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Pulse Code Modulation | ECE Tech Notes

PCM is an analog to digital converter where the information contained in the instantaneous samples of an analog signals are represented by digital codes in a serial bit stream manner.

Block Diagram Of PCM System

a) PCM Transmitter b) Transmission c) PCM Receiver

Working of PCM

The PCM system consist of 3 stages:
  • Transmitter (Tx)
  • Regenerative repeaters
  • Receiver (Rx)

PCM Tx:
In practice, the low pass filter(pre-alias filter) is used before samples in order to limit the frequency greater than 'w' Hz. Hence the message signal is bandlimited to 'w' Hz.

Sampler:
The incoming message signal is sampled with a train of narrow rectangular pulses. The sampling rate 'fs' is selected above the Nyquist rate to avoid aliasing. which is, fs ≥ 2w.

Quantization:
The sampled signals are fed to the quantizer, the quantizer approximates each input signal level to the nearest prefixed level. The output of the quantizer is discrete time discrete valued signal known as "quantized signal".

Encoding:
The quantized samples are then encoded in the encoder. The encoding process involves assigning some digital/binary codes to each level of a quantized signal. These code levels are transmitted as a bitstream of data, which are 0's and 1's.

Regenerative Repeaters:
The PCM signals are reconstructed by means of regenerative repeaters located at sufficiently closed spacing along the transmission path. The RR are used at intermediate points between Tx and Rx in order to enhance pulse amplitude.

PCM Rx

Decoder:
The 1st operation in the receiver is to generate the received pulse. The decoder converts binary coded signal to a approximated pulses of discrete magnitude.

Reconstruction Filter:
The final operation in the receiver is to recover the original analog/message signal. This is done by passing the decoder output through a LPF. Finally the o/p of LPF is analog signal.


Advantages:

1. Relatively inexpensive digital circuitry is involved in PCM.

2. PCM signals can be multiplexed & transmitted over a common high speed communication link.

3. In long distance transmission, clear waveforms can be regenerated using repeaters.

4. The noise performance of digital system is superior to that of an analog system.

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