Distortion in a Pulse Count Modulation System

In the further development of pulse time multiplex systems the form of modulation known as pulse count modulation (PCM) has been studied both theoretically and experimentally. This work reviews briefly the principles of pulse communication and its extension to PCM. Unlike the conventional types of modulation, PCM is represented by a discrete scanning of the amplitude of the modulating signal rather than a continuous amplitude scanning, in addition to the discrete scanning in time which is common to the other forms of pulse modulation. The discrete nature of such a system introduces a type of distortion which is a function of the number of amplitude levels used. The relation between the numbers of levels and this type of distortion has been investigated and is discussed. Several types of signals are analyzed mathematically; namely, a single-frequency signal, a 2-frequency signal, and finally a continuous band of frequencies. In connection with the multifrequency case, the application of the Fourier transform to pulse amplitude modulation and its extension to PCM is introduced. In addition, the cross talk introduced in different channels of the modulating frequency is shown graphically for several amplitude levels.