NTSC questions

Discussion in 'General Electronics Chat' started by upand_at_them, Sep 11, 2016.

  1. upand_at_them

    Thread Starter Active Member

    May 15, 2010
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    I just picked up Don Lancaster's "TV Typewriter Cookbook".

    It's pretty interesting, but I have some questions.

    Do horizontal blanking lines contain sync pulses? Or are they just 63.5 microseconds of blanking level with nothing else?

    If they contain sync pulses, then how long after a vertical sync pulse do I have to wait to send the horizontal sync pulse?
     
  2. bertus

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  3. upand_at_them

    Thread Starter Active Member

    May 15, 2010
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    Hi bertus,

    Thanks for looking that up. I do have that horizontal line timing diagram in my notes, but it still leaves me unclear about my questions above.

    In Don's book, he says you should "allow 4 horizontal lines or more after vertical sync before displaying characters [or graphics]". Explaining that, "Several blanked horizontal lines should be provided following the vertical sync pulse. This gives the TV set a chance to catch up and adjust for the several horizontal sync pulses it may have missed during the vertical inverval."

    So, given the purpose of the horizontal blanking lines, I'm unsure if they should have sync pulses or not. If the TV set has to catch up on missed H syncs, then I could see a reason for the H blanking lines not having sync pulses.

    And there doesn't seem to be any mention (yet) of when to send the H sync after the V sync. The V sync pulse width (400 us) is already several times longer than one H line.

    (I'm doing non-interlaced frames, by the way.)
     
  4. crutschow

    Expert

    Mar 14, 2008
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    You should send a horizontal sync pulse for the first horizontal line, immediately after the vertical sync.
    There is no reason to not send a sync pulse with the horizontal blanking lines.
    To "catch up" requires sync pulses, otherwise the horizontal oscillator is just oscillating at its free-run frequency (which is slower than the sync frequency).
     
    Last edited: Sep 11, 2016
  5. upand_at_them

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    May 15, 2010
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    Thanks guys.
     
  6. bertus

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    Hello,

    The blank lines are shown in this timeframe from the same page:
    NTSC_RS-170A.gif

    Bertus
     
  7. crutschow

    Expert

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    Note that the double frequency horizontal sync pulses during the equalizing and vertical sync pulse typically keep the horizontal oscillator in sync.
    It syncs on every other pulse during that interval.
     
  8. upand_at_them

    Thread Starter Active Member

    May 15, 2010
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    Is that just for interlacing? The image Bertus linked to has a lot of interlacing information.
     
  9. bertus

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    Hello,

    You may have noticed the half lines in the even frames.
    In the raster they show also:
    NTSC_raster.gif

    Bertus
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Is what "just for interlacing"?
    The double frequency sync before and after the vertical sync?
     
  11. upand_at_them

    Thread Starter Active Member

    May 15, 2010
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    Yes.
     
  12. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    To make life easier, you want to ignore the 1/2 lines when your trying to display text the "EASY WAY", The easy way also means non-interlaced.

    NTSC was once 60 Hz vertical which was sync'ed to the power line frequency. It, and the Horizontal frequency are now based on the color burst signal.

    https://en.wikipedia.org/wiki/NTSC

     
  13. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The vertical sync signal is "serrated" and contains BOTH the horizontal and vertical timing information. The leading and lagging duty cycles of the vertical sync also determine if an even or an odd field will commence. There's a LOT of information embedded in the vertical sync pulse!
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    What are you actually attempting to do?
    You only need to know NTSC if you are going to generate composite video.
    If you are trying to drive a computer monitor all you need is VSYNC, HSYNC and video or RGB.
     
  15. crutschow

    Expert

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    What is amazing to me, is that they were able to generate this complex digital sync signal way back in the late 1940's with only vacuum tubes.
    I believe it took about a 6 foot equipment rack of tubes to do it.
    I imagine keeping that running was a major concern in the TV studio. Likely they had a least one backup on hand that they could rapidly switch to in case of a tube failure.
    Fortunately the TV receiver could readily decode the sync using much simpler (mostly analog) circuits.
     
    RichardO likes this.
  16. upand_at_them

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    Yes, just NTSC for now. It's more of a "something to do" project. Maybe make a simple video game. And eventually add some data to the signal from my DVD player. That's later down the line.

    I'm thinking I need a cheap analog TV to practice with first; I don't want to break my [only] LCD TV.
     
  17. bertus

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  18. upand_at_them

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    I'll check them out. Thanks.
     
  19. RichardO

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    May 4, 2013
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