DSP Group Truespeech
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- Format tag: 0x22
- Company: DSP Group
This is the codec developed by DSP Group. It operates on 8000 Hz, 16-bit mono PCM and have some fixed bitrates, Windows version supports 15:1 compression (~1 Kbps) only.
Technical details (for standard scheme)
This codec employs LPC (Linear Predictive Coding) as almost every speech codec does.
Frame (240 samples) is divided into 4 subframes.
Packed frame data holds 8-order filter used in synthesis, 4x7 pulse positions and values, offset in previously decoded frame.
Decoding flow:
unpack frame data reconstruct packed filter merge this filter with previous to create 4 filters for each subframe for each subframe { apply twopoint filter on some saved data place pulses (7 pulses for each subframe max) update saved filter apply main filter to subframe data (mostly pulses) }