Adaptive Delta Modulation with a One-Bit Memory

Linear (or unadaptive) delta modulators, which work with a fixed step-size for the "staircase" approximation to an input signal, have the following basic limitation. Small values of the step size introduce slopeoverload distortion during bursts of large signal slope; large values of the step-size accentuate the granular noise during periods of small signal slope; and, even when the step-size is optimized, the performance of these modulators will be satisfactory only at sampling frequencies t h a t may be undesirably high. Equivalently, one encounters important ranges of operating frequency in which the performance of conventional delta modulation falls short of the standards attainable by conventional P C M or by cZ-level differential PCM, of which delta modulation is a special case {d = 2). With a view to employing delta modulation (which is inherently a very simple signal-processing strategy) at such relatively low operating frequencies, several types of adaptive delta modulation have been proposed. 1 - 3 In these schemes, the step size is changed in accordance with the time-varying slope characteristics of the input signal, as per a predetermined adaptation strategy. Such adaptation or "companding" can be either at a syllabic rate (long-term) or instantaneous (shortterm). Typical of syllabic-companding delta modulators are recently developed schemes for reproducing telephone quality speech at operating frequencies of the order of 50 kHz. 1 , 5 , 6 These systems are characterized by "continuous" adaptation of the step magnitude.