AAC USAC Mps212 support

[Artem S. Tashkinov]

Here's a live xHE-AAC radio station:

​https://www.maxradio.ca/UHD/MaxRadio/48k/Max.m3u8

It actually worked with ffmpeg 6.1.2 albeit audio quality was horrible.

ffmpeg 7.1 just bails out:

Error decoding audio.
[ffmpeg/audio] aac: AAC USAC Mps212 is not implemented. Update your FFmpeg version to the newest one from Git. If the problem still occurs, it means that your file has a feature which has not been implemented.

ffmpeg 7.1 also fails to mux it:

Input #0, hls, from 'https://www.maxradio.ca/UHD/MaxRadio/48k/Max.m3u8':
  Duration: N/A, start: 4164154.700045, bitrate: 48 kb/s
  Program 0 
    Metadata:
      variant_bitrate : 0
  Stream #0:0(und): Audio: aac (xHE-AAC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 48 kb/s (default)
      Metadata:
        variant_bitrate : 0
        compatible_brands: isomiso6hlsfmp42
        handler_name    : StreamS Encoder
        vendor_id       : [0][0][0][0]
        major_brand     : mp42
        minor_version   : 0
        creation_time   : 2024-08-13T07:32:00.000000Z
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
[adts @ 0x11498300] MPEG-4 AOT 42 is not allowed in ADTS
[out#0/mpegts @ 0x1076a9c0] Could not write header (incorrect codec parameters ?): Invalid data found when processing input
Conversion failed!


ERROR: ffmpeg exited with code 183

A few more xHE-AAC streams: ​https://www.hiresaudio.online/max-radio-society/

Assigning to Lynne if that's OK. :-)

[Lynne]

Mps212 is MPEG-H surround, not xHE-AAC. It's not part of the base spec.

Not really a priority right now.

The MPEG-H spec is a different dimension of cursed.

[Artem S. Tashkinov]

Got it, but then why ffmpeg 6.1.2 is able to decode this stream while ffmpeg 7.1 fails?

Looks like an unintended regression to me :-)

[Lynne]

It was a coincidence. xHE-AAC uses the same scalefactor coding as regular AAC, which could be mistaken by a very robust AAC decoder as valid, and result in a semi-horrible sounding output, as the per-band volume levels would be similar.

Decoding codecs with clearly incompatible decoders is not and has not been supported.