Subject : Principle of Communication Engineering

Topic : PCM and Multiplexing

Difficulty : Low

• Companding is the process of compression and then expansion. With companded system, the higher amplitude analog signals are compressed (amplified less than lower amplitude signals) prior to transmission and then expanded (amplified more than the lower amplitude signals) in the receiver.

  

  Fig. 1 Block diagram of Companded PCM system

• Companding is used in pulse code modulation (PCM). The process involves decreasing the number of bits used to record the strongest (loudest) signals.

• In the digital file format, companding improves the signal-to-noise ratio at reduced bit rates. For example, a 16-bit PCM signal may be converted to an eight-bit ".wav" or ".au" file.

There are two types of Companding techniques. They are −

A-law Companding Technique

Uniform quantization is achieved at A = 1, where the characteristic curve is linear and no compression is done.

A-law has mid-rise at the origin. Hence, it contains a non-zero value.

A-law companding is used for PCM telephone systems.

µ-law Companding Technique

Uniform quantization is achieved at µ = 0, where the characteristic curve is linear and no compression is done.

µ-law has mid-tread at the origin. Hence, it contains a zero value.

µ-law companding is used for speech and music signals.

µ-law is used in North America and Japan.

• Companding is the process of compression and then expansion. With companded system, the higher amplitude analog signals are compressed (amplified less than lower amplitude signals) prior to transmission and then expanded (amplified more than the lower amplitude signals) in the receiver.

  

  Fig. 1 Block diagram of Companded PCM system

• Companding is used in pulse code modulation (PCM). The process involves decreasing the number of bits used to record the strongest (loudest) signals.

• In the digital file format, companding improves the signal-to-noise ratio at reduced bit rates. For example, a 16-bit PCM signal may be converted to an eight-bit ".wav" or ".au" file.

There are two types of Companding techniques. They are −

A-law Companding Technique

Uniform quantization is achieved at A = 1, where the characteristic curve is linear and no compression is done.

A-law has mid-rise at the origin. Hence, it contains a non-zero value.

A-law companding is used for PCM telephone systems.

µ-law Companding Technique

Uniform quantization is achieved at µ = 0, where the characteristic curve is linear and no compression is done.

µ-law has mid-tread at the origin. Hence, it contains a zero value.

µ-law companding is used for speech and music signals.

µ-law is used in North America and Japan.