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Pulse code modulation

The pulse amplitude, pulse width, and pulse position modulations are not completely digital. A completely digital modulation is obtained by pulse code modulation (PCM). An analog signal is pulse code modulated by following three operations:
Sampling It is the process of generating pulses of zero width and of amplitude equal to the instantaneous amplitude of the analog signal. The number of samples taken per second is called sampling rate.
Quantization The process of dividing the maximum amplitude of the analog voltage signal into a fixed number of levels is called quantization. For example, amplitude 5 V of the analog voltage signal divides into six quantization levels, viz., 0, 1, 2, 3, 4, 5.
Pulses having amplitude between –0.5 V and 0.5 V are approximated (quantized) to a value of 0 V, amplitude between 0.5 V and 1.5 V are approximated to a value of 1 V, and so on.
Coding The process of digitizing the quantized pulses according to some code is called coding. For example, consider that voltage amplitude of an analog signal varies between 0 and 7 V.
Fig. 15

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