Variable bit rate speech codec with backward-type prediction and quantization
First Claim
1. A multiple quantizer, backward-type codec comprising:
- input signal receiving means;
a plurality of adaptive quantizers, each for producing quantized signals representing an input signal thereto;
means for causing a selected one of said plurality of adaptive quantizers to receive an input signal from said input signal receiving means and produce quantized signals representing said received input signal,means for causing the others of said plurality of adaptive quantizers to receive a quantized signal produced by the selected one of said plurality of quantizers and to up-date their respective quantizing step-size on the basis of the received quantized signal.
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Abstract
In an adaptive differential PCM (ADPCM) system transmitter, three quantizers provide, respectively, 3, 4, and 5-bit quantization at the signal sampling rate. Each quantizer produces, in response to an input signal sample, a coded signal I(n) in the form of a bit sequence the number of bits in the coded signal from each quantizer being different. The number of bits comprising the coded signal is proportional to the bit rate. The bit rate can thus be altered by switching the codec output to selected ones of the adaptive quantizers. To eliminate quantizing distortion during bit rate transitions, one adaptive quantizer is caused to operate in the dominant mode, to receive the input signal sample and produce the coded signal I(n). At the same time, the other adaptive quantizers operate in a secondary mode to update their step-size Δ(n) on the basis of the quantized signal produced by the dominant mode quantizer. When a secondary mode quantizer is switched to the dominant mode, minimal time is required for tracking the input signal thereby reducing the quantization distortion during bit rate transitions.
70 Citations
6 Claims
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1. A multiple quantizer, backward-type codec comprising:
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input signal receiving means; a plurality of adaptive quantizers, each for producing quantized signals representing an input signal thereto; means for causing a selected one of said plurality of adaptive quantizers to receive an input signal from said input signal receiving means and produce quantized signals representing said received input signal, means for causing the others of said plurality of adaptive quantizers to receive a quantized signal produced by the selected one of said plurality of quantizers and to up-date their respective quantizing step-size on the basis of the received quantized signal. - View Dependent Claims (2, 3, 4, 5)
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6. A variable bit rate, ADPCM transmission system comprising:
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signal sample S(n) receiving means; a plurality of adaptive quantizers, each of said quantizers including adaptive coder means producing a coded signal I(n) in the form of a bit sequence having a predetermined number of bits, said signal I(n) representing a quantized version of the input signal to the quantizer, decoder means for decoding coded signal I(n) to produce a quantized input signal dq (n), and control circuit means for altering the quantizing step-size in response to changes in the level of the quantizer input signal; at least one predictor receiving an output from a selected one of said plurality of quantizers to produce a predicted signal Se (n); a subtractor for subtracting the signal sample S(n) from the predicted signal Se (n) for producing the quantizer input signal d(n); means for connecting an output from a selected one of said plurality of quantizers to said at least one predictor and connecting the input of each of the others of said plurality of quantizers to receive the quantized input signal dq (n) from the selected one of said plurality of quantizers; and means for altering the quantizer step-size in said others of said plurality of quantizers in accordance with variations in the size of said quantized input signal dq (n).
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Specification