G.711 (μ-law and A-law)
More info
- Owner
- ITU-T (formerly CCITT)
- Released
- 1972
- Type
- Telephony audio codec
- Spec
- ITU-T G.711
The phone network's audio encoding: 8 bits per sample on a logarithmic scale, giving quiet sounds finer steps than loud ones.
Linear 8-bit PCM sounds rough, because quiet passages get the same coarse steps as loud ones. The fix is called companding: compress the amplitude logarithmically before quantizing, expand it again afterwards. For speech the result is roughly as good as 13–14 linear bits would have been, at 8 kHz and 64 kbit/s per call.
The standard comes in two flavours with nearly identical curves — μ-law in North America and Japan, A-law in Europe and most of everywhere else — so every intercontinental call gets converted at the border. G.711 is still the one codec every VoIP phone is guaranteed to speak, and what's left when negotiation over everything else falls through.