Undersized with xvid koepi

[Alain_French]

Hi,

There are a lot of post on the subject and i tried to understand the problem to get the right size.

Config : default settings + vhq=1 + bframe=2 (koepi 24062003)

Source : Tv acquistion MPEG2 from ati aiw 8500dv at 8mbps -> d2v -> avs...

Yesterday i decided to encode in xvid now. I made 6 jobs and 5 are ok size perfectly ok, but one is undersized.

I checked the STATS files and i noticed that the size is over for my 5 jobs ok, but the size is only 126Mo for the bad job...

I read in a thread, i would decrease the quantitizer offset from 100 to 30. The result is better : 146 Mo and the predicted size must be
160000kbytes and 1000/1100 for the bitrate.
And the job make 126Mo (with the first stat file) and 768kbps.

I don t understand why it doesn t work for my last job, whereas this is the same source file, but not the same part. Perhaps this is only a problem with my source... However i want to have the right size so what can i do ? Set the offset to 0 ? Set the bframe to -1 to have not bframe ?

I hope you ll understand my problem (my english).
Thank you
Alain

[Teegedeck]

As you have mentioned, there are several threads on this forum that explain how to increase filesize and gain quality if you get undersized results. I won't repeat those suggestions, as some have been made just recently and you should be able to find them. One of them is entitled Increasing Filesize and Pass 2 Questions (http://forum.doom9.org/showthread.php?s=&threadid=60646) and it's just a little down this page.

About why some videos compress better than others, dark and/or scarcely detailed and/or non-moving sequences are easier for a codec to 'describe'. Try to write down a description of various scenes yourself and you'll realize pretty much the same phenomenon; you'll need less paper for desribing a shot of a white wall than for a zoom into a crowded street.

[Alain_French]

Yes that s true my video is dark. All the action is in the night...
Thank you for your explanation and the link.

I changed the quantization type from H.263 to Mpeg and it seems to be ok. And I suppressed b-frame...
But I compared the two solutions and i haven't see differences... If i understood the b-frame allow to have a more compressive source ?
So it is rather for low bitrate ? Mine is around 1000/1100kbps.

[Teegedeck]

No, B-frames aren't only for low bitrates. Undersized 2nd passes are an exception; usually B-frames raise the general quality because they save bits without the viewer noticing much and allow for lower P-frame quantizers which are noticeable.

[Alain_French]

ah ok :D
thank you.