Distortion near peak level, specific to OGG, OPUS and AAC

[mayreg]

I noticed that some tracks converted to Vorbis, OPUS and AAC will become distorted at higher volume levels, especially when used with compressor and makeup gain (default compressor settings with Threshold -24 dB). Other formats (tested: WAV (pcm_s16le), FLAC, MP3, AC3, MP2) are OK and have no distortion at all with the same volume and compressor setting.

More details and audio samples in my bug report: https://www.rockbox.org/tracker/task/13375

Anyone experienced the same?

[dconrad]

I noticed in your bug report you're using 3.15, this is really pretty old at this point and I think the standard first step is to try a development build from here https://www.rockbox.org/daily.shtml, try that and see if you have any change in behavior.

I haven't noticed any distortion issues, though I don't typically use any of your problematic formats. It seems likely to me that the difference is just in how those formats compress the data results in a slightly louder way maybe? Just speculating.

[saratoga]

Lossy audio is expected to clip somewhat if you decode near-peak value waveforms, so testing with Ogg or OPUS is not so informative.  Some distortion is expected, so it is hard to know if what you hear is a bug or normal.  More interesting would be if you could replicate the problem with a lossless format, which will decode to exactly the same PCM values and so should not clip.  If that clips, then probably something is not setting the gain correctly (maybe compressor plugin?). 

[mayreg]

The same OGG, OPUS and AAC audio samples don't clip when played back on another player (e.g. VLC, mplayer, MPC-HC, etc.).

It doesn't clip either when the compressed file loaded into Audacity and checked visually (see attachment: noclipping.jpg). The peak is exactly at -1 dB, as generated.

It doesn't clip either when decoded with oggdec to WAV and played back by Rockbox+Compressor. That's why I suspect that something nasty is happening between the OGG/OPUS/AAC decoder and the compressor but not with other decoders.

As dconrad recommended, I also tried with build 38687821b2-221117 instead of 3.15. Same result.

[saratoga]

It doesn't clip either when decoded with oggdec to WAV and played back by Rockbox+Compressor. That's why I suspect that something nasty is happening between the OGG/OPUS/AAC decoder and the compressor but not with other decoders.

If you think the problem is the decoders themselves rather than the compressor, you can compile a build with the test_codec plugin enabled (or run the uisim which I believe has it by default) and output the decoded audio to .wav.  Then you can compare to the floating point decoder on PC.