[FFmpeg-user] File duration of MP3 generated by mp3_mf seems wrong

[Tomas Kalibera]

Hello,

when I try converting a wav file to mp3 using the mp3_mf codec, and then 
display the duration of the resulting file, it is surprisingly short. Am 
I doing or interpreting anything wrong, or is this a bug and should I 
submit a full bug report?

The output file otherwise plays as expected for the expected duration.

ffmpeg -i u.wav ## gives duration 5.22s
ffmpeg -i u.wav -acodec mp3_mf u_mf.mp3
ffmpeg -i u_mf.mp3 ## gives duration 1.49s

Now, when I use "-acodec libmp3lame", the resulting file duration is as 
expected. According "mp3info -x" on Ubuntu, "Length" is 5s in both 
cases, as it should be. But the surprisingly short duration is shown 
even on Ubuntu.

Not sure if it helps, but when converting using mp3_mf, the output includes:

Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16, 128 kb/s
size=      83KiB time=00:00:05.22 bitrate= 129.7kbits/s speed= 149x

When converting using libmp3lame, the output includes:

Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16p
size=      82KiB time=00:00:05.22 bitrate= 129.0kbits/s speed= 121x

The part of "ffmpeg -i" on the result with mp3_mf is:

Duration: 00:00:01.49, start: 0.011995, bitrate: 455 kb/s
Stream #0:0: Audio: mp3, 44100 Hz, stereo, fltp, 128 kb/s

And with libmp3lame it is:

Duration: 00:00:05.25, start: 0.025057, bitrate: 128 kb/s
Stream #0:0: Audio: mp3, 44100 Hz, stereo, fltp, 128 kb/s

This is on Windows 10 (build 19045) with ffmpeg 7.1 from Msys2/ucrt64.

Thanks,
Tomas