DSP Group Truespeech

• 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)
 }