[mythtv-users] Bit rate oddity?

[Joseph A. Caputo]

On Thursday 12 June 2003 09:46 pm, Pierre-Olivier Bouchard wrote:
> Albert Santoni wrote:
> > With the MJPEG codec, I've noticed that the size of shows are not
> > consistent, even if the length of the show is exactly the same. Is this a
> > feature of the MJPEG codec?
>
> How can something so unhelpful be a feature?
> See if both channels have the same amount of noise, the one that's
> noisier *will* be bigger. rtjpeg is more sensitive to signal noise than
> MPEG-4, and its file sizes aren't as constant. With my MPEG-4 settings,
> I get about 217Megs +/- 5 megs per half-hour, whatever the channel or
> the noise.
>
> Pete

I'm not sure about the difference between RTjpeg and MJPEG; but I believe one 
of the key differences between them and MPEG is that while MPEG is based on 
keyframes/interframes and a bitrate (constant, in Myth), RTjpeg and MJPEG 
compress each frame with JPEG compression (that's my understanding; I'm by no 
means an expert).  What this says to me is that, while with MPEG CBR file 
sizes will be more or less consistent (length in seconds * bits/second = 
total bits), RTjpeg and MJPEG are heavily dependent upon the content of each 
frame.  A recording with a large percentage of frames that don't compress 
well will be larger than one with frames that do compress well.

That, at least, is my layman's understanding of it all.

-JAC