DSP Group Truespeech

• Format tag: 0x22
• Company: DSP Group
• Description and binaries: http://www.rarewares.org/rrw/truespeech.php
• Samples: http://samples.mplayerhq.hu/A-codecs/truespeech/

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.

This codec employs LPC (Linear Predictive Coding) as almost every speech codec does.

Bitstream format

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.

All data is stored in 32-bit little-endian words and bits are read from the LSB.

Frame format

 // 1st word
 1 bit - previous filter selection mode
 the rest are 8 filter coefficients with the next bit allocation: 5, 5, 4, 4, 4, 3, 3, 3
 
 // 2nd word
 4x7 bits - offset2 values 0-3
 4 bits - high 4 bits of offset1[0]
 
 // 3d word
 2x14 bits - pulse values 0 and 1
 4 bits - low 4 bits of offset1[1]
 
 // 4th word
 2x14 bits - pulse values 2 and 3
 4 bits - high 4 bits of offset1[1]
 
 // 5th word
 4 bits - pulse offset 0
 27 bits - pulse position 0
 1 bit - offset1[0] bit 0
 
 // 6th word
 4 bits - pulse offset 1
 27 bits - pulse position 1
 1 bit - offset1[0] bit 1
 
 // 7th word
 4 bits - pulse offset 2
 27 bits - pulse position 2
 1 bit - offset1[0] bit 2
 
 // 8th word
 4 bits - pulse offset 3
 27 bits - pulse position 3
 1 bit - offset1[0] bit 3

previous filter selection mode flag - when set to one, first subframe uses filter average 2/3*prev_filt[] + 1/3*cur_filt[] and 1/3*prev_filt[]+2/3*cur_filt[] for the second subframe, otherwise previous filter coefficients are used for both; the third and fourth subframes always use current filter.

Technical details (for standard scheme)

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)
 }