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Collaboration, Voice & AV · Intermediate

Companding

Compressing and then expanding signal amplitude, as with mu-law and A-law in G.711, to improve quiet-speech quality.

Definition

Companding applies a logarithmic curve before quantization so that low-amplitude signals receive more quantization levels than loud ones, improving signal-to-quantization-noise ratio for speech with only 8 bits. G.711 defines two curves: mu-law, used in North America and Japan, and A-law, used in Europe and most other regions. When calls cross between regions the gateway converts mu-law to A-law.

Example

A call from a US mu-law system to a European A-law PRI is converted by the gateway without a separate transcoder.

Exam tip

Remember mu-law equals North America/Japan and A-law equals Europe/international; both are 64 kbps G.711.

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