Pulse code modulation (PCM).

1)Definition:-

Pulse-code modulation is known as a digital pulse modulation technique. Tn fact , the pulse-code modulation (PCM) is quite complex compared to the analog pulse modulation techniques (i ,e.,PAM, PWM , and PPM) in the senes that the message signal is subjected to a great number of operatins.

2)Elements of a PCM system:-

The basic elements of the PCM systems are consists of three main parts i , e . transmitter , transmission path and receiver. The essential operations in the transmitter of a PCM system are sampling , quantizing and encoding. The operations in which an analog signal is sampled according to the sampling theorem resulting in a discrete-time signal.The quantizing and encoding operations are usually performed in the same ciruit which is known as an analog-t-digital converter(ADC).The essential operations in the receiver are regeneration of impaired signals , decoding and demodulation of the train of quantized samples.These operations are usually performed in the same ciruit which is known as a digital-to-analog converter (DAC).

At intermediate points , analog the transmission route from the transmitter to the receiver , regenerative repeaters used to reconstruct the transmitted sequence of coded pulses in order to combat the accumulated effects of signal distortion and noise.

The quantization refers to use of a finite set of amplitude levels and the selection of a level nearest to a particular sample value of the message signal as the representation for it.

3)Few important points:-
 

Now , let us summarize PCM in the form of few points as under:-

1.PCM is a type of pulse modulation like PAM, PWM and PPM but there is an important difference between them PAM,PWM and PPM are analog pulse modulation systems where as PCM is a digital pulse modulation system.

2.This means that the PCM output is in the coded digital form. It is in the form of digital pulses of constant amplitude , width and position.

3.The information is transmitted in the form of code words. A PCM system consists of a PCM encoder and a PCM decoder.

4.The essential operations in the PCM transmitter are sampling , quantizing and encoding.

5.All the operations are usually performed in the same ciruit called as analog -to-digital converter.

6.It should be understood that the PCM is not modulation in the conventional sene.

7.Because in modulation , one of the characteristics of the carrier is varied in proportion with the amplitude of the modulating signal.