>I know what audio noise is, are you talking about that or video noise? If
>VN, what exactly is it?
I was talking about video noise and it's really the same thing as
audio noise. The most basic definition of video noise could probably
be stated as any visual artifact that isn't part of the "real" video
information. White flashes--or dropout, degradation of the signal
through generation loss, age/wear on the tape stock or problems
introduced by faulty time base (hence the use of TBC's or time base
correctors in professional edit suites) can all cause noise of one
sort or another.
The problem is that noise, being somewhat random by nature, is
impossible to compress. All compression schemes are based on the
notion that in any given series of video frames, many (most?) of the
pixels don't change. A red land-rover driving across the frame
against a "fixed" background is an example. So instead of storing all
of the information for frame one, frame two... thru frame five
million, four-hundred thousand and fifty-two the file just stores the
information for the first frame and then stores the changes (or
deltas) for the next series of frames. So our theoretical Red Land
rover driving across the frame will compress better then a tracking
shot that follows the red rover as it moves along the background.
(Assuming that is that the red rover isn't dominating the frame...) I
hope you get the idea.
Every so often a complete or "key" frame is stored. Some compression
schemes are very smart about when to do this. If the delta number
exceeds a certain level, store the entire frame instead. An example
of a new key frame might be a sudden cut from a dark shot of rovers
at night to one of the group wheeling the next morning. As there is
no relationship (from a visual perspective) between the two frames.
Visually similar frames compress well, visually diverse frames do not.
The problem with noise is that it introduces random patterns into the
frame that disrupt the frame-to-frame continuity. This increases the
number of deltas that need to be stored and may even cause each frame
(in a particularly noisey sequence) to be stored in it's entirety.
Result: Big files and "jerky" playback on slower machines.
For the more technically minded, there is a much "cleaner" definition
of compression in A. Michael Noll's book "Introduction to
Telecommunication Electronics, Second Edition pp340-341, Artech House
Publishers. (Disclaimer: I helped Dr. Noll with certain sections of
the second edition of the book and am credited in the preface.)
However, after reading it I think my "plain language" version might
be more accessible--if more wordy--to the majority of the group.
Sorry to those who got this far and are waiting for the Rover content...
jeff
--
Jeffrey A. Berg Purple Shark Media Rowayton, CT
jeff@purpleshark.com
==================
I believe in heavy thinking, I believe in heavy sound,
I believe in heavy images, To hold it all down.
--Chris Smither, Happier Blue
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