A Mid-Range/Mid-Tread Quantizer Switch for Improved Idle-Channel Performance in Adaptive Coders

A Mid-Rise/Mid-Tread Quantizer Switch for Improved Idle-Channel Performance in Adaptive Coders By R. E. CROCHIERE (Manuscript received November 17, 1977) A mid-rise/mid-tread switch is proposed for improving the idle channel performance of an adaptive quantizer. The method incorporates the advantages of both mid-rise and mid-tread quantizer characteristics. In adaptive waveform coding such as A D P C M (adaptive differential PCM), ADM (adaptive delta modulation),1-2 and sub-band coding,3-4 the quantizer step-size in the coder varies in accordance with the short-time energy of the signal being coded in order to take advantage of its nonstationary properties. In practice, these types of coders generally have minimum and maximum limits on their step-size. Furthermore, they often use a mid-rise quantizer characteristic as depicted in Fig. 1 where x denotes the input signal level, x denotes the discrete output signal levels, and A denotes the quantizer step-size. This mid-rise characteristic is desirable because of its symmetry and because it uses the 2B possible levels of a B-bit coder efficiently. A disadvantage of this mid-rise characteristic is that it cannot represent a zero output level. During very low, or zero, input signal intervals (such as silent regions in speech), the output of the coder must be ± Amin, where Amin is the minimum step-size in the coder. Generally, Amjn is chosen to be small enough so that this signal is very low. Unfortunately, in many coder designs the sign of the output signal varies in a systematic pattern which can be perceived even for very low values of Amjn.