Subjective Evaluation of an Adaptive Differential Voice Encoder with Oversampling and Entropy Coding

Abstract

An adaptive differential speech encoder was assessed using subjective evaluation procedures. The coder's adaptive quantizer was similar to the one used by Cohn and Melsa [1] and the predictor involved nonadaptive previous-sample feedback. The digital channel used to transmit the quantizer output levels was assumed error-free. Paired comparison tests were used to obtain scaled isopreference contours on the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N-f</tex> plane, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</tex> denote, respectively, the number of quantizer output levels and the sampling rate relative to the Nyquist rate. These contours were used to determine the subjective signal-to-noise ratio vs. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> , maximum subjective signal-signal-to-noise ratios vs. bit rate, optimum values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> , and bit-rate savings which occur when entropy coding is used instead of natural coding of the quantizer output levels. Entropy coding yielded a bit rate approximately equal to threequarters that for natural coding and Nyquist-rate sampling minimized the bit rate in each case. Savings of from one to two bits occurred when ADPCM was compared with nonadaptive DPCM. The fact that our system was better than others for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f \simeq 1.0</tex> but worse for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f \gsim 2.0</tex> indicates the need to modify our quantizer adaptation algorithm as the sampling rate increases relative to the Nyqusit rate.