Explain pulse code modulation (PCM) technique with diagram.

Pulse Code Modulation (PCM): Pulse Code Modulation (PCM) is based on the sampling theorem, which states:

“If a signal f(t) is sampled at regular intervals of time and at a rate higher than twice the highest significant signal frequency, then the samples contain all the information of the original signal. The function f(t) may be reconstructed from these samples by the use of a low-pass filter.”

Pulse Code Modulation (PCM) is the simplest form of waveform coding. Waveform coding is used to encode analog signals (for example speech) into a digital signal. PCM is an extension of Pulse Amplitude Modulation (PAM). PCM is the most frequently used analog to digital conversion technique.

Figure-1: Pulse Code Modulation (PCM)

Figure-2: Analog to Digital conversion

A PCM encoder performs three functions:

(i) Sampling

(ii) Quantizing

(iii) Encoding

(i) Sampling: Sampling is the process of reading the values of the filtered analog signal at discrete time intervals.

(ii) Quantizing: Quantizing is the process of assigning a discrete value from a range of possible values to each sample obtained. The number of possible values will depend on the number of bits used to represent each sample. The signal to noise ratio (SNR), including quantization noise, is the most important factor affecting voice quality in uniform quantization.

The signal to noise ratio (SNR) for quantizing noise can be expressed as,

(iii) Encoding: Encoding is the process of representing the sampled values as a binary number in the range 0 to n.