TIA/EIA Standard Vocoders
IS-641-A
The algorithm implemented is the
TIA/EIA recommendation IS-641-A TDMA, a fixed rate speech coder. The encoder
compresses speech input data in a 16-bit uniform Pulse Code Modulated (PCM)
format, at a sample rate of 8kHz to a fixed bit rate of 7 400 bps. The 20ms
speech frames are processed by a Code Excited Linear Prediction (CELP) encoding
process in which the fixed codebook employs the Algebraic Code Excited Linear
Prediction (ACELP) codebook structure. IS-641-A also uses short-term post-filtering.
The IS-641-A TDMA 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) algorithm and a Comfort Noise
Generator (CNG) algorithm that allows the insertion of Silence Insertion Descriptor
(SID) frames during the silence intervals. This also gives the additional
advantage of using lower processing loads and bandwidth resource of the DSP
during silence frames.
Integrated AMR-GSM and EFR-GSM functionality is also available.
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