So, finally there’s a post about some codec.
It is a specialised codec from Oxford Germanium Television (all names are changed just in case) that has 4:1 compression ratio and very niche use. It’s hard to find even a decoder for it so this analysis was done on ARM version of encoder (maybe I’ll be able to RE something more useful next time like VX).
The codec itself is rather simple: you take 4 samples from one channel, compress them, output the 16-bit result and repeat the same for the second channel. Encoding is rather simple too:
- feed input to 4-band QMF (with filter looking a lot like D4 wavelet to me);
- perform ADPCM on each band (this varies a bit for each band but it’s the same approach);
- generate output word (7 bits for band 0, 4 bits for band 1, 2 bits for band 2 and 3 plus a parity bit for them all).
Since I have no samples of it don’t expect a decoder from me any time soon (and I don’t have enough motivation to hook Android encoder directly to make it produce data). Not that anyone cares about it either.