Could you explain the signal differences (between RGB and YPrPb). I can
use either between my deck and Targa DTX - is one better than the other?
less processor intensive? look better?
Thanks
-Rohan
I reply:
I don't know about how 'processor intensive' they are, but I can tell
you what each is:
RGB is the raw Red, Green, and Blue signals from a video source such as
a camera or computer video board. Each of these includes luminance so
that if you view each one of them separately, you will see a form of B/W
image (I am ignoring the need for sync at the moment).
YPrPb as you call it (I'd prefer to call it Y,R-Y,B-Y) are MATRIXED.
That is, these components are derived from RGB by adding or subtracting
them to get the new components. Y (or luminance) is derived by adding
Red, Green, and Blue together in a specific formula to make the Black &
White signal we refer to as luminance (the formula is 30% Red, 59%
Green, and 11% Blue). R-Y is derived by taking the newly-created
luminance signal and subtracting it from the raw Red signal. Since we
already have a luminance signal (Y), we don't need the Y information in
the red component so we subtract it, leaving only the color difference
component for red we call R-Y (or Pr).
We do the same thing with the Blue channel, subtracting Y from it to
leave only the color component of blue.
We do not need to make a G-Y component because we have the others, so
we can matrix them to get what's left, which is Green.
Now I suppose you'd want to know why we do this. Here's one
explanation: Each component of R, G, and B uses a lot of bandwidth. For
proper component detail, they each need to be about 4.2Mhz or so. 4.2Mhz
times 3 equals 12.6Mhz. That's a lot to record on tape or digitize into
a computer. Since the luminance component is common to all 3, why send
it 3 times? Matrix it so that you only send it once (Y), and send the
color difference signals (R-Y and B-Y) separately. The result is more
efficient use of the bandwidth. Typically, the color components are each
about half the bandwidth of the luminance component. Therefore you have
saved 4.2Mhz of bandwidth out of the total of 12.6Mhz, giving you 8.4 (Y
4.2, R-Y 2.1, B-Y 2.1) with very little lost in the final image (only
the color channels are compromized, and they can get by with less
bandwidth without a noticable effect).
Dennis Degan, Video Editor (DennyD1@hotmail.com)
Studios USA "The Sally Jessy Raphael Show"
15 Penn Plaza OF2
New York, NY 10001 212-419-7400
"The difficult we do right away . . .
The impossible takes slightly longer . . ."
--Philo T. Farnsworth (the Father of Television)
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