Now my TrueSpeech decoder decodes compressed audio almost as well as original decoder. Why not 100% complete? It decodes audio slightly different from original decoder, so it may take some time in future to make my decoder identical.
I hope to see it in FFmpeg source tree soon.
Here are main differences between TrueSpeech and G.723:
1) One bitrate (8 Kb/s) vs. two bitrates (6.3 and 5.3 Kb/s)
2) Lower complexity (TrueSpeech uses 8-point LPC, G.723 – 10-point LPC)
Frame structure is also differs, TrueSpeech packs everything into 32 byte frame where some values are spread between different doublewords (3d bit is in one dword, 2nd is in another one).
DSP Group TrueSpeech audio codec is rumored to be a relative to ITU G.723. That may be partly true: DSP Group took part in developing G.723, but if you study recommendation G.723, you’ll see that there are two methods used in this rec., and they differ.
Studies showed that TrueSpeech may be one of predecessors of G.723 – it is simplier, needs higher bitrate than G.723 and such. But I hope to use some ideas to reconstruct TrueSpeech decoder using methods from G.723.