Question about "multi-pass" encoding

[c10250]

I am using CCE to convert an .avi file to MPEG2 format. I understand that the higher the bit rate is, the better the quality will be. So therefore, using a constant bit rate of, say, 8700 would be better than using a variable bit rate of 500 (low), 6000 (avg), 8700 (hi). So to preserve a quality .avi, and if file size does not matter, I am assuming I should just encode with a CBR that is around 9000.

Now, here is the question: What the heck does "multi-pass" get you in terms of quality with a constant bit rate. It is my understanding (probably wrong) that multiple passes in VBR will properly allocate high and low bit rates to your .avi for encoding. This can't be totally correct, if at all, since multi-pass is offered in CCE for constant bit rate. What does it do??

Thanks.

[andkiich]

Constant bitrate is not "technically" constant, it fluxuates some but stays more rigid than VBR.

[FOO]

If the encoder can read and analyze the whole video , it can
accurately allocate bits according to complexity. It also allows
hitting the target average bitrate exactly.

[DJ_Izumi]

Yes, Multi-pass VBR just allows the encoder to efficently allocate bitrate when space is an issue. However, if you're burning to DVD-R and have no issu with space, a 9000kbps singlepas CBR encode is ideal, VBR multipas at that bitrate is simply redundant.

Multipass is best suited for lower tasks, like SVCDs and MiniDVDs... Or trying to fit 10 episodes of a TV series on a single DVD-9 disk.

[FulciLives]

I am using CCE to convert an .avi file to MPEG2 format. I understand that the higher the bit rate is, the better the quality will be. So therefore, using a constant bit rate of, say, 8700 would be better than using a variable bit rate of 500 (low), 6000 (avg), 8700 (hi). So to preserve a quality .avi, and if file size does not matter, I am assuming I should just encode with a CBR that is around 9000.

If you can do a CBR of 8000kbps and be happy with it in terms of only fitting approximately 1 hour per single layer DVD recordable then that is the best you can get.

You really should limit yourself to a CBR of 8000kbps and no higher otherwise some DVD players may have trouble reading the disc.

- John "FulciLives" Coleman

[bugster]

Constant bitrate is not "technically" constant, it fluxuates some but stays more rigid than VBR.

Correct. With CBR (as far as I understand it) the bitrate is constant for each GOP, but as each GOP is made up of I,B and P frames, the number of bits within each frame varies, though the total number of bits for each GOP remains the same. So when CCE does multi-pass CBR it is trying to optimise the bit allocation within each GOP to get the best quality output. How effective it is I have no idea, not having used the latest CCE. Is it worth the extra encoding time? At 9000Kbps I doubt it, but in the end only you can tell