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VGA to NTSC line-for-line ?

Discussion in 'Electronic Design' started by Peter, Feb 2, 2009.

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  1. Peter

    Peter Guest

    Hi All,

    I am after a converter which does VGA 640x480 to NTSC video.

    There are loads of boxes which do this e.g. this Ebay reference:
    190280908219

    The catch is that I am looking for something which does not replicate
    (or eliminate) any pixels *vertically*.

    This is because this is for displaying TEXT on a product which has an
    NTSC video input which displays on an LCD, and I think that the video
    is mapped onto the LCD pixel for pixel, so if I keep this up in the
    converter box also, I should get the best quality.

    Obviously some lines will be lost because the displayed part of an
    NTSC frame is less than 480 lines...

    Any pointers would be appreciated.
     
  2. Nobody

    Nobody Guest

    It's actually 486 displayed lines (of 525 lines in total), although some
    of those may be not actually be visible (esp. for CRTs).
    The main issue is that NTSC is interlaced, VGA isn't. So VGA will have
    roughly twice the horizontal frequency of NTSC (e.g. 31.46875 kHz for
    [email protected] vs 15.734 kHz for NTSC).

    A simple converter would digitise and store every other input line, then
    display it at half speed. This doesn't actually eliminate lines; you would
    use the even lines from one frame and the odd lines from the next, so each
    line appears in one of the two fields.

    OTOH, most PC video hardware is capable of generating NTSC timings
    directly. The driver probably won't allow it if the card doesn't include
    a "video-out" (composite, S-Video, etc) connector, as VGA monitors
    typically can't handle it.
     
  3. Guest

    Oh wow, that is a problem that's more complicated than it first
    appears.
    Since there are no "pixels" as such in NTSC, the idea of a per-pixel
    mapping in a LCD is ambiguous.
    Even the lines themselves have no real meaning unless the LCD is
    exactly 525 lines (give or take).
    The best you can do is make the text large enough to be legible.
    For example, in the '80s, home computers used TVs as their displays.
    None of them had 80 columns on a TV though.
    The computers with 80 columns either used monochrome monitors with
    NTSC-like syncs but no color information in the video,
    or had dedicated RGBI/CGA-style graphics.
    (I don't include "80 column" software display modes with tiny 4 pixel
    wide fonts on s-video monitors, but that's one way of doing it)
    If you look at the fonts used back then, a low-res computer like a
    VIC-20 used single pixel wide text and it was legible on TVs.
    However, machines like the C64 with 320 pixels horizontally had to use
    fonts with lines two pixels wide.
    You'll have to try the same idea.
    Now modern TVs, even CRT-based TV from less than 10 years ago, use all
    kinds of processing to improve the video.
    Lots of sharpening and DSP magic lets my clunky old 27 inch Wega
    display re-scaled 800x600 (from my PC) almost as clearly as 800x600 on
    a monitor.
    But this won't happen with a small LCD or less-evolved tube TV.

    Now as the other poster mentioned, since VGA is progressive, you'll
    lose pixels in time. No way around that.
    Unless you're scrolling fine text, it shouldn't be a problem.

    Maybe your problem could be solved easier with a LCD display with a
    dedicated input with true per-pixel graphics.
    But that's a whole 'nother kettle of fish.
     
  4. Peter

    Peter Guest

    wrote
    I am quite happy to lose lines to make this work 1:1 in the vertical
    direction.

    Certainly I am not going to be building anything, because for a start
    the line frequencies are different. This job needs a proper frame
    buffer - just like the $50 boxes which are all over Ebay. They are all
    seemingly based on the same chip - in the same way that all the RGB to
    USB converters use a particular chip made by Philips (which has 3
    9-bit A-D converters, etc).

    I just wondered if any of these VGA-NTSC converters have the
    capability to do a vertically-direct pixel translation. This should be
    easy.
     
  5. JosephKK

    JosephKK Guest

    I have an old box from avermedia (an averkey imicro, KOC3) that
    converts VGA, SVGA, and XGA to NTSC. I can't vouch for line for line,
    but it works.
     
  6. Rich Grise

    Rich Grise Guest

    I was wondering about that - apparently NTSC is good for 486 lines
    displayed, so should cover 640x480 VGA.
    http://en.wikipedia.org/wiki/NTSC#Technical_details

    Cheers!
    Rich
     
  7. Jasen Betts

    Jasen Betts Guest

    set your VGA to run at the NTSC frame rate, and then use an RGB to
    NTSC converter, or get a VGA with a composite output.
     
  8. Greegor

    Greegor Guest

    Peter, I am curious as to why they made the LCD
    to use NTSC input? What were they thinking?

    What did they intend it to be connected to?
    Have you tried feeding Composite NTSC
    from an old VCR into it?
    Does it just throw away the parts of the NTSC signal
    it doesn't use?

    Are these LCD's available cheap on some surplus market?
    Got a lead?
     
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