Calculating delta modulator performance

Abstract

The various parameters which affect the performance of a single integration exponential delta modulator are discussed, viz., line rate, signal channel bandwidth and the time constant of the RC combination ("integrator") used in the feedback path of the coder (and in the forward path of the decoder). The performance of the delta modulator is described quantitatively in terms of its overload characteristic (level at which harmonic distortion occurs versus signal frequency), its coding threshold, and the level of quantizing noise. A novel form of diagram is constructed, showing these performance boundaries in a manner which enables the dynamic range of the coder as a function of signal frequency to be readily appreciated. At the same time, values of signal to quantizing noise ratio are easily ascertainable. The shape of the overload characteristic is discussed, and attention is drawn to the importance of relating this to the long-term spectrum of the input signal. A formula is derived for the effect of random errors occurring in the digital transmission path, and it is shown how this may be minimized. The delta-sigma modulator is treated in a similar way.