US20150332698A1 - Apparatus and method for selecting one of a first encoding algorithm and a second encoding algorithm - Google Patents

Apparatus and method for selecting one of a first encoding algorithm and a second encoding algorithm Download PDF

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US20150332698A1
US20150332698A1 US14/812,138 US201514812138A US2015332698A1 US 20150332698 A1 US20150332698 A1 US 20150332698A1 US 201514812138 A US201514812138 A US 201514812138A US 2015332698 A1 US2015332698 A1 US 2015332698A1
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audio signal
estimated
adaptive codebook
weighted
encoding
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Emmanuel RAVELLI
Stefan DOEHLA
Guillaume Fuchs
Eleni FOTOPOULOU
Christian Helmrich
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Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
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Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
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Publication of US20150332698A1 publication Critical patent/US20150332698A1/en
Assigned to FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E.V. reassignment FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E.V. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: RAVELLI, EMMANUEL, DOEHLA, STEFAN, FOTOPOULOU, Eleni, Helmrich, Christian, FUCHS, GUILLAUME
Priority to US16/148,993 priority patent/US10622000B2/en
Priority to US16/836,857 priority patent/US11521631B2/en
Priority to US18/058,389 priority patent/US11908485B2/en
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
    • G10L19/16—Vocoder architecture
    • G10L19/18—Vocoders using multiple modes
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
    • G10L19/08—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters
    • G10L19/12—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders
    • G10L19/125—Pitch excitation, e.g. pitch synchronous innovation CELP [PSI-CELP]
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
    • G10L19/16—Vocoder architecture
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
    • G10L19/16—Vocoder architecture
    • G10L19/18—Vocoders using multiple modes
    • G10L19/22—Mode decision, i.e. based on audio signal content versus external parameters
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M7/00—Conversion of a code where information is represented by a given sequence or number of digits to a code where the same, similar or subset of information is represented by a different sequence or number of digits
    • H03M7/30—Compression; Expansion; Suppression of unnecessary data, e.g. redundancy reduction
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/02—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders
    • G10L19/0212—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders using orthogonal transformation
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
    • G10L19/08—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters

Definitions

  • the present invention relates to audio coding and, in particular, to switched audio coding, where, for different portions of an audio signal, the encoded signal is generated using different encoding algorithms.
  • Switched audio coders which determine different encoding algorithms for different portions of the audio signal are known.
  • switched audio coders provide for switching between two different modes, i.e. algorithms, such as ACELP (Algebraic Code Excited Linear Prediction) and TCX (Transform Coded Excitation).
  • ACELP Algebraic Code Excited Linear Prediction
  • TCX Transform Coded Excitation
  • the LPD mode of MPEG USAC is based on the two different modes ACELP and TCX.
  • ACELP provides better quality for speech-like and transient-like signals.
  • TCX provides better quality for music-like and noise-like signals.
  • the encoder decides which mode to use on a frame-by-frame basis. The decision made by the encoder is critical for the codec quality. A single wrong decision can produce a strong artifact, particularly at low-bitrates.
  • the most-straightforward approach for deciding which mode to use is a closed-loop mode selection, i.e. to perform a complete encoding/decoding of both modes, then compute a selection criteria (e.g. segmental SNR) for both modes based on the audio signal and the coded/decoded audio signals, and finally choose a mode based on the selection criteria.
  • a selection criteria e.g. segmental SNR
  • Open-loop selection consists of not performing a complete encoding/decoding of both modes but instead choose one mode using a selection criteria computed with low-complexity.
  • the worst-case complexity is then reduced by the complexity of the least-complex mode (usually TCX), minus the complexity needed to compute the selection criteria.
  • the save in complexity is usually significant, which makes this kind of approach attractive when the codec worst-case complexity is constrained.
  • the AMR-WB+ standard (defined in the International Standard 3GPP TS 26.290 V6.1.0 2004-12) includes an open-loop mode selection, used to decide between all combinations of ACELP/TCX20/TCX40/TCX80 in a 80 ms frame. It is described in Section 5.2.4 of 3GPP TS 26.290. It is also described in the conference paper “Low Complex Audio Encoding for Mobile, Multimedia, VTC 2006, Makinen et al.” and U.S. Pat. No. 7,747,430 B2 and U.S. Pat. No. 7,739,120 B2 going back to the author of this conference paper.
  • U.S. Pat. No. 7,747,430 B2 discloses an open-loop mode selection based on an analysis of long term prediction parameters.
  • U.S. Pat. No. 7,739,120 B2 discloses an open-loop mode selection based on signal characteristics indicating the type of audio content in respective sections of an audio signal, wherein, if such a selection is not viable, the selection is further based on a statistical evaluation carried out for respectively neighboring sections.
  • the open-loop mode selection of AMR-WB+ can be described in two main steps.
  • the first main step several features are calculated on the audio signal, such as standard deviation of energy levels, low-frequency/high-frequency energy relation, total energy, ISP (immittance spectral pair) distance, pitch lags and gains, spectral tilt. These features are then used to make a choice between ACELP and TCX, using a simple threshold-based classifier. If TCX is selected in the first main step, then the second main step decides between the possible combinations of TCX20/TCX40/TCX80 in a closed-loop manner.
  • WO 2012/110448 A1 discloses an approach for deciding between two encoding algorithms having different characteristics based on a transient detection result and a quality result of an audio signal.
  • applying a hysteresis is disclosed, wherein the hysteresis relies on the selections made in the past, i.e. for the earlier portions of the audio signal.
  • an apparatus for selecting one of a first encoding algorithm having a first characteristic and a second encoding algorithm having a second characteristic for encoding a portion of an audio signal to acquire an encoded version of the portion of the audio signal may have: a first estimator for estimating a first quality measure for the portion of the audio signal, the first quality measure being associated with the first encoding algorithm, without actually encoding and decoding the portion of the audio signal using the first encoding algorithm; a second estimator for estimating a second quality measure for the portion of the audio signal, the second quality measure being associated with the second encoding algorithm, without actually encoding and decoding the portion of the audio signal using the second encoding algorithm; and a controller for selecting the first encoding algorithm or the second encoding algorithm based on a comparison between the first quality measure and the second quality measure, wherein the first and second quality measures are SNRs (signal to noise ratio) or segmental SNRs of the corresponding portion of a weighted version of the audio signal.
  • an apparatus for encoding a portion of an audio signal may have: the apparatus according to claim 1 , a first encoder stage for performing the first encoding algorithm and a second encoder stage for performing the second encoding algorithm, wherein the apparatus for encoding is configured to encode the portion of the audio signal using the first encoding algorithm or the second encoding algorithm depending on the selection by the controller.
  • a system for encoding and decoding may have an apparatus for encoding according to claim 11 and a decoder configured to receive the encoded version of the portion of the audio signal and an indication of the algorithm used to encode the portion of the audio signal and to decode the encoded version of the portion of the audio signal using the indicated algorithm.
  • a method for selecting one of a first encoding algorithm having a first characteristic and a second encoding algorithm having a second characteristic for encoding a portion of an audio signal to acquire an encoded version of the portion of the audio signal may have the steps of: estimating a first quality measure for the portion of the audio signal, the first quality measure being associated with the first encoding algorithm, without actually encoding and decoding the portion of the audio signal using the first encoding algorithm; estimating a second quality measure for the portion of the audio signal, the second quality measure being associated with the second encoding algorithm, without actually encoding and decoding the portion of the audio signal using the second coding algorithm; and selecting the first encoding algorithm or the second encoding algorithm based on a comparison between the first quality measure and the second quality measure, wherein the first and second quality measures are SNRs (signal to noise ratio) or segmental SNRs of the corresponding portion of a weighted version of the audio signal.
  • the first and second quality measures are SNRs (signal to noise
  • Another embodiment may have a computer program having a program code for performing, when running on a computer, the method of claim 13 .
  • Embodiments of the invention are based on the recognition that an open-loop selection with improved performance can be implemented by estimating a quality measure for each of first and second encoding algorithms and selecting one of the encoding algorithms based on a comparison between the first and second quality measures.
  • the quality measures are estimated, i.e. the audio signal is not actually encoded and decoded to obtain the quality measures.
  • the quality measures can be obtained with reduced complexity.
  • the mode selection may then be performed using the estimated quality measures comparable to a closed-loop mode selection.
  • an open-loop mode selection where the segmental SNR of ACELP and TCX are first estimated with low complexity is implemented. And then the mode selection is performed using these estimated segmental SNR values, like in a closed-loop mode selection.
  • Embodiments of the invention do not employ a classical features+classifier approach like it is done in the open-loop mode selection of AMR-WB+. But instead, embodiments of the invention try to estimate a quality measure of each mode and select the mode that gives the best quality.
  • FIG. 1 shows a schematic view of an embodiment of an apparatus for selecting one of a first encoding algorithm and a second encoding algorithm
  • FIG. 2 shows a schematic view of an embodiment of an apparatus for encoding an audio signal
  • FIG. 3 shows a schematic view of an embodiment of an apparatus for selecting one of a first encoding algorithm and a second encoding algorithm
  • FIGS. 4 a and 4 b possible representations of SNR and segmental SNR.
  • FIG. 1 shows an apparatus 10 for selecting one of a first encoding algorithm, such as a TCX algorithm, and a second encoding algorithm, such as an ACELP algorithm, as the encoder for encoding a portion of an audio signal.
  • the apparatus 10 comprises a first estimator 12 for estimating a first quality measure for the signal portion.
  • the first quality measure is associated with the first encoding algorithm.
  • the first estimator 12 estimates a first quality measure which the portion of the audio signal would have if encoded and decoded using the first encoding algorithm, without actually encoding and decoding the portion of the audio signal using the first encoding algorithm.
  • the apparatus 10 comprises a second estimator 14 for estimating a second quality measure for the signal portion.
  • the second quality measure is associated with the second encoding algorithm.
  • the second estimator 14 estimates the second quality measure which the portion of the audio signal would have if encoded and decoded using the second encoding algorithm, without actually encoding and decoding the portion of the audio signal using the second encoding algorithm.
  • the apparatus 10 comprises a controller 16 for selecting the first encoding algorithm or the second encoding algorithm based on a comparison between the first quality measure and the second quality measure.
  • the controller may comprise an output 18 indicating the selected encoding algorithm.
  • the first characteristic associated with the first encoding algorithm is better suited for music-like and noise-like signals
  • the second encoding characteristic associated with the second encoding algorithm is better suited for speech-like and transient-like signals.
  • the first encoding algorithm is an audio coding algorithm, such as a transform coding algorithm, e.g. a MDCT (modified discrete cosine transform) encoding algorithm, such as a TCX (transform coding excitation) encoding algorithm.
  • Other transform coding algorithms may be based on an FFT transform or any other transform or filterbank.
  • the second encoding algorithm is a speech encoding algorithm, such as a CELP (code excited linear prediction) coding algorithm, such as an ACELP (algebraic code excited linear prediction) coding algorithm.
  • the quality measure represents a perceptual quality measure.
  • a single value which is an estimation of the subjective quality of the first coding algorithm and a single value which is an estimation of the subjective quality of the second coding algorithm may be computed.
  • the encoding algorithm which gives the best estimated subjective quality may be chosen just based on the comparison of these two values. This is different from what is done in the AMR-WB+ standard where many features representing different characteristics of the signal are computed and, then, a classifier is applied to decide which algorithm to choose.
  • the respective quality measure is estimated based on a portion of the weighted audio signal, i.e. a weighted version of the audio signal.
  • the weighted audio signal can be defined as an audio signal filtered by a weighting function, where the weighting function is a weighted LPC filter A(z/g) with A(z) an LPC filter and g a weight between 0 and 1 such as 0.68. It turned out that good measures of perceptual quality can be obtained in this manner. Note that the LPC filter A(z) and the weighted LPC filter A(z/g) are determined in a pre-processing stage and that they are also used in both encoding algorithms.
  • the weighting function may be a linear filter, a FIR filter or a linear prediction filter.
  • the quality measure is the segmental SNR (signal to noise ratio) in the weighted signal domain. It turned out that the segmental SNR in the weighted signal domain represents a good measure of the perceptual quality and, therefore, can be used as the quality measure in a beneficial manner. This is also the quality measure used in both ACELP and TCX encoding algorithms to estimate the encoding parameters.
  • Another quality measure may be the SNR in the weighted signal domain.
  • Other quality measures may be the segmental SNR, the SNR of the corresponding portion of the audio signal in the non-weighted signal domain, i.e. not filtered by the (weighted) LPC coefficients.
  • Other quality measures may be the cepstral distortion or the noise-to-mask ratio (NMR).
  • SNR compares the original and processed audio signals (such as speech signals) sample by sample. Its goal is to measure the distortion of waveform coders that reproduce the input waveform. SNR may be calculated as shown in FIG. 4 a , where x(i) and y(i) are the original and the processed samples indexed by I and N is the total number of samples.
  • Segmental SNR instead of working on the whole signal, calculates the average of the SNR values of short segments, such as 1 to 10 ms, such as 5 ms. SNR may be calculated as shown in FIG. 4 b , where N and M are the segment length and the number of segments, respectively.
  • the portion of the audio signal represents a frame of the audio signal which is obtained by windowing the audio signal and selection of an appropriate encoding algorithm is performed for a plurality of successive frames obtained by windowing an audio signal.
  • portion and “frame” are used in an exchangeable manner.
  • each frame is divided into subframes and segmental SNR is estimated for each frame by calculating SNR for each subframe, converted in dB and calculating the average of the subframe SNRs in dB.
  • the respective quality measure is estimated based on the energy of a portion of the weighted audio signal and based on an estimated distortion introduced when encoding the signal portion by the respective algorithm, wherein the first and second estimators are configured to determine the estimated distortions dependent on the energy of a weighted audio signal.
  • an estimated quantizer distortion introduced by a quantizer used in the first encoding algorithm when quantizing the portion of the audio signal is determined and the first quality measure is determined based on the energy of the portion of the weighted audio signal and the estimated quantizer distortion.
  • a global gain for the portion of the audio signal may be estimated such that the portion of the audio signal would produce a given target bitrate when encoded with a quantizer and an entropy encoder used in the first encoding algorithm, wherein the estimated quantizer distortion is determined based on the estimated global gain.
  • the estimated quantizer distortion may be determined based on a power of the estimated gain.
  • D is the estimated quantizer distortion
  • G is the estimated global gain.
  • the quantizer distortion may be determined form the global gain in a different manner.
  • a quality measure such as a segmental SNR, which would be obtained when encoding and decoding the portion of the audio signal using the first encoding algorithm, such as the TCX algorithm, can be estimated in an appropriate manner by using the above features in any combination thereof.
  • the first quality measure is a segmental SNR and the segmental SNR is estimated by calculating an estimated SNR associated with each of a plurality of sub-portions of the portion of the audio signal based on an energy of the corresponding sub-portion of the weighted audio signal and the estimated quantizer distortion and by calculating an average of the SNRs associated with the sub-portions of the portion of the weighted audio signal to obtain the estimated segmental SNR for the portion of the weighted audio signal.
  • an estimated adaptive codebook distortion introduced by an adaptive codebook used in the second encoding algorithm when using the adaptive codebook to encode the portion of the audio signal is determined, and the second quality measure is estimated based on an energy of the portion of the weighted audio signal and the estimated adaptive codebook distortion.
  • the adaptive codebook may be approximated based on a version of the sub-portion of the weighted audio signal shifted to the past by a pitch-lag determined in a pre-processing stage, an adaptive codebook gain may be estimated such that an error between the sub-portion of the portion of the weighted audio signal and the approximated adaptive codebook is minimized, and an estimated adaptive codebook distortion may be determined based on the energy of an error between the sub-portion of the portion of the weighted audio signal and the approximated adaptive codebook scaled by the adaptive codebook gain.
  • the estimated adaptive codebook distortion determined for each sub-portion of the portion of the audio signal may be reduced by a constant factor in order to take into consideration a reduction of the distortion which is achieved by an innovative codebook in the second encoding algorithm.
  • the second quality measure is a segmental SNR and the segmental SNR is estimated by calculating an estimated SNR associated with each sub-portion based on the energy the corresponding sub-portion of the weighted audio signal and the estimated adaptive codebook distortion and by calculating an average of the SNRs associated with the sub-portions to obtain the estimated segmental SNR.
  • the adaptive codebook is approximated based on a version of the portion of the weighted audio signal shifted to the past by a pitch-lag determined in a pre-processing stage, an adaptive codebook gain is estimated such that an error between the portion of the weighted audio signal and the approximated adaptive codebook is minimized, and the estimated adaptive codebook distortion is determined based on the energy between the portion of the weighted audio signal and the approximated adaptive codebook scaled by the adaptive codebook gain.
  • the estimated adaptive codebook distortion can be determined with low complexity.
  • the quality measure such as a segmental SNR
  • the second encoding algorithm such as an ACELP algorithm
  • a hysteresis mechanism is used in comparing the estimated quality measures. This can make the decision which algorithm is to be used more stable.
  • the hysteresis mechanism can depend on the estimated quality measures (such as the difference therebetween) and other parameters, such as statistics about previous decisions, the number of temporally stationary frames, transients in the frames. As far as such hysteresis mechanisms are concerned, reference can be made to WO 2012/110448 A1, for example.
  • an encoder for encoding an audio signal comprises the apparatus 10 , a stage for performing the first encoding algorithm and a stage for performing the second encoding algorithm, wherein the encoder is configured to encode the portion of the audio signal using the first encoding algorithm or the second encoding algorithm depending on the selection by the controller 16 .
  • a system for encoding and decoding comprises the encoder and a decoder configured to receive the encoded version of the portion of the audio signal and an indication of the algorithm used to encode the portion of the audio signal and to decode the encoded version of the portion of the audio signal using the indicated algorithm.
  • FIG. 2 Before describing an embodiment of the first estimator 12 and the second estimator 14 in detail referring to FIG. 3 , an embodiment of an encoder 20 is described referring to FIG. 2 .
  • the encoder 20 comprises the first estimator 12 , the second estimator 14 , the controller 16 , a pre-processing unit 22 , a switch 24 , a first encoder stage 26 configured to perform a TCX algorithm, a second encoder stage 28 configured to perform an ACELP algorithm, and an output interface 30 .
  • the pre-processing unit 22 may be part of a common USAC encoder and may be configured to output the LPC coefficients, the weighted LPC coefficients, the weighted audio signal, and a set of pitch lags. It is to be noted that all these parameters are used in both encoding algorithms, i.e. the TCX algorithm and the ACELP algorithm. Thus, such parameters have not to be computed for the open-loop mode decision additionally.
  • the advantage of using already computed parameters in the open-loop mode decision is complexity saving.
  • An input audio signal 40 is provided on an input line.
  • the input audio signal 40 is applied to the first estimator 12 , the pre-processing unit 22 and both encoder stages 26 , 28 .
  • the pre-processing unit 22 processes the input audio signal in a conventional manner to derive LPC coefficients and weighted LPC coefficients 42 and to filter the audio signal 40 with the weighted LPC coefficients 42 to obtain the weighted audio signal 44 .
  • the pre-processing unit 22 outputs the weighted LPC coefficients 42 , the weighted audio signal 44 and a set of pitch-lags 48 .
  • the weighted LPC coefficients 42 and the weighted audio signal 44 may be segmented into frames or sub-frames. The segmentation may be obtained by windowing the audio signal in an appropriate manner.
  • quantized LPC coefficients or quantized weighted LPC coefficients may be used.
  • LPC coefficients is intended to encompass “quantized LPC coefficients” as well
  • weighted LPC coefficients is intended to encompass “weighted quantized LPC coefficients” as well.
  • the TCX algorithm of USAC uses the quantized weighted LPC coefficients to shape the MCDT spectrum.
  • the first estimator 12 receives the audio signal 40 , the weighted LPC coefficients 42 and the weighted audio signal 44 , estimates the first quality measure 46 based thereon and outputs the first quality measure to the controller 16 .
  • the second estimator 16 receives the weighted audio signal 44 and the set of pitch lags 48 , estimates the second quality measure 50 based thereon and outputs the second quality measure 50 to the controller 16 .
  • the weighted LPC coefficients 42 , the weighted audio signal 44 and the set of pitch lags 48 are already computed in a previous module (i.e. the pre-processing unit 22 ) and, therefore, are available for no cost.
  • the controller takes a decision to select either the TCX algorithm or the ACELP algorithm based on a comparison of the received quality measures. As indicated above, the controller may use a hysteresis mechanism in deciding which algorithm to be used. Selection of the first encoder stage 26 or the second encoder stage 28 is schematically shown in FIG. 2 by means of switch 24 which is controlled by a control signal 52 output by the controller 16 .
  • the control signal 52 indicates whether the first encoder stage 26 or the second encoder stage 28 is to be used. Based on the control signal 52 , the signals that may be used and are schematically indicated by arrow 54 in FIG.
  • the selected encoder stage applies the associated encoding algorithm and outputs the encoded representation 56 or 58 to the output interface 30 .
  • the output interface 30 may be configured to output an encoded audio signal which may comprise among other data the encoded representation 56 or 58 , the LPC coefficients or weighted LPC coefficients, parameters for the selected encoding algorithm and information about the selected encoding algorithm.
  • FIG. 3 shows the first estimator 12 and the second estimator 14 and the functionalities thereof in the form of flowcharts showing the respective estimation step-by-step.
  • the first (TCX) estimator receives the audio signal 40 (input signal), the weighted LPC coefficients 42 and the weighted audio signal 44 as inputs.
  • step 100 the audio signal 40 is windowed. Windowing may take place with a 10 ms low-overlap sine window.
  • the block-size may be increased by 5 ms, the left-side of the window may be rectangular and the windowed zero impulse response of the ACELP synthesis filter may be removed from the windowed input signal. This is similar as what is done in the TCX algorithm.
  • a frame of the audio signal 40 which represents a portion of the audio signal, is output from step 100 .
  • step 102 the windowed audio signal, i.e. the resulting frame, is transformed with a MDCT (modified discrete cosine transform).
  • step 104 spectrum shaping is performed by shaping the MDCT spectrum with the weighted LPC coefficients.
  • a global gain G is estimated such that the weighted spectrum quantized with gain G would produce a given target R, when encoded with an entropy coder, e.g. an arithmetic coder.
  • an entropy coder e.g. an arithmetic coder.
  • the term “global gain” is used since one gain is determined for the whole frame.
  • variables used in gain estimation are initialized by:
  • the result of the iteration is the offset value.
  • the specific manner in which the global gain is estimated may vary dependent on the quantizer and the entropy coder used.
  • a scalar quantizer with an arithmetic encoder is assumed.
  • Other TCX approaches may use a different quantizer and it is understood by those skilled in the art how to estimate the global gain for such different quantizers.
  • the AMR-WB+ standard assumes that a RE8 lattice quantizer is used.
  • estimation of the global gain could be estimated as described in chapter 5.3.5.7 on page 34 of 3GPP TS 26.290 V6.1.0 2004-12, wherein a fixed target bitrate is assumed.
  • step 108 After having estimated the global gain in step 106 , distortion estimation takes place in step 108 .
  • the quantizer distortion is approximated based on the estimated global gain. In the present embodiment it is assumed that a uniform scalar quantizer is used.
  • segmental SNR calculation is performed in step 110 .
  • the SNR in each sub-frame of the frame is calculated as the ratio of the weighted audio signal energy and the distortion D which is assumed to be constant in the subframes. For example the frame is split into four consecutive sub-frames (see FIG. 4 ).
  • the segmental SNR is then the average of the SNRs of the four sub-frames and may be indicated in dB.
  • This approach permits estimation of the first segmental SNR which would be obtained when actually encoding and decoding the subject frame using the TCX algorithm, however without having to actually encode and decode the audio signal and, therefore, with a strongly reduced complexity and reduced computing time.
  • the second estimator 14 receives the weighted audio signal 44 and the set of pitch lags 48 which is already computed in the pre-processing unit 22 .
  • the adaptive codebook is approximated by simply using the weighted audio signal and the pitch-lag T.
  • the adaptive codebook is approximated by
  • an adaptive codebook gain for each sub-frame is determined.
  • the codebook gain G is estimated such that it minimizes the error between the weighted audio signal and the approximated adaptive-codebook. This can be done by simply comparing the differences between both signals for each sample and finding a gain such that the sum of these differences is minimal.
  • step 116 the adaptive codebook distortion for each sub-frame is determined.
  • the distortion D introduced by the adaptive codebook is simply the energy of the error between the weighted audio signal and the approximated adaptive-codebook scaled by the gain G.
  • the distortions determined in step 116 may be adjusted in an optional step 118 in order to take the innovative codebook into consideration.
  • the distortion of the innovative codebook used in ACELP algorithms may be simply estimated as a constant value. In the described embodiment of the invention, it is simply assumed that the innovative codebook reduces the distortion D by a constant factor.
  • the distortions obtained in step 116 for each sub-frame may be multiplied in step 118 by a constant factor, such as a constant factor in the order of 0 to 1, such as 0.055.
  • step 120 calculation of the segmental SNR takes place.
  • the SNR is calculated as the ratio of the weighted audio signal energy and the distortion D.
  • the segmental SNR is then the mean of the SNR of the four sub-frames and may be indicated in dB.
  • This approach permits estimation of the second SNR which would be obtained when actually encoding and decoding the subject frame using the ACELP algorithm, however without having to actually encode and decode the audio signal and, therefore, with a strongly reduced complexity and reduced computing time.
  • the first and second estimators 12 and 14 output the estimated segmental SNRs 46 , 50 to the controller 16 and the controller 16 takes a decision which algorithm is to be used for the associated portion of the audio signal based on the estimated segmental SNRs 46 , 50 .
  • the controller may optionally use a hysteresis mechanism in order to make the decision more stable. For example, the same hysteresis mechanism as in the closed-loop decision may be used with slightly different tuning parameters.
  • Such a hysteresis mechanism may compute a value “dsnr” which can depend on the estimated segmental SNRs (such as the difference therebetween) and other parameters, such as statistics about previous decisions, the number of temporally stationary frames, and transients in the frames.
  • the controller may select the encoding algorithm having the higher estimated SNR, i.e. ACELP is selected if the second estimated SNR is higher less than the first estimated SNR and TCX is selected if the first estimated SNR is higher than the second estimated SNR.
  • the controller may select the encoding algorithm according to the following decision rule, wherein Decp_snr is the second estimated SNR and tcx_snr is the first estimated SNR:
  • embodiments of the invention permit for estimating segmental SNRs and selection of an appropriate encoding algorithm in a simple and accurate manner.
  • the segmental SNRs are estimated by calculating an average of SNRs estimated for respective sub-frames.
  • the SNR of a whole frame could be estimated without dividing the frame into sub-frames.
  • Embodiments of the invention permit for a strong reduction in computing time when compared to a closed-loop selection since a number of steps involved in the closed-loop selection are omitted.
  • aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus.
  • Embodiments of the apparatuses described herein and the features thereof may be implemented by a computer, one or more processors, one or more micro-processors, field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs) and the like or combinations thereof, which are configured or programmed in order to provide the described functionalities.
  • processors one or more processors, one or more micro-processors, field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs) and the like or combinations thereof, which are configured or programmed in order to provide the described functionalities.
  • FPGAs field-programmable gate arrays
  • ASICs application specific integrated circuits
  • Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus.
  • embodiments of the invention can be implemented in hardware or in software.
  • the implementation can be performed using a non-transitory storage medium such as a digital storage medium, for example a floppy disc, a DVD, a Blu-Ray, a CD, a ROM, a PROM, and EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable.
  • Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.
  • embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer.
  • the program code may, for example, be stored on a machine readable carrier.
  • inventions comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier.
  • an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.
  • a further embodiment of the inventive method is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein.
  • the data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitionary.
  • a further embodiment of the invention method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein.
  • the data stream or the sequence of signals may, for example, be configured to be transferred via a data communication connection, for example, via the internet.
  • a further embodiment comprises a processing means, for example, a computer or a programmable logic device, configured to, or programmed to, perform one of the methods described herein.
  • a processing means for example, a computer or a programmable logic device, configured to, or programmed to, perform one of the methods described herein.
  • a further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.
  • a further embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of the methods described herein to a receiver.
  • the receiver may, for example, be a computer, a mobile device, a memory device or the like.
  • the apparatus or system may, for example, comprise a file server for transferring the computer program to the receiver.
  • a programmable logic device for example, a field programmable gate array
  • a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein.
  • the methods are advantageously performed by any hardware apparatus.

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