An investigation of delta modulation of speech for mobile radio

A characteristic of a mobile radio channel is the occurrence of correlated signal fading that results in burst errors. The use of adaptive delta modulation (ADM) based on explicit transmission of the quantizer step size was proposed earlier for speech communication over such a channel. Two other variable step-size delta modulation (VSDM) schemes are presented, and their performance in a mobile radio environment is discussed. One of them is the constant factor delta modulation that uses one-bit memory and produces fast and instantaneous step-size changes. The other is the digitally controlled delta modulation (DCDM) that incorporates a new step-size adaptation strategy using seven bits of memory. In some cases, bit scrambling has been used. This is equivalent to scrambling (spreading out in time) the, clustered errors. Computer simulations providing values of coder parameters for satisfactory signal-to-noise ratios for band-limited speech signals and Gaussian noise are described. New hardware realizations are given that allow those parameters to vary smoothly for a wide range of sampling frequencies. Results of informal listening tests obtained with a mobile radio channel simulator are included. It is shown that for mobile radio, DCDM, as expected, is the better of the two coders. This is because it does not sacrifice its overload performance for the sake of error resistance.