Data processing: speech signal processing – linguistics – language – Speech signal processing – Recognition
Reexamination Certificate
2000-08-28
2001-07-31
Knepper, David D. (Department: 2645)
Data processing: speech signal processing, linguistics, language
Speech signal processing
Recognition
C704S222000
Reexamination Certificate
active
06269333
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to low data rate vocoders. More specifically, the present invention relates to low data rate vocoders using split vector processing whereby the coding efficiency of the vocoder is maximized. In particular, the present invention relates to low data rate encoder— decoder pairs employing split vector quantization and differential pitch and gain quantization processing. A codebook populating method for adaptively populating one of two codebooks used for encoding one sub-vector while maintaining ordered properties given the quantized value of the other sub-vector is also disclosed
BACKGROUND OF THE INVENTION
There has been an increasing interest in the development of low bit rate speech coding technologies that can operate at rates of 2400 bit per second (b/s) and below for both current military use and for future commercial applications. Although Government and industry have begun to pursue new coding methodologies which can yield high quality speech at bit rates in the 2400 b/s range, relatively less resources have been applied to efforts regarding development of a good quality 1200 b/s coding that can either be used as a stand-alone coder or as an embedded coder in a higher rate variable rate coder.
The use of Line Spectral Pair (LSP) or Line Spectral Frequency (LSF) representation for vector quantization of short-term spectral parameters is very well known. For example, U.S. Pat. Nos. 5,012,518 and 4,975,956 disclose techniques for vector quantization of LSP parameters. However, the technique described in these patents requires a significantly higher computational overhead than an alternative encoder employing split vector quantization encoding. For example, in split-vector quantization using 20 bits, a maximum of 2048 comparisons are needed to arrive at the optimal quantized LSF vector. In contrast, the conventional method of vector quantization using 20 bits requires more than a million comparisons to arrive at the optimal quantized LSF vector. LSF's are ideally suited for split vector quantization techniques due to its ordered and localized spectral sensitivity properties.
U.S. Pat. Nos. 5,187,745, 5,179,594, 5,173,941 and 5,086,475 disclose CELP vocoders.
SUMMARY OF THE INVENTION
The principal purpose of the present invention is to provide a vocoder achieving optimal coding efficiency for a given low bit transmission rate.
One object of the present invention is to provide a vocoder employing a novel populating method that improves the performance of the split-vector quantization coding.
Another object of the present invention is to provide a vocoder employing a highly efficient quantization method for encoding gain and pitch using a differential quantization method.
These and other objects, features and advantages of the present invention are provided by a 1200 b/s vocoder providing a high degree of speech intelligibility and natural voice quality. The 1200 b/s vocoder advantageously includes a tenth-order linear prediction analyzer, a split vector quantizer for line spectral frequencies, circuitry providing voicing classification and pitch estimation and a differential pitch and gain quantizer.
According to one aspect of the invention, the vocoder includes a multiplexer for producing an encoded word transmitted to a receptive demultiplexer. The vocoder provides a characteristic encoded word including a first codeword, a second codeword, a pitch codeword and a gain codeword, wherein the first and second codewords are selected from respective first and second codebooks having a equal number of codewords and wherein the first and second codewords represent unequal numbers of elements of respective first and second sub-vectors.
According to another aspect of the invention, a codebook populating method for a split vector quantizer vocoder includes the steps of (a) determining a first number of eligible codewords in original second codebook given a selected codeword from a first codebook, (b) when the first number is greater than a predetermined number, computing a second number of centroid of pairs of the codewords in the second codebook and (c) when the first number is less than the predetermined number, computing the second number of centroids by repeatedly calculating the centroids of all the pairs of codewords having a first form and then calculating the centroids of all the pairs of codewords having a second form until the second codebook is fully populated.
These and other objects, features and advantages of the invention are disclosed in or apparent from the following description of preferred embodiments.
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W. B. Kleijn et al, “An Efficient Stochastically Excited Linear Predictive Coding Algorithm for High Quality Low Bit Rate Transmission of Speech”, Speech Communication 7 (1988), Elsevier Science Publishers B.V., pp. 305-316.
K. K. Paliwal et al, Efficient Vector Quantization of LPC Parameters at 24 Bits/Frame, ICASSP 91, vol. 1, 1991 International Conference on Acoustics, Speech and Signal Processing, May 14-17, 1991, pp. 661-664.
Comsat Corporation
Knepper David D.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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