ITU-T Standard Vocoders
G.723.1
The algorithm implemented is the
ITU-T G.723.1 recommendation; dual rate speech coder for internet and multimedia
communications, The encoder compresses linear-PCM (Pulse Code Modulated),
speech input data at a sample rate of 8kHz, to 5 300 and 6 400 bps with an
audio bandwidth of 300-3400Hz. The encoder includes an optional High Pass
Filter and the decoder includes an optional Post Filter, which enhance performance.
The G.723.1 algorithm implements
silence compression techniques to reduce the transmitted bit rate during the
silent intervals of speech. Systems allowing discontinuous transmission (DTX)
are based on Voice Activity Detection (VAD) algorithms and Comfort Noise Generator
(CNG) algorithms that allows the insertion of Silence Insertion Descriptor
(SID) frames during the silence intervals. This also provides the additional
advantage of using lower processing loads and DSP bandwidth resource during
silence frames.
The G.723.1 specification encodes
speech and audio signals using linear predictive analysis-by-synthesis coding.
The higher 6 400 bps data rate requires the use of a Multiple Maximum Likelihood
Quantisation (MP-MLQ) excitation signal. The lower 5 300 bps data rate uses
Conjugate-Structure Algebraic-Code-Excited Linear Prediction (CS-ACELP).
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