Kostya's Boring Codec World

In the previous post I gave general overview of codec changes, now I’m going to look more deeply at the filter changes with time.

• 3950 — current mode with up to three layers of IIR filters
• 3930 — simpler filters: no third layer (there was no insane compression level back then) and the difference between predicted and actual value was not used.

For the older versions there are differences in the implementations of the filters for the different compression modes.

Fast compression:

• 3200 — order 2 adaptive prediction (i.e. previously decoded and adjustable prediction value are used in prediction)
• 0000 — almost the same but with different rules for adjustment factor updating

Normal compression:

• 3800 — two layers of filters: order 4 adaptive prediction and order 2 afterwards
• 3200 — the same structure, different rules for updating
• 0000 — three layers with orders 3, 2 and 1 and different updating rules

High compression:

• 3700 — first it tries first order adaptive prediction with the delay of 2-16 (i.e. the next to previous element is used for prediction) and normal mode decompression afterwards (different decoding for 3800 of course)
• 3600 — the same but delays are 2-13
• 3200 — the same but delays are 2-7
• 0000 — orders 5 and 4 and different updating rules

Extra high compression:

• 3830 — an IIR filter resembling the one used in the newer Monkey’s Audio versions
• 3800 — some filter parameters were half as much as in 3830 and there was no delay 2-8 filtering
• 3600 — delay filtering plus high filtering (which is delay filtering plus normal filtering, which can be expressed as a layer of filtering over fast filtering)
• 0000 — essentially the same but with different prefiltering

While preparing for working on old APE versions support I finally got courage to try and trace all changes for different versions. So here’s the list of internal versions and the changes they introduced:

• 0000 — the reference version for all prehistoric version. Before version 0000 it was fine, then it all got worse IMO.
• 3320 — changes in the filters
• 3600 — changes in the filters
• 3700 — changes in the filters
• 3800 — blocks per frame changed for extra high compression level; changes in the filters (yawn)
• 3810 — frame start at byte boundaries now
• 3820 — special codes extension (signalled by top bit of CRC set to one)
• 3830 — filter lengths and some implementation details changed
• 3840 — CRC calculation algorithm changed a bit
• 3870 — significant changes in residue coding
• 3890 — small changes in residue coding
• 3900 — residue coding format has changed seriously.
• 3910 — small change in the residue coding (more than 16 bit values can be coded now)
• 3930 — significantly changed format introduced (both filtering and coding scheme were changed)
• 3950 — filter format changed a bit (and insane compression mode is added somewhere after that), blocks per frame is changed too
• 3960 — some small and compatible change in the bitstream (consuming two last bytes or not)
• 3980 — file format is changed a bit; filtering process has changed a bit too.
• 3990 — the latest (known) format. Residue coding has changed.

Do you still wonder why I strongly dislike this format?

By the request of one guy (he has provided samples as well) I shall work on supporting old Monkey’s Audio versions (before 3.95).

Why? Because the latest official version of Monkey’s Audio has dropped support for those files, because I wanted to support such files since really long time (just didn’t have a good opportunity to do that) and because I definitely need a distraction from Go2Insanity codec (I shan’t blog about it anymore).

Well, let’s see what the old versions of the worst (known) designed lossless audio codec have to offer me.