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Strange NTSC problem

Discussion in 'General Electronics' started by Caj Zell, Aug 16, 2003.

  1. Caj Zell

    Caj Zell Guest

    Hello!

    I have a video encoder, an ADI 7183, that is behaving quite strange.
    It is supposed to output NTSC and when I hook it up to a TV it
    displays the picture perfectly. But, on LCD/TFT devices I have (and I
    have tested three different LCD screens!), the picture is

    1. lowered about a tenth of the height and

    2. in the remaining upper gap the picture is black with occasional
    white boxes (concentrated to the left half)

    i.e.

    looks (very approximately) like

    * * * * * ***********
    * * * * * ***********
    *********************
    original picture con-
    tent shifted down 1/10
    and consequently the
    bottom 10th is missing

    I think this may have to do with the LCD screens misinterpreting the
    vertical sync and start displaying the top lines to early during the
    vertical blanking interval. But, why is this only happening on LCD
    screens and everythings fine on a Sony TV (with automatic NTSC/PAL
    detection)? Has anyone come across this phenomenon?

    Caj Zell
     
  2. Caj Zell

    Caj Zell Guest

    Hello again,

    just a small correction.
    I meant an ADV7179 from Analog Devices.


    Caj Zell
     
  3. I bet the blanking (sync position) is bad always but the overscan of
    the tube monitor hides it. Can the tube monitor underscan or do pulse
    cross? Have you looked at the composite output with a scope? If you're
    really NTSC, the H blanking should be 10.8 to 11.4 uS and active video
    should start on line 21. What is the video input to the encoder? What
    is the relation between video start of frame and vertical sync?

    See page 5 of the PDF for H blanking specs.

    http://www.tek.com/Measurement/App_Notes/25_7049/eng/section2.pdf

    GG
     
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