FFmpeg audio dither creates unwanted DC offset

[mstgrv]

I ran a test of FFmpeg's audio dithering and I noticed that it creates a DC offset in the resulting audio file, which shouldn't be there. Doing the same dithering process with other tools produces no DC offset in the resulting audio.

Here's what I did:

1. Create a silent WAV file (44.1khz, 32-bit floating point) using iZotope RX.
2. Convert that file to 16-bit with triangular dither, using iZotope RX
3. Convert the same file to 16-bit with triangular dither, using FFmpeg. The command I used was:

   ffmpeg -i input.wav -c:a pcm_s16le -dither_method triangular output.wav
   

Because the file in step 1 is completely silent, the files created in steps 2 & 3 contain just the dither by itself.

FFmpeg's conversion results in a file with DC offset, but iZotope RX's conversion has none. The expected result is no DC offset.

I have attached all the WAV files, plus screenshots of the analysis of each file (using iZotope RX) which shows the DC offset result.

[mstgrv]

A similar bug existed in Reaper, which was fixed in 2021 ​https://forum.cockos.com/showpost.php?p=2425940&postcount=25

The developer clarified what the issue was and how it was fixed ​https://forum.cockos.com/showpost.php?p=2801990&postcount=33

The bug (fixed in v6.27) was using floor() instead of rounding when reducing floating point samples to the desired bit depth. For example if you have a variable in [-1,1] with mean=0, floor(x) will have a mean of -0.5, but floor(x+0.5) will have a mean of 0.

Perhaps this could be the fix for the issue in FFmpeg?

[Balling]

Paul that worked on this moved to his own fork of ffmpeg. Open the issue there.

​https://github.com/librempeg/librempeg

[MasterQuestionable]

͏    "-dither_method triangular"
͏    Does it happen with other methods?
͏    Or the default..?? 0?

[MasterQuestionable]

͏    Your files can be deleted.