Explaination on the difference of two pass divx / xvid

[drewzor]

Hi guys, help me understand why this is so.

I have a vob file that i open in vdubmod and have done two pass xvid

I then demuxed the audio and converted to mp3 190kps and remuxed into the xvid file

I then opened the new xvid file with muxed mp3 audio in vdubmod and added a resize filter
scaling it down from 704x528 to 576x432.

I then decided i would do a two pass divx5.2.0 instead of xvid.

Here is the thing i get a larger file with divx5.2.0 at 576x432 then xvid at 704x528 ???

can anyone explain that? noise / artifacts / whatever or is it the version of divx just too old?

[jimmalenko]

Bitrate.

But of course that's just a wild stab in the dark since you make no mention of the settings you used for the video.

[drewzor]

Well yeah the biterate differs hence the file size differs, however they are both 2nd pass encodes therefore i assumed the first pass would calculate the same biterate ?? or at least the smaller 576x432 should have a smaller biterate. But it doesn't, it has come out larger?

Still i ask why?

=======================================
Settings
Xvid
First pass
Qpel=on
GMC=on
BVOP selected
Second pass target size set at 750mb
outcome = 559mb VideoSize = 704x528 biterate = 2715
====================================
Divx
1st pass
Settings on default
nth pass target 750mb
outcome = 617mb VideoSize = 576x432 biterate = 2996
====================================

[jimmalenko]

Those bitrates are massive for XVID and DIVX, and notice how your target size is 750MB yet your outputs are only 559MB and 617MB ?

I'd say that what is happening is saturation - that is, the bitrate required to achieve your given target size is more than ample for each second, and the codecs are only using as much bitrate as they need to to describe each second, which is less than the average bitrate you have allowed.

As for the question why the DivX file is larger, yet a smaller frame size - maybe XVID with the settings you've used is more efficient, hence less bitrate required, hence smaller file size ?

[drewzor]

Thanks, I guessed that if i used a target size that was surely not going to constrain the quality it would just use what was needed and no more... maybe this is not the case. I have lowered file size to 350mb just to see how damaging / effective target size is.

[jimmalenko]

Thanks, I guessed that if i used a target size that was surely not going to constrain the quality it would just use what was needed and no more... maybe this is not the case.

I'd say that that's exactly what has happened.

For your clip of run time 28 min 47 sec, you have said to use an average bitrate of approx 3642kbps, but the codec has ascertained that it only needs on average 2715kbps when using XVID or 2996kbps when using DivX. So therefore it has only used the values it thought necessary and has "ignored" the value you gave it so to speak.

[drewzor]

Right,
An xvid 2 pass re-encode of the xvid 704x576 to 576x432 has produced a file of 356mb while maintaining quality, a huge improvement!

divx is still substancially bigger and can only conlcude its is because it lacks features like bvop?

Thank you for helping with this one jimmalenko, it is much appreciated.