Pulse modulation systems are not entirely digital. In a pulse modulation system, amplitude, width and position of the pulses transmitted changes with base band signal variation. For achieving complete digitization, pulse amplitude modulation is preferred since it is a simple system where a signal which is discrete on time axis and continuous along amplitude axis, is used. For realizing this, pulse amplitude modulation signal is quantized. This results in pulse code modulation. Here a time slot is allotted to each bit. A portion of this time slot is referred as guard time. Here information lies in the presence or absence of pulse and nothing to do with any property of the pulse. As a result, noise distortion makes no difference as long as the decision regarding the presence or absence of the pulse is correct. Repeaters used here are not consists of any amplifiers. They use only regenerators which generates pulses in time slots according to the presence of 0 or 1. This eliminates noise related issues. A pulse code modulation system consists of a PCM transmitter (with sampler, quantizer and encoder) and a PCM receiver (with quantizer, decoder, holding circuit and low pass filter).
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