ITU-T Standard Vocoders
G.722
The algorithm implemented is the
ITU-T G.722 recommendation, variable rate audio coder. The encoder compresses
linear-PCM (Pulse Code Modulated), wideband audio input data, at a sample
rate of 16kHz, to 64 000 bps and decodes one of three bit rates: 48 000, 56
000, or 64 000 bps. The specification uses a sub-band ADPCM (adaptive differential
pulse code modulation) algorithm.
G.722 is a mandatory coding scheme
for wideband audio under the ITU specification H.320 for videoconferencing
applications.
Like G.711, G.722 is also often
used at rates less than 64 000 bps, namely 56 000 and 48 000 bps with a slight
reduction in speech quality. This allows an 8 000 bps or 16 000 bps auxiliary
data channel. However unlike G.711 is it necessary to inform the decoder of
the received bit rate, as it uses different quantisation tables at the different
rates, this selection is performed by the decoder initialisation function.
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