[FFmpeg-user] MP3 enconding bitrate

[Carl Eugen Hoyos]

Paulo Fidalgo <paulo.fidalgo.pt <at> gmail.com> writes:

> I have trouble to get the mp3 encoded right to play 
> on my Volvo Infotainment System (Sensus 3.0), so I 
> have contacted the costumer support, and they told 
> me that the files are being encoding with frames 
> with more than 320Kbps of bitrate.

Please ask them to explain which frame has >320kb, I 
don't think it's true, see below.

> Since I don't have a tool to debug it, I've tried to 
> see in codec statistics in VLC, which for the files 
> I'm showing as example I've seen 380Kbps near 3:31 
> mins.

I played the encoded file you uploaded with vlc, 
it does not show a bitrate of >320 here at any 
time.

I then analyzed the file you uploaded:
$ ffmpeg -i outff.mp3 -acodec copy -f image2 -map 0 out%5d
The largest mp3 frame in your encoded file is 960 bytes 
which corresponds to 320kb/s.
Maybe you should test the following to verify:
$ lame -b 320 input.wav out320.mp3
$ lame -b 256 input.wav out256.mp3
Do both files play in your car?
Or only the second?

> I believe it's a bug on ffmpeg, although I'm not sure, 
> so I'm asking for advice here, since encoding with the 
> lame it works like a charm.

lame documentation states that the default for lame's 
-q option is 5 but if you use -V, the default is lower 
(4 or smaller for your input sample).
This may be a bug, either in the documentation of 
FFmpeg (because it states that FFmpeg's option -q 
corresponds to -V which is mostly does but not exactly) 
or because FFmpeg sets lame's -q to 5 if you don't 
specify anything else.

> I've encoded with this command:
> 
> ffmpeg -i 2L38_01_96kHz.flac -codec:a libmp3lame 
> -qscale:a 0 2L38_01_96kHz.mp3

Please test with -compression_level 4, this should 
produce output files more similar with lame.

Carl Eugen