B-Frames and undersized files

[MetaHiro]

I'm encoding my DVD set of My So Called Life. I encode without b-frames and it reaches my specified size of ~500MB but once i turn on b-frames (3) the end resulting file shoots down to ~300MB. Does this happen to anyone else when using b-frames?

[digitize]

@MetaHiro
1st: I suggest you read up on xvid before posting questions like this.
2nd: b-frames = p-frames (delta frames), but look at frames before & after them (p-frames just look at frames before) which in turn gives better effieciency (spelling? i can't really spell :confused: ) anyway... lower files sizes are expected because b-frames are more effecient and compress better, and like I said read up on xvid.

[gldblade]

It's more than just higher efficiency. B-frames are encoded at a higher quantizer, thus using fewer bits on a particular frame. What's supposed to happen is that these extra bits will be spent on other frames to improve image quality.

What often happens is that the other frames have already maxed out, and can't put the extra bits to good use anyway. So you have a lot of extra bits lying around, and they can't be used for any other scene. Basically, you end up with an undersized file.

If you have an undersized file, don't use B-frames. B-frames wouldn't really help in that situation anyway.

[MetaHiro]

I guess i just naturally assumed that all the other frames would get a higher bitrate to compensate for the added compressibility of the b-frames. I asked for 500MB and I thought i would get it no matter what.

[Assault]

@MetaHiro
Did you do a two pass encode? If you did what's the size of your first pass? If it's smaller than 500mb XviD can't reach your desired filesize because you maxed out the codec. Next time you can uncheck "discard first pass" and keep the first pass .avi if it's already smaller than your desired size.

Regards
Assault