[MPlayer-users] Bitrates and File Size

[Ethan John]

Hi all,

(New to the list) I've been ripping movies from DVD with MEncoder for 
about a year now, but typically just for backup purposes, maybe 
time-shifting rentals, or things like that. Since I rarely watch them, 
or keep them, quality so far hasn't been an issue. But it occurred to 
me recently that if I was going to have *good* backups, I should 
probably learn a little more about this whole MEncoder thing.

So after reading encoding_tips.txt, the MPlayer/MEncoder man page, and 
a number of posts to this list in the archive, I've still got this one 
lingering question:

Why don't the bitrate numbers match the file size numbers?

That might not make sense. Example:
I have a copy of US Marshals. It's 131 minutes, and when I play it with 
MEncoder, it tells me 618.6 kb/s video, and 96 kb/s audio, for a total 
of 714.6 kb/s. However, when you total these numbers up, you come out 
with an expected file size of 685 MB, but the file is only 600 MB. How 
is this possible? aren't mpeg4 movies constant bit rate unless you 
specify otherwise?

For a while I thought this had something to do with cutting out 
duplicate frames -- shaving a movie back down to 24 fps instead of 29 
or 30 on the DVD, but even adding in that ratio (1.25 or .8), the file 
sizes don't come out correctly.

In my head, the ideal bitrate for a movie would be calculated like this:
(30 / 24) * ((bits for total desired file size) - (bits reserved for 
audio)) / (number of seconds in movie)

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

Right now, I calculate bitrate based on trial and error from the past 
-- typically something like:
(bits for 725 MB file) / (seconds in movie)

i.e., I calculate the video bitrate based upon wanting a 725 MB movie 
of _just_ video. This typically yields file sizes between 660 and 700 
MB.

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

(I have a million more questions, but I'll try to keep it to one a day 
for a while. =- )

ethan john
http://students.washington.edu/thaen/
Apple Campus Representative
University of Washington
206.841.4157

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