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[AUDITORY] Converting audio file from WAV to MP3 changes file duration. Why?



Dear Members,

An audio file in WAV can be converted to MP3 using following two utilities in unix terminal (both work, and there may be many more also):

$ ffmpeg -i <input.wav> -codec:a libmp3lame -b:a 320k <output.mp3> </dev/null
$ lame -q0 -b128 <input.wav> <output.wav>

But the issue is that the duration of <output.mp3> is more than duration of <input.wav>. This is true with other utilities which I have tried, like sox. Can anyone give insight on:

a. why the duration is increasing? In the attached image below, the duration variation is plotted for 410 sound files. The increase in duration appears to be WAV file dependent (although it is within 140ms in this case)

b. is there option in the above utilities which can reduce this difference in duration?  I haven't been able to figure this out.

Similar issue has been reported by few others also.
Example: https://www.sweetwater.com/forums/showthread.php?42631

Best regards,
Neeks


Attachment: duration_var_wav_mp3.png
Description: PNG image