[MPlayer-users] Bitrates and File Size

[Vladimir Mosgalin]

On Mon, 25 Aug 2003, Ethan John wrote:

EJ>That might not make sense. Example:
EJ>I have a copy of US Marshals. It's 131 minutes, and when I play it with 
EJ>MEncoder, it tells me 618.6 kb/s video, and 96 kb/s audio, for a total 
EJ>of 714.6 kb/s. However, when you total these numbers up, you come out 
EJ>with an expected file size of 685 MB, but the file is only 600 MB. How 

Afair mencoder reports not average bitrate, but bitrate at the end or
something like that. You have to calculate average bitrate yourself,
it's easy with cbr/abr audio. Don't forget that avi isn't raw format and
space is wasted for the container structure and index too.

EJ>is this possible? aren't mpeg4 movies constant bit rate unless you 
EJ>specify otherwise?

No, with lavc, they are vbr.

EJ>But in practice, this number is too small -- it yields files in the 
EJ>600-650 MB range.

Very easy. Calculate desired video bitrate and use second pass, it's
designed for it.

EJ>What's going on here? My many thanks in advance.

Be happy that libavcodec even does quality vbr coding on the first pass
and don't ask bitrate to be very correct ;) In days of divx3, you always
had to do second pass to have good vbr video.

-- 

Vladimir