Variable frame rate, fixed bit rate vocoding method
First Claim
1. A method of operating a vocoder to compress a multiplicity of voice data samples for application to a constant rate channel, said method comprising the steps of:
- selecting one of a plurality of subframe rates;
analyzing at lest a portion of the samples to produce a predictive code representative of the analyzed samples;
quantizing the predictive code into a quantized code having a bit length defined by the selected subframe rate and a rate parameter of the constant rate channel; and
said steps of selecting, analyzing and quantizing being performed at a bit rate of said constant rate channel.
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Abstract
A method of operating a vocoder so that a variable frame rate results while maintaining a constant bit rate is disclosed. A base frame rate remains constant. However, spectral change is measured to determine an appropriate subframe rate within the base frame. Only one of a plurality of separate vector quantization processes is selected in response to spectral change for performance of a vector quantization operation on LPC spectrum coefficients. Each of the plurality of vector quantization operations utilizes its own codebook that contains a different quantity of reference patterns from that contained in the other codebooks. Each vector quantization operation produces a reference pattern descriptor code that contains a different number of bits from that produced by the other vector quantization processes. Vector quantization operations producing smaller, less spectrally accurate outputs are selected when more subframes are included with a base frame, and vector quantization operations producing larger, more spectrally accurate outputs are selected when fewer subframes are included within a base frame.
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Citations
16 Claims
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1. A method of operating a vocoder to compress a multiplicity of voice data samples for application to a constant rate channel, said method comprising the steps of:
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selecting one of a plurality of subframe rates; analyzing at lest a portion of the samples to produce a predictive code representative of the analyzed samples; quantizing the predictive code into a quantized code having a bit length defined by the selected subframe rate and a rate parameter of the constant rate channel; and said steps of selecting, analyzing and quantizing being performed at a bit rate of said constant rate channel. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A method of operating a vocoder to decompress a base frame of data which represents a multiplicity of voice data samples and which is received from a constant rate channel, said method comprising the steps of:
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obtaining information describing one of a plurality of subframe rates from the constant rate base frame of data; separating the base frame of data into a least one subframe of data in response to the subframe rate from said obtaining step; expanding each subframe of data from said separating step into an independent predictive code; synthesizing the multiplicity of voice data samples from the predictive code for each subframe; and said steps of obtaining, separating, expanding and synthesizing being performed at a bit rate of said constant rate channel. - View Dependent Claims (11, 12, 13, 14)
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15. A method of operating a vocoder to compress a multiplicity of voice data samples for application to a fixed rate channel, said method comprising the steps of:
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measuring spectral change occurring between the multiplicity of voice data samples and a prior multiplicity of voice data samples; modulating a subframe rate in response to the spectral change so that increasing spectral change causes increasing subframe rate; performing a linear predictive coding of at least a portion of the multiplicity of voice data samples to generate excitation, energy and spectrum data; making first and second vector quantization codebooks available for searching; choosing one of the first and second vector quantization codebooks in response to the subframe rate of said modulating step; searching within only the one codebook chosen in said choosing step for a nearest matching reference pattern to the spectrum data of said performing step; and retrieving a reference pattern descriptor code which uniquely describes the nearest matching reference pattern, wherein the reference pattern descriptor code contains N bits and the one codebook chosen in said choosing step contains less than 2N unique reference patterns. - View Dependent Claims (16)
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Specification