There’s one codec I’d like to have reverse-engineered and implemented as an opensource decoder (well, lots of other codecs as well but this one particularly). Its name is VoxWare MetaSound, that’s an old codec which was used as an alternative to MP3 in old days of DiVX 3 😉 and its clones.
It’s definitely based on TwinVQ and is probably closer to the variant that got into MPEG-4 Audio standard (I suspect that mostly to make that standard even more bloated than before). That figures from having such modes like 8kHz/6kbps which is not present in VQF but present in ISO 14496-3 draft.
This codec probably has more data tables than TwinVQ (in binary decoder the section with codebooks is more than 256kB large, in TwinVQ it’s about 200kB) and should set a new record if we ever get a decoder for it.
Decoding looks very simple in theory: decoder initialises codebooks for given samplerate and bitrate (it’s actually signaled in extradata: VOXq for 44.1kHz/32kbps, VOXk for 16kHz/16kbps, VOXz for 44.1kHz/48kbps), for every frame it reads window type and an array of some values and performs reconstruction.
So far I was able to identify only some codebook information. Bark tables seems to be identical, but shape and whatever codebooks seem to be different.
I’ve spent a couple of evenings finding out that information and I dare someone (especially you, Vitor!) to write a decoder for it. I don’t know a thing about TwinVQ except one fact and it’s stated in the title.