G.711: Кодек импульсно-кодовой модуляции голоса

ITU T recommendation

G.711 is a narrowband audio codec originally designed for use in telephony that provides toll quality audio at 64 kbit/s. It is an ITU T standard (Recommendation) for audio encoding, titled Pulse code modulation (PCM) of voice frequencies released for use in 1972. G.711 passes audio signals in the frequency band of 300–3400 Hz and samples them at the rate of 8000 Hz, with the tolerance on that rate of 50 parts per million (ppm). It uses one of two different logarithmic companding algorithms: μ law, which is used primarily in North America and Japan, and A law, which is in use in most other countries outside North America. Each companded sample is quantized as 8 bits, resulting in a 64 kbit/s bit rate. G.711 is a required standard in many technologies, such as in the H.320 and H.323 standards. It can also be used for fax communication over IP networks (as defined in T.38 specification). Two enhancements to G.711 have been published: G.711.0 utilizes lossless data compression to reduce the bandwidth usage and G.711.1 increases audio quality by increasing bandwidth.

G.711 defines two main companding algorithms, the μ law algorithm and A law algorithm. Both are logarithmic, but A law was specifically designed to be simpler for a computer to process. The standard also defines a sequence of repeating code values which defines the power level of 0 dB. The μ law and A law algorithms encode 14 bit and 13 bit signed linear PCM samples (respectively) to logarithmic 8 bit samples. Thus, the G.711 encoder will create a 64 kbit/s bitstream for a signal sampled at 8 kHz.

G.711.1 "Wideband embedded extension for G.711 pulse code modulation" is a higher fidelity extension to G.711, ratified in 2008 and further extended in 2012. G.711.1 allows a series of enhancement layers on top of a raw G.711 core stream (Layer 0): Layer 1 codes 16 bit audio in the same 4kHz narrowband, and Layer 2 allows 8kHz wideband using MDCT; each uses a fixed 16 kbps in addition to the 64 kbps core. They may be used together or singly, and each encodes the differences from the previous layer. Ratified in 2012, Layer 3 extends Layer 2 to 16kHz "superwideband," allowing another 16 kbps for the highest frequencies, while retaining layer independence. Peak bitrate becomes 96 kbps in original G.711.1, or 112 kbps with superwideband. No internal method of identifying or separating the layers is defined, leaving it to the implementation to packetize or signal them. A decoder that doesn't understand any set of fidelity layers may ignore or drop non core packets without affecting it, enabling graceful degradation across any G.711 (or original G.711.1) telephony system with no changes. Also ratified in 2012 was G.711.0 lossless extended to the new fidelity layers. Like G.711.0, full G.711 backward compatibility is sacrificed for efficiency, though a G.711.0 aware node may still ignore or drop layer packets it doesn't understand.

The patents for G.711, released in 1972, have expired, so it may be used without the need for a licence.