Low cost adaptation of bass post filter

Abstract

A method and decoder for audio decoding in which an encoded primary signal is decoded to form a decoded primary signal, then the decoded primary signal is post-filtered to form a post-filtered signal, in which an output signal of the decoder is one of the decoded primary signal and the post-filtered signal. An energy estimate of at least a portion of the spectrum of the primary signal being reconstructed by the decoder and an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal are obtained. Decision variables are generated based on the obtained energy estimates and analysis of discontinuities. The decision variable is compared to a threshold value, and the output signal is set as a decoded primary signal or a post-filtered signal based on the comparison of the decision variable to the threshold value.

Description

Description of the case

[0001] This application is a divisional application of the invention patent application with application date of April 23, 2021, application number 202180030319.7, and invention name “Low-cost adaptation of bass post-filter”. Technical Field

[0002] The present disclosure relates generally to communications and, more particularly, to methods and apparatus for mono, stereo, or multi-channel audio encoding and decoding. Background Art

[0003] Although the capacity of telecommunication networks is constantly increasing, it is still of great interest to limit the bandwidth required for each communication channel. In mobile networks, a smaller transmission bandwidth for each call results in lower power consumption in both mobile devices and base stations. This saves energy and costs for mobile operators, while end users experience extended battery life and increased talk time. In addition, consuming less bandwidth per user, mobile networks can serve a larger number of users in parallel.

[0004] In the field of speech coding, the ACELP (Algebraic Code Excited Linear Prediction) algorithm has been the leading technology for providing high-quality sound at low bit rates. In short, the ACELP model consists of a linear predictor (LP) filter that models the vocal tract and provides a rough spectral shape of the reconstructed speech. The LP filter is driven by two codebooks: a pitch codebook (or adaptive codebook) that models the periodic components of speech and an innovative codebook (or fixed codebook) that generates non-periodic speech segments and builds the pitch codebook. The core algorithm of the ACELP algorithm has been further enhanced to include post-processing tools such as post filters. Two main such filters are the formant post filter and the pitch post filter, both of which use parameters that are part of the ACELP speech model. The formant post filter enhances the rough spectral shape using the linear predictor filter (LP), and the pitch post filter reduces interharmonic distortion by emphasizing the pitch period. A variant of the pitch post filter for the low frequency range is the bass post filter (BPF). This tool appears in recent speech codec standards such as ITU-T G.718 and 3GPP EVS, as shown in 3GPP TS 26.445 V16.0.0, Codec for Enhanced Voice Services (EVS); Detailed Algorithm Description, 6.1.4.2 Bass Post Filter [1].

[0005] Although the bass post-filter generally improves the quality of the decoded audio, it may have a negative impact on some signals. Several adaptation methods have been used to control the post-filter strength. In 3GPP EVS [1], the post-filter strength is adapted to the degree of correlation between the post-filtered signal and the input signal. Low correlation indicates that the filter may have a degrading effect and, therefore, the filter output is attenuated. The post-filter strength is also applied to LP filter stability, where low stability results in an attenuated filter.

[0006] US 9,224,403 describes another adaptation method for a bass post filter. Here, US 9,224,403 has taken into account that the codec can use multiple modes, wherein the CELP or ACELP algorithm is one of these modes. Since the bass post filter is only valid for the ACELP mode, when the filter is enabled and disabled in the presence of frequent mode switching, the strength of the bass post filter can be adapted to avoid artifacts. US Patent 9,224,403 also considers how the input signal is represented by the ACELP or CELP coding model. If there is a significant energy loss, it is possible that the signal is not well modeled, and the bass post filter may be harmful. In order to reduce the artifacts caused by switching the filter on and off, the post filter strength can also be gradually adapted to provide a smoother transition. The filter difference signal can be analyzed for the filter effect, and the filter difference signal describes the difference between the filtered signal and the unfiltered signal. The filter effect analysis can also be performed on the approximate difference signal to reduce the computational complexity of the method.

[0007] In H. Chiba et al., "Adaptive Post-Filtering Controlled by Pitch Frequency for CELP-based Speech Coder", 2014 48th Asilomar Conference on Signals, Systems and Computers, it is recognized that the suitability of a bass post-filter can depend on the fundamental pitch or base frequency of the signal. Here, the strength of the post-filter is limited as a function of the fundamental pitch, so that the post-filter is attenuated for lower frequencies. The output of the filter is also low-pass filtered with a cutoff frequency that depends on the fundamental frequency, resulting in a lower operating bandwidth for lower fundamental frequencies. Summary of the invention

[0008] Post filters are intended to reduce noise, but in some cases they may introduce new artifacts. Specifically, abrupt changes in parameters (e.g., pitch period parameter or post filter strength) may introduce discontinuities that become audible in low energy regions of the spectrum.

[0009] As suggested in US 9224403 B2, gradual activation and deactivation does not address the fact that discontinuities may occur inside the filter due to the influence of the switching parameters. Experience has shown that attempts to smooth the transition of parameter switching can lead to slower filter adaptation, which reduces the performance of the post-filter, while artifacts are still not completely removed.

[0010] In one aspect, a method for audio decoding is provided, wherein an encoded primary signal is decoded to form a decoded primary signal, and then the decoded primary signal is post-filtered to form a post-filtered signal, wherein an output signal of the decoder is one of the decoded primary signal and the post-filtered signal. An energy estimate of at least a portion of the spectrum of the primary signal being reconstructed by the decoder and an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal are obtained. A decision variable is generated based on the obtained energy estimate and the analysis of the discontinuity. The decision variable is compared to a threshold value, and based on the comparison of the decision variable to the threshold value, the output signal is set to the decoded primary signal or the post-filtered signal.

[0011] In another aspect, a decoder for audio decoding is provided, wherein an encoded primary signal is decoded to form a decoded primary signal, and then the decoded primary signal is post-filtered to form a post-filtered signal, wherein an output signal of the decoder is one of the decoded primary signal and the post-filtered signal. The decoder includes a processing circuit and a memory coupled to the processing circuit, wherein the memory includes instructions that, when executed by the processing circuit, cause the decoder to perform operations including: obtaining an energy estimate of at least a portion of a spectrum of the primary signal being reconstructed by the decoder and an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal; generating a decision variable based on the obtained energy estimate and the obtained analysis of the discontinuities; comparing the decision variable to a threshold, and setting the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable to the threshold.

[0012] On the other hand, a decoder is provided that is suitable for performing operations, the operations comprising: obtaining an energy estimate of at least a portion of a spectrum of a primary signal that is being reconstructed by the decoder to form a decoded primary signal; obtaining an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal; generating a decision variable based on the obtained energy estimate and the obtained analysis of discontinuities; comparing the decision variable to a threshold; and setting the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable to the threshold.

[0013] On the other hand, a computer program is provided, the computer program comprising program code to be executed by a processing circuit of a decoder, whereby execution of the program code causes the decoder to perform operations comprising: obtaining an energy estimate of at least a portion of a spectrum of a primary signal, the primary signal being reconstructed by the decoder to form a decoded primary signal; obtaining an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal; generating a decision variable based on the obtained energy estimate and the obtained analysis of discontinuities; comparing the decision variable to a threshold; and setting the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable to the threshold.

[0014] On the other hand, a computer program product is provided comprising a non-transitory storage medium, the non-transitory storage medium comprising program code to be executed by a processing circuit of a decoder, whereby execution of the program code causes the decoder to perform operations comprising: obtaining an energy estimate of at least a portion of a spectrum of a primary signal being reconstructed by the decoder; obtaining an analysis of discontinuities in the time domain caused by post-filtering of the primary signal; generating a decision variable based on the obtained energy estimate and the obtained analysis of the discontinuities; comparing the decision variable to a threshold; and setting the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable to the threshold.

[0015] One advantage that can be obtained using the inventive concept described herein is to increase the adaptation of the post filter so that the benefits of the post filter are maintained while problematic situations are mitigated by attenuating or disabling the post filter. In addition, this advantage is achieved by a low complexity method that has a limited impact on the overall computational complexity of the audio decoder. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings illustrate certain non-limiting embodiments of the inventive concept, and are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this application. In the drawings:

[0017] Figure 1 is a diagram illustrating an example of a decoder system operating in a network according to some embodiments;

[0018] Figure 2 is a block diagram illustrating an example of a decoder with a pitch post filter according to some embodiments;

[0019] Figure 3 is an illustration of discontinuities in the pitch postfilter adjustment signal that may occur at subframe boundaries;

[0020] Figure 4is a graphical representation of the power spectrum of a signal before and after applying a pitch post filter called a "bass post filter (BPF)";

[0021] Figure 5 is a diagram showing a decoder with a pitch post filter according to some embodiments, wherein the decoder reconstructs a signal in the frequency domain;

[0022] Figure 6 is a block diagram of elements of an adaptive post filter according to some embodiments of the present inventive concept;

[0023] Figure 7 is a block diagram illustrating elements of an adaptive post filter according to some embodiments of the inventive concept;

[0024] Figure 8 is a flow chart illustrating operations performed by an adaptive post-filter according to some embodiments of the inventive concept;

[0025] Fig. 9 is a block diagram illustrating elements of an alternative adaptive post-filter according to some embodiments of the present inventive concept;

[0026] Fig.10 is a block diagram illustrating a decoder according to some embodiments of the inventive concept;

[0027] Fig.11 is a flowchart illustrating the operation of a decoder according to some embodiments of the inventive concept;

[0028] Fig.12 is a flowchart illustrating the operation of a decoder according to some embodiments of the inventive concept;

[0029] Fig.13 is a flowchart illustrating the operation of a decoder according to some embodiments of the inventive concept. DETAILED DESCRIPTION

[0030] Hereinafter, the inventive concept will be described more fully with reference to the accompanying drawings, in which examples of embodiments of the inventive concept are shown. However, the inventive concept can be embodied in a variety of different forms and should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that the disclosure will be comprehensive and complete, and the scope of the inventive concept will be fully conveyed to those skilled in the art. It should also be noted that these embodiments are not mutually exclusive. A component from one embodiment can be assumed by default to be present in / used in another embodiment.

[0031] The following description presents various embodiments of the disclosed subject matter. These embodiments are presented as teaching examples and are not to be construed as limiting the scope of the disclosed subject matter. For example, some details of the embodiments may be modified, omitted or expanded without departing from the scope of the subject matter.

[0032] Figure 1 An example of an operating environment of a decoder 100 is shown, which can be used to decode a mono, stereo, or multi-channel bitstream as described herein. The decoder 100 can be part of a media player, a mobile device, a set-top device, a desktop computer, etc. The decoder 100 receives an encoded bitstream. The bitstream can be sent from an encoder, a storage device 104, or a device on the cloud via a network 102, etc. During operation, the decoder 100 receives and processes frames of a bitstream as described herein. The decoder 100 outputs an audio signal (e.g., a mono, stereo, or multi-channel audio signal) and sends the audio signal to an audio player 106 having at least one speaker to play back the mono, stereo, or multi-channel audio signal. The storage device 104 can be a repository (e.g., a repository of a store or streaming music service) of a mono, stereo, or multi-channel audio signal, a separate storage component, a component of a mobile device, etc. The audio player can be a Bluetooth speaker, a device having at least one speaker, a mobile device, a streaming music service, etc.

[0033] Fig.10 1 is a block diagram illustrating elements of a decoder device 100 configured to provide wireless communication according to an embodiment of the inventive concept. The decoder 100 may be part of a mobile terminal, a mobile communication terminal, a wireless communication device, a wireless terminal, a wireless communication terminal, a user equipment UE, a user equipment node / terminal / device, etc. As shown, the decoder 100 may include a network interface circuit 1005 (also referred to as a network interface) configured to provide communication with other devices / entities / functions / etc. The decoder 100 may also include a processor circuit 1001 (also referred to as a processor) operably coupled to the network interface circuit 1005 and a memory circuit 1003 (also referred to as a memory) operably coupled to the processor circuit. The memory circuit 1003 may include a computer-readable program code that, when executed by the processor circuit 1001, causes the processor circuit to perform operations according to the embodiments disclosed herein.

[0034] According to other embodiments, the processor circuit 1001 can be defined to include a memory so that a separate memory circuit is not required. As discussed herein, the operation of the decoder 100 can be performed by the processor 1001 and / or the network interface 1005. For example, the processor 1001 can control the network interface 1005 to send communications to a multi-channel audio player and / or receive communications from one or more other network nodes / entities / servers (e.g., encoder nodes, repository servers, etc.) through the network interface 1005. In addition, modules can be stored in the memory 1003, and these modules can provide instructions so that when the instructions of the modules are executed by the processor 1001, the processor 1001 performs the corresponding operations.

[0035] Figure 2 An audio decoding system including a pitch post filter is shown. The decoder 220 receives a bit stream 210 from, for example, a transmission network or a storage medium. The decoder generates a reconstructed time domain signal ,in, is the sample index, and is the frame number. The reconstructed time domain signal In the following description, it may also be referred to as the primary signal or the decoded primary signal. The pitch period can also be used The pitch post filter 230 further enhances the performance.

[0036] Pitch period It may be obtained by performing a pitch analysis on the decoded audio, or it may come from analysis in the encoder or decoder of a target signal or a related audio signal which may have the same or similar dominant pitch as the post-filter input signal.

[0037] Post-filtered signal The pitch postfilter can be derived using the following form:

[0038]

[0039] in, is the fundamental pitch period in samples, and Controls the postfilter strength. The equivalent expression for the postfiltered signal is

[0040]

[0041] in, is the effect of the filter, expressed as a negative difference signal or correction signal or error signal. and It is usually updated every subframe, where a subframe can be 4 or 5 ms long. If a full audio frame is 20 ms, this means that the full frame is divided into 5 or 4 subframes, respectively. In 3GPP EVS [1], the post-filter strength Adapts based on the spectral stability of the signal. It also has a built-in fail-safe mechanism by measuring the correlation of the post-filtered audio with the input signal. If the correlation is low, it means the filter may be negatively affecting the quality and the filter is inhibited or turned off.

[0042] Post-filtered signal Output from the decoder system for playback by an audio player or for storage or transmission, possibly in a decoded PCM format. Note that the decoder system may include further processing of the post-filtered signal before outputting the final signal, such as additional enhancement or combination with other signals or signal components. The reconstructed signal may in this case correspond to a difference signal or residual signal, as outlined in ITU-T G.718 "Frame error robust narrow-band and wideband embedded variable bit-rate coding of speech and audio from 8-32 kbit / s", section 7.14.1.2 Double bass post filter. The residual signal may be combined with another signal to provide an enhanced output signal.

[0043] Bass Post Filter Adaptation—Example A

[0044] Figure 2 A disadvantage of the bass post-filter is that abrupt changes in the bass post-filter parameters at sub-frame boundaries may lead to undesirable discontinuities in the filtered signal, such as Figure 3 300 in FIG. 1 . These discontinuities can produce distortions across a range of frequencies. Depending on the frequency spectrum of the input signal, these distortions may be noticeable and become annoying. Consider, for example, Figure 4 FIG. 4 is a spectrum 400 in which the post filter operation generates audible noise 402 above about 1 kHz. Note that the filter still has the desired effect below 200 Hz, where interharmonic distortion is reduced.

[0045] The issue of subframe transitions is not addressed by the filter’s fail-safe mechanism, which measures the correlation of the filter output with the filter input signal. Since the correlation is calculated within each subframe, the transitions between subframes and their potential impact are not taken into account.

[0046] Possible techniques to reduce the effects of discontinuities are to apply smoothing by low-pass filtering the parameters or by crossfading the post-filter output between subframes. Although it has been found that this operation can reduce artifacts, it also slows down the adaptation of the filter, so that the positive effects of the filter are reduced. In addition, although low-pass filtering of the parameters reduces artifacts, it is found that it is better to turn off the post-filter for these critical sections. Therefore, it seems desirable to keep the post-filter unaffected for areas with positive effects, and to turn it off completely when it has a negative impact. The adaptation of the post-filter, which can foresee distortion and disable the filter when necessary, can reduce and in some embodiments eliminate the effects of discontinuities.

[0047] Figure 5 The decoder outlined in provides this adaptation. The decoder 520 receives the bitstream 510 and generates a reconstructed signal in the frequency domain , where m is the frame number and k is the frequency bin index. A transform often used in audio encoder and decoder systems is the MDCT (Modified Discrete Cosine Transform). It should be noted that the concepts presented herein are applicable to any transform domain where energy calculation is possible, such as DFT (Discrete Fourier Transform), QMF (Quadrature Mirror Filter Bank) or Hybrid QMF Filter Bank. Processing block 530 performs an inverse MDCT (IMDCT) transform and applies a post filter. This can be accomplished by using Figure 6 The adaptive post filter block 600 is replaced by Figure 5 The processing block 530 is used to describe the post-filter adaptation method according to some embodiments of the present invention. The resulting time domain signal is input to the post filter difference generation block 610. 630 and reconstructed signal in frequency domain 620 is input to a post filter adapter 640 which forms a decision 650 whether the post filter should be applied. The decision 650 is used to control the output 660 of the adaptive post filter block by activating or deactivating the subtraction of the post filter difference from the reconstructed primary signal.

[0048] Fig. 9 An alternative method is shown, in which the post filter outputs a filtered signal instead of a filter difference signal. Here, the decision mechanism of the alternative adaptive post filter block 900 decides whether to use the filtered signal 902 or the unfiltered signal 904. The time domain analysis of the filtered signal is performed on the filtered signal 902 instead of the difference signal, which will obtain similar results.

[0049] Figure 6 The post filter adapter 640 can be composed of Figure 7 Execution Figure 8The elements of the steps outlined in are further described. Figure 3 and Figure 4 As shown in the analysis of the problematic items, the post-filter adaptation method can be performed based on detecting two conditions:

[0050] 1. The spectrum has strong dips or deep valleys, which can expose potential distortions in the post-filter, such as Figure 4 , where the signal 400 has a valley starting at around 1000 Hz, where the distortion 402 from the post filter is exposed, and

[0051] 2. The discontinuity at the subframe boundary is large, such as Figure 3 The discontinuity 300 in FIG.

[0052] Detecting strong dips or deep valleys in the spectrum can be done by measuring the energy of the spectrum in certain critical bands. Low energy in a critical band can then indicate that a deep valley is found in a perceptually sensitive part of the spectrum. The energy measurement for each frame m The reconstructed signal in the MDCT domain can be The MDCT domain energy estimator 710 performs block 800 by measuring the energy of critical bands.

[0053]

[0054] Frequency range restriction ,and can be set to match the frequency range of the critical band. For example, if the MDCT frame length , then the sampling rate is 8000Hz, and the critical frequency range is 1000Hz to 1600Hz, a suitable value can be and For strict high-pass filtering operation, the upper limit should be 4000Hz and In the above description, the critical bands may be adaptive and, for example, depend on the reconstructed signal. The critical bands may, for example, be centered around identified low energy areas measured on a perceptually weighted spectrum. The perceptually weighted spectrum may be generated based on the spectrum of the reconstructed signal and transformed in the frequency and level dimensions such that perceptually important areas are emphasized. Adaptive critical bands may also take into account for which frequency ranges the post filter may produce distortion.

[0055] because The MDCT synthesis of may involve an overlap-add operation, so it may be desirable to simulate overlap-add in the energy estimate. This may be done by applying a low-pass FIR filter 720 to the energy estimate in block 810:

[0056] .

[0057] here, is the low-pass filter coefficient, which depends, for example, on the shape of the MDCT synthesis window and the length of the overlap. Suitable values ​​can be .

[0058] The size of the discontinuity is determined by using the subframe discontinuity analyzer 730 to filter the difference signal at block 820. The step size at the subframe boundary is averaged to measure:

[0059]

[0060] Where m represents the frame number, is the subframe number, is the number of subframes, and is the sample index of the subframe boundary marking the start of each new subframe. And the frame length , then the subframe boundary index can be . Note that for the first sample , will cite the sample . However, this is the last sample from the previous frame The same sample. In a practical implementation, this sample value will be stored in memory between frames.

[0061] The decision variable is formed at block 830 at multiplier 740 as and The ratio between them.

[0062]

[0063] Alternatively, with the optional low-pass filtering step 810 omitted, the decision variables are formed as and The ratio between is the energy estimate calculated in block 800 .

[0064] In order to make the decision stable, Low pass filtering may be performed by applying a low pass filter 760 between frames at block 840, e.g.

[0065]

[0066] in, is the low-pass filter coefficient, and a suitable value can be .

[0067] It may also be beneficial to limit the range of the low-pass filtered energy ratio via limiter 750 in block 840, in which case the expression may be written as

[0068]

[0069]

[0070] in, An upper limit will be set for the energy ratio, and a suitable value is found to be . Note that when When it is 1, the energy ratio is no longer low-pass filtered.

[0071] The post filter activation decision in various embodiments is made by comparing the low pass filtered energy ratio to a threshold at the threshold comparator 770 in block 850 and determining whether to use (e.g., activate) the post filter in block 860. In one embodiment of the inventive concept, the threshold is set to 1.

[0072]

[0073] Where "active" indicates that the post-filter is activated, and "inactive" indicates that the post-filter is disabled. It should be noted that if optional block 840 is omitted, the decision variables calculated in block 830 will be Compare with threshold As described above, when the post filter is activated, the output of the post filter is the reconstructed primary signal minus the post filter difference. When the post filter is inactive, the output of the post filter is the reconstructed primary signal.

[0074] Note that the filter output signal can be Rather than a difference signal Perform a similar analysis of discontinuities, such as Fig. 9 This may result in changes in filter constants such as , , and The above-mentioned concepts will remain the same although different options may be used.

[0075] In some embodiments, if the low-pass filtered energy ratio hovers around a threshold, it may be useful to add some hysteresis to the switching to reduce the switching back and forth. One way to implement hysteresis is to have two thresholds: one for activation and one for deactivation. If the activation threshold is slightly higher than the deactivation threshold, this creates a "dead zone" for the decision variable and reduces the switching back and forth when the variable hovers around the threshold. Another way to implement hysteresis is to determine a count of the number of times the low-pass filtered energy ratio is below (or alternatively, above) the threshold over a period of time, and activate (or deactivate) the post filter after the low-pass filtered energy ratio is below (or alternatively, above) the threshold a predetermined number of times over the period of time.

[0076] In the above-described embodiments, critical bands are used. In various other embodiments of the inventive concept, there may be more than one critical band corresponding to more than one spectral valley. In one embodiment, the critical band selected for analysis is the most sensitive area, and the decision whether to use a post filter is performed for the selected critical band. In other embodiments, there may be multiple areas where the noise is just below the threshold for noise to be noticeable, and combining many of these areas may cause the user to hear noise, while the analysis for each area indicates that the noise should not be noticeable. One way to solve this problem may be to sum the contributions from several critical bands and decide based on the above-described embodiments whether to set the output to a primary signal or a post-filtered signal. An alternative approach is to analyze the bands individually and then disable the post filter if a threshold is triggered for any of the bands being analyzed.

[0077] Example B

[0078] An alternative method for deciding whether to mask noise is to compare the energy of the signal in the critical region before and after the post filter. This alternative method was found to give similar results to the inventive concept described under embodiment A, but at the expense of higher latency and complexity. The energy of the critical band of the reconstructed signal can be measured in the time domain:

[0079]

[0080]

[0081] in, is a high-pass filter or a band-pass filter that matches the critical band. Similarly, the energy of the critical band of the post-filtered reconstructed signal can be written as

[0082]

[0083]

[0084] For frame Decision to activate or deactivate the post filter The decision threshold can be obtained by comparing the energy ratio of the critical bands of the signal before and after applying the post filter to the decision threshold shown below The comparison is formed, wherein "active" indicates that the post-filter is activated, and "inactive" indicates that the post-filter is disabled. In one embodiment of the present invention, the threshold is set to 1. In other words, when the energy above a certain cutoff frequency is higher after applying the post filter, the energy increase is considered to be caused by noise, and the post filter is disabled.

[0085]

[0086] For frame Disabling the post filter can in some embodiments be accomplished by using the decoded signal Instead of the post-filtered version If the filter routine generates a filter difference signal , the filter can be disabled by skipping the decoded signal This is accomplished by subtracting the filter difference signal from the

[0087] In some embodiments, if the energy ratio hovers near a threshold, it may be useful to add some hysteresis to the switching between the primary signal and the post-filtered signal to reduce the switching back and forth. One way to implement hysteresis is to have two thresholds: one for activation and one for deactivation. If the activation threshold is slightly higher than the deactivation threshold, this creates a "dead zone" for the decision variable and reduces the switching back and forth when the variable hovers near the threshold. Another way to implement hysteresis is to determine a count of the number of times the low-pass filtered energy ratio is below (or alternatively, above) the threshold over a period of time, and activate (or deactivate) the post filter after the low-pass filtered energy ratio is below (or alternatively, above) the threshold a predetermined number of times over the period of time.

[0088] Reference will now be made to some embodiments according to the present invention. Fig.11 The decoder 100 (using Fig.10 For example, a module can be stored in Figure 3 The modules are stored in the memory 1003, and these modules can provide instructions, so that when the corresponding communication device processing circuit 1001 executes the instructions of the modules, the processing circuit 1001 performs the corresponding operations of the flowchart.

[0089] Now turn to Fig.11In block 1101, the processing circuit 1001 obtains an energy estimate for at least a portion of a spectrum of a primary signal being reconstructed (i.e., decoded) by the decoder 100. The primary signal reconstruction may be performed in the frequency domain. The operation of block 1101 is similar to the operation of block 800 described above. In various embodiments of the inventive concept, the processing circuit 1001 may obtain the energy estimate by summing energy coefficients of at least a portion of the spectrum in the frequency domain. For example, in some embodiments, the processing circuit 1001 obtains the energy estimate by measuring the energy of a critical band of the reconstructed signal according to the following formula:

[0090]

[0091] Where m is the frame number, is the energy of the critical band of the reconstructed signal, is the reconstructed signal, and the frequency interval is limited and is set to match the frequency range of the critical band.

[0092] Processing circuit 1001 may reconstruct the energy of the critical band of the signal by applying a low pass filter according to the following equation To further process the measurements:

[0093]

[0094] in, and is the low-pass filter coefficient, which depends on the shape of the modified discrete cosine transform MDCT synthesis window and the length of the overlap.

[0095] In block 1103, the processing circuit 1001 obtains an analysis of the discontinuity in the time domain caused by the post filtering of the primary signal. The operation of block 1103 is similar to the operation of block 820 described above. In various embodiments of the inventive concept, the processing circuit 1001 may obtain the analysis of the discontinuity in the time domain by measuring the average energy of the magnitude of the discontinuity. For example, in some embodiments, the processing circuit 1001 performs a filter difference signal according to the following formula: The average energy of the discontinuity is measured by averaging the step size at the subframe boundary.

[0096]

[0097] Where m is the frame number, i is the subframe number, is the average energy of the step size at the subframe boundary, is the number of subframes, and is the sample index of the subframe boundary marking the start of each subframe.

[0098] In block 1105, the processing circuit 1001 generates a decision variable based on the obtained energy estimate and the obtained discontinuity analysis. The operation of block 1105 is similar to the operation of block 830 described above. In various embodiments of the inventive concept, the processing circuit 1001 may generate a decision variable according to the following formula:

[0099]

[0100] in, yes and The energy ratio between is the average energy of the step size at the subframe boundary, is the low-pass filtered energy of the critical band of the reconstructed signal .

[0101] Steering Fig.12 In some embodiments of the present inventive concept, the processing circuit 1001 may limit the decision variable to a maximum value in block 1201 and perform low-pass filtering on the decision variable in block 1203. The operations of blocks 1201 and 1203 are similar to the operations of block 840 described above. In some embodiments, the processing circuit 1001 limits the decision variable and performs low-pass filtering on the decision variable according to the following formula:

[0102]

[0103]

[0104]

[0105] Where m is the number of frames, yes and The energy ratio between is the average energy of the step size at the subframe boundary, is the low-pass filtered energy of the critical band of the reconstructed signal , are the low-pass filter coefficients, and is the upper limit of the energy ratio.

[0106] return Fig.11 , in block 1107, the processing circuit 1001 compares the decision variable with a threshold. For example, as described above, when the decision variable is and When the energy ratio is between Make a comparison.

[0107] In block 1109, the processing circuit 1001 sets the output signal of the decoder 100 to the decoded primary signal or the post-filtered signal (formed by post-filtering) based on the comparison of the decision variable with the threshold. For example, as described above, in some embodiments, the processing circuit 1001 compares the decision variable according to the following formula:

[0108]

[0109] In various embodiments of the present inventive concept, the threshold energy Can be set to a value of 1.

[0110] Fig.13 An example of setting the output signal is shown in FIG. Fig.13 In block 1301 , the processing circuit 1001 compares an energy ratio between an average energy of a step size at a subframe boundary and an energy estimate of at least a portion of a spectrum of a primary signal with a threshold.

[0111] In block 1303 , in response to an energy ratio between an average energy of the step size at a subframe boundary and an energy estimate of at least a portion of the spectrum of the primary signal being less than a threshold, the processing circuit 1001 sets the output signal to a post-filtered signal.

[0112] In block 1305 , in response to an energy ratio between the average energy of the steps at the subframe boundary and the energy of at least a portion of the spectrum of the primary signal being equal to or above a threshold, the processing circuit 1001 sets the output signal to the decoded primary signal.

[0113] In some embodiments of the present inventive concept, if the energy ratio hovers around a threshold, hysteresis may be added to setting the output between the decoded primary signal and the post-filtered signal to reduce the back-and-forth switching.

[0114] Example embodiments are discussed below.

[0115] Embodiment 1. A method for audio decoding, wherein an encoded primary signal is decoded to form a decoded primary signal, and then the decoded primary signal is post-filtered to form a post-filtered signal, wherein the output signal of the decoder is one of the decoded primary signal and the post-filtered signal, the method comprising: obtaining (1101, 800) an energy estimate of at least a portion of the spectrum of the primary signal being reconstructed by the decoder; obtaining (1103, 820) an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal; generating (1105, 830) a decision variable based on the obtained energy estimate and the obtained analysis of the discontinuity; comparing the decision variable with a threshold (1107, 850); and setting (1109) the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable with the threshold.

[0116] Embodiment 2. The method according to embodiment 1, wherein the primary signal reconstruction is performed in the frequency domain.

[0117] Embodiment 3. A method according to embodiment 2, wherein obtaining the energy estimate comprises: summing energy coefficients of the at least a portion of the spectrum in the frequency domain.

[0118] Embodiment 4. The method according to embodiment 3, wherein obtaining the energy estimate comprises: measuring the energy of the critical band of the reconstructed signal according to the following formula

[0119]

[0120] Where m is the frame number, is the energy of the critical band of the reconstructed signal, is the reconstructed signal, and the frequency interval is limited and is set to match the frequency range of the critical band.

[0121] Embodiment 5. The method according to embodiment 4 further comprises: applying a low-pass filter to the energy of the critical frequency band of the reconstructed signal according to the following formula:

[0122]

[0123] in, and is the low-pass filter coefficient, which depends on the shape of the modified discrete cosine transform MDCT synthesis window and the length of the overlap.

[0124] Embodiment 6. The method according to any one of embodiments 1 to 5, wherein obtaining the analysis of the discontinuity in the time domain comprises: measuring an average energy of the magnitude of the discontinuity.

[0125] Embodiment 7. The method according to embodiment 6, wherein measuring the average energy of the magnitude of the discontinuity comprises: calculating the filter difference signal according to the following formula: The step size at the subframe boundary is averaged

[0126]

[0127] Where m is the frame number, i is the subframe number, is the average energy of the step size at the subframe boundary, is the number of subframes, and is the sample index of the subframe boundary marking the start of each subframe.

[0128] Embodiment 8. The method according to any one of embodiments 1 to 7, wherein generating the decision variable comprises: limiting (1201, 840) the decision variable to a maximum value.

[0129] Embodiment 9: The method according to any one of embodiments 1 to 8 further comprises: performing low-pass filtering on the decision variables (1203, 840).

[0130] Embodiment 10. The method according to embodiment 9, wherein generating the decision variable comprises: generating the decision variable according to the following formula

[0131]

[0132]

[0133]

[0134] Where m is the frame number, yes and The energy ratio between is the average energy of the step size of the subframe boundary, is the low-pass filtered energy of the critical band of the reconstructed signal , is the low-pass filter coefficient, and is the upper limit of the energy ratio.

[0135] Embodiment 11. A method according to any one of embodiments 1 to 10, wherein setting the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable with the threshold comprises: in response to an energy ratio between the average energy of the step size at the subframe boundary and the energy estimate of at least a portion of the spectrum of the primary signal being less than a threshold (1301), setting (1303) the output signal to the post-filtered signal; and in response to an energy ratio between the average energy of the step size at the subframe boundary and the energy of at least a portion of the spectrum of the primary signal being equal to the threshold or higher than the threshold (1301), setting (1305) the output signal to the decoded primary signal.

[0136] Embodiment 12. The method according to embodiment 11 also includes: providing hysteresis for the comparison of the decision variable with the threshold to prevent the output signal from being set back and forth between the primary signal and the post-filtered signal when the energy ratio is above and below the threshold level within a specified time period.

[0137] Embodiment 13. A decoder (100) for audio decoding, wherein a primary signal is decoded to form a decoded primary signal, and then the decoded primary signal is post-filtered to form a post-filtered signal, wherein an output signal of the decoder is one of the decoded primary signal and the post-filtered signal, the decoder comprising: a processing circuit (1001); and a memory (1003) coupled to the processing circuit, wherein the memory comprises instructions that, when executed by the processing circuit, cause the decoder to perform operations, the operations comprising: obtaining (1 101, 800) an energy estimate of at least a portion of the spectrum of a primary signal being reconstructed by the decoder; obtaining (1103, 820) an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal; generating (1105, 830) a decision variable based on the obtained energy estimate and the obtained analysis of the discontinuities; comparing the decision variable with a threshold (1107, 850); and setting (1109) the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable with the threshold.

[0138] Embodiment 14. The decoder of embodiment 13, wherein the primary signal reconstruction is performed in the frequency domain.

[0139] Embodiment 15. A decoder according to embodiment 14, wherein, when obtaining the energy estimate, the memory includes additional instructions, which, when executed by the processing circuit, cause the decoder to perform an operation comprising summing the energy coefficients of the at least a portion of the spectrum in the frequency domain.

[0140] Embodiment 16. The decoder of embodiment 15, wherein, when obtaining the energy estimate, the memory includes further instructions that, when executed by the processing circuit, cause the decoder to perform operations comprising measuring the energy of a critical band of the reconstructed signal according to

[0141]

[0142] Where m is the frame number, is the energy of the critical band of the reconstructed signal, is the reconstructed signal, and the frequency interval is limited and is set to match the frequency range of the critical band.

[0143] Embodiment 17. A decoder according to embodiment 16, wherein the memory includes further instructions that, when executed by the processing circuit, cause the decoder to perform further operations, the further operations comprising applying a low pass filter to the energy of the critical bands of the reconstructed signal according to the following equation

[0144]

[0145] in, and is the low-pass filter coefficient, which depends on the shape of the modified discrete cosine transform MDCT synthesis window and the length of the overlap.

[0146] Embodiment 18. A decoder according to any one of embodiments 13 to 17, wherein, when obtaining the analysis of the discontinuity in the time domain, the memory includes further instructions, which, when executed by the processing circuit, cause the decoder to perform operations including measuring the average energy of the magnitude of the discontinuity.

[0147] Embodiment 19. A decoder according to any one of embodiments 13 to 18, wherein when measuring the average energy of the magnitude of the discontinuity, the memory includes further instructions that, when executed by the processing circuit, cause the decoder to perform operations comprising: The step size at the subframe boundary is averaged

[0148]

[0149] Where m is the frame number, i is the subframe number, is the average energy of the step size at the subframe boundary, is the number of subframes, and is the sample index of the subframe boundary marking the start of each subframe.

[0150] Embodiment 20. A decoder according to any one of embodiments 13 to 19, wherein when generating the decision variable, the memory includes further instructions, which when executed by the processing circuit cause the decoder to perform an operation, the operation comprising limiting (1201, 840) the decision variable to a maximum value.

[0151] Embodiment 21. A decoder according to any one of embodiments 13 to 20, wherein the memory includes further instructions, which when executed by the processing circuit cause the decoder to perform operations including low pass filtering the decision variables (1203, 840).

[0152] Embodiment 22. The decoder of embodiment 21, wherein, when generating the decision variable, the memory includes further instructions that, when executed by the processing circuit, cause the decoder to perform operations comprising generating the decision variable according to

[0153]

[0154]

[0155]

[0156] Where m is the frame number, yes and The energy ratio between is the average energy of the step size at the subframe boundary, is the low-pass filtered energy of the critical band of the reconstructed signal , is the low-pass filter coefficient, and is the upper limit of the energy ratio.

[0157] Embodiment 23. A decoder according to any one of embodiments 13 to 22, wherein, when the output signal is set to the primary signal or the post-filtered signal based on a comparison of the decision variable with the threshold, the memory includes additional instructions which, when executed by the processing circuit, cause the decoder to perform operations comprising: setting (1303) the output signal to the post-filtered signal in response to an energy ratio between an average energy of a step size at a subframe boundary and an energy estimate of at least a portion of the spectrum of the primary signal being less than a threshold (1301); and setting (1305) the output signal to the decoded primary signal in response to an energy ratio between an average energy of a step size at a subframe boundary and an energy estimate of at least a portion of the spectrum of the primary signal being equal to or greater than the threshold (1301).

[0158] Embodiment 24. A decoder according to embodiment 23, wherein the memory includes additional instructions, which when executed by the processing circuit cause the decoder to perform additional operations, the additional operations including: providing hysteresis for the comparison of the decision variable with the threshold to prevent the output signal from being set back and forth between the primary signal and the post-filtered signal when the energy ratio is above and below the threshold level within a specified time period.

[0159] Embodiment 25. A decoder (100) adapted to perform operations comprising: obtaining (1101, 800) an energy estimate of at least a portion of a spectrum of a primary signal, the primary signal being reconstructed by a decoder to form a decoded primary signal; obtaining (1103, 820) an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal; generating (1105, 830) a decision variable based on the obtained energy estimate and the obtained analysis of the discontinuities; comparing the decision variable with a threshold (1107, 850); and setting (1109) the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable with the threshold.

[0160] Embodiment 26. The decoder (100) according to embodiment 25, wherein the decoder (100) is adapted to perform the operations according to any one of embodiments 2 to 12.

[0161] Embodiment 27. A computer program comprising program code to be executed by a processing circuit (1001) of a decoder (100), whereby execution of the program code causes the decoder (100) to perform operations comprising: obtaining (1101, 800) an energy estimate of at least a portion of a spectrum of a primary signal, the primary signal being reconstructed by a decoder to form a decoded primary signal; obtaining (1103, 820) an analysis of discontinuities in the time domain caused by post-filtering of the decoded primary signal; generating (1105, 830) a decision variable based on the obtained energy estimate and the obtained analysis of the discontinuities; comparing the decision variable with a threshold value (1107, 850); and setting (1109) the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable with the threshold value.

[0162] Embodiment 28. The computer program according to embodiment 27, comprising further program code, whereby execution of the program code causes the decoder (100) to perform the operations according to any one of embodiments 2 to 12.

[0163] Embodiment 29. A computer program product comprising a non-transitory storage medium, the non-transitory storage medium comprising program code to be executed by a processing circuit (1001) of a decoder (100), whereby execution of the program code causes the decoder (100) to perform operations comprising: obtaining (1101, 800) an energy estimate of at least a portion of a spectrum of a primary signal being reconstructed by the decoder; obtaining (1103, 820) an analysis of discontinuities in the time domain caused by post-filtering of the primary signal; generating (1105, 830) a decision variable based on the obtained energy estimate and the obtained analysis of the discontinuities; comparing the decision variable with a threshold (1107, 850); and setting (1109) the output signal to the decoded primary signal or the post-filtered signal based on the comparison of the decision variable with the threshold.

[0164] Embodiment 30. The computer program product according to embodiment 29, wherein the non-transitory storage medium comprises further program code, whereby execution of the further program code causes the decoder (100) to perform the operations according to any one of embodiments 2 to 12.

[0165] Provided below are explanations of various abbreviations / acronyms used in this disclosure.

[0166] Abbreviations

[0167] BPF Bass Post Filter

[0168] DFT Discrete Fourier Transform

[0169] MDCT modified discrete cosine transform

[0170] EVS Enhanced Voice Service

[0171] QMF Quadrature Mirror Filter Bank

[0172] Additional instructions are provided below.

[0173] Generally, unless clearly given and / or different meanings are implied from the context, all terms used in this article will be interpreted according to their common meaning in the relevant technical field. Unless otherwise clearly stated, all references to "one / an / element, equipment, assembly, device, step, etc." should be openly interpreted as referring to at least one example in an element, equipment, assembly, device, step, etc. Unless a step must be clearly described as after or before another step and / or a step must be after or before another step implicitly, the steps of any method disclosed herein do not have to be performed in the exact order disclosed. Where appropriate, any feature of any embodiment disclosed herein may be applied to any other embodiment. Similarly, any advantage of any embodiment may be applicable to any other embodiment, and vice versa. By the following description, other purposes, features and advantages of the attached embodiments will be apparent.

[0174] Further definitions and embodiments are discussed below.

[0175] In the above description of various embodiments of the inventive concept, it is to be understood that the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the inventive concept. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meanings as commonly understood by ordinary technicians in the field to which the inventive concept belongs. It should also be understood that terms such as those defined in general dictionaries should be interpreted as having meanings consistent with their meanings in the context of this specification and the relevant technology, and not interpreted as ideal or overly superficial meanings, unless so clearly defined herein.

[0176] When an element is referred to as being "connected", "coupled", "responsive" or a variation thereof relative to another element, it may be directly connected, coupled to or responsive to other elements, or there may be an intermediate element. On the contrary, when an element is referred to as being "directly connected", "directly coupled", "directly responsive" or a variation thereof relative to another element, there is no intermediate element. Throughout the text, similar reference numerals represent similar elements. In addition, "coupling", "connection", "responsiveness" or variations thereof used herein may include wireless coupling, connection or response. As used herein, the singular forms "one", "an" and "said" are intended to also include plural forms, unless the context clearly indicates otherwise. For the sake of brevity and / or clarity, known functions or structures may not be described in detail. The term "and / or" (abbreviated as " / ") includes any and all combinations of one or more related listed items.

[0177] It will be understood that, although the terms first, second, third, etc. can be used herein to describe each element / operation, these elements / operations should not be limited by these terms. These terms are only used to distinguish one element / operation from another element / operation. Therefore, the first element / operation in some embodiments can be referred to as the second element / operation in other embodiments without departing from the teaching of the inventive concept. Throughout the specification, the same reference numerals or the same reference signs represent the same or similar elements.

[0178] As used herein, the terms "comprise, comprising, comprises, include, including, includes," "have, has, having," or variations thereof, are open ended and include one or more stated features, integers, elements, steps, components, or functions, but do not preclude the presence or addition of one or more other features, integers, elements, steps, components, functions, or combinations thereof. In addition, as used herein, the commonly used abbreviation "eg," derived from the Latin phrase "exempli gratia," may be used to introduce or specify one or more general examples of a previously mentioned item without being intended as a limitation of that item. The commonly used abbreviation "ie," derived from the Latin phrase "idest," may be used to specify a specific item of a more general reference.

[0179] This article describes example embodiments with reference to block diagrams and / or flowchart illustrations of computer-implemented methods, devices (systems and / or equipment), and / or computer program products. It should be understood that the blocks of the block diagrams and / or flowchart illustrations and the combination of blocks in the block diagrams and / or flowchart illustrations can be implemented by computer program instructions executed by one or more computer circuits. These computer program instructions can be provided to processor circuits of general-purpose computer circuits, special-purpose computer circuits, and / or other programmable data processing circuits to produce a machine, so that instructions executed by processors of computers and / or other programmable data processing devices convert and control transistors, values ​​stored in memory locations, and other hardware components within such circuits to implement the functions / actions specified in the block diagrams and / or flowcharts, and thereby create a device (functional body) and / or structure for implementing the functions / actions specified in the block diagrams and / or flowcharts.

[0180] These computer program instructions may also be stored in a tangible computer-readable medium that can direct a computer or other programmable data processing device to act in a specific manner so that the instructions stored in the computer-readable medium produce an article of manufacture, the article of manufacture including instructions for implementing the functions / actions specified in the blocks of the block diagrams and / or flow charts. Therefore, embodiments of the present inventive concept may be implemented in hardware and / or in software (including firmware, resident software, microcode, etc.) running on a processor such as a digital signal processor, which may be collectively referred to as a "circuit", "module" or variations thereof.

[0181] It should also be noted that in some alternative implementations, the function / action marked in the frame may not occur in the order marked in the flow chart. For example, depending on the function / action involved, the two frames shown in succession can actually be performed substantially simultaneously, or the frame can sometimes be performed in the opposite order. In addition, the function of a given frame of a flow chart and / or block diagram can be divided into multiple frames and / or the function of two or more frames of a flow chart and / or block diagram can be integrated at least in part. Finally, without departing from the scope of the inventive concept, other frames can be added / inserted between the frames shown, and / or the frame / operation can be omitted. In addition, although some figures include arrows on the communication path to illustrate the main direction of communication, it should be understood that communication can occur in the direction opposite to the arrows depicted.

[0182] Under the premise of not departing from the principle of the inventive concept basically, many changes and modifications can be made to the embodiment. All these changes and modifications are intended to be included in the scope of the inventive concept herein. Therefore, the above-mentioned subject matter should be understood as exemplary and not restrictive, and the examples of the embodiments are intended to cover all these modifications, improvements and other embodiments that fall within the spirit and scope of the inventive concept. Therefore, to the maximum extent permitted by law, the scope of the inventive concept should be determined by the widest permissible interpretation of the present disclosure including the examples of the embodiments and their equivalents, and should not be limited to or restricted to the previous specific embodiments.

Claims

1. A method for audio decoding, wherein: Decoding an encoded signal to form a decoded signal, the method comprising: obtaining an energy estimate of at least a portion of a frequency spectrum of the decoded signal; Generate post-filter difference signal , the post-filter difference signal Describe the difference between the post-filtered decoded signal and the unfiltered decoded signal in the time domain; By applying the post-filter difference signal The size of the discontinuity is measured by averaging the step sizes at the subframe boundaries; forming a decision variable based on a ratio between an average energy of the step at the subframe boundary and the energy estimate; comparing the decision variable to a threshold; and According to the result of the comparison, it is decided whether the output signal is set as the decoded signal or the post-filtered signal.

2. The method according to claim 1, wherein: Obtaining the energy estimate comprises summing energy coefficients of the at least a portion of the spectrum.

3. The method according to claim 2, wherein: Obtaining the energy estimate comprises: measuring the energy of the critical band of the decoded signal according to the following formula Where m is the frame number, is the energy of the critical band of the decoded signal, is the spectrum of the decoded signal, and the frequency interval is limited and is set to match the frequency range of the critical band.

4. The method according to claim 3, further comprising: A low-pass filter is applied to the energy of the critical bands of the decoded signal according to in, And is the low-pass filter coefficient.

5. The method according to any one of claims 1 to 4, wherein: Measuring the size of the discontinuity at the subframe boundary includes measuring an average energy of the size of the discontinuity.

6. The method according to claim 5, wherein: Measuring the average energy of the magnitude of the discontinuity includes: calculating the average energy of the post-filter difference signal according to the following formula: The step size at the subframe boundary is averaged Where m is the frame number, i is the subframe number, is the average energy of the step size at the subframe boundary, is the number of subframes, and is the sample index of the subframe boundary marking the start of each subframe.

7. The method according to any one of claims 1 to 6, wherein: Forming the decision variable includes limiting the decision variable to a maximum value.

8. The method according to any one of claims 1 to 7, further comprising: The decision variables are low-pass filtered.

9. The method according to claim 8, wherein: Forming the decision variables includes: generating the decision variables according to the following formula Where m is the frame number, yes and The energy ratio between is the average energy of the step size at the subframe boundary, is the low-pass filtered energy of the critical band of the decoded signal , are the low-pass filter coefficients, and is the upper limit of the energy ratio.

10. The method according to any one of claims 1 to 9, wherein: Setting the output signal to the decoded signal or the post-filtered signal based on a comparison of the decision variable with the threshold comprises: in response to a ratio between the size of the discontinuity and the energy estimate being less than a threshold, setting the output signal to the post-filtered signal; and in response to an energy ratio between the size of the discontinuity and the energy estimate being equal to or higher than the threshold, setting the output signal to the decoded signal.

11. A decoder (100) for audio decoding, wherein: decoding an encoded signal to form a decoded signal, the decoder comprising a processing circuit and a memory coupled to the processing circuit, wherein the memory comprises instructions that, when executed by the processing circuit, cause the decoder to: obtaining an energy estimate of at least a portion of a frequency spectrum of the decoded signal; Generate post-filter difference signal , the post-filter difference signal Describe the difference between the post-filtered decoded signal and the unfiltered decoded signal in the time domain; By applying the post-filter difference signal The size of the discontinuity is measured by averaging the step sizes at the subframe boundaries; forming a decision variable based on a ratio between an average energy of the step at the subframe boundary and the energy estimate; comparing the decision variable to a threshold; and According to the result of the comparison, it is decided whether the output signal is set as the decoded signal or the post-filtered signal.

12. The decoder according to claim 11, wherein: Obtaining the energy estimate comprises summing energy coefficients of the at least a portion of the spectrum.

13. The decoder according to claim 12, wherein: Obtaining the energy estimate comprises: measuring the energy of the critical band of the decoded signal according to the following formula Where m is the frame number, is the energy of the critical band of the decoded signal, is the spectrum of the decoded signal, and the frequency interval is limited and is set to match the frequency range of the critical band.

14. The decoder according to claim 13, wherein: The memory includes further instructions which, when executed by the processing circuit, cause the decoder to: apply a low pass filter to the energy of the critical bands of the decoded signal according to in, And is the low-pass filter coefficient.

15. The decoder according to any one of claims 11 to 14, wherein: Measuring the size of the discontinuity at the subframe boundary includes measuring an average energy of the size of the discontinuity.

16. The decoder according to claim 15, wherein: Measuring the average energy of the magnitude of the discontinuity includes: calculating the average energy of the post-filter difference signal according to the following formula: The step size at the subframe boundary is averaged Where m is the frame number, i is the subframe number, is the average energy of the step size at the subframe boundary, is the number of subframes, and is the sample index of the subframe boundary marking the start of each subframe.

17. A decoder according to any one of claims 11 to 16, wherein: Forming the decision variable includes limiting the decision variable to a maximum value.

18. A decoder according to any one of claims 11 to 17, wherein: The memory comprises further instructions which, when executed by the processing circuit, cause the decoder to: low pass filter the decision variable.

19. The decoder according to claim 18, wherein: Forming the decision variables includes: generating the decision variables according to the following formula Where m is the frame number, yes and The energy ratio between is the average energy of the step size at the subframe boundary, is the low-pass filtered energy of the critical band of the decoded signal , are the low-pass filter coefficients, and is the upper limit of the energy ratio.

20. A decoder according to any one of claims 11 to 19, wherein: Setting the output signal to the decoded signal or the post-filtered signal based on a comparison of the decision variable with the threshold comprises: in response to a ratio between the size of the discontinuity and the energy estimate being less than a threshold, setting the output signal to the post-filtered signal; and in response to an energy ratio between the size of the discontinuity and the energy estimate being equal to or higher than the threshold, setting the output signal to the decoded signal.

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