[Libav-user] [libavcodec] requested bitrate is too low, when using VBR qscale

Nicolas George nicolas.george at normalesup.org
Tue Jul 24 17:28:18 CEST 2012

Le septidi 7 thermidor, an CCXX, Richard H Lee a écrit :
> (I'm not sure if this post belongs one of the other lists like
> ffmpeg-users, so please let me know if this is the wrong list.)

Since you are using the command-line tool and not the API, ffmpeg-user would
have been a better choice. But I do not think it is worth changing now that
the thread is started.

> The command line I use for 1st pass is:
> ffmpeg -i trailer_480p.mov -vcodec mpeg4 -q:v 4 -pass 1 -an test.mp4
> For second pass:
> ffmpeg -i trailer_480p.mov -vcodec mpeg4 -q:v 4 -pass 2 -an test.mp4

I do not think that having both -q and -pass options makes sense.

Multi-pass encoding is designed to achieve a certain average bitrate, and
manage it over the whole file to give more bits to the parts that need more.

The -q option sets the quantizer value (which, for any given frame, is
directly related to the quality level), and does not let any choice to the
encoder to set the bitrate.

If you care about the total size of your resulting file, you need to use -b
to set the average bitrate and -pass to spend it as efficiently as possible.
If not, you set -q and do only a single pass.


  Nicolas George
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