Kostya's Boring Codec World

Since currently there’s a preparation phase for the next Ukrainian counteroffensive and I don’t know what other features to add to NihAV (beside improving video player), I was bored enough to do this.

After a comment from Paul, I’ve looked at some random lossless audio codec from 90’s, namely a-Pac. While the codec by itself it very simple (no LPC or IIR filter, no variable-length codes except in block header, no arithmetic coder either) as you can see from its description in the Wiki, it was a bit tricky to RE.

The main issues being: apac.exe is in NE format (which means 16-bit code and Ghidra sucks at figuring out which segment registers should be used when) and, which is worse, written in Delphi with all the quirks of its object model implementation. So it’s after some search I’ve found the virtual table corresponding to the APAC format handler (both coding and decoding) and after trying one function after another I’ve finally found the one responsible for encoding—after which finding a decoder function was easy.

The code itself (beside the weirdness introduced by Delphi compiler like using both positive and negative offsets in vtable calls) is very old-fashioned too: bit reading is done by keeping bitreader state as global variables, reading n-bit values by reading a single bit in a loop and refilling that bit buffer every byte (with a callback function).

It’s still fun to see how we’ve moved from simple formats like this to overcomplicated formats like LA or OptimFrog while settling down on in-between complexity FLAC, Monkey’s Audio and WavPack.

Recently The Mike asked if I can look at this format. In case you didn’t know, The Multimedia Mike is one of the under-appreciated founders of opensource multimedia, involved both in reverse engineering codecs and maintaining infrastructure for about two decades (for example this particular blog has been here for fifteen years thanks to him and his maintenance efforts). So of course I had to look at it even if out of sheer respect.

Sonarc is probably the first known lossless audio codec as the copyright mentions year 1992 as the first date (Shorten and VocPack appeared in 1993). Spoiler: it turned out to be closer to Shorten in design.

This was harder to RE because it was larger (decompressor was three to four times larger than VocPack) and the original was written in Borland Pascal with all the peculiarities it brings. By those peculiarities I mean mostly Pascal strings. Well, the code for manipulating them is annoying to parse but not too bad, the main problem is that they are put in the same segment with code right before the function that uses it and that confuses Ghidra which for some reason selects the segment with standard library routines for them instead (and uninitialised variables are not assigned to any segment at all). The write() implementation is also no fun.

Side note: back in the day Turbo Pascal was probably the best programming language for DOS and back in school at least two my schoolmates were doing crazy things with it (and Delphi later) which I couldn’t (and I was writing in C as I still do today). Yet somehow the popularity of the language vanished and I haven’t heard anything about them becoming famous programmers (neither did I but they had better chances). And the only modern project written in Pascal that I’m aware of is Hedgewars.

Anyway, let’s talk about the format itself. Sonarc can compress raw PCM, .voc and .wav into either its own format or into .wav and it supports both 8- and 16-bit audio.

From what I saw it uses the same approach: optionally applying the LPC filter and coding the residues. Residues can be coded with two different approaches: old one for 8-bit audio and new one for 8- and 16-bit audio. Old 8-bit audio coding uses one of eight different static Huffman codebooks or can code residues as raw bytes (and I can’t remember that many other codecs doing the same except for MLP and DT$-HD Lossless probably because why compress audio in that case). New 8-/16-bit coding still uses fixed codebooks but in a different fashion: now they simply code the number of bits for the residue. It does not look like the data is split into segments but I may be wrong (I/O is still not the easiest thing to get around there).

Overall it’s not a bad codec for its time and e.g. FLAC has not come that far away from it in concepts (except that it uses Rice codes and has independent frames plus partitioning inside individual frame for better compression). I hope though there are no older lossless audio codecs out there to be discovered (CCITT G.711 infinite-law with its fixed 1:1 compression does not count).

One of the issues with On2 VPx family is that they started it from VP3 while having four different TrueMotion codecs before that (it’s like the company was called Valve and not Duck at that time). But I wanted to look at some lossless audio codecs and there’s VocPack or VP for short which has versions 1 and 2. Bingo!

This is a very old lossless audio codec that appeared in 1993 along with Shorten and, as it turns out, originated the second approach to lossless audio compression. While Shorten was a simple format oriented on fast decoding and thus used fixed prediction (either LPC filter or even fixed prediction scheme) and Rice codes for residues (the same scheme used in FLAC and TAK), VocPack employed adaptive filter and arithmetic coding (the approach carried by LA, Monkey’s Audio, OptimFROG and such). And it was made for DOS and 8-bit audio! Well, version 2 added support for 16-bit but it seems to compress only high 8 bits of the sample anyway while transmitting low bits verbatim.

And it turned out to be my first real experience of using Ghidra with DOS executables. The main troubles were identifying library functions and dealing with pointers. Since it was compiled with Borland C++ 3.0 (who doesn’t remember it?) it was rather easy to decompile but library functions were not recognized (DOS executables don’t get much love these days…) but searching the disassembly for int 21h with Ralf Brown’s interrupts list at hand it was easy to identify calls for file operations (open/read/write/seek) and from those infer the stdio library functions using them and finally the code using all those getc()s. And of course segmented model makes decompiling fun, especially when decompiler can’t understand segment/offset variables being used separately. In result sometimes you recognize offset but you have to look at the data segment yourself to see what it refers to; even worse, for some local variables Ghidra seemed to assume wrong segment which resulted in variables in disassembly and decompiled output pointing to non-existent locations. Despite all of that it was rather easy to understand what unpacker for VP1 does. VP2 has only packer and no unpacker publicly available (feel free to trace the author and buy a copy from him that supports unpacking) plus it depends on those wrongly understood global variables more which prevented me from understanding how encoding a residue works there. In theory you should be able to set data segment manually but I don’t see a point on spending more than a couple of hours on REing the format.

It was a nice distraction though.

You know, the greatest reverse engineer I know is Derek B. He’s managed to RE such codecs as Canopus HQX and Cineform HD in the most efficient manner ever—saying he’ll do it and patiently waiting until somebody else does it.

So here are some words about his favourite lossless audio codec. The most interesting thing about it is that it was actively developed in 2001-2006 and then it was suddenly resurrected in 2015. Also it’s one of few non-standard codecs (i.e. not made into standard) that has several articles written about it.

The codec actually consists of two different formats, seemingly an old one and a newer one (that looks like it supports all range of sample type). The former is notable for having signal reconstruction stage using floating point math (a thing you don’t see in codecs every day), the latter seems to employ various parameter reading and reconstruction methods. Coding is done using low precision range coder (large values are decoded using chunks of 8 or 12 bits). So nothing really interesting there.

P.S. I’m definitely not going to write a decoder for it. There are too many lossless audio codecs already, let all proprietary ones (in custom containers too) die in peace.

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.