Subject: Priniciples of Communication Engineering

Difficulty : Medium

Marks : 05

Companding:

Companding is the process of compressing and expanding. With companded systems, the higher amplitude analog signals are compressed (amplified less the lower amplitude signals) prior to transmission and then expanded (amplified more than the lower amplitude signals).

Companding means to improve dynamic range of communication system.

In a uniform or linear PCM system the size of every quantization interval is determined by the SQR (signal to quantization ratio) requirement of the lowest signal to be encoded. This interval is also for the largest signal - which therefore has a much better SQR.

Example: A 26 dB SQR for small signals and a 30 dB dynamic range produces a 56 dB SQR for the maximum amplitude signal.

In this way a uniform PCM system provides unneeded quality for large signals. In speech the max amplitude signals are the least likely to occur. The code space in a uniform PCM system is very inefficiently utilized.

There are 2 companding schemes to describe the curve above:

µ-Law Companding (also called log-PCM):

This is used in North America and Japan. It uses a logarithmic compression curve which is ideal in the sense that quantization intervals and hence quantization noise is directly proportional to signal level (and so a constant SQR).

A- Law Companding

This is the ITU-T standard. It is used in Europe and most of the rest of the world. It is very similar to the m-Law coding. It is represented by straight line segments to facilitate digital companding.

a) Analog Companding: Analog compression was obtained using non linear devices such as special diodes in PCM transmitter prior to sample and hold circuit. Analog expansion was implemented just after low pass filtering in PCM receiver.

b) Digital Companding: It involves compression after input sample is converted to PCM code and then expansion in the receiver prior to PCM decoding.