[MEncoder-users] video bitrate 1.25 smaller then expected

[Ivo Maljevic]

Hi,
I posted this same message on openSUSE forum, so I'll just copy it and
edit where necessary:

Did anyone try mencoder to convert a dvd to xvid? It works fine, but
there seems to be a bug, which I do not know if it is caused by
mencoder or xvid.

I don't have the exact script right now, but it is a slightly modified
version of the following two lines from Gentoo's two pass encoding
howto:

Code:

mencoder dvd:// -oac mp3lame -ovc xvid -xvidencopts pass=1 -o /dev/null

mencoder dvd:// -oac mp3lame -ovc xvid -xvidencopts
pass=2:bitrate=1200 -o <filename.avi>

I've added cropping, scaling, and changed audio parameters, but that
is irrelevant for the behaviour in question. The resulting video bit
rate is exactly and consistently 1.25 smaller than the bitrate
setting, regardless of what the bitrate setting is (1200 is just an
example, I tried several numbers).

I guess I can see where the error is coming from, and it is probably
related to bit to byte rate conversion, and a factor of 1/8 is
somewhere applied incorrectly.

I've googled a bit, and I've seen some old reports where people were
not able to go beyond a certain rate, but that was related to default
minimal quantization level set in mencoder. That does not appear to be
the case here.

Also, if I set the negative value to indicate the size, I see the same
bitrat (and file size) reduction 1.25 factor (I have my own bitrate
calculator).

This is fine by me, I can always set the bit rate to 1.25x desired
value, but just wanted to share the experience, and see if somebody
knows if this is a bug or something else.