avcodec: add ac-4 decoder

[gnattu]

Changes

Added the AC-4 decoder for ATSC 3.0 audio. Experimental. Decoder only. Remuxing is limited to mpegts containers and in my opinion should not be used and always convert into a more compatible format for jellyfin use cases.

Should not be enabled until at least Jellyfin 10.10 in my opinion.

Tested with sample files provided on dolby's website: https://ott.dolby.com/OnDelKits/AC-4/Dolby_AC-4_Online_Delivery_Kit_1.5/help_files/topics/kit_wrapper_MP4_multiplexed_streams.html

Support currently limited to common channel configurations. Dolby Atmos features like 5.1.4 channel configuration (5.1 channel with additional overhead channels) and AC4-IMS is not supported.

Issues

Fixes #311
Closes #128

[nyanmisaka]

Can this be easily ported to FFmpeg 7.0+? Will this cause existing fate tests to fail?

[gnattu]

Can this be easily ported to FFmpeg 7.0+? Will this cause existing fate tests to fail?

The demuxer and encoder is already in the 7.0, so the only thing need to be ported is the decoder itself. Is there even fate tests for AC4 yet?

[nyanmisaka]

Can this be easily ported to FFmpeg 7.0+? Will this cause existing fate tests to fail?

The demuxer and encoder is already in the 7.0, so the only thing need to be ported is the decoder itself. Is there even fate tests for AC4 yet?

I think you mean de/muxer (libavformat). Then this should be easy enough to maintain for decoding purposes, hopefully there won't be more breaking changes.

Raw AC-4 muxer and demuxer

I would have thought there might be some tests in richardpl's fork. But due to some personal reasons, he deleted the repo.

https://github.com/richardpl/FFmpeg/tree/ac4

[gnattu]

I think you mean de/muxer (libavformat). Then this should be easy enough to maintain for decoding purposes, hopefully there won't be more breaking changes.

Yes, the libavformat one. I did not port the muxer because I just want to enable transcode to prevent all kinds of compatibility issues.

Also, I just finished the 7.0 port and I do have a ready-to-go working tree based on the release/7.0 branch.

I would have thought there might be some tests in richardpl's fork. But due to some personal reasons, he deleted the repo.

I checked on upstream ffmpeg, there is none in the 7.0 branch yet.