EP0573398A2
Expired / Lapsed
EP
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C.E.L.P. Vocoder
Assignee: The AT&T MVPD Group LLC
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Filing: 1993-05-28
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Granted: —
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Expires: 2013-05-28
Abstract
Abstract A high quality low bit rate audio codec having a reproduced voice quality that is comparable to that of a full rate codec compresses audio data sampled at 8 Khz, e.g., 64 Kbps PCM, to 4.2 Kbps or decompresses it back to the original audio or both. The accompanying degradation in voice quality is comparable to the standard 8.0 Kbps voice codes. This is accomplished by using the same parametric model used in traditional CELP coders but determining, quantizing, encoding, and updating these parameters differently. The low bit rate audio decoder is like most CELP decoders except that it operates in two modes depending on the received mode bit. Both pitch prefiltering and global postfiltering are employed for enhancement of the synthesized audio. In addition, built-in error detection and error recovery schemes are used that help mitigate the effects of any uncorrectable transmission errors.
Inventors
- 👤 Kumar Swaminathan
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-10-30 17:01
CPC Classifications
G; G10; G10L; G10L19/00; G10L19/04; G10L19/26; G10L19/08; G10L19/12; G10L25/00; G10L25/90; G10L2019/0001; G10L2019/0002; G10L2019/0003; G10L25/93
Claims Analysis
Total Claims: 15
Claim 1
Independent Claim
193 words
A system for compressing audio data comprising: means (31) for receiving audio data and dividing the data into audio frames; a linear predictive code analyzer and quantizer (32) operative on data in each audio frame for performing linear predictive code analysis on first and second audio windows, the first window being centered substantially at the middle and the second window being centered substantially at the edge of an audio frame, to generate first and second sets of filter coefficients and line spectral frequency pairs; a codebook including a vector quantization index; a pitch estimator (33) for generating two estimates of pitch using third and fourth audio windows which, like the first and second windows, are respectively centered substantially at the middle and edge of the audio frame; a mode determiner (34) responsive to the first and second filter coefficients and the two estimates of pitch for classifying the audio frame into a first predominantly voiced mode; and a transmitter (16) for transmitting the second set of line spectral frequency vector quantization codebook indices from the codebook and the second pitch estimate to guide the closed loop pitch estimation for the first mode audio.
Claim 2
Dependent Claim
137 words
The system of Claim 1 further comprising: a CELP excitation analyzer for guiding a closed loop pitch search in the first mode; delayed decision means for refining the excitation model parameters in the first mode in such a manner that the overall delay is not affected; and encoder means (26) for the first mode dividing a received audio frame into a plurality of subframes and for each subframe determining a pitch index, a pitch gain index, a fixed codebook index, a fixed codebook gain index, and a fixed codebook gain sign using a closed loop analysis by synthesis approach, the encoder means performing a closed loop pitch index search centered substantially around the quantized pitch estimate derived from the second pitch analysis window of a current audio frame as well as that of the previous audio frame.
Claim 3
Independent Claim
172 words
A system for compressing audio data comprising: means (31) for receiving audio data and dividing the data into audio frames; a linear predictive code analyzer and quantizer (32) operative on data in each audio frame for performing linear predictive code analysis on first and second audio windows, the first window being centered substantially at the middle and the second window being centered substantially at the edge of an audio frame, to generate first and second sets of filter coefficients and line spectral frequency pairs; a codebook including a vector quantization index; a pitch estimator (33) for generating two estimates of pitch using third and fourth audio windows which, like the first and second windows, are respectively centered substantially at the middle and edge of the audio frame; a mode determiner (34) responsive to the first and second filter coefficients and the two estimates of pitch for classifying the audio frame into a second predominantly voiced mode; and a transmitter (16) for transmitting both sets of line spectral frequency vector quantization codebook indices.
Claim 4
Dependent Claim
117 words
The system of Claim 3 further comprising: delayed decision means for refining the excitation model parameters in such a manner that the overall delay is not affected; and encoder means (26) for dividing a received audio frame into a plurality of subframes and for each subframe determining a pitch index, a pitch gain index, a fixed codebook index, and a fixed codebook gain index using a closed loop analysis by synthesis approach, the encoder means performing a closed loop pitch index search centered substantially around the quantized pitch estimate derived from the second pitch analysis window of a current audio frame as well as that of the pitch of the last subframe of the previous audio frame.
Claim 5
Independent Claim
21 words
The system of either one of Claims 1 or 2, incorporating the system of either one of Claims 3 or 4.
Claim 6
Dependent Claim
121 words
The system of Claim 1, 2 or 5 wherein the pitch estimator comprises: first and second computing means (331,332), respectively, receiving data from the third and fourth audio windows for computing an error function; means (331) receiving computed error functions from the first and second computing means for refining past pitch estimates; a look back and look ahead pitch tracker (337) responsive to the refined past pitch estimates for producing optimum first and second optimum pitch estimates; a pitch selector for selecting one of the first and second optimum pitch estimates as one of the two estimates of pitch; and a look back pitch tracker responsive to the pitch selector for outputting a second one of the two estimates of pitch.
Claim 7
Dependent Claim
129 words
The system of claim 6 wherein the mode determination means comprises: a first tester (342) receiving an interpolated set of filter coefficients for the second window and filter coefficients for the first window for comparing a cepstral distortion measure against a threshold value; a second tester (343) for comparing the refined pitch estimate for window four and the pitch estimate for window three; a third tester (344) for comparing the pitch estimate for window four and the pitch estimate for window three; and a mode selector (345) for selecting the first mode if the comparisons made by the second or third tester are close but selecting the second mode if the comparison made by the first tester exceeds the threshold whose value is a function of the previous mode.
Claim 8
Independent Claim
156 words
A method of compressing audio data comprising the steps of: receiving audio data (31) and dividing the data into audio frames; performing linear predictive code analysis (32) on the data in first and second audio windows in each audio frame, the first window being centered substantially at the middle and the second window being centered substantially at the edge of an audio frame, and generating first and second sets of filter coefficients; generating two estimates of pitch (33) using third and fourth audio windows which, like the first and second windows, are respectively centered substantially at the middle and edge of the audio frame; classifying the audio frame (34) into a first predominantly voiced mode based on the first and second filter coefficients and the two estimates of pitch; transmitting in the first mode the second set of line spectral frequency vector quantization indices and the second pitch estimate to guide the closed loop pitch estimation.
Claim 9
Dependent Claim
130 words
The method of Claim 8 further comprising the steps of: refining the excitation model parameters in the first mode using a delayed decision means in such a manner that the overall delay is not affected; dividing an audio frame into a first plurality of subframes and for each subframe determining a pitch index, a pitch gain index, a fixed codebook index, a fixed codebook gain index, and a fixed codebook gain sign using a closed loop analysis by synthesis approach when the frame is identified as a first mode transmission; and performing a closed loop pitch index search (26) using CELP excitation analysis centered around the quantized pitch estimate derived from the second pitch analysis window of a current audio frame as well as that of the previous audio frame.
Claim 10
Independent Claim
143 words
A method of compressing audio data comprising the steps of: receiving audio data (31) and dividing the data into audio frames; performing linear predictive code analysis (32) on the data in first and second audio windows in each audio frame, the first window being centered at the middle and the second window being centered substantially at the edge of an audio frame, and generating first and second sets of filter coefficients; generating two estimates of pitch (33) using third and fourth audio windows which, like the first and second windows, are respectively centered substantially at the middle and edge of the audio frame; classifying the audio frame (311) into a second predominantly voiced mode based on the first and second filter coefficients and the two estimates of pitch; and transmitting in the second mode both sets of line spectral frequency vector quantization indices.
Claim 11
Dependent Claim
129 words
The method of Claim 10 further comprising the steps of: refining the excitation model parameters in the second mode using a delayed decision means in such a manner that the overall delay is not affected; dividing an audio frame into a first plurality of subframes and for each subframe determining a pitch index, a pitch gain index, a fixed codebook index, a fixed codebook gain index, using a closed loop analysis by synthesis approach when the frame is identified as a second mode transmission; and performing a closed loop pitch index search (26) centered around the quantized pitch estimate derived from the second pitch analysis window of a current audio frame as well as that of the previous audio frame of the last subframe of the previous audio frame.
Claim 12
Independent Claim
21 words
The method of either one of Claims 8 or 9, incorporating the method of either one of Claims 10 or 11.
Claim 13
Dependent Claim
76 words
The method of claim 8, 9 or 12 wherein the step of generating pitch estimates comprises the steps of: receiving data from the third and fourth audio windows for computing an error function; receiving computed error functions from the first and second computing means for refining past pitch estimates; producing optimum first and second optimum pitch estimates; and selecting one of the first and second optimum pitch estimates as one of the two estimates of pitch.
Claim 14
Dependent Claim
121 words
The method of claim 8, 9 or 12 wherein the step of classifying the audio frame comprises the steps of: receiving an interpolated set of filter coefficients for the second window and filter coefficients for the first window for comparing a cepstral distortion measure against a threshold value; comparing the refined pitch estimate for window four and the pitch estimate for window three; comparing the pitch estimate for window four and the pitch estimate for window three; and selecting the first mode if the comparisons made by the pitch estimate comparing steps are close but selecting the second mode if the comparison made by the cepstral distortion comparing step exceeds the threshold whose value is a function of the previous mode.
Claim 15
Dependent Claim
390 words
The method of Claim 9, 11 or 12 wherein the step of performing a closed loop pitch index search using CELP excitation analysis comprises the steps of: dividing the audio frame into a plurality of subframes and for each subframe using an interpolated set of filter coefficients for use in the closed loop pitch search and the fixed codebook search, the interpolation being carried out in the lag domain using an optimum set of interpolation weights; for each subframe, determining excitation model parameters corresponding to the two best sets of excitation model parameters determined in the previous subframes thus far; and further comprising any one or more of the following steps: for each subframe in the first mode and for each set of excitation model parameters determined thus far, determining two optimum closed loop pitch estimates by searching a range of pitch values from a non-uniform pitch delay table that is derived from the quantized open loop pitch value; for each subframe in the second mode and for each set of excitation model parameters determined thus far, determining two optimum closed loop pitch estimates by searching only integer pitch delays; for each subframe in the first mode and for each set of excitation model parameters determined thus far and for each optimum closed loop pitch value, searching a glottal pulse codebook for the optimum glottal pulse vector and its gain, and exploiting the codebook's special structure during the search as well as the fact that the energy terms used in the search need to be calculated only once; for each subframe in the second mode and for each set of excitation model parameters determined thus far, and for each optimum closed loop pitch value, searching a multi-innovation codebook to yield the optimum innovation sequence and its gain; at the end of each subframe, except the first, pruning the four sets of excitation model parameters, resulting from the combination of two previous sets and the two optimum pitch estimates, to two sets of excitation model parameters using cumulative SNR as the criteria; for the first subframe selecting the two sets of excitation model parameters corresponding to the best set of previously determined excitation model parameters only; or at the end of each frame, using delayed decision means, determining the optimum excitation model parameter indices in each subframe by doing a traceback.
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