[mythtv-users] Slightly OT: Huge files and RTJpeg tech question

[Jay R. Ashworth]

On Fri, Nov 05, 2004 at 07:35:42AM -0600, Michael J. Lynch wrote:
> I've been experiencing some strange behavior with recorded programs.
> What I've noticed is that some of my recorded programs are huge
> (9G/hour) and others are the size I expect them to be. All recording
> is performed using the same recording profile which is set to 544x480
> RTJpeg (slow machine w/bttv) quality set to 205.
>
> I've noticed a pattern to this behavior however which leads me to a
> tech question concerning RTJpeg. All of the recordings that are the
> size expected are recordings from channels were reception (off air
> analog antenna) is excellent and all recordings where the file size is
> huge, reception is not perfect. These "imperfect reception" channels
> exhibit snow which appears to be electrical noise. Anyway, here is the
> question. Could this noise result in RTJpegs compression algorithm
> not being able to do a very good job of compressing the video? If I
> were to lower my quality setting could I get more consistant (accross
> channels) recording files sizes?

Indeed.  MPEG and other related types of compression do not deal well
with random analog noise in the input signal at *all*.  This is one
reason why, for example, the Hauppauge cards have onboard noise
reduction.

How *much* bigger your files ought to get is unknown.  But there's a
problem with transcoding that can cause the output file to contain raw
uncompressed frames; not sure what causes it, but we get it here.

Get Gavin's nuvtools package, and run nuvscan on your big files, look
at the frame types.

Cheers,
-- jra
-- 
Jay R. Ashworth                                                jra at baylink.com
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	"You know: I'm a fan of photosynthesis as much as the next guy,
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