Kostya's Boring Codec World

Today I (not really) want to talk about some weird coding methods employed by two codecs.

Monstrous Go2Meeting. To my very deep surprise its compression method 2 uses ELS coder, a curious binary arithmetic coder replacement. In essence it operates on fraction of bits (called jots by its autor) and uses something like state machine for model (i.e. depending on state and decoded jot value — 0 or 1 — move to one of two possible other states and subtract some state-defined value from input value). This implementation uses 36 jots per byte, has ladder with 174 rungs and operates on 24-bit state instead of 16-bit in the paper.

From a cursory glance on TAK it seems to be more or less ordinary lossless audio codec — i.e. LPC plus residue coding. The only peculiar thing is that residue coding. While other codecs use mostly adaptive coding, this one seems to employ fixed coding parameters for segments of residues and bitstream also contain parameter set indices for all these segments.

Scheme is rather simple: read predefined number of bits, if it’s not the escape value then reinterpret code as signed. For escape value get unary code, if it’s not equal to the secondary escape value then scale that value and reinterpret as signed. Else just read some additional number of bits, scale them and reinterpret as signed. Number of bits to read and escape values make those coding parameters (about 52 total).

Also even if it might provoke small flame war, I publicly say that I’d rather not see TAK supported in opensource. We have enough lossless codecs already, especially with their own containers. And they cover all possible uses already (don’t tell me about insignificantly higher compression ratio).

This entry was posted on Friday, July 13th, 2012 at 4:23 pm and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.