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G.711

ITU-T recommendation

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G.711 passes audio signals in the frequency band of 300–3400 Hz and samples them at the rate of 8000 Hz, within a tolerance of 50 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 to eight bits, resulting in a 64 kbit/s bit rate.

G.711 is a required standard in many technologies, such as the H.320 and H.323 standards. It can also be used for fax communication over IP networks (as defined in the T.38 specification).

Two enhancements to G.711 have been published: G.711.0 utilizes lossless data compression to reduce bandwidth usage, and G.711.1 increases audio quality by increasing bandwidth usage.

Features

  • 8 kHz sampling frequency
  • 64 kbit/s bit rate (8 kHz sampling frequency × 8 bits per sample)
  • Typical algorithmic delay is 0.125 ms, with no look-ahead delay
  • G.711 is a waveform speech coder
  • G.711 Appendix I defines a packet loss concealment (PLC) algorithm to help hide transmission losses in a packetized network
  • G.711 Appendix II defines a discontinuous transmission (DTX) algorithm which uses voice activity detection (VAD) and comfort noise generation (CNG) to reduce bandwidth usage during silence periods
  • PSQM testing under ideal conditions yields mean opinion scores of 4.45 for G.711 μ-law, 4.45 for G.711 A-law
  • PSQM testing under network stress yields mean opinion scores of 4.13 for G.711 μ-law, 4.11 for G.711 A-law

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