What is Pulse Code Modulation

Pulse Code Modulation (PCM) is an encoding technique that is widely used to convert analog signals in to their digital counter parts. In this coding scheme, analog signal is converted in to a series of pulses that are of fixed length and fixed amplitude. Each pulse represents the level ‘1’ while each lack of pulse represents the level ‘0’ in the prescribed time slot in this coding system.

As most of the analog signals processed today are in the form of continuous wave forms, the encoding process is first of all proceeded by a sample and hold circuitry. This is because of the fact that the encoding circuitry takes a certain amount of time for conversion of an analog value in to a digital one, and therefore it can’t respond to a continuously varying analog signal. Therefore the analog signal is first of all converted it to a series of samples separated from each other by hold intervals. These samples are then fed in to the encoding circuitry rather then the continuous analog waveform.

This sample and hold circuitry can easily be constructed by using a MOSFET and feeding the impulse sampling train to the gate of the MOSFET while applying the signal to its source. For proper sampling, the frequency of the impulse train should be at least twice as large as the highest frequency present in the signal. Though in practical applications, this frequency is usually kept five times and higher than the highest frequency in the signal spectrum. To filter out unwanted frequencies, a band pass filter, commonly known as anti-aliasing filter, is used before the sample and hold circuitry.

The encoding circuitry responds to a certain range varying from V to +V volts. This range is divided in to a large number of steps, with each step corresponding to a particular PCM code.The most prevalent PCM coding uses 8 bits to represent a step and therefore the total number of different steps possible within the range are 28  i.e. 256 steps.

The value of the sample that is fed in to the encoding circuitry is first of all rounded off to the nearest step value before its conversion. This process is known as quantization. It introduces a small error in the shape of the signal when it is reproduced back from the digital code. This error is known as quantization noise.

Thus the whole process of PCM coding is divided in to three main steps of sampling, quantizing and encoding. This coding is widely used in communication systems such as T1 carrier line, digital radio etc.

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