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RE: Green light



Mike Reichel wrote in a message to JC Soret:

>.............................................I was told by Philips
themselves that the 
>reason why the chroma sensors are only half resolution of the luminance
>sensors is that it allows them to be two times bigger in surface in
>consequence get two times more light.

That is primarily because in television Including HDTV the bandwidth of the
chroma is half that of the luminance in the horizontal. With MPEG that can
be also half in the vertical. The sensors, I beleive, are the same size
just that one is for luminance and the other for chromanace has every other
sensor filtered in red and blue. The lower resolution red and blue are then
matrixed with a half resolution luminance signal to get the green. The high
passed luminance signal is then added to the red and blue in the same
amounts so that there is no color difference to the color encoder. Mike's
description in 1. below is correct but they don't use two different size
CCD sensors only one for Luminance and one for Red/Blue.

Actually, by making the sensors double the width - four times more light
hit
them as we are talking about a surface here. A line is finite and must have
both length and a width in practical life. So this is a very clever
compromise that in turn build on two all-important factors.

1. The eye is far less acute to color detail than to luminance.
The relationship is about 1/3. Not hard to understand if you think about
that when you mix RGB electrically to get white, the intensity goes up
three
times. In the Spirit case it is not a matter of "getting away" with
something more than actually understanding and making best use of how we
humans actually operate.

Jim Mendrala
Real Image Technology, Inc.
805-294-1049    Fax:805-294-0705
mailto:J_Mendrala at compuserve.com

---
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