Also known as: G.711, PCMU, PCMA
G.711 is the ITU-T standard that defines how the telephone network digitises voice: 8 kHz sampled pulse-code modulation at 64 kbps, using 8-bit samples that are logarithmically companded rather than linearly quantised.1 It is the oldest and most widely deployed digital speech coder in the world — the single 64 kbps DS0 channel is literally defined by it — and it remains the “toll-quality” baseline every VoIP endpoint supports.
How it works
G.711 samples the 300–3400 Hz telephone band at 8000 samples per second and codes each sample in 8 bits, giving 64 kbps. The clever part is the quantization: instead of uniform steps, G.711 uses a logarithmic companding curve so that small-amplitude signals — where the ear is most sensitive — get fine step sizes while loud signals get coarse ones. The effect is that an 8-bit companded sample delivers a signal-to-quantization-noise ratio roughly equivalent to 12–13 bits of linear PCM across the whole speech dynamic range.
Two companding laws exist: mu-law (μ = 255), used in North America and Japan, and A-law (A = 87.6), used in most of the rest of the world; they are piecewise-linear approximations of the logarithmic curve and are trivially transcoded between each other. Because G.711 is straight sample-by-sample coding with essentially no algorithmic delay and negligible computation, it is the lowest-latency, highest-interoperability voice codec available — the cost is that it does not compress speech the way modern vocoders do; it merely digitises it efficiently.
Relevance to SDR
G.711 is a wireline/VoIP telephony codec, not an over-the-air radio format, so a scanner never decodes it off RF. It matters here as the canonical example of PCM companding and as the reference “uncompressed” baseline against which low-bitrate radio vocoders are measured — a 64 kbps G.711 channel versus, say, a sub-5 kbps land-mobile vocoder shows exactly how much compression digital voice radio buys. GopherTrunk does not use G.711 in its RF decode chain, though PCM audio it produces could of course be carried over a G.711 link downstream.