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From: grege@gold.gvg.tek.com (Greg Ebert)
Newsgroups: sci.electronics
Subject: Re: video chroma circuit
Message-ID: <1863@gold.gvg.tek.com>
Date: 10 Jan 91 23:23:58 GMT
References: <5497@exodus.Eng.Sun.COM>
Distribution: usa
Organization: Grass Valley Group, Grass Valley, CA
Lines: 25

In article <5497@exodus.Eng.Sun.COM> bender@oobleck.Eng.Sun.COM (Michael Bender) writes:
> [...]
>I think this is a simple lowpass filter, but would it also introduce a
>phase shift?  Any help on what this really does would be appreciated!
>

Yes, it will low-pass filter, but I'm not about to analyze the beast.

>also, is chroma information phase encoded, i.e. the phase of the
>chroma signal determines the particular hue that you will see?
>

Yes. Chroma information consists of amplitude and phase. There is a reference
burst between Hsync and active video which locks a PLL once each video line.
The phase difference between the PLL and the instantaneous chroma signal
corresponds to hue (ie, color). The amplitude of the chroma signal
corresponds to the saturation (or 'brightness') of that particular hue.
Color uses RGB, and the information is encoded as luminance, chroma_phase, 
and chroma_amplitude.

It's more fun when done digitally :) !

Tidbit: The NTSC standard was developed in the 1940's to be compatible with
monochrome, and still fit inside 6Mhz. Considering the state of technology
back then, those guys were damn clever!