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Look-Ahead Transition Detection for Quantizer Adaptation in Sub-Band Coders

IP.com Disclosure Number: IPCOM000039693D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Galand, C: AUTHOR

Abstract

The quantization of non-stationary signals like speech requires the continuous adaptation of the quantizers. This adaptation can be implemented either in a forward or a backward way. This general rule applies to sub-band coding of speech, where the sub-band quantizers are adapted either forwardly or backwardly. In forwardly-adapted quantizers, like block companded PCM (BCPCM), one determines the quantizer characteristics C(i) and, optionally, the bit allocation n(i) for sub-band i. This information is encoded and transmitted along with the coded sub-band samples as side information. This technique gives good performances but introduces a long end-to-end delay due to the necessity of processing the samples by blocks. Attempts to avoid this drawback have been made by using quantizers with one sample backward adaptation.

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Look-Ahead Transition Detection for Quantizer Adaptation in Sub-Band Coders

The quantization of non-stationary signals like speech requires the continuous adaptation of the quantizers. This adaptation can be implemented either in a forward or a backward way. This general rule applies to sub-band coding of speech, where the sub-band quantizers are adapted either forwardly or backwardly. In forwardly-adapted quantizers, like block companded PCM (BCPCM), one determines the quantizer characteristics C(i) and, optionally, the bit allocation n(i) for sub-band i. This information is encoded and transmitted along with the coded sub-band samples as side information. This technique gives good performances but introduces a long end-to-end delay due to the necessity of processing the samples by blocks. Attempts to avoid this drawback have been made by using quantizers with one sample backward adaptation. In this case, the quantizer characteristics and, optionally, the bit allocation n(i) for sub-band i are derived from past samples available at the emitter and at the receiver, avoiding the transmission of a side information and of a block delay. The delay is thus reduced to the delay inherent to the sub-band filter bank (QMF) filtering. However, this type of adaptation gives lower performances than block-companded coders, because of the high clipping noise occuring in speech transitions. We propose hereafter a method allowing to look ahead to the speech transients, without delay, thus quantizing the sub-band samples without clipping noise. This method applies both to forwardly-adapted quantizers and to backwardly-adapted quantizers. The attached figure shows the implementation of the proposed algorithm applied to backward adapted quantizer. The look-ahead transition detection (LATD) building block determines a transition ratio G which avoids any overflow (clipping noise) or underflow (granular noise) in the quantizers. This detector takes advantage of the delay to introduced by the QMF analysis f...