FFmpeg 4.4: Missing decoded samples

[Uwe Klotz]

Starting with FFmpeg 4.4 I experience missing samples when decoding MP3 and M4A files. It seems to affect many files while the circumstances are yet unknown.

MP3: 48 missing samples, starting at offset 0
warning [AnalyzerThread 1 #2] ReadAheadFrameBuffer - Missing range [0 -> 47) between output buffer [0 -> 4096) and input buffer [47 -> 1199)

M4A: 64 missing samples, starting at offset 896
warning [AnalyzerThread 1 #2] ReadAheadFrameBuffer - Missing range [896 -> 960) between output buffer [896 -> 4096) and input buffer [960 -> 1984)

The missing samples are replaced with silence which is usually not noticeable right after the start of a track.

[Uwe Klotz (uklotzde-deactivatedaccount)]

This issue might shift markers by 64 samples.

[Be (be.ing)]

Does FFmpeg not have tests upstream that could detect such a regression? It might be worth contributing that upstream.

[Uwe Klotz (uklotzde-deactivatedaccount)]

Doesn't look like they do have proper regression tests in place:

1st AAC packet: [-64, 896] ( 960 samples)
2nd AAC packet: [960, 1984] (1024 samples)
all following packets: 1024 samples

The 2nd and all following samples do not account for the lead-in of 64 samples from the 1st packet. It is unclear if those samples need to be ignored (starting from 0) or if the offset of the 1st packet is wrong.

It is definitely a bug in the decoder.

[Uwe Klotz (uklotzde-deactivatedaccount)]

The first audible frame delivered by the decoder should contain 960 valid samples. The first 64 samples of this frame belong to the 1024 + 1024 + 64 = 2112 priming samples that are needed by the decoder.

The length of the first audible frame in AVFrame::nb_samples doesn't seem to account for the priming samples and only counts the audible samples! Unfortunately, this is not documented properly.

[Uwe Klotz (uklotzde-deactivatedaccount)]

https://github.com/mixxxdj/mixxx/pull/4245

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/10464