ETSI Standard Vocoders
ETSI HR-GSM
The algorithm implemented is the ETSI, Half-Rate (HR) GSM recommendation,
digital mobilie telephone system. The encoder compresses linear-PCM (pulse
code modulated), speech input data, at a sample rate of 8kHz, using a codebook
search to process 20ms speech frames into 112 transmission bits comprised
of 3 general sets of parameters: energy, spectral and excitation. The decoder
generates the synthesized speech using the energy and excitation parameters
and weights the excitation to match the orginal input speech by filtering
the spectral parameters.
The HR-GSM algorithm implements silence compression techniques to reduce
the transmission 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.
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