Gstreamer - horribly distorted mp3 playback (solved)

[causality]

On my system, gstreamer (and anything using it such as clementine) plays any format I throw at it flawlessly, except any mp3 files. It will play them without errors but the actual sound is horrible. It sounds heavily clipped and full of distortion. I can go play a .flac file and it sounds beautiful. Video files in various formats works fine including the audio. It's only mp3 files. This is an example command:

[causality]

I did some further experimentation and found something strange to me.

This command plays a given mp3 file and it sounds flawless:

[causality]

I did some digging by running the failing command with --gst-debug-level=4 and examining the logs. When told to auto-select a plugin (which is what decodebin does) it picks gst-plugins-libav for mp3 files (as opposed to the working command where it was told to use mpg123 instead). So the problem must be with the libav gstreamer plugin.

I previously had -libav USE flag for media-plugins/gst-plugins-libav. I enabled libav USE flag for this package and rebuilt it. Now all mp3 files play flawlessly regardless of how gstreamer plays them. This means that now I can use Clementine again since it uses gstreamer.

On my system I have media-video/ffmpeg installed but I do not have media-video/libav installed. This remains the case because gst-plugins-libav uses a bundled ffmpeg library and not the system media-video/ffmpeg if the libav USE flag is enabled. The description of the flag (as produced by "equery uses gst-plugins-libav") is: "Use bundled ffmpeg to support systems using media-video/libav globally instead of media-video/ffmpeg".

I still don't understand how this happened or why. Everything else built against my system media-video/ffmpeg plays all files flawlessly including mp3s. The ffplay command plays them just fine. I have no idea why this affected only gstreamer but I'm glad to have it fixed. I have a penchant for finding oddball problems that apparently no one else is having.

[agroth]

I'm posting to this 5-years old topic, because I had the same problem in 2024, and found another solution which I like to share.

TL;DR: Compiling ffmpeg with the "-floop-nest-optimize" flag produces a buggy MP3 decoder at least on some architectures.

I had the same problem when playing MP3 with quodlibet. Quodlibet is also using gstreamer, which in turn used ffmpeg for MP3. I could work around this by changing USE flags for gst-plugins-meta. Disabling "ffmpeg" and enabling "mp3" made gstreamer use mpg123 for MP3s instead. This is similar to causality's solution. It worked for me, but might not work for others, if gstreamer needs ffmpeg for different formats they might want to play.

I looked into this again recently when I wanted Vivaldi to play proprietary media. There is a "proprietary-media" flag for this, which either pulls in ffmpeg-chromium or ensures ffmpeg was installed with the "chromium" USE flag. It seems there is still some work in progress, as recently "ffmpeg-chromium" seems to be a "forced" flag. Anyway, the outcome is always that Vivaldi uses a libffmpeg.so built on the system instead of the one that comes shipped with it. And my system ffmpeg had broken MP3 support.

As a side-note: ffmpeg has two MP3 decoders. On my system, only the "fixed point" one was broken; the "float" one played well.

[sam_]

Please do file a bug on bugs.gentoo.org. I'm interested in such issues and handle them. If you would be so kind to, please include what you've said above, as well as what -march=native expands to on your machine (resolve-march-native may help), as well as some simple way to generate a bad file (using whatever example.mp3 fetched from somewhere). I'd much appreciate it.

It's especially useful given that such issues are often latent on other hardware and may resurface later.

[agroth]

@sam_ thank you for your interest. I filed a bug now.