The Real Advantage of Variable Bitrate

[jchunter]

Torsten,
Remember that most of us don't give a hoot what Procoder does because we are using Video Studio for everything. What matters most to me is to know that Video Studio is not screwing up the picture quality by using VBR. My tests show no difference in PQ between CBR and VBR.

[2Dogs]

Either the average is a certain bitrate, and the bitrate multiplied by the duration of the video yields a certain size video, or the average bitrate is something else, but the file size will still be predictable. In any event, the file size will be essentially the same as for a CBR encoded video.

And nobody who has participated in this thread has tried to back up the statement that I took issue with -- not 2Dogs, or anyone else: VBR encoding itself will not in and by itself reduce the file size when compared to CBR encoding.

The point is I was checking out VBR vs CBR in VS9 - with it's limited implementation of VBR. I know what you're trying to say - that a CBR 8000 encoded clip will be the same filesize as a VBR AVERAGE 8000 encoded one. That's pretty much a no-brainer. But VS does not allow me to set an average bitrate.

I presume it would be fair to say that if you had a CBR encoded clip and the same clip encoded in VBR, and that both files were the same size - that the average VBR bitrate was the same as the CBR bitrate.

My purpose was to try to determine what settings would give the best quality output, for a given file size. If CBR is better, why does VS have VBR as the default in it's DVD templates? Since there are all kinds of illogical VS default values, a more pertinent question would be why almost if not ALL commercial DVD's use VBR. I know you say Hollywood has those 20-pass machines, but are you suggesting that those alone are responsible for their use of VBR?

[sjj1805]

Perceived wisdom has it that you get the greatest reduction in MPEG-2 file size when using variable bitrate encoding rather than constant bitrate for footage that features lots of movement.

Actually I always thought it was the other way round and your tests have proven that fact. The more movement the larger the file size - now to me that makes sense.

[2Dogs]

Hi sjj1805,

as I said in my first (rather snappy) response to THoff, I probably made the wrong choice of words.

Actually I always thought it was the other way round and your tests have proven that fact. The more movement the larger the file size - now to me that makes sense.

I can pull up quite a few quotes from various forum posts that imply that VBR will show the greatest advantage over CBR when used with video containing lots of movement. As far as filesize is concerned, the opposite appears to be the case.

The actual output quality is something else, that I need to look into for my own satisfaction at least.

Thanks for your contribution!

[sjj1805]

Hi sjj1805,
The actual output quality is something else, that I need to look into for my own satisfaction at least.

I look forward to reading the results of your tests.
I also think that the best way of finding out how something works is to take the lid off and put it through its paces.

Regards
Steve J

[skier-hughes]

VBR 9,000 max 8,000 av, 0 min = 1,954kb

same as 8,000cbr.