Audio signal processing in high frequency reconstruction.

Abstract

To provide a method and system for performing HFR (high frequency reconstruction / regeneration) of audio signals having large variations in energy level across the low frequency range.SOLUTION: Provided is a system configured to generate a plurality of high frequency subband signals from a plurality of low frequency subband signals, including: means for receiving the plurality of low frequency subband signals; means for receiving a set of target energies, each of the target energies covering a different target interval within a high frequency interval and being indicative of a desired energy of one or more high frequency subband signals lying within the target interval; means for generating the plurality of high frequency subband signals from the plurality of low frequency subband signals and from a plurality of spectral gain coefficients associated with the plurality of low frequency subband signals, respectively; and means for adjusting the energy of the plurality of high frequency subband signals using the set of target energies.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This application relates to High Frequency Reconstruction / Regeneration (HFR) of audio signals. In particular, this application relates to methods and systems for performing HFR of audio signals that have large variations in energy levels across the low frequency range that are used to reconstruct the high frequencies of the audio signal. [Background technology]

[0002] HFR techniques, such as Spectral Band Replication (SBR), allow for significant improvements in the coding efficiency of traditional perceptual audio codecs. In combination with MPEG-4 Advanced Audio Coding (AAC), HFR forms a highly efficient audio codec already used in the XM Satellite Radio system and Digital Radio Mondiale, and is also standardized by 3GPP®, the DVD Forum, and others. The combination of AAC and SBR is called aacPlus. It is part of the MPEG-4 standard, where it is called the High Efficiency AAC Profile (HE-AAC). In general, HFR technology can be combined with any perceptual audio codec in a compatible manner, both legacy and future, thus offering the potential to upgrade established broadcast systems, such as MPEG Layer 2, used in the Eureka DAB system. HFR methods can also be combined with audio codecs to allow wideband speech at very low bit rates.

[0003] The basic idea behind HFR is the observation that there is 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. Therefore, a good approximation of the representation of the original input high frequency range of a signal can be achieved by signal transposition from the low frequency range to the high frequency range.

[0004] This transfer concept was established in WO 98 / 57436, which is incorporated by reference, as a method for regenerating high frequency bands from lower frequency bands of an audio signal. Substantial bit rate savings 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 unified speech and audio coding (USAC).

[0005] High-frequency reconstruction can be performed in the time domain or the frequency domain using a selected filter bank or transform. This process typically involves several steps. Two main operations are first generating a high-frequency excitation signal and then shaping the high-frequency excitation signal to approximate the spectral envelope of the original high-frequency spectrum. The step of generating a high-frequency excitation signal may be based on, for example, single sideband modulation (SSB). In this case, a sine wave of frequency ω is mapped to a sine wave of frequency ω + Δω with Δω as a fixed frequency shift. In other words, a high-frequency signal can be generated from a low-frequency signal by a "copy up" operation from a low-frequency subband to a high-frequency subband. A further approach to generating a high-frequency excitation signal may involve harmonic transposition of the low-frequency subband. A harmonic transposition of order T is typically designed to map a sine wave of frequency ω of the low-frequency signal to a sine wave of frequency Tω of the high-frequency signal, where T>1.

[0006] HFR techniques may be used as part of a source coding system, where miscellaneous control information that guides the HFR process is transmitted from the encoder to the decoder along with a representation of the narrowband / low-frequency signal. For systems where additional control signals cannot be transmitted, the process may be applied at the decoder side using suitable control data that is inferred from available information at the decoder side.

[0007] The envelope adjustment of the high-frequency excitation signal described above aims to achieve a spectral shape that resembles the spectral shape of the original high-band. To do so, the spectral shape of the high-frequency signal needs to be modified. In other words, the adjustment applied to the high-band is a function of the existing spectral envelope and the desired target spectral envelope.

[0008] For systems operating in the frequency domain, e.g., HFR systems implemented in pseudo-QMF filter banks, prior art methods are suboptimal in this regard because generating a high band by combining several contributions from the source frequency range introduces an artificial spectral envelope in the high band to be envelope adjusted. In other words, the high band or high frequency signal generated from the low frequency signal during the HFR process typically exhibits an artificial spectral envelope (typically with spectral discontinuities). This presents challenges for the spectral envelope adjuster because not only must the adjuster be able to apply the desired spectral envelope with adequate time and frequency resolution, but the adjuster must also be able to undo the spectral characteristics artificially introduced by the HFR signal generator. This presents difficult design constraints for the envelope adjuster. As a result, these challenges tend to lead to a perceived loss of high frequency energy and, particularly for speech-type signals, audible discontinuities in the spectral shape of the high band signal. In other words, conventional HFR signal generators tend to introduce discontinuities and level fluctuations in the high-band signal for signals that have large variations in level across the low-band range, such as sibilants. When the envelope adjuster then encounters this high-band signal, it cannot rationally and consistently separate the newly introduced discontinuities from any natural spectral characteristics of the low-band signal. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] ISO / IEC14496-3 Information Technology―Coding of audio-visual objects―Part3: Audio [Non-patent document 2] MPEG-D USAC: ISO / IEC23003-3 United Speech and Audio Coding Summary of the Invention [Problem to be solved by the invention]

[0010] This paper outlines solutions to the above-mentioned problems that lead to improved perceived audio quality. In particular, this paper describes a solution to the problem of generating a high-band signal from a low-band signal, in which the spectral envelope of the high-band signal is effectively adjusted to resemble the original spectral envelope in the high-band without introducing undesirable artifacts. [Means for solving the problem]

[0011] This paper proposes an additional correction step as part of the generation of a high frequency reconstructed signal, which results in an improved audio quality of the high frequency components or high band signal. This additional correction step can 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 to regenerate the high frequencies of an audio signal.

[0012] According to one aspect, a system configured to generate a plurality of high-frequency subband signals covering a high-frequency range is described. The system may be configured to generate the plurality of high-frequency subband signals from a plurality of low-frequency subband signals. The plurality of low-frequency subband signals may be subband signals of a low-band or narrow-band audio signal that can be determined using a decomposition filter bank or transform. In particular, the plurality of low-frequency subband signals may be determined from a low-band time-domain signal using a decomposition QMF (quadrature mirror filter) filter bank or an FFT (Fast Fourier Transform). The plurality of generated high-frequency subband signals may correspond to approximations of the high-frequency subband signals of the original audio signal from which the plurality of low-frequency subband signals were derived. In particular, the plurality of low-frequency subband signals and the plurality of (re)generated high-frequency subband signals may correspond to the subbands of a QMF filter bank and / or an FFT transform.

[0013] The system may have means for receiving the plurality of low-frequency subband signals. Thus, the system may be downstream of the analysis filterbank or transform that generates the plurality of low-frequency subband signals from a low-band signal. The low-band signal may be an audio signal decoded in a core decoder from a received bitstream. The bitstream may be stored on a storage medium, such as a compact disc or DVD, or the bitstream may be received at the decoder via a transmission medium, such as an optical or radio wave transmission medium.

[0014] The system may include means for receiving a set of target energies, which may also be referred to as scale factor energies. Each target energy may cover a different target interval, which may also be referred to as a scale factor band. Typically, the set of target intervals corresponding to the set of target energies covers the entire high-frequency interval. The target energies included in the set of target energies typically indicate the desired energy of one or more high-frequency subband signals within the corresponding target interval. In particular, the target energies may correspond to the average desired energy of the one or more high-frequency subband signals within the corresponding target interval. The target energy of a target interval is typically derived from the energy of a high-band signal of the original audio signal within the target interval. In other words, the set of target energies typically describes the spectral envelope of the high-band portion of the original audio signal.

[0015] The system may comprise means for generating the plurality of high frequency subband signals from the plurality of low frequency subband signals, and to this end the means for generating the plurality of high frequency subband signals may be configured to perform a copy transfer onto the plurality of low frequency subband signals and / or to perform a harmonic transformation of the plurality of low frequency subband signals.

[0016] Furthermore, the means for generating the plurality of high-frequency subband signals may take into account a plurality of spectral gain factors during the process of generating the plurality of high-frequency subband signals. The plurality of spectral gain factors may be associated with the plurality of low-frequency subband signals, respectively. In other words, each low-frequency subband signal of the plurality of low-frequency subband signals may have a corresponding spectral gain coefficient from the plurality of spectral gain coefficients. A spectral gain coefficient from the plurality of spectral gain coefficients may be applied to a corresponding low-frequency subband signal.

[0017] The plurality of spectral gain factors may be associated with the energy of the respective plurality of low-frequency subband signals. In particular, each spectral gain factor may be associated with the energy of its corresponding low-frequency subband signal. In an embodiment, the spectral gain factor is determined based on the energy of the corresponding low-frequency subband signal. For this purpose, a frequency-dependent curve may be determined based on the plurality of energy values ​​of the plurality of low-frequency subband signals. In this case, the method for determining the plurality of gain factors may rely on the frequency-dependent curve determined from a representation (e.g., a logarithmic representation) of the energy of the plurality of low-frequency subband signals.

[0018] In other words, the plurality of spectral gain factors may be derived from a frequency-dependent curve fitted to the energies of the plurality of low-frequency subband signals. In particular, the frequency-dependent curve may be a polynomial of a predetermined degree. Alternatively or additionally, the frequency-dependent curve may comprise different curve segments, which are fitted to the energies of the plurality of low-frequency subband signals in different frequency intervals. The different curve segments may be different polynomials of a predetermined degree. In one embodiment, the different curve segments are polynomials of degree 0, which represent average energy values ​​of the energies of the plurality of low-frequency subband signals in corresponding frequency intervals. According to a further embodiment, the frequency-dependent curve is fitted to the energies of the plurality of low-frequency subband signals by performing a moving average filtering operation along the different frequency intervals.

[0019] In an embodiment, a gain factor in the plurality of gain factors is derived from the difference between an average energy of the plurality of low-frequency subband signals and a corresponding value of the frequency-dependent curve, which may be a value of the curve at a frequency lying within the frequency range of the low-frequency subband signal to which the gain factor corresponds.

[0020] Typically, the energies of the low-frequency subband signals are determined on a time grid, e.g., frame by frame. That is, the energy of a low-frequency subband signal within a time interval defined by the time grid corresponds to the average energy of the samples of the low-frequency subband signal within that time interval, e.g., within a frame. Thus, different spectral gain factors may be determined on a chosen time grid. For example, different spectral gain factors may be determined for each frame of an audio signal. In some embodiments, the spectral gain factors may be determined sample by sample, e.g., by determining the energies of the low-frequency subband signals using a floating window spanning the samples of each low-frequency subband signal. It should be noted that the system may comprise means for determining the spectral gain factors from the low-frequency subband signals. These means may be configured to perform the above-described methods for determining the spectral gain factors.

[0021] The means for generating a plurality of high-frequency subband signals may be configured to amplify the low-frequency subband signals using the respective plurality of spectral gain factors. Although reference is made hereinafter to "amplifying" or "amplifying," the "amplifying" operation may be replaced by other operations, such as a "multiplying" operation, a "rescaling" operation, or an "adjusting" operation. Amplification may be performed by multiplying samples of the low-frequency subband signals by their corresponding spectral gain factors. In particular, the means for generating a plurality of high-frequency subband signals may be configured to determine samples of the high-frequency subband signals at a given time point from samples of the low-frequency subband signals at the given time point and at least one preceding time point. Furthermore, samples of the low-frequency subband signals may be amplified by the respective spectral gain factors of the plurality of spectral gain factors. In one embodiment, the means for generating a plurality of high-frequency subband signals is configured to generate the plurality of high-frequency subband signals from the plurality of low-frequency subband signals according to the "copy up" algorithm specified in MPEG-4 SBR. The low-frequency subband signals used in this "copy up" algorithm may be amplified using the spectral gain factors, where the "amplify" operation may be performed as outlined above.

[0022] The system may include means for adjusting the energies of the high-frequency subband signals using the set of target energies. This operation is typically referred to as spectral envelope adjustment. The spectral envelope adjustment is performed by adjusting the energies of the high-frequency subband signals in a target interval so that the average energies of the high-frequency subband signals correspond to the corresponding target energies. This may be achieved by determining an envelope adjustment value from the energy values ​​of the high-frequency subband signals in the target interval and the corresponding target energies. In particular, the envelope adjustment value may be determined from the ratio between the target energies and the energy values ​​of the high-frequency subband signals in the corresponding target interval. The envelope adjustment value may be used to adjust the energies of the high-frequency subband signals.

[0023] In one embodiment, the energy adjusting means comprises means for limiting the adjustment of the energy of the high-frequency subband signal within a limiter section. Typically, a limiter section covers two or more target sections. The limiting means is typically used to avoid undesired amplification of noise in certain high-frequency subband signals. For example, the limiting means may be configured to determine an average envelope adjustment value of envelope adjustment values ​​corresponding to the target sections covered by or within the limiter section. Furthermore, the limiting means may be configured to limit the adjustment of the energy of the high-frequency subband signal within the limiter section to a value proportional to the average envelope adjustment value.

[0024] Alternatively or additionally, the means for adjusting the energy of the plurality of high frequency subband signals may include means for ensuring that the adjusted high frequency subband signals within the particular target interval have the same energy. This means is often referred to as "interpolation" means. In other words, the "interpolation" means ensures that the energy of each high frequency subband signal within the particular target interval corresponds to the target energy. The "interpolation" means may be implemented by separately adjusting each high frequency subband signal within the target interval so that the energy of the adjusted high frequency subband signal corresponds to the target energy associated with the particular target interval. This may be achieved by determining a different envelope adjustment value for each high frequency subband signal within the particular target interval. The different envelope adjustment value may be determined based on the energy of the particular high frequency subband signal and the target energy corresponding to the particular target interval. In some embodiments, the envelope adjustment value for a particular high frequency subband signal is determined based on the ratio between the target energy and the energy of the particular high frequency subband signal.

[0025] The system may further comprise means for receiving control data. The control data may indicate whether to apply the plurality of spectral gain factors to generate the plurality of high-frequency subband signals. In other words, the control data may indicate whether additional gain adjustment of the low-frequency subband signals should be performed. Alternatively or additionally, the control data may indicate a method to be used to determine the plurality of spectral gain factors. For example, the control data may indicate a predetermined order of a polynomial to be used to determine the frequency-dependent curve to be fitted to the energy of the plurality of low-frequency subband signals. The control data is typically received from a corresponding decoder or encoder that analyzes the original audio signal and informs a corresponding decoder or HFR system how to decode the bitstream.

[0026] According to another aspect, an audio decoder configured to decode a bitstream including a low-frequency audio signal and a set of target energies describing the spectral envelope of a high-frequency audio signal is described. In other words, an audio decoder configured to decode a bitstream representing a low-frequency audio signal and a set of target energies describing the spectral envelope of a high-frequency audio signal is described. The audio decoder may include a core decoder and / or a transform unit configured to determine a plurality of low-frequency subband signals associated with the low-frequency audio signal from the bitstream. Alternatively or additionally, the audio decoder may include a high-frequency generation unit based on the system outlined herein, the system configured to determine a plurality of high-frequency subband signals from the plurality of low-frequency subband signals and the set of target energies. Alternatively or additionally, the decoder may include a merge and / or inverse transform unit configured to generate an audio signal from the plurality of low-frequency subband signals and the plurality of high-frequency subband signals. The merge and inverse transform unit may comprise a synthesis filter bank or a transform, for example an inverse QMF filter bank or an inverse FFT.

[0027] According to a further aspect, an encoder configured to generate control data from an audio signal is described. The audio encoder may include means for analyzing the spectral shape of the audio signal and determining a degree of spectral envelope discontinuity introduced when regenerating high-frequency components of the audio signal from low-frequency components of the audio signal. Accordingly, the encoder may include certain elements of a corresponding decoder. In particular, the encoder may include an HFR system as outlined herein, which enables the encoder to determine a degree of discontinuity in the spectral envelope that may be introduced into high-frequency components of the audio signal at the decoder side. Alternatively or additionally, the encoder may include means for generating control data for controlling the regeneration of high-frequency components based on the degree of discontinuity. In particular, the control data may correspond to control data received by a corresponding decoder or the HFR system. The control data may indicate whether to use the plurality of spectral gain factors during the HFR process and / or which of predetermined polynomial orders to use to determine the plurality of spectral gain factors. To determine this information, the ratio of the low-frequency interval, i.e., selected portions of the frequency range covered by the low-frequency subband signals, can be determined. This ratio information can be determined, for example, by examining the lowest frequencies of the low band and the highest frequencies of the low band. This provides access to the spectral variability of the low-band signal, which will later be used for high-frequency reconstruction in the decoder. A large ratio can indicate an increased degree of discontinuity. Control data can also be determined using a signal type detector. For example, detection of a speech signal can indicate an increased degree of discontinuity. On the other hand, detection of a prominent sine wave in the original audio signal can lead to control data indicating that the spectral gain factors should not be used during the HFR process.

[0028] According to another aspect, a method for generating a plurality of high-frequency subband signals covering a high-frequency interval from a plurality of low-frequency subband signals is described. The method may include receiving the plurality of low-frequency subband signals and / or receiving a set of target energies. Each target energy may cover a different target interval within the high-frequency interval. Furthermore, each target energy may indicate a desired energy of one or more high-frequency subband signals within the target interval. The method may include generating the plurality of high-frequency subband signals from the plurality of low-frequency subband signals and a plurality of spectral gain factors associated with each of the plurality of low-frequency subband signals. Alternatively or additionally, the method may include adjusting the energy of the plurality of high-frequency subband signals using the set of target energies. Adjusting the energy may include limiting the adjustment of the energy of the high-frequency subband signals within a limiter interval. Typically, the limiter interval covers two or more target intervals.

[0029] According to a further aspect, a method for decoding a bitstream representing or including a set of target energies describing the spectral envelopes of a low-frequency audio signal and a corresponding high-frequency audio signal is described. Typically, the low-frequency and high-frequency audio signals correspond to low-frequency and high-frequency components of the same original audio signal. The method may include determining a plurality of low-frequency subband signals associated with the low-frequency audio signal from the bitstream. Alternatively or additionally, the method may include determining a plurality of high-frequency subband signals from the plurality of low-frequency subband signals and the set of target energies. This step is typically performed based on the HFR method outlined herein. The method may then include generating an audio signal from the plurality of low-frequency subband signals and the plurality of high-frequency subband signals.

[0030] According to another aspect, a method of generating control data from an audio signal is described. The method may include analyzing a spectral shape of the audio signal to determine a degree of discontinuity introduced when regenerating high frequency components of the audio signal from low frequency components of the audio signal. The method may further include generating control data for controlling the regeneration of the high frequency components based on the degree of discontinuity.

[0031] According to a further aspect, a software program is described, which may be adapted for execution on a processor and, when executed on a computing device, to perform the method steps outlined herein.

[0032] According to another aspect, a storage medium is described, which may have a software program adapted for execution on a processor and, when executed on a computing device, to perform the method steps outlined herein.

[0033] According to a further aspect, a computer program product is described, which may have executable instructions for performing the method steps outlined herein when executed on a computer.

[0034] It should be noted that the methods and systems, including the preferred embodiments, outlined in this patent application may be used alone or in combination with other methods and systems described herein. Furthermore, all aspects of the methods and systems outlined in this patent application may be combined in any manner. In particular, the features of each claim may be combined with each other in any manner. [Brief explanation of the drawings]

[0035] The present invention will now be described by way of illustrative example with reference to the accompanying drawings, in which: [Figure 1a] FIG. 1 illustrates the absolute spectrum of an exemplary high-band signal prior to spectral envelope adjustment. [Figure 1b] FIG. 10 illustrates an exemplary relationship between time frames of audio data and envelope time boundaries of a spectral envelope. [Figure 1c] FIG. 1 illustrates the absolute spectrum of an exemplary high-band signal prior to spectral envelope adjustment, along with the corresponding scale factor bands, limiter bands, and HF (high frequency) patches. [Figure 2] FIG. 10 illustrates an embodiment of an HFR system in which the up-copy process is complemented by an additional gain adjustment step. [Figure 3] FIG. 1 illustrates an approximation of the coarse spectral envelope of an exemplary low-band signal. [Figure 4] FIG. 10 illustrates an embodiment of an additional gain adjuster that operates based on optional control data, QMF subband samples, and outputs a gain curve. [Figure 5] 5 shows a more detailed embodiment of the additional gain adjuster of FIG. 4. [Figure 6] FIG. 1 illustrates an embodiment of an HFR system that has a narrowband signal as input and a wideband signal as output. [Figure 7] FIG. 1 illustrates an embodiment of an HFR system incorporated into an SBR module of an audio decoder. [Figure 8] FIG. 1 illustrates an embodiment of a high-frequency reconstruction module of an exemplary audio decoder. [Figure 9] FIG. 2 illustrates an embodiment of an exemplary encoder. [Figure 10a] 1 is a spectrogram of an exemplary voice segment decoded using a conventional decoder. [Figure 10b] 10 is a spectrogram of an exemplary voice segment decoded using a decoder that applies additional gain adjustment processing. [Figure 10c]10b is a spectrogram of the voice segment of FIG. 10a for the original uncoded signal. DETAILED DESCRIPTION OF THE INVENTION

[0036] The following embodiments merely illustrate the principles of the present invention "Audio Signal Processing During High Frequency Reconstruction." It is understood that modifications and variations of the configurations and details described herein will be apparent to those skilled in the art. Therefore, it is intended to be limited only by the scope of the appended claims, and not by the specific details presented by the description and explanation of the embodiments herein.

[0037] As outlined above, an audio decoder using HFR techniques typically has an HFR unit for generating a high-frequency audio signal and a subsequent spectral envelope adjustment unit for adjusting the spectral envelope of that high-frequency audio signal. When adjusting the spectral envelope of an audio signal, this is typically done by a filter bank implementation or by time-domain filtering. The adjustment can seek to correct the absolute spectral envelope, or it can be performed by filtering that also corrects the phase characteristics. In either case, the adjustment is typically a combination of two steps: removal of the current spectral envelope and application of a target spectral envelope.

[0038] It is important to note that the methods and systems outlined herein are not simply directed to removing the spectral envelope of an audio signal. They seek to provide suitable spectral correction of the spectral envelope of a low-band signal as part of the high-frequency regeneration step, so as not to introduce spectral envelope discontinuities in the high-frequency spectrum that are generated by combining different segments of a low-band, i.e., low-frequency signal, that have been shifted or translated into different frequency ranges of a high-band, i.e., high-frequency signal.

[0039] In Figure 1a, stylized spectra 100, 110 of the output of the HFR unit are shown before entering the envelope adjuster. In the top panel, an up-copy method (with two patches) is used to generate the high-band signal 105 from the low-band signal 101, such as the up-copy method used in MPEG-4 SBR (Spectral Band Replication) outlined in non-patent document 1, which is incorporated by reference. The up-copy method shifts portions of the lower frequencies 101 to higher frequencies 105. In the bottom panel, a harmonic translation method (with two patches) is used to generate the high-band signal 115 from the low-band signal 111, such as the MPEG-D USAC harmonic translation method described in non-patent document 2, which is incorporated by reference.

[0040] In a subsequent envelope adjustment stage, a target spectral envelope is applied to the high-frequency components 105, 115. As can be seen from the spectra 105, 115 entering the envelope adjuster, discontinuities (especially at patch boundaries) can be observed in the spectral shape of the high-band excitation signals 105, 115, i.e., the high-band signals entering the envelope adjuster. These discontinuities result from the fact that several contributions from low frequencies 101, 111 are used to generate the high-band 105, 115. As can be seen, the spectral shape of the high-band signals 105, 115 is related to the spectral shape of the low-band signals 101, 111. As a result, a particular spectral shape of the low-band signals 101, 111, such as the slope shape shown in FIG. 1a, can lead to discontinuities in the overall spectra 100, 110.

[0041] In addition to the spectra 100 and 110, FIG. 1a shows exemplary frequency bands 130 of spectral envelope data representing the target spectral envelope. These frequency bands 130 are referred to as scale factor bands or target intervals. Typically, a target energy value, i.e., scale factor energy, is specified for each target interval, i.e., scale factor band. In other words, since there is typically only a single target energy per target interval, the scale factor band defines the effective frequency resolution of the target spectral band. Using the scale factor or target energy specified for the scale factor band, the subsequent envelope adjuster seeks to adjust the highband signal so that the energy of the highband signal within the scale factor band is equal to the energy, i.e., target energy, of the received spectral envelope data for that respective scale factor band.

[0042] A more detailed description is provided using an example audio signal in Figure 1c. In this plot, the spectrum of a real-world audio signal 121 entering the envelope adjuster is depicted along with the corresponding original signal 120. In this particular example, the SBR range, i.e., the range of the high-frequency signal, begins at 6.4 kHz and consists of three different replicas of the low-band frequency range. The frequency ranges of these different replicas are indicated by "Patch 1," "Patch 2," and "Patch 3." From the spectrogram, it is clear that this patch configuration introduces discontinuities in the spectral envelope at approximately 6.4 kHz, 7.4 kHz, and 10.8 kHz. In this example, these frequencies correspond to the patch boundaries.

[0043] 1c further shows a scale factor band 130 and a limiter band 135, the function of which is outlined in more detail below. In the illustrated embodiment, an MPEG-4 SBR envelope adjuster is used. This envelope adjuster operates using a QMF filter bank. The main aspects of the operation of such an envelope adjuster are as follows:

[0044] Calculate the average energy across the scale factor bands 130 of the input signal to the envelope adjuster, i.e., the signal coming out of the HFR unit. In other words, the average energy of the regenerated high band signal is calculated within each scale factor band / target interval 130.

[0045] Determine a gain value, also called an envelope adjustment value, for each scale factor band 130. The envelope adjustment value is the square root of the energy ratio between the target energy (i.e., the energy target received from the encoder) and the average energy of the regenerated highband signal 121 within the respective scale factor band 130.

[0046] Applying each envelope adjustment value to the regenerated highband signal 121 in a frequency band corresponding to each scale factor band 130.

[0047] Furthermore, the envelope adjuster may have additional steps and variations, specifically:

[0048] A limiter function limits the maximum allowable envelope adjustment value to be applied to a frequency band, i.e., a limiter band 135. The maximum allowable envelope adjustment value is a function of the envelope adjustment values ​​determined for the various scale factor bands 130 that fall within the limiter band 135. Specifically, the maximum allowable envelope adjustment value is a function of the average of the envelope adjustment values ​​determined for the various scale factor bands 130 that fall within the limiter band 135. By way of example, the maximum allowable envelope adjustment value may be the average of the associated envelope adjustment values ​​multiplied by a limiter factor (e.g., 1.5). The limiter function is typically applied to limit the introduction of noise into the regenerated highband signal 121. This is particularly important for audio signals that contain prominent sinusoids, i.e., audio signals whose spectra have clear peaks at certain frequencies. Without the use of the limiter function, meaningful envelope adjustment values ​​would be determined for scale factor bands 130 in which the original audio signal contained such clear peaks. As a result, the spectrum of the complete scale factor band 130 (not just the sharp peak) will be modulated, thereby introducing noise.

[0049] An interpolation function. This allows envelope adjustment values ​​to be calculated for each individual QMF subband within a scale factor band, rather than calculating a single envelope adjustment value for the entire scale factor band. Because a scale factor band typically contains more than one QMF subband, an envelope adjustment value can be calculated as the ratio of the energy of a particular QMF subband within the scale factor band to the target energy received from the encoder, rather than calculating the ratio of the average energy of all QMF subbands within the scale factor band to the target energy received from the encoder. Thus, a different envelope adjustment value may be determined for each QMF subband within a scale factor band. Note that the received target energy value for a scale factor band typically corresponds to the average energy of that frequency range in the original signal. It is up to the decoder 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 the scale factor band of the regenerated highband signal, or by applying individual envelope adjustment values ​​to each QMF subband. The latter approach can be thought of as if the received envelope information (i.e., one target energy per scale factor band) is "interpolated" through the QMF subbands within the scale factor band to provide higher frequency resolution, hence the term "interpolation" in MPEG-4 SBR.

[0050] 1c, it can be seen that the envelope adjuster must apply a high envelope adjustment value to match the spectrum 121 of the signal entering the envelope adjuster to the spectrum 120 of the original signal. It can also be seen that, due to discontinuities, large fluctuations in the envelope adjustment value occur within the limiter band 135. As a result of such large fluctuations, the envelope adjustment value corresponding to the local minimum in the regenerated spectrum 121 is limited by the limiter function of the envelope adjustment value. As a result, the discontinuities in the regenerated spectrum 121 remain even after the envelope adjustment operation is performed. On the other hand, if the limiter function is not used, undesirable noise may be introduced, as outlined above.

[0051] Thus, any signal with large level variations across the low-band range will present a problem for regenerating the high-band signal. This problem is due to discontinuities introduced during high-frequency regeneration of the high-band. When the envelope adjuster then encounters this regenerated signal, it will not be able to rationally and consistently separate the newly introduced discontinuities from any "real-world" spectral characteristics of the low-band signal. The impact of this problem is twofold. First, a spectral shape is introduced into the high-band signal that the envelope adjuster cannot compensate for. As a result, the output will have an incorrect spectral shape. Second, an instability effect is perceived due to the fact that this effect comes and goes depending on the low-band spectral characteristics.

[0052] This paper addresses the above-mentioned problem by describing a method and system for providing an HFR high-band signal at the input of an envelope adjuster that does not exhibit spectral discontinuities. To this end, it is proposed to remove or reduce the spectral envelope of the low-band signal when performing high-frequency reconstruction. By doing so, the introduction of any spectral discontinuities in the high-band signal is avoided before performing envelope adjustment. As a result, the envelope adjuster does not need to address such spectral discontinuities. In particular, a conventional envelope adjuster may be used, in which the envelope adjuster's limiter function is used to avoid introducing noise into the reconstructed high-band signal. In other words, the described method and system can be used to regenerate an HFR high-band signal with little or no spectral discontinuities and a low noise level.

[0053] It should be noted that the time resolution of the envelope adjuster may differ from the time resolution of the proposed processing of the spectral envelope during high-band signal generation. As mentioned above, the processing of the spectral envelope during high-band signal regeneration is intended to modify the spectral envelope of the low-band signal to reduce subsequent processing within the envelope adjuster. This processing, i.e., modifying the spectral envelope of the low-band signal, may be performed, for example, once per audio frame. Here, the envelope adjuster may adjust the spectral envelope over several time intervals, i.e., using several received spectral envelopes. This is overviewed in Figure 1b, where a time grid 150 of spectral envelope data is depicted in the upper panel, and a time grid 155 for processing the spectral envelope of the low-band signal during high-band signal regeneration is depicted in the lower panel. As can be seen in the example of Figure 1b, the time boundaries of the spectral envelope data change over time, while the processing of the spectral envelope of the low-band signal operates on a fixed time grid. It can also be seen that several envelope adjustment cycles (represented by time boundaries 150) may be performed during one cycle of processing the spectral envelope of the low-band signal. In the illustrated example, the processing of the spectral envelope of the low-band signal operates on a frame-by-frame basis, i.e., different spectral gain factors are determined for each frame of the signal. It should be noted that the processing of the low-band 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.

[0054] FIG. 2 illustrates a filterbank-based HFR system 200. The HFR system 200 operates using a pseudo-QMF filterbank, which may be used to generate the high- and low-band signals 100 shown in the top panel of FIG. 1a. However, an additional step of gain adjustment is added as part of the high-frequency generation process. The high-frequency generation process is an upward copy process in the illustrated example. A low-frequency input signal is analyzed by a 32-subband QMF 201 to generate multiple low-frequency subband signals. Some or all of the low-frequency subband signals are patched to higher-frequency locations based on a high-frequency (HF) generation algorithm. The multiple low-frequency subbands are then directly input to a synthesis filterbank 202. The aforementioned synthesis filterbank 202 is a 64-subband inverse QMF 202. For the specific implementation illustrated in FIG. 2, the use of a 32-subband QMF analysis filterbank 201 and a 64-subband QMF synthesis filterbank 202 results in an output sampling rate of the output signal that is twice the input sampling rate of the input signal. However, the systems outlined herein are not limited to systems with different input and output sampling rates, as many different sampling rate relationships are contemplated by those skilled in the art.

[0055] As shown in FIG. 2, subbands from lower frequencies are mapped to higher frequency subbands. A gain adjustment stage 204 is introduced as part of this up-copying process. The resulting high-frequency signal, i.e., the plurality of high-frequency subband signals, is input to an envelope adjuster 203 (possibly with limiter and / or interpolator functions) before being combined with the plurality of low-frequency subband signals in a synthesis filter bank 202. Using such an HFR system 200, and in particular the gain adjustment stage 204, can avoid the introduction of the spectral envelope discontinuities shown in FIG. 1. For this purpose, the gain adjustment stage 204 modifies the spectral envelope of the low-band signal, i.e., the plurality of low-frequency subband signals, so that the modified low-band signal can be used to generate a high-band signal, i.e., the plurality of high-frequency subband signals, that does not exhibit discontinuities, particularly at patch boundaries. Referring to FIG. 1c, an additional gain adjustment stage 204 ensures that the spectral envelopes 101, 111 of the low-band signals are modified so that the generated high-band signals 105, 115 have no or only limited discontinuities.

[0056] Modification of the spectral envelope of the low-band signal can be achieved by applying a gain curve to the spectral envelope of the low-band signal. Such a gain curve can be determined by a gain curve determination unit 400 shown in FIG. 4. Module 400 takes as input QMF data 402 corresponding to the frequency range of the low-band signal used to regenerate the high-band signal. In other words, the plurality of low-frequency subband signals are input to gain curve determination unit 400. As previously mentioned, only a subset of the available QMF subbands of the low-band signal can be used to generate the high-band signal. That is, only a subset of the available QMF subbands can be input to gain curve determination unit 400. Furthermore, module 400 may receive optional control data 404, e.g., control data sent from a corresponding encoder. Module 400 outputs a gain curve 403 to be applied during the high-frequency regeneration process. In one embodiment, gain curve 403 is applied to the QMF subbands of the low-band signal used to generate the high-band signal. That is, gain curve 403 can be used in a copy-on-HFR process.

[0057] The optional control data 404 may include information about the resolution of the coarse spectral envelope estimated within the module 400 and / or information about the suitability of the gain adjustment process to be applied. Thus, 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 bypass of the additional gain adjustment process when a signal that does not lend itself well to coarse spectral envelope estimation occurs, for example, a signal having a single sine wave.

[0058] FIG. 5 provides a more detailed view of module 400 of FIG. 4. QMF data 402 of the low-band signal is input to an envelope estimation unit 501, which estimates the spectral envelope, e.g., on a logarithmic energy scale. The spectral envelope is then input to module 502, which estimates the coarse spectral envelope from the high- (frequency) resolution spectral envelope received from envelope estimation unit 501. In one embodiment, this is done by fitting a low-order polynomial, e.g., a polynomial of order 1, 2, 3, or 4, to the spectral envelope data. 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 the coarse spectral envelope 301 of the low-band signal is visualized in FIG. 3. It can be seen that the absolute spectrum 302 of the low-band signal, i.e., the energy 302 of the QMF bands, is approximated by a coarse spectral envelope 301, i.e., a frequency-dependent curve fitted to the spectral envelopes of the low-frequency subband signals. Furthermore, it is shown that only 20 QMF subband signals are used to generate the high-band signal, i.e., only a portion of the 32 QMF subband signals are used in the HFR process.

[0059] The method used to determine the coarse spectral envelope from the high-resolution spectral envelope, and in particular the order of the polynomial fitted to the high-resolution spectral envelope, can be controlled by optional control data 404. The order of the polynomial may be a function of the size of the frequency range 302 of the low-band signal for which the coarse spectral envelope 301 is determined, and / or may be a function of other parameters important to the overall coarse spectral shape of the relevant frequency range 302 of the low-band signal. The polynomial fitting calculates a polynomial that approximates the data in a least-squares error sense. A preferred embodiment is outlined below by Matlab code.

[0060] [Table 1] In the code above, the input is the spectral envelope of the low-band signal (LowEnv), obtained by averaging the QMF subband samples for each subband over a time interval corresponding to the current time frame of the data that will subsequently be acted upon by the envelope adjuster. As noted above, the gain adjustment process for the low-band signal may be performed on a variety of other time grids. In the above example, the estimated absolute spectral envelope is expressed in the logarithmic domain. A low-order polynomial, in the above example a third-order polynomial, is fit to the data. Given the polynomial, the gain curve (GainVec) is calculated from the difference in average energy between the low-band signal and the curve obtained from the polynomial fitted to the data (lowBandEnvSlope). In the above example, the operation to determine the gain curve is performed in the logarithmic domain.

[0061] The gain curve calculation is performed by the gain curve calculation unit 503. As described above, the gain curve may be determined from the average energy of the portion of the low-band signal used to regenerate the high-band signal and from the spectral envelope of the portion of the low-band signal used to regenerate the high-band signal. In particular, the gain curve may be determined from the difference between the average energy and a coarse spectral envelope, e.g., represented by a polynomial. That is, the calculated polynomial may be used to determine the gain curve. The gain curve includes separate gain values ​​for all significant QMF subbands of the low-band signal. The gain values ​​are also referred to as spectral gain coefficients. This gain curve including the gain values ​​is then used in the HFR process.

[0062] As an example, the HFR generation process based on MPEG-4 SBR is described below: The HF generated signal is derived by the following formula (see document MPEG-4 Part 3 (ISO / IEC 14496-3), sub-part 4, section 4.6.18.6.2, which is incorporated herein by reference):

[0063]

number

[0064]

number

[0065] Further details about the copy-up process, such as the relationship between p and k, are specified in the MPEG-4 Part 3 document mentioned above. Low (p,l) denotes the sample at time l of the low-frequency subband signal with subband index p. This sample is combined with the preceding samples to produce the high-frequency subband signal X with subband index k. High is used to generate the (k,l) sample.

[0066] The gain adjustment aspect can be used in any filterbank-based high-frequency reconstruction system. This is shown in FIG. 6. Here, the present invention is part of a stand-alone HFR unit 601 that operates on a narrowband or lowband signal 602 and outputs a wideband or highband signal 604. Module 601 may receive additional control data 603 as input, which may specify, among other things, the amount of processing to be used for the described gain adjustment, as well as information about, for example, the target spectral envelope of the highband signal. However, these parameters are merely examples of optional control data 603. In some embodiments, the relevant information may be derived from the narrowband signal 602 input to module 601 or by other means. That is, control data 603 may be determined within module 601 based on information available to module 601. It should be noted that the stand-alone HFR unit 601 may receive the plurality of low-frequency subband signals and output the plurality of high-frequency subband signals. That is, the analysis / synthesis filter bank or transform may be located outside the HFR unit 601.

[0067] As already mentioned above, it may be beneficial to signal the enablement of gain adjustment processing in the bitstream from the encoder to the decoder. For certain signal types, e.g., single sine waves, gain adjustment processing may not be significant, and therefore it may be beneficial for the encoder / decoder system to be able to turn off additional processing in order not to introduce undesirable behavior for such edge cases. For this purpose, the encoder may be configured to analyze the audio signal and generate control data to turn on or off gain adjustment processing in the decoder.

[0068] In FIG. 7, the proposed gain adjustment stage is included in a high-frequency reconstruction unit 703, which is part of an audio codec. An example of such an HFR unit 703 is an MPEG-4 spectral band replication tool used as part of a high-efficiency AAC codec or MPEG-D USAC (Unified Speech and Audio Codec). In this embodiment, a bitstream 704 is received by an audio decoder 700. The bitstream 704 is demultiplexed in a demultiplexer 701. An SBR-related portion 708 of the bitstream is provided to an SBR module or HFR unit 703, and a core decoder-related bitstream 707, e.g., AAC data or USAC core decoder data, is sent to a core coder module 702. Furthermore, a lowband or narrowband signal 706 is passed from the core decoder 702 to the HFR unit 703. The present invention can be incorporated as part of the SBR process in the HFR unit 703, for example, based on the system outlined in FIG. 2. The HFR unit 703 outputs a wideband or highband signal 705 using the processing outlined in this document.

[0069] FIG. 8 provides a more detailed overview of one embodiment of the high frequency reconstruction module 703. FIG. 8 illustrates that HF ​​(high frequency) signal generation may be derived from different HF generation modules at different times. HF generation may be based on a QMF-based up-copy shifter 803, or the HF generation may be based on an FFT-based harmonic shifter 804. For either HF signal generation module, the low-band signal is processed as part of the HF generation (801, 802) to determine the gain curve used in the up-copy 803 or harmonic shifter 804 process. The outputs from the two shifters are selectively input to an envelope adjuster 805. The decision as to which shifter signal to use is controlled by the bitstream 704 or 708. It should be noted that due to the up-copy nature of the QMF-based shifter, the shape of the spectral envelope of the low-band signal is more clearly preserved than when using a harmonic shifter. This typically leads to a more pronounced discontinuity in the spectral envelope of the high-band signal when using an up-copy shifter. This is shown in the top and bottom panels of Figure 1a. As a result, it may be sufficient to incorporate gain adjustment for the QMF-based up-copy method performed in module 803. Nevertheless, it may also be beneficial to apply gain adjustment for the harmonic transformation performed in module 804.

[0070] FIG. 9 outlines a corresponding encoder module. The encoder 901 may be configured to analyze a particular input signal 903 and determine the amount of gain adjustment processing appropriate for the particular type of input signal 903. In particular, the encoder 901 may determine the degree of discontinuities in the high-frequency subband signal that will be caused by the HFR unit 703 in the decoder. For this purpose, the encoder 901 may include the HFR unit 703 or at least a relevant part of the HFR unit 703. Based on the analysis of the input signal 903, control data 905 can be generated for the corresponding decoder. Information 905 regarding the gain adjustment to be performed in the decoder is combined with the audio bitstream 906 in the multiplexer 902, thereby forming the complete bitstream 904 that is transmitted to the corresponding decoder.

[0071] In Figure 10, the output spectrum of a real-world signal is displayed. In Figure 10a, the output of an MPEG USAC decoder decoding a 12 kbps mono bitstream is depicted. This section of the real-world signal is the vocal portion of an a cappella recording. The horizontal axis corresponds to the time axis, and the vertical axis corresponds to the frequency axis. Comparing the spectrogram in Figure 10a with the corresponding spectrogram in Figure 10c of the original signal, it is clear that there are holes (see references 1001 and 1002) appearing in the spectrum for the fricative portion of the vocal segment. In Figure 10b, the spectrogram of the output of an MPEG USAC decoder incorporating the present invention is depicted. From this spectrogram, it can be seen that the holes in the spectrogram have disappeared (see references 1003 and 1004, which correspond to references 1001 and 1002).

[0072] The complexity of the proposed gain adjustment algorithm was calculated as weighted MOPS. Functions such as POW / DIV / TRIG are weighted as 25 operations, and all other operations are weighted as 1 operation. Given these assumptions, the calculated complexity is approximately 0.1 WMOPS and negligible RAM / ROM usage. In other words, the processing and memory requirements of the proposed gain adjustment process are low.

[0073] This paper has described a method and system for generating a high-band signal from a low-band signal. The method and system are adapted to generate a high-band signal with few or no spectral discontinuities, thereby improving the perceptual performance of high-frequency reconstruction methods and systems. The method and system can be easily integrated into existing audio encoding / decoding systems. In particular, the method and system can be integrated into existing audio encoding / decoding systems without the need to modify the envelope adjustment process. This applies in particular to the limiter and interpolation functions of the envelope adjustment process, which can perform their intended tasks. Thus, the described method and system can be used to regenerate a high-band signal with few or no spectral discontinuities and a low noise level. Furthermore, the use of control data has been described. The control data may be used to adapt the parameters (and computational complexity) of the described method and system to the type of audio signal.

[0074] The methods and systems described herein may be implemented as software, firmware, and / or hardware. Certain components may be implemented, for example, as software running on a digital signal processor or microprocessor. Other components may be implemented, for example, as hardware and / or application-specific integrated circuits. Signals encountered in the described methods and systems may be stored on media such as random access memory or optical storage media. Such signals may be transmitted over a network such as an airwave, satellite, wireless, or wired network, e.g., the Internet. Typical devices utilizing the methods and systems described herein are portable electronic devices or other consumer devices used to store and / or play audio signals. The methods and systems may be used on computer systems, e.g., Internet web servers, that store and provide audio signals, e.g., music signals, for download.

[0075] Several aspects will be described. [Aspect 1] 1. A system configured to generate a plurality of high-frequency subband signals covering a high frequency interval from a plurality of low-frequency subband signals, the system comprising: means for receiving the plurality of low frequency subband signals; means for receiving a set of target energies, each target energy covering a different target interval within the high frequency interval and indicating a desired energy of one or more high frequency subband signals within the target interval; means for generating the plurality of high-frequency subband signals from the plurality of low-frequency subband signals and a plurality of spectral gain factors respectively associated with the plurality of low-frequency subband signals; means for adjusting the energies of the plurality of high frequency subband signals using the set of target energies; system. [Aspect 2] 2. The system of claim 1, wherein the means for adjusting the energy comprises means for limiting the adjustment of the energy of the high frequency subband signal within a limiter section (135), the limiter section covering two or more target sections (130). Aspect 3 3. The system of aspect 1 or 2, wherein the plurality of spectral gain factors are associated with energies of the respective plurality of low frequency subband signals. Aspect 4 4. The system of aspect 3, wherein the plurality of spectral gain factors are derived from a frequency-dependent curve fitted to the energy of the plurality of low-frequency subband signals. Aspect 5 5. The system of embodiment 4, wherein the frequency dependent curve is a polynomial of a predetermined degree. Aspect 6 6. The system of claim 4 or 5, wherein a spectral gain factor included in the plurality of spectral gain factors is derived from a difference between an average energy of the plurality of low-frequency subband signals and a corresponding value of the frequency-dependent curve. Aspect 7 7. The system of any one of aspects 1-6, wherein the means for generating the plurality of high-frequency subband signals is configured to amplify the plurality of low-frequency subband signals with the respective plurality of spectral gain factors. Aspect 8 the means for generating the plurality of high frequency subband signals comprises: performing an on-copy transition of said plurality of low-frequency subband signals; and / or configured to perform a harmonic transformation of the plurality of low-frequency subband signals; 8. The system of any one of embodiments 1 to 7. Aspect 9 9. The system of claim 8, wherein the means for generating the plurality of high-frequency subband signals comprises: multiplying samples of the low frequency subband signal by each spectral gain factor of the plurality of spectral gain factors, thereby providing modified samples; configured to determine a sample of a corresponding high-frequency subband signal at a particular time instant from modified samples of the low-frequency subband signal at said particular time instant and at least one preceding time instant, system. Aspect 10 10. The system of claim 9, wherein the samples of the corresponding high frequency subband signal at the particular time instant are determined from the modified samples of the low frequency subband signal using an MPEG-4 SBR copy-over algorithm. Aspect 11 11. The system of any one of aspects 1 to 10, wherein the means for adjusting the energy of the plurality of high frequency subband signals further comprises means for ensuring that the adjusted high frequency subband signals within a specific target interval have the same energy. Aspect 12 the plurality of low frequency subband signals and the plurality of high frequency subband signals QMF filter banks and / or FFT 12. The system of any one of aspects 1 to 11, wherein the subbands correspond to: Aspect 13 13. The system of any one of aspects 1 to 12, further comprising means for receiving control data, the control data comprising: whether to apply the plurality of spectral gain factors to generate the plurality of high-frequency subband signals; and / or a method for determining the plurality of spectral gain factors; system. Aspect 14 A system according to claim 13 when relying on the description of claim 5, wherein the control data indicates the predetermined degree of the polynomial. Aspect 15 1. An audio decoder configured to decode a bitstream representing a low frequency audio signal and a set of target energies describing a spectral envelope of a corresponding high frequency audio signal, the bitstream comprising: a core decoder and transform unit configured to determine from the bitstream a plurality of low-frequency sub-band signals associated with the low-frequency audio signal; a high-frequency generation unit based on the system of any one of aspects 1 to 14, configured to determine a plurality of high-frequency subband signals from the plurality of low-frequency subband signals and the set of target energies; a merging and inverse transform unit configured to generate an audio signal from the plurality of low-frequency subband signals and the plurality of high-frequency subband signals, decoder. Aspect 16 1. An encoder configured to generate control data from an audio signal, the audio encoder comprising: means for analyzing the spectral shape of the audio signal and determining the degree of spectral envelope discontinuity introduced when regenerating high frequency components of the audio signal from low frequency components of the audio signal; means for generating control data for controlling the regeneration of the high frequency components based on the degree of discontinuity; Encoder. Aspect 17 1. A method for generating a plurality of high-frequency subband signals covering a high frequency range from a plurality of low-frequency subband signals, comprising: receiving the plurality of low frequency subband signals; receiving a set of target energies, each target energy covering a different target interval within the high frequency interval and indicating a desired energy of one or more high frequency subband signals within the target interval; generating the plurality of high-frequency subband signals from the plurality of low-frequency subband signals and a plurality of spectral gain factors respectively associated with the plurality of low-frequency subband signals; adjusting the energies of the plurality of high-frequency subband signals using the set of target energies; method. Aspect 18 1. A method of decoding a bitstream representing a low frequency audio signal and a set of target energies describing the spectral envelope of a corresponding high frequency audio signal, comprising: determining from the bitstream a plurality of low frequency subband signals associated with the low frequency audio signal; determining a plurality of high-frequency subband signals from the plurality of low-frequency subband signals and the set of target energies according to the method of aspect 17; generating an audio signal from the plurality of low frequency subband signals and the plurality of high frequency subband signals, method. Aspect 19 1. A method for generating control data from an audio signal, comprising: analyzing the spectral shape of the audio signal to determine a degree of spectral envelope discontinuity introduced when regenerating high frequency components of the audio signal from low frequency components of the audio signal; generating control data for controlling the regeneration of the high frequency components based on the degree of discontinuity; method. Aspect 20 20. A software program adapted for execution on a processor and to perform the steps of the method of any one of aspects 17 to 19 when executed on a computing device. Aspect 21 20. A storage medium having a software program adapted for execution on a processor and, when executed on a computing device, to perform the steps of the method of any one of aspects 17 to 19. Aspect 22 20. A computer program product having executable instructions for performing the method of any one of aspects 17 to 19 when executed on a computer.

Claims

1. A system (601, 703) configured to generate a wideband output signal (604) from a narrowband input signal (602), the system comprising: receiving the narrowband input signal and control data; generating a plurality of low-frequency subband signals (602) from the narrowband input signal by a quadrature mirror filter (QMF) decomposition filterbank; receiving a set of target energies, each target energy covering a different target interval (130) within the high frequency interval and indicating a desired energy of one or more high frequency subband signals within the target interval; generating a plurality of high-frequency subband signals (604) from the plurality of low-frequency subband signals (602) by performing an upward copy transition (803) of the plurality of low-frequency subband signals (602) or a harmonic transformation (804) of the narrowband input signal in the FFT domain, and when the upward copy transition (803) is performed, by using a plurality of spectral gain factors respectively associated with the plurality of low-frequency subband signals (602) if the control data indicates that a plurality of spectral gain factors respectively associated with the plurality of low-frequency subband signals should be used to generate a plurality of high-frequency subband signals; adjusting (203) the energies of the plurality of high-frequency subband signals using the set of target energies; combining the low frequency subband signals and the energy adjusted high frequency subband signals; generating the wideband output signal from the combined subband signals by a QMF synthesis filter bank. system.

2. 1. A method for generating a wideband output signal from a narrowband input signal, the method comprising: receiving the narrowband input signal and control data; generating a plurality of low-frequency subband signals (602) from the narrowband input signal by a quadrature mirror filter (QMF) decomposition filterbank; receiving a set of target energies, each target energy covering a different target interval (130) within the high frequency interval and indicating a desired energy of one or more high frequency subband signals (604) within the target interval; generating a plurality of high-frequency subband signals (604) from the plurality of low-frequency subband signals (602) by performing an upward copy transition (803) of the plurality of low-frequency subband signals (602) or a harmonic transformation (804) of the narrowband input signal in the FFT domain, and when the upward copy transition (803) is performed, by using a plurality of spectral gain factors respectively associated with the plurality of low-frequency subband signals (602) if the control data indicates that a plurality of spectral gain factors respectively associated with the plurality of low-frequency subband signals should be used to generate a plurality of high-frequency subband signals; adjusting the energy of the plurality of high-frequency subband signals (604) using the set of target energies; combining the low frequency subband signals and the energy adjusted high frequency subband signals; generating the wideband output signal from the combined subband signals by a QMF synthesis filter bank; method.

3. A software program adapted for execution on a processor to perform the method steps of claim 2 when executed on a computing device.

4. 3. A storage medium having a software program adapted for execution on a processor to perform the method steps of claim 2 when executed on a computing device.

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