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Key Frame Interval?

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#1 robinc48


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Posted 12 December 2009 - 04:06 AM

Hi ...

What does the "Key Frame Interval" setting on the SV3 interface mean? The default setting is 10 seconds ... what happens to the quality / file size, etc. if you raise or lower that?


#2 Olivier Debon

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Posted 12 December 2009 - 04:34 AM

Key Frame definition is needed to undestand impact on compressed movies.

All recent video compression formats use the same technique that is based
on encoding an image is compressing differences between the current image
and the previous one.
Of course you need a reference so the very first image is always a complete
one, this is a Key Frame.
We could compress an entire movie based on this very first Key Frame, but,
what if you wanted to seek to the middle on the movie? Your computer would
have to decode all images before being able to decode the image at the middle.
That's why Key Frames are inserted from time to time to be able to seek into
the entire movie w/o having to decode everything before.

The problem with Key Frames is that they are much bigger than the others
(called Inter Frames). So too many key frames may result in file size increase
or quality degradation (if you select constant bitrate encoding).

It's a tradeoff.

Short key frame interval = bigger files (or lesser quality movies) but easy
to seek into.
Long key frame intervale = small files (or better quality movies) but harder
to seek into.

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#3 Roger Onslow

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Posted 12 December 2009 - 10:30 AM

So basically, a keyframe has the entire frame image, frames between keyframes have only differences (so smaller)
More keyframes means larger file size, but easier to seek to.
Roger the RED - SWiSH developer

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