Kostya's Boring Codec World

If there are still people interested in my work on AAC encoder (I doubt though), here is a status report for you.

Tasks completed:

• Make encoder provide lookahead samples
• Make psy model use these samples to produce block switching decision
• M/S case handling (at least I hope so)
• Multichannel coding (from mono to 5.1, seems to work fine)

So there are only two bits left: tune model to produce better sound and commit that all to FFmpeg SVN.

The problem with current implementation is that it follows given bitrate too closely, so some frames may be coded too badly resulting in audible artifacts. I will add ABR mode and quality-based mode to make it better. Another issue is how to use codec options to turn them on, so Robert Swain can add profiles for them ;).

In related news: AAC decoder is slowly migrating to FFmpeg SVN, so I should be able to submit my encoder for review and inclusion as well.

I’ve not abandoned work on AAC encoder completely.
For example, I think now my psy model calculates and handles bitrate better. Now the only goals I set to myself are:

• Make encoder provide psy model lookahead samples for block switching
• Finetune psy model, or more specifically:
  • block switching decision
  • finish M/S case handling (for now it does not update all psy model information and result in artifacts on several test samples)
  • adjust scalefactors to reduce quantisation distortion
  • some other tricks from 3GPP TS26.403?

After that there are several ways to go: improve quality until it beats other encoders (at least libfaac), implement SBR/PS encoding (the latter is easier to do), introduce multichannel coding.

I suspect that at least one person will suggest to do it all.

I’m battling with psychoacoustic model. 3GPP TS26.403 seemed clear at the start but then problems have begun.

The main problem is perceptual entropy estimation. Since MDCT coefficients differ by magnitude in FFmpeg and 3GPP, perceptual entropy estimated by formulae from 3GPP differs much from actual number of bits to code. Also 3GPP encoder always makes scalefactor lie in range 104…164, beats me why (and its center does not correspond to scale = 1.0 either).

I have to investigate further before continuing work on psy model. I also hope to see AAC decoder in FFmpeg SVN soon and push my work on encoder there as well.

I’m working on creating psychoacoustic model from recommendations presented in 3GPP TS26.403. Implementation is very rough but at least it can produce the files with desired bitrate (not quite that bitrate but ~2kbps around it).

Now the tasks are to eliminate noise from encoded material and add block switching. Maybe window switching as well.
Oh, and commit that all to FFmpeg SVN.

Looks like Gabriel Bouvigne of mp3-tech.org (how many information I got from there!) and Lame fame took interest in AAC encoder. For now I’m following his advise and trying to implement psychoacoustic model after 3GPP TS 26.403 document. It should be simple yet effective enough.

In the other news: AAC decoder mutates to become fit for FFmpeg SVN inclusion. I hope that will happen soon. Keep going, Robert, and keep reviewing, Michael!

If you are interested in what happens with my encoder, here’s a piece of report.

Simple encoding works. That means you can encode files with it now and they can be played back and you’ll be able to recognize the sound. Also I’ve separated psychoacoustic model and encoder itself, so it calls model to ask what windowing to use and what scaling/coefficients to encode.
Can I say this concludes the task for this summer of code? Technically yes but there are few points I ought to finish.

Encoder side:

• MDCT for the cases different from simple 1024-point window (8 short windows sequence and two transition windows)
• correct bitstream writing for 8SS case
• probably multichannel encoding (it’s useless until we have defined multichannel audio API though)

Psychoacoustic model(s) side:

• good psychoacoustic model 🙂
• quantizer which allows rate control
• something else?

I can add some models after the work is complete too and probably tune it for my ears and music I like to listen to. Reading papers I got on psychoacoustic models should help.

Back to work then.