VBR

TuffGong

TuffGong

Well-known member
I've been doing a bunch of experimenting with Vegas 5 and Windows Media 9 for audio and video compression.

I know that VBR means Variable Bit Rate, but what does that mean for those folks viewing a stream of my video clip?

If I set the VBR for for a Peak of 400Kbps and an average of 100Kbps, what does that do for the viewer? In this example, I always pull just above the average, even though I have 1.5 megs download. Even when I change the average bitrate, I pull just above that mark.

My results have been great btw, small file size and crisp quality, which is the ultimate goal. 8)
 
Re: VBR

The reason for that is because your movie most likely has titles and black screen. The bit rate at this time drops dramatically... because the average bit rate includes this time on your video, it allows the rest of the video to have an above average bit rate...

Bottom line though, if it looks good, it looks good...
 
Re: VBR

I have a single title in the beginning, but no black other than the fade to black at the end. All transitions are crossfades from one moving shot to the next.

I'll look around online to see what the effect is by choosing a peak VBR and an average bitrate. One would think it's obvious - Peak VBR is the top end download speed, average is...well..the average.
 
Re: VBR

Not sure about WM9, but with MPEG2, if you have a static shot, the bit rate will go down considerably, as well.

For example, if your frame is a perfectly static "extreme long shot," and the only thing moving in the frame takes up a height of twenty pixels, and a width of ten pixels, then the MPEG2 codec only needs to pay attention to the different pixels in a successive sequence.

If thinking of the footage like a grid of colour dots - which it basically is - then there may be times when a perfectly static shot can be compressed at full *apparent* quality, and be at a very low bitrate. If pixel at grid point # 036, 024 is always a certain shade of BLUE, then the MPEG codec only needs that information for *one* frame's worth.



This was far too long winded, and definitely more unclear, than it needed to be. But all I was meaning was that if you have many static shots, too, it'll allow for a higher bitrate whenever you *don't* have a static shot. It's an average, remember.
 
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