of the present invention, the inter-channel phase difference calculator is configured such that the inter-channel phase difference is a member of a closed interval bounded by the global inter-channel phase difference of the current time segment and the inter-channel phase difference of the immediately preceding time segment. A closed interval is an interval that includes an upper and lower bound. Using such an interval can reduce the variability of the inter-channel phase difference.

[0026] According to some embodiments of the present invention, the inter-channel phase difference calculator is configured to use the inter-channel phase difference of the previous time segment as the inter-channel phase difference of the current time segment if the average band-specific inter-channel phase difference change is smaller than a preset value, thereby further reducing the fluctuation of the inter-channel phase difference.

[0027] According to some embodiments of the present invention, the inter-channel phase difference calculator comprises a difference of global inter-channel phase difference calculator configured to calculate, for each time segment, the absolute value of the difference between the inter-channel phase difference of the immediately preceding time segment and the global inter-channel phase difference of the current time segment; The inter-channel phase difference calculator is configured to use the global inter-channel phase difference of the current time segment as the inter-channel phase difference of the current time segment if the average band-specific inter-channel phase difference change is equal to or greater than a preset value, and if the absolute value of the difference between the inter-channel phase difference of the previous time segment and the global inter-channel phase difference of the current time segment is equal to or less than the average band-specific inter-channel phase difference change.

[0028] The difference of global inter-channel phase difference calculator is a device that can calculate, for each time segment, the absolute value of the difference between the inter-channel phase difference of the previous time segment and the global inter-channel phase difference of the current time segment. The difference of global inter-channel phase difference calculator may comprise a processor or may be a processor.

[0029] Using the global inter-channel phase difference for the current time segment as the inter-channel phase difference for the current time segment in this particular case further reduces the variation in the inter-channel phase difference.

[0030] According to some embodiments of the present invention, the inter-channel phase difference calculator comprises: If the global inter-channel phase difference for the current time segment is greater than the inter-channel phase difference for the previous time segment, the sum of the inter-channel phase difference for the previous time segment and the average band-specific inter-channel phase difference change; or If the global inter-channel phase difference of the current time segment is smaller than the inter-channel phase difference of the immediately preceding time segment, the difference between the inter-channel phase difference of the immediately preceding time segment and the average band-specific inter-channel phase difference change is calculated as: If the average band-specific inter-channel phase difference change is equal to or greater than the preset value, and if the absolute value of the difference between the inter-channel phase difference of the previous time segment and the global inter-channel phase difference of the current time segment is greater than the average band-specific inter-channel phase difference change, the inter-channel phase difference of the current time segment is set to It is configured to use

[0031] In this particular case, using the sum or difference of the inter-channel phase difference of the previous time segment and the average band-wise inter-channel phase difference change as the inter-channel phase difference of the current time segment further reduces the fluctuations in the inter-channel phase difference.

[0032] According to some embodiments of the present invention, the inter-channel phase difference calculator device comprises: a band-wise average inter-channel phase difference calculator configured to calculate a band-wise average inter-channel phase difference for each of a subset of sub-bands for each time segment of a plurality of consecutive time segments based on a plurality of previous band-wise inter-channel phase differences of the respective sub-band; The band-specific inter-channel phase difference change calculator is configured to calculate a band-specific inter-channel phase difference change for each of a subset of subbands for each time segment of the plurality of consecutive time segments based on the band-specific inter-channel phase difference of the current time segment and based on the band-specific average inter-channel phase difference of each subband.

[0033] The band-specific average inter-channel phase difference calculator is a device capable of calculating a band-specific average inter-channel phase difference for each of a plurality of subbands for each time segment of a plurality of consecutive time segments based on a plurality of previous band-specific inter-channel phase differences for the respective subband. The band-specific average inter-channel phase difference calculator may comprise a processor or may be a processor.

[0034] By calculating the band-specific inter-channel phase difference variation as specified herein, the fluctuations in inter-channel phase difference are further reduced.

[0035] According to some embodiments of the present invention, the inter-channel phase difference calculator is configured to have a preset value greater than or equal to 0.2 and less than or equal to 0.4, thereby further reducing the variation in inter-channel phase difference.

[0036] In a second aspect, the present invention provides a method for operating an encoder for generating an audio bitstream from a stereo audio signal, the method comprising: using a downmixer of the encoder to downmix the stereo audio signal to generate a mono audio signal; using an inter-channel phase difference calculator device of the encoder to calculate an inter-channel phase difference for each time segment of a plurality of consecutive time segments of the stereo audio signal; using a bitstream generator of the encoder to generate an audio bitstream such that the mono audio signal and inter-channel phase differences for a plurality of consecutive time segments are embedded in the audio bitstream; using a global inter-channel phase difference calculator of the inter-channel phase difference calculator device to calculate a global inter-channel phase difference for each time segment of a plurality of consecutive time segments based on a frequency band of the stereo audio signal, the frequency band including a plurality of sub-bands; using a band-specific inter-channel phase difference calculator of the inter-channel phase difference calculator device to calculate a band-specific inter-channel phase difference for each of the subset of subbands for each time segment of the plurality of consecutive time segments; using a band-wise inter-channel phase difference change calculator of the inter-channel phase difference calculator device to calculate a band-wise inter-channel phase difference change for each of the subset of sub-bands for each time segment of the plurality of consecutive time segments based on an inter-channel phase difference of a current time segment of the plurality of consecutive time segments and a band-wise inter-channel phase difference of at least one previous time segment of the plurality of consecutive time segments of a respective sub-band; using an average per-band inter-channel phase difference change calculator of the inter-channel phase difference calculator device to calculate an average per-band inter-channel phase difference change for each time segment of a plurality of consecutive time segments based on the per-band inter-channel phase difference change of each of the subset of subbands; using an inter-channel phase difference calculator of the inter-channel phase difference calculator device to calculate an inter-channel phase difference according to the global inter-channel phase difference for the current time segment and according to the average band-specific inter-channel phase difference change for the current time segment; Includes.

[0037] In a third aspect, the present invention provides a computer program for, when executed on a processor, carrying out the method according to the present invention.

[0038] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0039] [Figure 1] 1 shows a schematic diagram of an embodiment of an encoder for generating an audio bitstream from a stereo audio signal according to the present invention; [Figure 2] 1 shows a schematic diagram of an embodiment of an inter-channel phase difference calculator device configured to calculate an inter-channel phase difference for each time segment of a plurality of consecutive time segments of a stereo audio signal according to the present invention; [Figure 3]1 shows an exemplary graph of global inter-channel phase difference over time derived from frequency bands of a stereo audio signal. [Figure 4] 1 shows an exemplary graph of band-wise inter-channel phase differences over time, each derived from one of the sub-bands of the frequency band of an audio signal. [Figure 5] 1 shows an exemplary graph of inter-channel phase difference over time, where the value of the inter-channel phase difference for the current time segment is derived from the global inter-channel phase difference for the current time segment, from the global inter-channel phase difference for the immediately preceding time segment, and from the average band-specific inter-channel phase difference change for a subset of frequency bands for the current time segment. [Figure 6] 1 shows the results of a listening test illustrating the perceived quality of reproduction of a stereo audio signal encoded with a prior art encoder and with an encoder according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0040] Identical or equivalent elements, or elements having identical or equivalent functions, are designated in the following description by identical or equivalent reference numerals.

[0041] In the following description, numerous 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, to avoid obscuring embodiments of the present invention. Furthermore, features of different embodiments described below may be combined with each other unless otherwise noted.

[0042] 1 shows in a schematic diagram an embodiment of an encoder 1 for generating an audio bitstream BS from a stereo audio signal SAS according to the invention. a downmixer 2 configured to downmix the stereo audio signal SAS to generate a mono audio signal MAS; an inter-channel phase difference calculator device 3 configured to calculate an inter-channel phase difference ICPD for each time segment of a plurality of consecutive time segments of the stereo audio signal SAS; a bitstream generator 4 configured to generate an audio bitstream BS such that a mono audio signal MAS and an inter-channel phase difference ICPD for a plurality of consecutive time segments are embedded in the audio bitstream; Equipped with the inter-channel phase difference calculator device 3 comprises a global inter-channel phase difference calculator 5 configured to calculate a global inter-channel phase difference GICPD for each time segment of a plurality of consecutive time segments based on a frequency band of the stereo audio signal SAS; the frequency band comprises a plurality of sub-bands, and the inter-channel phase difference calculator device 3 comprises a band-specific inter-channel phase difference calculator 6 configured to calculate a band-specific inter-channel phase difference BICPD for each of a subset of the sub-bands for each time segment of a plurality of consecutive time segments; the inter-channel phase difference calculator device 3 comprises a band-specific inter-channel phase difference change calculator 7 configured to calculate a band-specific inter-channel phase difference change BICPD for each of a subset of sub-bands for each time segment of the plurality of consecutive time segments based on a band-specific inter-channel phase difference BICPD of a current time segment of the plurality of consecutive time segments and a band-specific inter-channel phase difference BICPD of at least one previous time segment of the plurality of consecutive time segments of a respective sub-band; the inter-channel phase difference calculator device 3 comprises an average band-specific inter-channel phase difference change calculator 8 configured to calculate an average band-specific inter-channel phase difference change MBICPDC for each time segment of a plurality of consecutive time segments based on the band-specific inter-channel phase difference change BICPDC for each of the subset of subbands; The inter-channel phase difference calculator device 3 comprises an inter-channel phase difference calculator 9 configured to calculate an inter-channel phase difference ICPD for the current time segment in response to the global inter-channel phase difference GICPD for the current time segment and in response to the average band-specific inter-channel phase difference change MBICPDC for the current time segment.

[0043] According to some embodiments of the present invention, the inter-channel phase difference calculator 9 is configured such that the inter-channel phase difference ICPD is an element of a closed interval bounded by the global inter-channel phase difference GICPD of the current time segment and the inter-channel phase difference ICPD of the immediately preceding time segment.

[0044] In a further aspect, the invention provides a method for operating an encoder 1 for generating an audio bitstream BS from a stereo audio signal SAS, the method comprising the steps of: using a downmixer 2 of the encoder 1 to downmix the stereo audio signal SAS to generate a mono audio signal MAS; using an inter-channel phase difference calculator device 3 of the encoder 1 to calculate an inter-channel phase difference ICPD for each time segment of a plurality of consecutive time segments of the stereo audio signal SAS; using a bitstream generator 4 of the encoder 1 to generate an audio bitstream BS such that a mono audio signal MAS and inter-channel phase differences ICPD for a plurality of consecutive time segments are embedded in the audio bitstream BS; using a global inter-channel phase difference calculator 5 of the inter-channel phase difference calculator device 3 to calculate a global inter-channel phase difference GICPD for each time segment of a plurality of consecutive time segments based on a frequency band of the stereo audio signal SAS, the frequency band comprising a plurality of sub-bands; using a band-wise inter-channel phase difference calculator 6 of the inter-channel phase difference calculator device 3 to calculate a band-wise inter-channel phase difference BICPD for each of a subset of sub-bands for each time segment of a plurality of consecutive time segments; using a band-specific inter-channel phase difference change calculator 7 of the inter-channel phase difference calculator device 3 to calculate a band-specific inter-channel phase difference change BICPDC for each of a subset of sub-bands for each time segment of the plurality of consecutive time segments based on a band-specific inter-channel phase difference BICPPD of a current time segment of the plurality of consecutive time segments and a band-specific inter-channel phase difference BICPPD of at least one previous time segment of the plurality of consecutive time segments of a respective sub-band; using an average band-specific inter-channel phase difference change calculator 8 of the inter-channel phase difference calculator device 3 to calculate an average band-specific inter-channel phase difference change MBICPDC for each time segment of a plurality of consecutive time segments based on the band-specific inter-channel phase difference change BICPDC for each of the subset of sub-bands; using an inter-channel phase difference calculator 9 of the inter-channel phase difference calculator device 3 to calculate an inter-channel phase difference ICPD for the current time segment in response to the global inter-channel phase difference GICPD for the current time segment and in response to the average band-specific inter-channel phase difference change MBICPDC for the current time segment; Includes.

[0045] In a further aspect, the present invention provides a computer program for performing the method according to the invention when the computer program is run on a processor.

[0046] FIG. 2 shows in a schematic diagram an embodiment of an inter-channel phase difference calculator device 3 according to the present invention configured to calculate an inter-channel phase difference ICPD for each time segment of a plurality of consecutive time segments of a stereo audio signal SAS.

[0047] According to some embodiments of the present invention, the inter-channel phase difference calculator 9 is configured to use the inter-channel phase difference ICPD of the previous time segment as the inter-channel phase difference ICPD of the current time segment if the average band-specific inter-channel phase difference change MICPDC is less than a preset value.

[0048] According to some embodiments of the present invention, the inter-channel phase difference calculator 9 comprises a difference of global inter-channel phase difference calculator 10 configured to calculate, for each time segment, an absolute value of the difference MOD between the inter-channel phase difference ICPD of the immediately preceding time segment and the global inter-channel phase difference GICPD of the current time segment; The inter-channel phase difference calculator 9 is configured to use the global inter-channel phase difference GICPD of the current time segment as the inter-channel phase difference ICPD of the current time segment if the average band-specific inter-channel phase difference change MICPDC is equal to or greater than a preset value, and if the absolute value MOD of the difference between the inter-channel phase difference ICPD of the immediately preceding time segment and the global inter-channel phase difference GICPD of the current time segment is equal to or less than the average band-specific inter-channel phase difference change MICPDC.

[0049] According to some embodiments of the present invention, the inter-channel phase difference calculator 9 comprises: If the global inter-channel phase difference GICPD of the current time segment is greater than the inter-channel phase difference ICPD of the immediately preceding time segment, the sum of the inter-channel phase difference ICPD of the immediately preceding time segment and the average band-specific inter-channel phase difference change MICPDC, or If the global inter-channel phase difference GICPD of the current time segment is smaller than the inter-channel phase difference ICPD of the immediately preceding time segment, the difference between the inter-channel phase difference ICPD of the immediately preceding time segment and the average band-specific inter-channel phase difference change MICPDC is calculated as follows: If the average band-specific inter-channel phase difference change MICPDC is equal to or greater than the preset value, and if the absolute value of the difference between the inter-channel phase difference ICPD of the previous time segment and the global inter-channel phase difference GICPD of the current time segment is greater than the average band-specific inter-channel phase difference change MICPDC, the inter-channel phase difference ICPD of the current time segment is set as It is configured to use

[0050] According to some embodiments of the present invention, the inter-channel phase difference calculator device 3 comprises a band-wise average inter-channel phase difference calculator 11 configured to calculate a band-wise average inter-channel phase difference BMICPD for each of a subset of sub-bands for each time segment of a plurality of consecutive time segments based on a plurality of band-wise inter-channel phase differences BICPD of previous time segments of the respective sub-band; The band-specific inter-channel phase difference change calculator 7 is configured to calculate a band-specific inter-channel phase difference change BICPDC for each of the subset of subbands for each time segment of the plurality of consecutive time segments based on the band-specific inter-channel phase difference BICPPD of the current time segment and based on the band-specific average inter-channel phase difference BMICPD of the respective subband.

[0051] According to some embodiments of the present invention, the inter-channel phase difference calculator 9 is configured such that the preset value is greater than or equal to 0.2 and the preset value is less than or equal to 0.4.

[0052] The invention can be used for different coding schemes. In particular, the invention may be used for the upcoming audio codec IVAS (Immersive Speech and Audio Services), which includes, among other input and output configurations, a parametric stereo coder as described in

[11] . In the encoder 1, this parametric coder performs a downmix of a given input stereo audio signal SAS to a single mono audio signal MAS and the extraction of stereo parameters, both of which are transmitted in the bitstream. In the decoder, the mono audio signal MAS is upmixed to stereo using the stereo parameters.

[0053] For each time segment (frame), these parameters can include band-specific information on channel panning via inter-channel loudness difference and decorrelation via inter-channel coherence, as well as one single inter-channel time difference and one single inter-channel phase difference ICPD, respectively. In this embodiment, a newly devised method for stabilizing the single inter-channel phase difference ICPD will be described in detail.

[0054] The global inter-channel phase difference GICPD can be calculated in the global inter-channel phase difference calculator 5 of the encoder 1 over a large frequency band, for example over a large range of DFT bins starting from the first complex bin (excluding the DC component) up to a certain maximum bin, via the following formula: TIFF2025539898000002.tif1390 where, TIFF2025539898000003.tif1383 and TIFF2025539898000004.tif1383, where gIPD denotes the global inter-channel phase difference GICPD, L denotes the left channel of the stereo audio signal SAS, and R denotes the right channel of the stereo audio signal SAS.

[0055] In the prior art, this global inter-channel phase difference GICPD was simply quantized and transmitted directly as the inter-channel phase difference ICPD for the current time frame in the audio bitstream BS without further processing.

[0056] For the new stabilization of the inter-channel phase difference ICPD, the estimated values ​​of the band-wise inter-channel phase differences BICPD of the sub-bands in the frequency band, calculated by the band-wise inter-channel phase difference calculator 6, are also taken into account. b and can be calculated for each subband b as one of the bins It can be represented as TIFF2025539898000005.tif1398, where: TIFF2025539898000006.tif1390 and The file is TIFF2025539898000007.tif1389.

[0057] Additionally, for each subband, a band-wise average inter-channel phase difference BMICPD over the previous time segment (e.g., five time segments in this implementation) can be calculated by the band-wise average inter-channel phase difference calculator 11. Because the distance between phases is ambiguous (two possible directions on a circle), a meaningful band-wise average inter-channel phase difference BMICPD cannot always be calculated by standard averaging (only when all phases are in the same semicircle). Instead, The subband band-wise average inter-channel phase difference BMICPD, shown as TIFF2025539898000008.tif721, is initialized to 0 and then TIFF2025539898000009.tif1392, where TIFF2025539898000010.tif719 is the index over the inter-channel phase difference values ​​before the band. After each iteration, TIFF2025539898000011.tif719 Distance to next value in buffer IPD diff is calculated, TIFF2025539898000012.tif778. TIFF2025539898000013.tif716 If it is larger than TIFF2025539898000014.tif73, i.e., if it is larger than a semicircular rotation in the given direction, TIFF2025539898000015.tif721 tells you which side of the circle it is on, i.e. TIFF2025539898000016.tif797 or 2 according to TIFF2025539898000017.tif797 By adding or subtracting TIFF2025539898000018.tif73, It needs to be temporarily shifted outside the range of TIFF2025539898000019.tif714.

[0058] next, The distance to the next value in TIFF2025539898000020.tif719 is This shifted version, which is less than TIFF2025539898000021.tif73, is used to update the band-specific average inter-channel phase difference BMICPD. After updating, TIFF2025539898000022.tif721 is still If it is outside TIFF2025539898000023.tif715, the shift is reversed before the next iteration.

[0059] Here, the current band-specific inter-channel phase difference BICPD( TIFF2025539898000024.tif711) and band-specific average inter-channel phase difference BMICPD( TIFF2025539898000025.tif721) and IPD change,b The band-specific inter-channel phase difference change BICPDC shown as For each sub-band having TIFF2025539898000026.tif765, the phase difference change between the channels by band is calculated by the phase difference change calculator 7, where The file is TIFF2025539898000027.tif7105.

[0060] The average inter-channel phase difference change MBICEPD(IPD) across all subbands is calculated from the individual inter-channel phase difference change BICPDC in each subband. change ) is calculated by the average band-specific channel phase difference change calculator 8, The file is TIFF2025539898000028.tif1363.

[0061] This average band-wise inter-channel phase difference change MBICEPD is considered an overall indicator of the stability of the band-wise inter-channel phase difference BICPD in the current time segment, and is used here to enforce a similar level of stability on the inter-channel phase difference ICPD.

[0062] For very small values ​​of the average band-specific inter-channel phase difference change MBICEPD (less than 0.3 in an embodiment implementation), the inter-channel phase difference ICPD of the immediately preceding time segment is used as the inter-channel phase difference ICPD and embedded in the audio bitstream BS for the current time segment; TIFF2025539898000029.tif774, where IPD is the inter-channel phase difference ICPD for the current time segment, and IPD prev is the inter-channel phase difference ICPD for the current time segment and is the inter-channel phase difference ICPD for the immediately preceding time segment.

[0063] If the average band-specific inter-channel phase difference change MBICEPD is larger, gIPD diffThe absolute value of the difference MOD between the inter-channel phase difference ICPD of the previous time segment and the global inter-channel phase difference GICPD of the current time segment, denoted as TIFF2025539898000030.tif757, where The file is TIFF2025539898000031.tif792.

[0064] This means that the absolute value MOD of the difference between the inter-channel phase difference ICPD of the previous time segment and the global inter-channel phase difference GICPD of the current time segment is greater than the average band-specific inter-channel phase difference change MBICEPD. TIFF2025539898000032.tif745, the maximum allowable change in the inter-channel phase difference ICPD for the immediately preceding time segment is limited to the average band-specific inter-channel phase difference change MBICEPD, and as a result, the inter-channel phase difference ICPD is TIFF2025539898000033.tif798 or Calculated as TIFF2025539898000034.tif7101.

[0065] However, this means that the absolute value MOD of the difference between the inter-channel phase difference ICPD of the previous time segment and the global inter-channel phase difference GICPD of the current time segment is less than or equal to the average band-specific inter-channel phase difference change MBICPD. TIFF2025539898000035.tif745, then the global inter-channel phase difference GICPD of the current time segment is used as the inter-channel phase difference ICPD of the current time frame, and therefore the inter-channel phase difference ICPD is calculated as follows: TIFF2025539898000036.tif724 The inter-channel phase difference ICPD stabilized as described above can now be quantized and transmitted as a side parameter in the audio bitstream BS.

[0066] 3 shows an exemplary graph of the global inter-channel phase difference GICPD over time, derived from the entire frequency band or at least a wide frequency band of the stereo audio signal SAS. The graph shows values ​​of the global inter-channel phase difference GICPD estimated over a wide frequency band. It is clear that this global inter-channel phase difference GICPD is stable, but fluctuates significantly between 0.3 and −π. Therefore, simply using this global inter-channel phase difference GICPD as the inter-channel phase difference ICPD embedded in the audio bitstream BS without further processing will result in a low-quality reproduction of the reconstructed stereo audio signal at the decoder side.

[0067] Figure 4 shows an exemplary graph of band-specific inter-channel phase differences BICPD over time, each derived from one of the eight sub-bands of the larger frequency band of the audio signal SAS shown in Figure 3. Here, it can be seen that the band-specific inter-channel phase differences BICPD vary between sub-bands, but are generally much more stable over time than the global inter-channel phase difference GICPD for the complete frequency band.

[0068] 5 shows an exemplary graph of inter-channel phase difference ICPD over time, where the value of inter-channel phase difference ICPD for a current time segment is derived from the global inter-channel phase difference GICPD for the current time segment and from the average band-by-band inter-channel phase difference change MBICPDC for a subset of frequency bands for the current time segment. Due to the stability of the band-by-band inter-channel phase difference BICPD, the inter-channel phase difference ICPD is also constrained here to remain stable compared to the global inter-channel phase difference GICPD shown in FIG.

[0069] FIG. 6 shows the results of a listening test illustrating the perceived quality of reproduction of a stereo audio signal encoded with a prior art encoder and with an encoder according to the invention.

[0070] The listening test was conducted as the MUSHRA listening test using binaurally-encoded clean speech input coded with the IVAS stereo coder at 24.4 kbps. MUSHRA stands for "Multiple Stimuli with Hidden Reference and Anchor," and is a method for performing codec listening tests to assess the perceived quality of the output from lossy audio compression algorithms. It is defined by ITU-R Recommendation BS.1534-3.

[0071] Seven expert listeners evaluated the quality of audio reproduction from an audio bitstream coded using a non-stabilized wideband inter-channel phase difference according to the prior art and the quality of audio reproduction from an audio bitstream coded using a stabilized inter-channel phase difference according to the present invention.

[0072] The results clearly show that the stabilized version is a clear improvement over the version without inter-channel phase difference stabilization.

[0073] Depending on particular implementation requirements, device and system embodiments of the present invention can be implemented in hardware and / or software. Implementations can be performed using digital storage media, such as floppy disks, DVDs, Blu-ray disks, CDs, ROMs, PROMs, EPROMs, EEPROMs, or flash memories, on which electronically readable control signals are stored, which cooperate (or are capable of cooperating) with a programmable computer system to perform one or more or all of the functions of the device or system of the present invention.

[0074] In some embodiments, a programmable logic device (e.g., a field programmable gate array) may be used to perform one or more or all of the functionality of the devices and systems described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor to implement one or more or all of the functionality of the devices and systems described herein.

[0075] While some aspects have been described in the context of an apparatus, it will be apparent that these aspects also represent a description of a corresponding method, where a block or device corresponds to a method step or feature of a method step, and similarly, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus.

[0076] Depending on particular implementation requirements, embodiments of the methods of the present invention can be implemented using apparatuses comprising hardware and / or software. Implementations can be performed using digital storage media, such as floppy disks, DVDs, Blu-ray disks, CDs, ROMs, PROMs, EPROMs, EEPROMs or flash memories, on which electronically readable control signals are stored, which cooperate (or can cooperate) with a programmable computer system to perform the respective methods.

[0077] Depending on particular implementation requirements, method embodiments of the present invention may be implemented using apparatus comprising hardware and / or software.

[0078] Some or all of the method steps may be performed by (or using) hardware devices such as microprocessors, programmable computers, or electronic circuits, and one or more of some of the most important method steps may be performed by such devices.

[0079] Some embodiments according to the present invention include a data carrier having electronically readable control signals that can cooperate with a programmable computer system to perform one of the methods described herein.

[0080] Generally, embodiments of the present invention can be implemented as a computer program product having program code operable to perform one of the methods when the computer program product is run on a computer, which program code may for example be stored on a machine-readable carrier.

[0081] Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier or a non-transitory storage medium.

[0082] A further embodiment comprises a processing means, for example a computer, or a programmable logic device, in particular a processor comprising hardware, configured to or adapted to perform one of the methods described herein.

[0083] A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.

[0084] In general, the method is advantageously performed by any apparatus comprising hardware and / or software.

[0085] While the present invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents that fall within the scope of the present invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents that fall within the true spirit and scope of the present invention. [Explanation of symbols]

[0086] 1 Encoder 2 Downmixer 3-Channel Phase Difference Calculator Device 4 Bitstream Generator 5 Global inter-channel phase difference calculator 6 Band-specific channel phase difference calculator 7. Calculation of phase difference between channels by band 8 Average band-specific inter-channel phase difference change calculator 9 Inter-channel phase difference calculator 10 Global Inter-Channel Phase Difference Calculator 11 Band-specific average inter-channel phase difference calculator BS Audio Bitstream SAS stereo audio signal MAS mono audio signal ICPD Inter-channel phase difference GICPD Global inter-channel phase difference BICPD Band-specific inter-channel phase difference BICPDC band-specific inter-channel phase difference change MBICPDC Average band-specific inter-channel phase difference change MOD The absolute value of the difference between the inter-channel phase difference of the previous time segment and the global inter-channel phase difference of the current time segment. BMICPD Band-specific average inter-channel phase difference

[0087] References [1] F. Baumgarte and C. Faller, “Binaural Cue Coding-Part I:Psycho-acoustic fundamentals and design principles,” IEEE Trans.on Speech and Audio Proc.,,vol.11,no.6,pp.pp.509-519,2003

[0088] [2] F.Baumgarte and C.Faller,“Binaural Cue Coding-Part II:Schemes and applications,”IEEE Trans.on Speech and Audio Proc.,vol.11,no.6,pp.pp.520-531,2003

[0089] [3] E.Schuijers,W.Oomen,B.Brinker and J.Breebaart,“Advances in Parametric Coding for High-Quality Audio,”in Preprint 5852,114th AES convention,Amsterdam,2003

[0090] [4] J.Breebaart,S.v.d.Par,A.Kohlrausch and E.Schuijers,“Parametric Coding of Stereo Audio,”EURASIP Journal on Applied Signal Processing,pp.1305-1322,September 2005

[0091] [5] J.Blauert,Spatial Hearing:The Psychoacoustics of Human Sound Localization,Cambridge,USA:MIT Press,1997

[0092] [6] T.Hoang,S.Ragot,B.Kovesi and P.Scalart,“Parametric stereo extension of ITU-T G.722 based on a new downmixing scheme,”in Proc.IEEE MMSP,St Malo,France,2010

[0093] [7] C.Tournery and C.Faller,“Improved time delay analysis / synthesis for parametric stereo audio coding,”in Preprint 120th Conv.Aud.Eng.Soc.,2006

[0094] [8] W.Wu,L.Miao,Y.Lang and D.Virette,“Parametric stereo coding scheme with a new downmix method and whole band inter channel time / phase differences,”in IEEE International Conference on Acoustics,Speech and Signal Processing,Vancouver,BC,Canada,2013

[0095] [9] M.Neusinger,J.Hilpert,B.Grill,J.Robilliard and M.Luis Valero,“Efficient Use Of Phase Information In Audio Encoding And Decoding”.WO Patent WO10003575,30 06 2009

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[10] S.Bayer,E.Fotopoulou,M.Multrus,G.Fuchs,E.Ravelli,M.Schnell,S.Doehla,W.Jaegers,M.Dietz and G.Markovic,“Apparatus and method for encoding or decoding a multi-channel signal using a broadband alignment parameter and a plurality of narrowband alignment parameters”.WO Patent WO2017125558A1,27 07 2017

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[11] S.Bayer,M.Dietz,S.Doehla,E.Fotopoulou,G.Fuchs,W.Jaegers,G.Markovic,M.Multrus,E.Ravelli and M.Schnell,“APPARATUS AND METHOD FOR ESTIMATING AN INTER-CHANNEL TIME DIFFERENCE”.Patent WO17125563,27 07 2017.

Claims

1. An encoder for generating an audio bitstream (BS) from a stereo audio signal (SAS), the encoder (1) comprising: a downmixer (2) configured to downmix said stereo audio signal (SAS) to generate a mono audio signal (MAS); an inter-channel phase difference calculator device (3) configured to calculate an inter-channel phase difference (ICPD) for each time segment of a plurality of consecutive time segments of the stereo audio signal (SAS); a bitstream generator (4) configured to generate the audio bitstream (BS) such that the mono audio signal (MAS) and the inter-channel phase difference (ICPD) for the plurality of consecutive time segments are embedded in the audio bitstream; Equipped with the inter-channel phase difference calculator device (3) comprises a global inter-channel phase difference calculator (5) configured to calculate a global inter-channel phase difference (GICPD) for each time segment of the plurality of consecutive time segments based on a frequency band of the stereo audio signal (SAS); the frequency band includes a plurality of sub-bands, and the inter-channel phase difference calculator device (3) comprises a band-specific inter-channel phase difference calculator (6) configured to calculate a band-specific inter-channel phase difference (BICPD) for each of a subset of the sub-bands for each time segment of the plurality of consecutive time segments; the inter-channel phase difference calculator device (3) comprises a band-specific inter-channel phase difference change calculator (7) configured to calculate a band-specific inter-channel phase difference change (BICPDC) for each of the subset of subbands for each time segment of the plurality of consecutive time segments based on the band-specific inter-channel phase difference (BICPD) of a current time segment of the plurality of consecutive time segments and the band-specific inter-channel phase difference (BICPD) of at least one previous time segment of the plurality of consecutive time segments of the respective subband; the inter-channel phase difference calculator device (3) comprises an average band-specific inter-channel phase difference change calculator (8) configured to calculate an average band-specific inter-channel phase difference change (MBICPDC) for each time segment of the plurality of consecutive time segments based on the band-specific inter-channel phase difference change (BICPDC) of each of the subset of subbands; the inter-channel phase difference calculator device (3) comprises an inter-channel phase difference calculator (9) configured to calculate the inter-channel phase difference (ICPD) for the current time segment in response to the global inter-channel phase difference (GICPD) for the current time segment and in response to the average band-specific inter-channel phase difference change (MBICPDC) for the current time segment.

2. 2. The encoder of claim 1, wherein the inter-channel phase difference calculator (9) is configured such that the inter-channel phase difference (ICPD) is a member of a closed interval bounded by the global inter-channel phase difference (GICPD) of the current time segment and the inter-channel phase difference (ICPD) of the previous time segment.

3. 3. The encoder of claim 1, wherein the inter-channel phase difference calculator (9) is configured to use the inter-channel phase difference (ICPD) of the previous time segment as the inter-channel phase difference (ICPD) of the current time segment if the average band-wise inter-channel phase difference change (MICPDC) is smaller than a preset value.

4. the inter-channel phase difference calculator (9) comprises a difference of global inter-channel phase difference calculator (10) configured to calculate, for each time segment, the absolute value of the difference (MOD) between the inter-channel phase difference (ICPD) of the previous time segment and the global inter-channel phase difference (GICPD) of the current time segment; 4. The encoder of claim 3, wherein the inter-channel phase difference calculator (9) is configured to use the global inter-channel phase difference (GICPD) of the current time segment as the inter-channel phase difference (ICPD) of the current time segment if the average per-band inter-channel phase difference change (MICPDC) is greater than or equal to the preset value and if the absolute value (MOD) of the difference between the inter-channel phase difference of the previous time segment and the global inter-channel phase difference of the current time segment is less than or equal to the average per-band inter-channel phase difference change (MICPDC).

5. The inter-channel phase difference calculator (9) if the global inter-channel phase difference (GICPD) for the current time segment is greater than the inter-channel phase difference (ICPD) for the immediately preceding time segment, the sum of the inter-channel phase difference (ICPD) for the immediately preceding time segment and the average band-specific inter-channel phase difference change (MICPDC); or If the global inter-channel phase difference (GICPD) of the current time segment is smaller than the inter-channel phase difference (ICPD) of the immediately preceding time segment, the difference between the inter-channel phase difference (ICPD) of the immediately preceding time segment and the average band-specific inter-channel phase difference change (MICPDC) is calculated by: If the average per-band inter-channel phase difference change (MICPDC) is equal to or greater than the preset value, and if the absolute value of the difference between the inter-channel phase difference (ICPD) of the previous time segment and the global inter-channel phase difference (GICPD) of the current time segment is greater than the average per-band inter-channel phase difference change (MICPDC), then:

5. An encoder according to claim 4, adapted for use.

6. the inter-channel phase difference calculator device (3) comprises a band-wise average inter-channel phase difference calculator (11) configured to calculate a band-wise average inter-channel phase difference (BMICPD) for each of the subset of sub-bands for each time segment of the plurality of consecutive time segments based on the band-wise inter-channel phase differences (BICPD) of the plurality of previous time segments of the respective sub-band; 6. The encoder of claim 1, wherein the band-specific inter-channel phase difference change calculator (7) is configured to calculate the band-specific inter-channel phase difference change (BICPDC) for each of the subset of subbands for each time segment of the plurality of consecutive time segments based on the band-specific inter-channel phase difference (BICPD) of the current time segment and based on the band-specific average inter-channel phase difference (BMICPD) of the respective subband.

7. The encoder according to any one of claims 3 to 6, wherein the inter-channel phase difference calculator (9) is configured such that the preset value is equal to or greater than 0.2 and the preset value is equal to or less than 0.

4.

8. A method for operating an encoder (1) for generating an audio bitstream (BS) from a stereo audio signal (SAS), said method comprising the steps of: using a downmixer (2) of said encoder (1) to downmix said stereo audio signal (SAS) to generate a mono audio signal (MAS); using an inter-channel phase difference calculator device (3) of said encoder (1) to calculate an inter-channel phase difference (ICPD) for each time segment of a plurality of consecutive time segments of said stereo audio signal (SAS); using a bitstream generator (4) of said encoder (1) to generate an audio bitstream (BS) such that said mono audio signal (MAS) and said inter-channel phase difference (ICPD) for said plurality of consecutive time segments are embedded in said audio bitstream (BS); using a global inter-channel phase difference calculator (5) of the inter-channel phase difference calculator device (3) to calculate a global inter-channel phase difference (GICPD) for each time segment of the plurality of consecutive time segments based on a frequency band of the stereo audio signal (SAS), the frequency band comprising a plurality of sub-bands; using a band-specific inter-channel phase difference calculator (6) of said inter-channel phase difference calculator device (3) to calculate a band-specific inter-channel phase difference (BICPD) for each of said subset of sub-bands for each time segment of said plurality of consecutive time segments; using a band-specific inter-channel phase difference change calculator (7) of the inter-channel phase difference calculator device (3) to calculate a band-specific inter-channel phase difference change (BICPDC) for each of the subset of subbands for each time segment of the plurality of consecutive time segments based on the band-specific inter-channel phase difference (BICPD) of a current time segment of the plurality of consecutive time segments and the band-specific inter-channel phase difference (BICPD) of at least one previous time segment of the plurality of consecutive time segments of the respective subband; using an average band-specific inter-channel phase difference change calculator (8) of the inter-channel phase difference calculator device (3) to calculate an average band-specific inter-channel phase difference change (MBICPDC) for each time segment of the plurality of consecutive time segments based on the band-specific inter-channel phase difference change (BICPDC) of each of the subset of subbands; using an inter-channel phase difference calculator (9) of the inter-channel phase difference calculator device (3) to calculate the inter-channel phase difference (ICPD) for the current time segment according to the global inter-channel phase difference (GICPD) for the current time segment and according to the average band-specific inter-channel phase difference change (MBICPDC) for the current time segment; A method comprising:

9. A computer program product which, when run on a processor, performs the method of claim 8.

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