a question for the experts

[kdiddy]

Having recently switched back to CCE over TMPG....Ive noticed something odd....on VBR, Ive set as so, 1150,1500,2250 = min,avg,max well while the high motion scenes look fantastic, I did notice bad low motion scenes, I thought hmmm, those dont look like 1150 bps scenes...well I open the video file with "bitrate viewer"....and in fact, SEVERAL of my low motion scenes fall as low as 600!?!?!?....this happen in both mpeg1 & mpeg2, with 1 pass & multipass....even with CBR, according to bitrate viewer CBR is more like a very narrow VBR, which I expected, but even here, I still had few scenes drop to 900 & peak at 1900 when I had 1450 set as the CBR..any thoughts, ideas, comments?

[Kedirekin]

You know, I've never really understood how bitrate viewers work. How do they calculate the bitrate at any given point? Is there some established standard?

If you think about it, even CBR mpg is variable. I-frames are bigger than P-frames, and P-frames are bigger than B-frames, so on a frame-by-frame basis all mpg streams are variable bitrate.

If you were to look at each GOP, then I could see where CBR could be viewed as constant, but I don't think bitrate viwers do that. The only bitrate viewer I know of (can't remember the name) reports the bitrate in one-second intervals. But most of us accept the default in CCE, which is GOPs 15 frames long, even though we're usually encoding at 23.97fps. That means each GOP is ~0.625 seconds long. Some one-second intervals will end up with one I-frame and some will end up with two, which would completely mess up the bitrate calculation.

Well, I don't know if these ruminations shed any light in your case, but it gives you something to think about.

The one other comment I have is about dropping below the minimum bitrate. If the encoder is processing a completely black scene, I doubt it could manage to compress it to anything but near-zero. Encoders must be built to handle this special case and allow 'violations' of minimums. And it follows that there must be scenes that are 'in-between', where the encoder can't get all the way up to the minimum.

As for going above the maximum, I don't have a clue.