[Theora-dev] Black frames need higher bitrates than moving ones?!

Ralph Giles giles at xiph.org
Sun Oct 3 09:52:52 PDT 2004

On Sun, Oct 03, 2004 at 03:56:30PM +0200, Robert Huitl wrote:

> I noticed that black frames (webcam lens shut) produce 45 KB/s (target bitrate 
> is 400 kbps), while moving video is around 8-20 KB/s. How can that be? Is the 
> encoder fooled by the background noise the webcam produces?
> Maybe it has to do with the quality setting, though I don't know what it's for 
> (there's already a bitrate setting) and what i.e. 30 means to it, but I could 
> imagine that a quality calculation in the codec has problems with black 
> frames...

It's quite possible this is the problem. Noise is expensive to encode, 
and without any actual signal to calibrate its efforts the encoder is 
trying very, very hard to represent the camera noise accurately. 
The Vorbis reference encoder behaves exactly this way when fed 
low-intensity analog noise vs. actual digital silence.

The quality setting tells the codec what 'quality level' you'd the like 
the output video to have; it then uses the minimum number of bits to 
achieve this quality, resulting in more efficient encoding since it 
doens't have to waste bits maintaining a constant bitrate when the scene 
isn't demanding. Generally, you should use this mode in preference to 
seting an actual bitrate.


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