Archive for the ‘Audio’ Category

Playing with trellis and encoding

Sunday, August 8th, 2021

I said before I want to play with encoding algorithms within NihAV and here’s another step (a previous major step was vector quantisation and a simple Cinepak encoder using it). Now it’s time for trellis search for encoding.

The idea by itself is very simple: you want to encode a sequence optimally (or decode transmitted sequence with distortions), so you represent your data as a set of possible states for each sample and search a path from one state to another with the minimum error. Since each state of the sample is connected with all states of the previous samples, its graph looks like a trellis:

The search itself is performed by selecting for each state a transition from a previous state that gives minimal error, then selecting a state with the least error for the last sample and tracing back the path that lead to it from the beginning. You just need to store the pointer to the previous state, error value and whatever decoder state you require.

I’ve chosen IMA ADPCM encoder as a test playground since it’s simple but useful. The idea of the format is very simple: you have a state consisting of current sample value and step size used as a multiplier for the transmitted 4-bit difference value; you reconstruct the difference, add it to the previous stored value, and correct step size (small delta—decrease step, large delta—increase step). You have 16 possible states for each sample which makes the search take not so long time.

There’s another tricky question of selecting initial step size (it will adapt to the samples but you need to start with something). I select it to be close to the difference between first and second samples and actually abuse first state to store not the index of the previous state but rather a step index. This way I start with (ideally) 16 different step sizes around the current one and can use the one that gives slightly lower error in the end.

And another fun fact: this way I can use just the code for decompression of single ADPCM sample and I don’t require actual code for compression—it traverses through all possible compressed codes already.

I hope this demonstrates that it’s an easy method that improves quality significantly (I have not conducted proper testing but a from a quick test it reduced mean squared error for me by 10-20%).

It should also come in handy for video compression but unfortunately rate distortion optimisation does not look that easy…

Why codecs are designed like this and why they are not very interchangeable

Monday, August 2nd, 2021

Sometimes I have to explain the role of various codecs and why it’s pointless in most cases to adapt compression tricks from image codecs to audio codecs (and vice versa) and even from lossy to lossless codecs in the same content. If you understand that already then you’ll find no new information here.

Yours truly
Captain Obvious
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A quick glance at Bink Audio

Tuesday, February 2nd, 2021

Since my attention was drawn to this format (and binary specification was provided as well) I’ve briefly looked at it—and a brief look should be enough.

From what I see it’s the same Bink Audio but in its own container instead of Bink. It has 24-byte header, a table of 16-bit audio block sizes and actual audio data (each frame may be prefixed with 0x99 0x99 but I’m not sure since I’ve not seen a single file in that format).

Frame header:

  • 1FCB magic;
  • one byte of version (version 2 groups audio frames together, previous one does not);
  • one byte with number of blocks per frame;
  • two-byte sampling rate;
  • four-byte variable, probably frame length in samples;
  • four-byte unknown variable, maybe suggested input buffer size?
  • four-byte unknown variable
  • four-byte variable, number of frames in seek table.

So as expected it’s nothing special.

Looking at XVD

Saturday, January 30th, 2021

A week ago a certain XviD developer made a request to look at something more compressed called XVD and so I did.
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A look on weird audio codec

Thursday, January 7th, 2021

Since I still have nothing better to do I decided to look at ALF2CD audio codec. And it turned out to be weird.

The codec is remarkable since while it seems to be simple transform+coefficient coding it does that in its own unique way: transform is some kind of integer FFT approximation and coefficient coding is done with CABAC-like approach. Let’s review all details for the decoder as much as I understood them (so not much).

Framing. Audio is split into sub-frames for middle and side channels with 4096 samples per sub-frame. Sub-frame sizes are fixed for each bitrate: for 512kbps it’s 2972 bytes each, for 384kbps it’s 2230 bytes each, for 320kbps it’s 2230/1486 bytes, for 256kbps it’s 1858/1114 bytes. Each sub-frame has the following data coded in it: first and last 16 raw samples, DC value, transform coefficients.

Coding. All values except for transform coefficients are coded in this sequence: non-zero flag, sign, absolute value coded using Elias gamma code. Transform coefficient are coded in bit-slicing mode: you transmit the lengths of region that may have 0x100000 set in their values plus bit flags to tell which entries in that actually have it set, then the additional length of region that may have 0x80000 set etc etc. The rationale is that larger coefficients come first so only first N coefficients may be that large, then N+M coefficients may have another bit set down below to bit 0. Plus this way you can have coarse or fine approximation of the coefficients to fit the fixed frame size without special tricks to change the size.

Speaking of the coder itself, it is context-adaptive binary range coder but not exactly CABAC you see in ITU H.26x codecs. It has some changes, especially in the model which is actually a combination of several smaller models in the same space and in the beginning of each sub-model you have to flip MPS value and maybe transition to some other sub-model. I.e. a single model is a collection of fixed probabilities of one/zero appearing and depending on what bit we decoded we move to another probability that more suits it (more zeroes to expect or more ones to expect). In H.26x there’s a single model for it, in ALF2CD there are several such models so when you hit the edge state aka “expect all ones or all zeroes” you don’t simply remain in the state but may transition to another sub-model with a different probabilities for expected ones-zeroes. A nice trick I’d say.

Coder also maintains around 30 bit states: state 0 is for coding non-zero flags, state 1 is for coding value sign, states 2-25 are for coding value exponent and state 26 is for coding value mantissa (or it’s states 2-17 for exponent and state 18 for mantissa bits when we code lengths of transform coefficient regions).

Reconstruction. This is done by performing inverse integer transform (which looks like FFT approximation but I’ve not looked at it that close), replacing first and last 16 coefficients with previously decoded ones (probably to deal with effects of windowing or imperfect reconstruction), and finally undoing mid/stereo for both sub-frames.

Overall it’s an interesting codec since you don’t often see arithmetic coding employed in lossy audio codecs unless they’re very recent ones of BSAC. And even then I can’t remember any audio codec using binary arithmetic coder instead of multi-symbol models. Who knows, maybe this approach will be used once again as something new. Most of those new ideas in various codecs have been implemented before after all (e.g. spatial prediction in H.264 is just a simplified version of spatial prediction in WMV2 X8-frames and quadtrees were used quite often in the 90s before reappearing in H.265; the same way Opus is not so modern if you know about ITU G.722.1 and heard that WMA Voice could have WMA Pro-coded frames in its stream).

A brief look at Sonarc

Tuesday, September 29th, 2020

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).

A look at VP1 and VP2

Saturday, September 26th, 2020

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.

Lossless audio codecs were more advanced than I thought

Wednesday, September 23rd, 2020

As I’d mentioned in a previous post on lossless audio codecs, I wanted to look at some of them that are still not reverse engineered for documentation sake. And I did exactly that so now entries on LA, OptimFROG and RK Audio are not stubs any more but rather contain some information on how the codecs work.

And if you look at LA structure you see a lot of filters of various sizes and structure. Plus an adaptive weight used to select certain parameters. If you look at other lossless audio codecs with high compression and slow decoding like OptimFROG or Monkey's Audio you’ll see the same picture: several filters of different kinds and sizes layered over each other plus adaptive weights also used in residuals coding. Of course that reminded me of AV2 and more specifically about neural networks. And what do you know, Monkey's Audio actually calls its longer filters neural networks (hence the name NNFilter.h in the official SDK and you can spot it in the version history as well leaving no doubts that it’s exactly the neural networks it is named after).

Which leads me to the only possible conclusion: lossless audio codecs had been using neural networks for compression before it became mainstream and it gave them the best compression ratios in the class.

And if we apply all this knowledge to video coding then maybe in AV4 we’ll finally see some kind of convolution filters processing whole tiles and then the smaller blocks removing spatial redundance maybe with some compaction layers like many neural network designs have (or transforms for largest possible block size in H.265/AV1/AVS2) and expansion layers (well, what do you think motion interpolation actually does?) and using RNNs to code residues left from all the prediction.

Revisiting lossless codecs…

Sunday, September 6th, 2020

I’ve decided to add a couple of lossless audio formats in a preparation for a long-term goal of having a NihAV-based player (the debug tool nihav-player that I currently have can’t really count for one especially considering how it does not play pure audio files and tends to deadlock in SDL audio thread).

So I’ve added nihav-llaudio crate with four most common formats for music I have, namely FLAC, Monkey’s Audio, TTA and WavPack. And I guess it’s time to revisit my opinion about various lossless audio formats now that I’ve (re)implemented support for some of them (I tried to summarise my views about them almost ten years ago). Let’s see what has changed since then:

  • I had a closer look at MPEG-4 ALS and it turned out to be rather interesting (and probably the only lossless audio codec with P-frames) but it also has somewhat insane options (like maximum prediction order of 1023 for LPC; or coding the whole file with just one I-frame and the rest being P-frames so no seeking is possible) and RLSLMS being broken (the reference decoder can’t decode the official reference samples) and it got no popularity at all;
  • TTA turned out to be very simple with a baffling rationale

    The sample count in a TTA1 frame is a multiple to 576 (sound buffer granule). Based on this, the “frame time” is defined as a constant 1.04489795918367346939. Thus, the sample count in a regular TTA1 frame determined as: regular TTA1 frame length = frame time * sample rate.

    I’m no mathematician so this does not form a coherent logical chain for me, I’d use something like “frame length in samples is sample rate rounded up to multiple of 576” instead of “sample rate multiplied by 256/245”. The main irritating point is that last frame contains less samples and you need to signal that it’s last frame (or merely check if you have enough bits left to decode a full frame after you decoded enough samples for the last frame). Oh, and TTA2 seems to be still in development.

  • And speaking about codecs in development, I don’t see new lossless audio codecs appearing after 2010. Either I got too old and don’t spot them or the interest has finally faded out. This might be because most people don’t buy music any more but rather rent it in some online store or use some streaming service. And those who still do probably use one of the old established codecs like FLAC.
  • And since I’ve mentioned it, my opinion on it has not changed and only got a bit more refined not that I have a decoder for it as well. Previously I thought FLAC is a simple format with a bad bitstream format that makes seeking hard. Now I know that FLAC is a simple format (fixed predictor or LPC up to order 32 and fixed Rice codes; the only thing that improves compression is splitting residues into partitions where optimal k for coding them can be selected) with horribly designed bitstream format.

    Normally lossless audio formats either store offsets for each frame or have an easily recognizable header, but FLAC is different. It’s obvious that the author was inspired by MPEG audio header design but those actually had frame sizes coded. Here in order to find where the frame ends you need either to decode it or calculate CRC for the data you read (and in the likely case of false positives also check that the data is followed by a valid header). One could argues that there’s often a seek table in FLAC file but for e.g. in luckynight.flac those entries are for multiples of ten seconds positions, making seeking to a more precise position a task of skipping frames (which is fun—see above).

  • WavPack is still the best designed format in my opinion though it would be nicer to have some initial header with various metadata instead of having it stored in the first block. Other than that still no objections.
  • And it turns out there’s lossless AAC compression that employs wavelet transform before LPC (it’s Chinese AAC though so who cares).

I remember reading somewhere (on Hydrogenaudio most likely) a brief story about development of several popular lossless audio codecs (even told by the author of one but I might be wrong). Essentially it’s not a NIH syndrome but very close: somebody develops a format, another guy finds a minor flaw the original developer refuses to address (my memory is hazy but I think there were such things mentioned as no plugin for some player or not supporting some tags) and develops another format. The amount of formats that came to existence because somebody wanted to create a format and could not keep it to himself is pretty large too.

But those days seem to be over and maybe I’ll reverse engineer some of those old codecs for documenting reasons as there’s very little risk that somebody would pick them up and make widespread now. Alternatively I can rant on newer formats sucking as well. Though why wait, let’s do it now:

  • AAC sucks because of the countless extensions and attempts to bundle various coding approaches under the same name (fun fact – “xHE-AAC” is actually pronounced as “MPEG-D you-suck”);
  • AV1 sucks because of the organisational structure and their decisions during (and after) the design stage;
  • AV2 is not here yet but it sucks for the same reason;
  • BlueTooth audio codecs suck in various ways (except SBC, it’s okay for the purpose), especially because of marketing them as high-definition and robust while in reality they rarely are;
  • Chinese codecs suck for being rip-offs of better-known codecs. It’s especially gross that one of them got standardised as IEEE 1857.2 AAC;
  • H.264 sucks because of countless extensions;
  • H.265 inherited some from H.264 and added the licensing situation on top of that;
  • MPEG-5 EVC sucks because it’s a Frankenstein monster constructed from bits from H.263-H.265;
  • Opus sucks for being designed for streaming case and used elsewhere;
  • Vector-based codecs suck because current tools are still not good enough to autovectorise complex shapes and recognize gradients.

Now back to doing nothing.

Weird Audio in VMD

Sunday, April 19th, 2020

Spoilers: as it turns out, a French company needs some Belgian technology in their life.

In last post I mentioned that NihAV can decode all of VMDs except maybe from the latest generation. Since I was provided with the samples I looked what changed there.
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