Processing of audio signals during high-frequency reconstruction

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a European divisional application of European patent application EP 22 151 584.4 (reference: D10060EP10), for which EPO Form 1001 was filed 14 January 2022.TECHNICAL FIELD

[0002] The application relates to HFR (High Frequency Reconstruction / Regeneration) of audio signals. In particular, the application relates to a method and system for performing HFR of audio signals having large variations in energy level across the low frequency range which is used to reconstruct the high frequencies of the audio signal.BACKGROUND OF THE INVENTION

[0003] HFR technologies, such as the Spectral Band Replication (SBR) technology, allow to significantly improve the coding efficiency of traditional perceptual audio codecs. In combination with MPEG-4 Advanced Audio Coding (AAC) HFR forms a very efficient audio codec, which is already in use within the XM Satellite Radio system and Digital Radio Mondiale, and also standardized within 3GPP, DVD Forum and others. The combination of AAC and SBR is called aacPlus. It is part of the MPEG-4 standard where it is referred to as the High Efficiency AAC Profile (HE-AAC). In general, HFR technology can be combined with any perceptual audio codec in a back and forward compatible way, thus offering the possibility to upgrade already established broadcasting systems like the MPEG Layer-2 used in the Eureka DAB system. HFR methods can also be combined with speech codecs to allow wide band speech at ultra low bit rates.

[0004] The basic idea behind HFR is the observation that usually a strong correlation between the characteristics of the high frequency range of a signal and the characteristics of the low frequency range of the same signal is present. Thus, a good approximation for the representation of the original input high frequency range of a signal can be achieved by a signal transposition from the low frequency range to the high frequency range.

[0005] This concept of transposition was established in WO 98 / 57436 as a method to recreate a high frequency band from a lower frequency band of an audio signal. A substantial saving in bit-rate can be obtained by using this concept in audio coding and / or speech coding. In the following, reference will be made to audio coding, but it should be noted that the described methods and systems are equally applicable to speech coding and in unified speech and audio coding (USAC). WO 02 / 41301 A1 discloses an audio decoder with linear-prediction based spectral whitening after high-frequency reconstruction and before envelope adjustment.

[0006] High Frequency Reconstruction can be performed in the time-domain or in the frequency domain, using a filterbank or transform of choice. The process usually involves several steps, where the two main operations are to firstly create a high frequency excitation signal, and to subsequently shape the high frequency excitation signal to approximate the spectral envelope of the original high frequency spectrum. The step of creating a high frequency excitation signal may e.g. be based on single sideband modulation (SSB) where a sinusoid with frequency ω is mapped to a sinusoid with frequency ω + Δω where Δω is a fixed frequency shift. In other words, the high frequency signal may be generated from the low frequency signal by a "copy - up" operation of low frequency subbands to high frequency subbands. A further approach to creating a high frequency excitation signal may involve harmonic transposition of low frequency subbands. Harmonic transposition of order T is typically designed to map a sinusoid of frequency ω of the low frequency signal to a sinusoid with frequency Tω, with T > 1, of the high frequency signal.

[0007] The HFR technology may be used as part of source coding systems, where assorted control information to guide the HFR process is transmitted from an encoder to a decoder along with a representation of the narrow band / low frequency signal. For systems where no additional control signal can be transmitted, the process may be applied on the decoder side with the suitable control data estimated from the available information on the decoder side.

[0008] The aforementioned envelope adjustment of the high frequency excitation signal aims at accomplishing a spectral shape that resembles the spectral shape of the original highband. In order to do so, the spectral shape of the high frequency signal has to be modified. Put differently, the adjustment to be applied to the highband is a function of the existing spectral envelope and the desired target spectral envelope.

[0009] For systems that operate in the frequency domain, e.g. HFR systems implemented in a pseudo-QMF filterbank, prior art methods are suboptimal in this regard, since the creation of the highband signal, by means of combining several contributions from the source frequency range, introduces an artificial spectral envelope into the highband to be envelope adjusted. In other words, the highband or high frequency signal generated from the low frequency signal during the HFR process typically exhibits an artificial spectral envelope (typically comprising spectral discontinuities). This poses difficulties for the spectral envelope adjuster, since the adjuster not only has to have the ability to apply the desired spectral envelope with proper time and frequency resolution, but the adjustor also has to be able to undo the artificially introduced spectral characteristics by the HFR signal generator. This poses difficult design constraints on the envelope adjuster. As a result, these difficulties tend to lead to a perceived loss of high frequency energy, and audible discontinuities in the spectral shape in the highband signal, particularly for speech type signals. In other words, conventional HFR signal generators tend to introduce discontinuities and level variations into the highband signal for signals which have large variations in level over the lowband range, e.g. sibilants. When subsequently the envelope adjuster is exposed to this highband signal, the envelope adjuster cannot with reasonability and consistence separate the newly introduced discontinuity from any natural spectral characteristic of the low band signal.

[0010] The present document outlines a solution to the aforementioned problem, which results in an increased perceived audio quality. In particular, the present document describes a solution to the problem of generating a highband signal from a lowband signal, wherein the spectral envelope of the highband signal is effectively adjusted to resemble the original spectral envelope in the highband without introducing undesirable artifacts.SUMMARY OF THE INVENTION

[0011] According to the present invention, there are provided a system as set forth in claim 1, a method as set forth in claim 2, a storage medium as set forth in claim 3 and a computer program product as set forth in claim 4.

[0012] The present document proposes an additional correction step as part of the high frequency reconstruction signal generation. As a result of the additional correction step, the audio quality of the high frequency component or highband signal is improved. The additional correction step may be applied to all source coding systems that use high frequency reconstruction techniques, as well as to any single ended post processing method or system that aims at re-creating high frequencies of an audio signal.

[0013] It should be noted that the methods and systems including their preferred embodiments as outlined in the present patent application may be used stand-alone or in combination with the other methods and systems disclosed in this document. Furthermore, all aspects of the methods and systems outlined in the present patent application may be arbitrarily combined.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention is explained below by way of illustrative examples with reference to the accompanying drawings, wherein Fig. 1aillustrates the absolute spectrum of an example high band signal prior to spectral envelope adjustment; Fig. 1billustrates an exemplary relation between time-frames of audio data and envelope time borders of the spectral envelopes; Fig. 1cillustrates the absolute spectrum of an example high band signal prior to spectral envelope adjustment, and the corresponding scalefactor bands, limiter bands, and HF (high frequency) patches; Fig. 2illustrates an embodiment of a HFR system where the copy-up process is complemented with an additional gain adjustment step; Fig. 3illustrates an approximation of the coarse spectral envelope of an example lowband signal; Fig. 4illustrates an embodiment of an additional gain adjuster operating on optional control data, the QMF subbands samples, and outputting a gain curve; Fig. 5illustrates a more detailed embodiment of the additional gain adjuster of Fig. 4; Fig. 6illustrates an embodiment of an HFR system with a narrowband signal as input and a wideband signal as output; Fig. 7illustrates an embodiment of an HFR system incorporated into the SBR module of an audio decoder; Fig. 8illustrates an embodiment of the high frequency reconstruction module of an example audio decoder; Fig. 9illustrates an embodiment of an example encoder; Fig. 10aillustrates the spectrogram of an example vocal segment which has been decoded using a conventional decoder; Fig. 10billustrates the spectrogram of the vocal segment of Fig. 10a, which has been decoded using a decoder applying the additional gain adjustment processing; and Fig. 10cillustrates the spectrogram of the vocal segment of Fig. 10a for the original un-coded signal. DESCRIPTION OF PREFERRED EMBODIMENTS

[0015] The below-described embodiments are merely illustrative for the principles of the present invention PROCESSING OF AUDIO SIGNALS DURING HIGH FREQUENCY RECONSTRUCTION. 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.

[0016] As outlined above, audio decoders using HFR techniques typically comprise an HFR unit for generating a high frequency audio signal and a subsequent spectral envelope adjustment unit for adjusting the spectral envelope of the high frequency audio signal. When adjusting the spectral envelope of the audio signal, this is typically done by means of a filterbank implementation, or by means of time-domain filtering. The adjustment can either strive to do a correction of the absolute spectral envelope, or it can be performed by means of filtering which also corrects phase characteristics. Either way, the adjustment is typically a combination of two steps, the removal of the current spectral envelope, and the application of the target spectral envelope.

[0017] It is important to note, that the methods and systems outlined in the present document are not merely directed at the removal of the spectral envelope of the audio signal. The methods and systems strive to do a suitable spectral correction of the spectral envelope of the lowband signal as part of the high frequency regeneration step, in order to not introduce spectral envelope discontinuities of the high frequency spectrum created by combining different segments of the lowband, i.e. of the low frequency signal, shifted or transposed to different frequency ranges of the highband, i.e. of the high frequency signal.

[0018] In Fig. 1a a stylistically drawn spectrum 100, 110 of the output of an HFR unit is displayed, prior to going into the envelope adjuster. In the top-panel, a copy-up method (with two patches) is used to generate the highband signal 105 from the lowband signal 101, e.g. the copy-up method used in MPEG-4 SBR (Spectral Band Replication) which is outlined in "ISO / IEC 14496-3 Information Technology - Coding of audio-visual objects - Part 3: Audio". The copy-up method translates parts of the lower frequencies 101 to higher frequencies 105. In the lower panel, a harmonic transposition method (with two patches) is used to generate the highband signal 115 from the lowband signal 111, e.g. the harmonic transposition method of MPEG-D USAC which is described in "MPEG-D USAC: ISO / IEC 23003-3 - Unified Speech and Audio Coding".

[0019] In the subsequent envelope adjustment stage, a target spectral envelope is applied onto the high frequency components 105, 115. As can be seen from the spectrum 105, 115 going into the envelope adjuster, discontinuities (notably at the patch borders) can be observed in the spectral shape of the highband excitation signal 105, 115, i.e. of the highband signal entering the envelope adjuster. These discontinuities originate from the fact that several contributions of the low frequencies 101, 111 are used in order to generate the highband 105, 115. As can be seen, the spectral shape of the highband signal 105, 115 is related to the spectral shape of the lowband signal 101, 111. Consequently, particular spectral shapes of the lowband signal 101, 111, e.g. a gradient shape illustrated in Fig. 1a, may lead to discontinuities in the overall spectrum 100, 110.

[0020] In addition to the spectrum 100, 110, Fig. 1a illustrates example frequency bands 130 of the spectral envelope data representing the target spectral envelope. These frequency bands 130 are referred to as scalefactor bands or target intervals. Typically, a target energy value, i.e. a scalefactor energy, is specified for each target interval, i.e. scalefactor band. In other words, the scalefactor bands define the effective frequency resolution of the target spectral envelope, as there is typically only a single target energy value per target interval. Using the scalefactors or target energies specified for the scalefactor bands, the subsequent envelope adjuster strives to adjust the highband signal so that the energy of the highband signal within the scalefactor bands equals the energy of the received spectral envelope data, i.e. the target energy, for the respective scalefactor bands.

[0021] In Fig. 1c a more detailed description is provided using an example audio signal. In the plot the spectrum of a real-world audio signal 121 going into the envelope adjuster is depicted, as well as the corresponding original signal 120. In this particular example, the SBR range, i.e. the range of the high frequency signal, starts at 6.4kHz, and consists of three different replications of the lowband frequency range. The frequency ranges of the different replications are indicated by "patch 1", "patch 2", and "patch 3". It is clear from the spectrogram that the patching introduces discontinuities in the spectral envelope at around 6.4kHz, 7.4kHz, and 10.8kHz. In the present example, these frequencies correspond to the patch borders.

[0022] Fig. 1c further illustrates the scalefactor bands 130 as well as the limiter bands 135, of which the function will be outlined in more detail in the following. In the illustrated embodiment, the envelope adjuster of the MPEG-4 SBR is used. This envelope adjuster operates using a QMF filterbank. The main aspects of the operation of such an envelope adjuster are: to calculate the mean energy across a scalefactor band 130 of the input signal to the envelope adjuster, i.e. the signal coming out of the HFR unit; in other words, the mean energy of the regenerated highband signal is calculated within each scalefactor band / target interval 130; to determine a gain value, also referred to as envelope adjustment value, for each scalefactor band 130, wherein the envelope adjustment value is the square root of the energy ratio between the target energy (i.e. the energy target received from an encoder), and the mean energy of the regenerated highband signal 121 within the respective scalefactor band 130; to apply the respective envelope adjustment value to the frequency band of the regenerated highband signal 121, wherein the frequency band corresponds to the respective scalefactor band 130.

[0023] Furthermore, the envelope adjuster may comprise additional steps and variations, in particular: a limiter functionality, which limits the maximum allowed envelope adjustment value to be applied over a certain frequency band, i.e. over a limiter band 135. The maximum allowed envelope adjustment value is a function of the envelope adjustment values determined for the different scalefactor bands 130 which fall within a limiter band 135. In particular, the maximum allowed envelope adjustment value is a function of the mean of the envelope adjustment values determined for the different scalefactor bands 130 which fall within a limiter band 135. By way of example, the maximum allowed envelope adjustment value may be the mean value of the relevant envelope adjustment values multiplied by a limiter factor (such as 1.5). The limiter functionality is typically applied in order to limit the introduction of noise into the regenerated highband signal 121. This is particularly relevant for audio signals comprising prominent sinusoids, i.e. audio signals having a spectrum with distinct peaks at certain frequencies. Without the use of the limiter functionality, significant envelope adjustment values would be determined for the scalefactor bands 130 for which the original audio signal comprises such distinct peaks. As a result, the spectrum of the complete scalefactor band 130 (and not only the distinct peak) would be adjusted, thereby introducing noise. an interpolation functionality, which allows the envelope adjustment values to be calculated for cach individual QMF subband within a scalefactor band, instead of calculating a single envelope adjustment value for the entire scalefactor band. Since the scalefactor bands typically comprise more than one QMF subband, a envelope adjustment value can be calculated as the ratio of the energy of a particular QMF subband within the scalefactor band and the target energy received from the encoder, instead of calculating the ratio of the mean energy of all QMF subbands within the scalefactor band and the target energy received from the encoder. As such, a different envelope adjustment value may be determined for each QMF subband within a scalefactor band. It should be noted that the received target energy value for a scalefactor band typically corresponds to the average energy of that frequency range within the original signal. It is up to the decoder operation how to apply the received average target energy to the corresponding frequency band of the regenerated highband signal. This can be done by applying an overall envelope adjustment value to the QMF subbands within a scalefactor band of the regenerated highband signal or by applying an individual envelope adjustment value to each QMF subband. The latter approach can be thought of as if the received envelope information (i.e. one target energy per scalefactor band) was "interpolated" across the QMF subbands within a scalefactor band in order to provide a higher frequency resolution. Hence, this approach is referred to as "interpolation" in MPEG-4 SBR.

[0024] Returning to Fig. 1c it can be seen that the envelope adjuster would have to apply high envelope adjustment values in order to match the spectrum 121 of the signal going into the envelope adjuster with the spectrum 120 of the original signal. It can also be seen that due to the discontinuities, large variations of envelope adjustment values occur within the limiter bands 135. As a result of such large variations, the envelope adjustment values which correspond to the local minima of the regenerated spectrum 121 will be limited by the limiter functionality of the envelope adjuster. As a result, the discontinuities within the re-generated spectrum 121 will remain, even after performing the envelope adjustment operation. On the other hand, if no limiter functionality is used, undesirable noise may be introduced as outlined above.

[0025] Hence, a problem for the re-generation of a highband signal occurs for any signal that has large variations in level over the lowband range. This problem is due to the discontinuities introduced during the high frequency re-generation of the highband. When subsequently the envelope adjuster is exposed to this re-generated signal, it cannot with reasonability and consistence separate the newly introduced discontinuity from any "real-world" spectral characteristic of the lowband signal. The effects of this problem are twofold. First, spectral shapes are introduced in the highband signal that the envelope adjuster cannot compensate for. Consequently, the output has the wrong spectral shape. Second, an instability effect is perceived, due to the fact that this effect comes and goes as a function of the lowband spectral characteristics.

[0026] The present document addresses the above mentioned problem by describing a method and system which provide an HFR highband signal at the input of the envelope adjuster which does not exhibit spectral discontinuities. For this purpose, it is proposed to remove or reduce the spectral envelope of the lowband signal when performing high frequency regeneration. By doing this, one will avoid to introduce any spectral discontinuities into the highband signal prior to performing envelope adjustment. As a result, the envelope adjuster will not have to handle such spectral discontinuities. In particular, a conventional envelope adjuster may be used, wherein the limiter functionality of the envelope adjuster is used to avoid the introduction of noise into the regenerated highband signal. In other words, the described method and system may be used to re-generate an HFR highband signal having little or no spectral discontinuities and a low level of noise.

[0027] It should be noted that the time-resolution of the envelope adjuster may be different from the time resolution of the proposed processing of the spectral envelope during the highband signal generation. As indicated above, the processing of the spectral envelope during the highband signal re-generation is intended to modify the spectral envelope of the lowband signal, in order to alleviate the processing within the subsequent envelope adjuster. This processing, i.e. the modification of the spectral envelope of the lowband signal, may be performed e.g. once per audio frame, wherein the envelope adjuster may adjust the spectral envelope over several time intervals, i.e. using several received spectral envelopes. This is outlined in Fig. 1b where the time-grid 150 of the spectral envelope data is depicted in the top panel, and the time-grid 155 for the processing of the spectral envelope of the lowband signal during highband signal re-generation is depicted in the lower panel. As can be seen in the example of Fig. 1b, the time-borders of the spectral envelope data varies over time, while the processing of the spectral envelope of the lowband signal operates on a fixed time-grid. It can also be seen that several envelope adjustment cycles (represented by the time-borders 150) may be performed during one cycle of processing of the spectral envelope of the lowband signal. In the illustrated example, the processing of the spectral envelope of the lowband signal operates on a frame by frame basis, meaning that a different plurality of spectral gain coefficients is determined for each frame of the signal. It should be noted that the processing of the lowband signal may operate on any time-grid, and that the time-grid of such processing does not have to coincide with the time-grid of the spectral envelope data.

[0028] In Fig. 2, a filterbank based HFR system 200 is depicted. The HFR system 200 operates using a pseudo-QMF filterbank and the system 200 may be used to produce the highband and lowband signal 100 illustrated on the top panel of Fig. 1a. However, an additional step of gain adjustment has been added as part of the High Frequency Generation process, which in the illustrated example is a copy-up process. The low frequency input signal is analyzed by a 32 subband QMF 201 in order to generate a plurality of low frequency subband signals. Some or all of the low frequency subband signals are patched to higher frequency locations according to a HF (high frequency) generation algorithm. Additionally, the plurality of low frequency subbands is directly input to the synthesis filterbank 202. The aforementioned synthesis filterbank 202 is a 64 subband inverse QMF 202. For the particular implementation illustrated in Fig. 2, the use of a 32 subband QMF analysis filterbank 201 and the use of a 64 subband QMF synthesis filterbank 202 will yield an output sampling rate of the output signal of twice the input sampling rate of the input signal. It should be noted, however, that the systems outlined in the present document are not limited to systems with different input and output sampling rates. A multitude of different sampling rate relations can be envisioned by those skilled in the art.

[0029] As outlined in Fig. 2, the subbands from the lower frequencies are mapped to subbands of higher frequencies. A gain adjustment stage 204 is introduced as part of this copy-up process. The created high frequency signal, i.e. the generated plurality of high frequency subband signals, is input to the envelope adjuster 203 (possibly comprising a limiter and / or interpolation functionality), prior to combination with the plurality of low frequency subband signals in the synthesis filterbank 202. By using such an HFR system 200, and in particular by using a gain adjustment stage 204, the introduction of spectral envelope discontinuities as illustrated in Fig. 1 can be avoided. For this purpose, the gain adjustment stage 204 modifies the spectral envelope of the lowband signal, i.e. the spectral envelope of the plurality of low frequency subband signals, such that the modified lowband signal can be used to generate a highband signal, i.e. a plurality of high frequency subband signals, which does not exhibit discontinuities, notably discontinuities at the patch borders. Referring to Fig. 1c, the additional gain adjustment stage 204 ensures that the spectral envelope 101, 111 of the lowband signal is modified such that there are no, or limited, discontinuities in the generated highband signal 105, 115.

[0030] The modification of the spectral envelope of the lowband signal can be achieved by applying a gain curve to the spectral envelope of the lowband signal. Such a gain curve can be determined by a gain curve determination unit 400 illustrated in Fig. 4. The module 400 takes as input the QMF data 402 corresponding to the frequency range of the lowband signal used for re-creating the highband signal. In other words, the plurality of low frequency subband signals is input to the gain curve determination unit 400. As already indicated, only a subset of the available QMF subbands of the lowband signal may be used to generate the highband signal, i.e. only a subset of the available QMF subbands may be input to the gain curve determination unit 400. In addition, the module 400 may receive optional control data 404, e.g. control data sent from a corresponding encoder. The module 400 outputs a gain curve 403 which is to be applied during the high frequency regeneration process. In an embodiment, the gain curve 403 is applied to the QMF subbands of the lowband signal, which are used to generate the highband signal. I.e. the gain curve 403 may be used within the copy-up process of the HFR process.

[0031] The optional control data 404 may comprise information on the resolution of the coarse spectral envelope which is to be estimated in the module 400, and / or information on the suitability of applying the gain-adjustment process. As such, the control data 404 may control the amount of additional processing involved during the gain-adjustment process. The control data 404 may also trigger a by-pass of the additional gain adjustment processing, if signals occur that do not lend themselves well to coarse spectral envelope estimation, e.g. signals comprising single sinusoids.

[0032] In Fig 5 a more detailed view of the module 400 in Fig. 4 is outlined. The QMF data 402 of the lowband signal is input to an envelope estimation unit 501 that estimates the spectral envelope, e.g. on a logarithmic energy scale. The spectral envelope is subsequently input to a module 502 that estimates the coarse spectral envelope from the high (frequency) resolution spectral envelope received from the envelope estimation unit 501. In one embodiment, this is done by fitting a low order polynomial to the spectral envelope data, i.e. a polynomial of an order in the range of e.g. 1, 2, 3, or 4. The coarse spectral envelope may also be determined by performing a moving average operation of the high resolution spectral envelope along the frequency axis. The determination of a coarse spectral envelope 301 of a lowband signal is visualized in Fig. 3. It can be seen that the absolute spectrum 302 of the lowband signal, i.e. the energy of the QMF bands 302, is approximated by a coarse spectral envelope 301, i.e. by a frequency dependent curve fitted to the spectral envelope of the plurality of low frequency subband signals. Furthermore, it is shown that only 20 QMF subband signals are used for generating the highband signal, i.e. only a part of the 32 QMF subband signals are used within the HFR process.

[0033] The method used for determining the coarse spectral envelope from the high resolution spectral envelope and in particular the order of the polynomial which is fitted to the high resolution spectral envelope can be controlled by the optional control data 404. The order of the polynomial may be a function of the size of the frequency range 302 of the lowband signal for which a coarse spectral envelope 301 is to be determined, and / or it may be a function of other parameters relevant for the overall coarse spectral shape of the relevant frequency range 302 of the lowband signal. The polynomial fitting calculates a polynomial that approximates the data in a least square error sense. In the following, a preferred embodiment is outlined, by means of Matlab code:

[0034] In the above code, the input is the spectral envelope (LowEnv) of the lowband signal obtained by averaging QMF subband samples on a per subband basis over a time-interval corresponding to the current time frame of data operated on by the subsequent envelope adjuster. As indicated above, the gain-adjustment processing of the lowband signal may be performed on various other time-grids. In the above example, the estimated absolute spectral envelope is expressed in a logarithmic domain. A polynomial of low order, in the above example a polynomial of order 3, is fitted to the data. Given the polynomial, a gain curve (GainVec) is calculated from the difference in mean energy of the lowband signal and the curve (lowBandEnvSlope)) obtained from the polynomial fitted to the data. In the above example, the operation of determining the gain curve is done in the logarithmic domain.

[0035] The gain curve calculation is performed by the gain curve calculation unit 503. As indicated above, the gain curve may be determined from the mean energy of the part of the lowband signal used to re-generate the highband signal, and from the spectral envelope of the part of the lowband signal used to re-generate the highband signal. In particular, the gain curve may be determined from the difference of the mean energy and the coarse spectral envelope, represented e.g. by a polynomial. I.e. the calculated polynomial may be used to determine a gain curve which comprises a separate gain value, also referred to as a spectral gain coefficient, for every relevant QMF subband of the lowband signal. This gain curve comprising the gain values is subsequently used in the HFR process.

[0036] As an example, an HFR generation process in accordance to MPEG-4 SBR is described next. The HF generated signal may be derived by the following formula (see document MPEG-4 Part 3 (ISO / IEC 14496-3), sub-part 4, section 4.6.18.6.2): X High k , l + t HFAdj = X Low p , l + t HFAdj + bwArray g k ⋅ α 0 p ⋅ X Low p , l − 1 + t HFAdj + bwArray g k 2 ⋅ α 1 p ⋅ X Low p , l − 2 + t HFAdj , wherein p is the subband index of the lowband signal, i.e. p identifies one of the plurality of low frequency subband signals. The above HF generation formula may be replaced by the following formula which performs a combined gain adjustment and HF generation: X High k , l + t HFAdj = preGain p ⋅ X Low p , l + t HFAdj + bwArray g k ⋅ α 0 p ⋅ X Low p , l − 1 + t HFAdj + bwArray g k 2 ⋅ α 1 p ⋅ X Low p , l − 2 + t HFAdj , wherein the gain curve is referred to as preGain(p).

[0037] Further details of the copy-up process, e.g. with regards to the relation between p and k, are specified in the above mentioned MPEG-4, Part 3 document. In the above formula, X Low (p,l) indicates a sample at time instance l of the low frequency subband signal having a subband index p. This sample in combination with preceding samples is used to generate a sample of the high frequency subband signal X High (k,l) having a subband index k.

[0038] It should be noted that the aspect of gain adjustment can be used in any filterbank based high frequency reconstruction system. This is illustrated in Fig. 6 where the present invention is part of a standalone HFR unit 601 that operates on a narrowband or lowband signal 602 and outputs a wideband or highband signal 604. The module 601 may receive additional control data 603 as input, wherein the control data 603 may specify, among other things, the amount of processing used for the described gain adjustment, as well as e.g. information on the target spectral envelope of the highband signal. However, these parameters are only examples of optional control data 603. In an embodiment, relevant information may also be derived from the narrow band signal 602 input to the module 601, or by other means. I.e. the control data 603 may be determined within the module 601 based on the information available at the module 601. It should be noted that the standalone HFR unit 601 may receive the plurality of low frequency subband signals and may output the plurality of high frequency subband signals, i.e. the analysis / synthesis filterbanks or transforms may be placed outside the HFR unit 601.

[0039] As already indicated above, it may be beneficial to signal the activation of the gain adjustment processing in the bitstream from an encoder to a decoder. For certain signal types, e.g. a single sinusoid, the gain adjustment processing may not be relevant and it may therefore be beneficial to enable the encoder / decoder system to turn the additional processing off in order to not introduce an unwanted behaviour for such corner case signals. For this purpose, the encoder may be configured to analyze the audio signals and to generate control data which turns on and off the gain adjustment processing at the decoder.

[0040] In Fig. 7 the proposed gain adjustment stage is included in a high frequency reconstruction unit 703 which is part of an audio codec. One example of such a HFR unit 703 is the MPEG-4 Spectral Band Replication tool used as part of the High Efficiency AAC codec or the MPEG-D USAC (Unified Speech and Audio Codec). In this embodiment a bitstream 704 is received at an audio decoder 700. The bitstream 704 is demultiplexed in de-multiplexer 701. The SBR relevant part of the bitstream 708 is fed to the SBR module or HFR unit 703, and the core coder relevant bitstream 707, e.g. AAC data or USAC core decoder data, is sent to the core coder module 702. In addition, the lowband or narrow band signal 706 is passed from the core decoder 702 to the HFR unit 703. The present invention is incorporated as part of the SBR-process in HFR unit 703, e.g. in accordance to the system outlined in Fig. 2. The HFR unit 703 outputs a wideband or highband signal 705 using the processing outlined in the present document.

[0041] In Fig. 8, an embodiment of the high frequency reconstruction module 703 is outlined in more detail. Fig. 8 illustrates that in an embodiment which is covered by the claims the HF (high frequency) signal generation is derived from different HF generation modules at different instances in time. The HF generation is based either on a QMF based copy-up transposer 803, or the HF generation is based on a FFT based harmonic transposer 804. For both HF signal generation modules, the lowband signal is processed 801, 802 as part of the HF generation in order to determine a gain curve which is used in the copy-up 803 or harmonic transposition 804 process. The outputs from the two transposers are selectively input to the envelope adjuster 805. The decision on which transposer signal to use is controlled by the bitstream 704 or 708. It should be noted that, due to the copy-up nature of the QMF based transposer, the shape of the spectral envelope of the lowband signal is maintained more clearly than when using a harmonic transposer. This will typically result in more distinct discontinuities of the spectral envelope of the highband signal when using copy-up transposers. This is illustrated in the top and bottom panels of Fig. 1a. Consequently, it may be sufficient to only incorporate the gain adjustment for the QMF-based copy-up method performed in module 803. Nevertheless, applying the gain adjustment for the harmonic transposition performed in module 804 may be beneficial as well.

[0042] In Fig. 9, a corresponding encoder module that is not covered by the claimed invention is outlined. The encoder 901 may be configured to analyse the particular input signal 903 and determine the amount of gain adjustment processing which is suitable for the particular type of input signal 903. In particular, the encoder 901 may determine the degree of discontinuity on the high frequency subband signal which will be caused by the HFR unit 703 at the decoder. For this purpose, the encoder 901 may comprise an HFR unit 703, or at least relevant parts of the HFR unit 703. Based on the analysis of the input signal 903, control data 905 can be generated for the corresponding decoder. The infonnation 905, which concerns the gain adjustment to be performed at the decoder, is combined in multiplexer 902 with audio bitstream 906, thereby forming the complete bitstream 904 which is transmitted to the corresponding decoder.

[0043] In Fig. 10, the output spectra of a real world signal are displayed. In Fig. 10 a, the output of a MPEG USAC decoder decoding a 12kbps mono bitstream is depicted. The section of the real world signal is a vocal part of an a cappella recording. The abscissa corresponds to the time axis, whereas the ordinate corresponds to the frequency axis. Comparing the spectrogram of Fig. 10a to Fig. 10c which displays the corresponding spectrogram of the original signal, it is clear that there are holes (see reference numerals 1001, 1002) appearing in the spectrum for the fricative parts of the vocal segment. In Fig. 10b the spectrogram of the output of the MPEG USAC decoder including the present invention is depicted. It can be seen from the spectrogram that the holes in the spectrum have disappeared (see the reference numerals 1003, 1004 corresponding to the reference numerals 1001, 1002.

[0044] The complexity of the proposed gain adjustment algorithm was calculated as weighted MOPS, where functions like POW / DIV / TRIG are weighted as 25 operations, and all other operations are weighted as one operation. Given these assumptions, the calculated complexity amounts to approximately 0.1WMOPS and insignificant RAM / ROM usage. In other words, the proposed gain adjustment processing requires low processing and memory capacity.

[0045] In the present document a method and system for generating a highband signal from a lowband signal have been described. The method and system are adapted to generate a highband signal with little or no spectral discontinuities, thereby improving the perceptual performance of high frequency reconstruction methods and systems. The method and system can be easily incorporated into existing audio encoding / decoding systems. In particular, the method and system can be incorporated without the need to modify the envelope adjustment processing of existing audio encoding / decoding systems. Notably this applies to the limiter and interpolation functionality of the envelope adjustment processing which can perform their intended tasks. As such, the described method and system may be used to re-generate highband signals having little or no spectral discontinuities and a low level of noise. Furthermore, the use of control data has been described, wherein the control data may be used to adapt the parameters of the described method and system (and the computational complexity) to the type of audio signal.

[0046] The methods and systems described in the present document may be implemented as software, firmware and / or hardware. Certain components may e.g. be implemented as software running on a digital signal processor or microprocessor. Other components may e.g. be implemented as hardware and or as application specific integrated circuits. The signals encountered in the described methods and systems may be stored on media such as random access memory or optical storage media. They may be transferred via networks, such as radio networks, satellite networks, wireless networks or wireline networks, e.g. the internet. Typical devices making use of the methods and systems described in the present document are portable electronic devices or other consumer equipment which are used to store and / or render audio signals. The methods and systems may also be used on computer systems, e.g. internet web servers, which store and provide audio signals, e.g. music signals, for download.

Claims

1. A system (601, 703) configured to generate a wideband output audio signal (604) covering a high frequency interval from a narrow band input audio signal (602), wherein the system (601, 703) is configured to: receive the narrow band input audio signal (706); generate, by a first quadrature mirror filter (QMF) analysis filterbank, a plurality of low frequency audio subband signals (602) from the narrow band input audio signal; receive a set of target energies, each target energy covering a different target interval (130) within the high frequency interval and being indicative of the desired energy of one or more high frequency audio subband signals lying within the target interval (130); generate, in response to control information indicating whether to use a copy-up transposer or a harmonic transposer, a plurality of high frequency audio subband signals by either: applying an Fast Fourier Transform, FFT, to the narrow band input signal to obtain a plurality of FFT coefficients; performing a harmonic transposition on the plurality of FFT coefficients to obtain transposed FFT coefficients (804); applying an inverse FFT to the transposed FFT coefficients to obtain a high frequency audio signal; and applying a second QMF analysis filterbank to the high frequency audio signal; or: performing a copy-up transposition (803) of the plurality of low frequency subband signals (602) using a plurality of spectral gain coefficients associated with the plurality of low frequency audio subband signals (602), respectively; adjust the energy (203) of the plurality of high frequency audio subband signals (604) using the set of target energies; combine the low frequency audio subband signals and the energy-adjusted high frequency audio subband signals; and generate, by a QMF synthesis filterbank, the wideband output audio signal from the combined audio subband signals.

2. A method for generating a wideband output audio signal covering a high frequency interval from a narrow band input audio signal, the method comprising: receiving the narrow band input audio signal; generating, by a first quadrature mirror filter (QMF) analysis filterbank, a plurality of low frequency audio subband signals (602) from the narrow band input audio signal; receiving a set of target energies, each target energy covering a different target interval (130) within the high frequency interval and being indicative of the desired energy of one or more high frequency audio subband signals (604) lying within the target interval (130); generating, in response to control information indicating whether to use a copy-up transposer or a harmonic transposer, a plurality of high frequency audio subband signals by either: applying an Fast Fourier Transform, FFT, to the narrow band input signal to obtain a plurality of FFT coefficients; performing a harmonic transposition on the plurality of FFT coefficients to obtain transposed FFT coefficients (804) applying an inverse FFT to the transposed FFT coefficients to obtain a high frequency audio signal; and applying a second QMF analysis filterbank to the high frequency audio signal; or: performing a copy-up transposition (803) of the plurality of low frequency subband signals (602) using a plurality of spectral gain coefficients associated with the plurality of low frequency audio subband signals (602), respectively; adjusting the energy of the plurality of high frequency audio subband signals (604) using the set of target energies; combining the low frequency audio subband signals and the energy-adjusted high frequency audio subband signals; and generating, by a QMF synthesis filterbank, the wideband output audio signal from the combined audio subband signals.

3. A storage medium comprising a software program adapted for execution on a processor and for performing the method steps of claim 2 when carried out on a computing device.

4. A computer program product comprising executable instructions for performing the method of claim 2 when executed on a computer.