View source for 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)
  }

[[Category: Audio Codecs]]