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From: bender@oobleck.Eng.Sun.COM (Michael Bender)
Newsgroups: sci.electronics
Subject: video chroma circuit
Message-ID: <5497@exodus.Eng.Sun.COM>
Date: 8 Jan 91 22:26:24 GMT
Sender: news@exodus.Eng.Sun.COM
Distribution: usa
Organization: Sun Microsystems, Mt. View, Ca.
Lines: 42

I'm modifying my Panasonic WJ-MX12 video switcher to do a different
type of video insert keying, and while designing on the modification
I came across this circuit stub in the chroma signal path (the MX12
seperates Y and C and processes them seperately):


                                                          +5V
            +VRef                                          ^
              ^                                            |
              |       +------------------------------------+
              X       |                                    |
              X 10K   |                            +VRef   |
 chroma       X       |                              ^     |
 in           |      |/c                             |     |
   >---||-----+------|  npn                          X     |
      0.1           b|\e                         10K X     |
                      |         100                  X    |/c
                      +---||---/\/\/\---+------||----+----|  npn
                      |  0.1            |     0.1        b|\e
                      X                --- 10              |
                      X 820            --- pF              +---||---> chroma
                      X                 |                  |  0.1     out
                      |                 |                  X
                    -----             -----                X 820
                    |||||             |||||                X
                                                           |
                                                         -----
                                                         |||||
    Note: VRef is about 2.8V

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!

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

thanks,
mike
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