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PAL video VSYNC timing

Discussion in 'Electronic Design' started by bonzer, Oct 8, 2007.

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  1. bonzer

    bonzer Guest

    I am putting together an 8-bit CPU-based computer which is to have a
    colour video display. In my research on PAL video timings, I notice
    that the vertical sync is a series of pulses. If I am just using a non-
    interlace display, do I strictly need to have a series of pulses?

    I have made a TV-based video display in the past and just used a 192uS
    pulse every 20ms as VSYNC. This seemed to work ok but I am wondering
    if it's actually important to have the series of long and short VSYNC
  2. Guest

    This series of pulses are called serration pulses in the NTSC world.
    They are meant to keep the horizontal oscillator locked during the
    lengthy vertical retrace. Since TVs don't use analog circuits anymore
    for sync, the pulses are not strictly necessary. If you low-pass
    filter this waveform enough, you end up with a VSYNC without the

    You will note that the serration pulses are at 2H, so that the
    interlaced display doesn't affect the lock of the H oscillator.

    Judging by the amount of 10 year old TVs I see in the garbage every
    week, I think the amount of TVs older than 10 years is negligeable.

    Modern TVs are very robust when it comes to digesting video signals.
    They lock on to anything and scan double for you. (In the case of the
    LCD TVs I see all over the place).

    Anyways, sounds like you are doing a fun project.
  3. Bob

    Bob Guest

    I suggest asking comp.sys.sinclair
    The old guys over there are still obsessed with the inner workings
    of 1980's home computers that use TV's as displays.

  4. Only if interlace.
    Just xor H and V.
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