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Propeller VGA adapter. — Parallax Forums

Propeller VGA adapter.

CumQuaTCumQuaT Posts: 156
edited 2011-02-21 18:25 in Propeller 1
Also while I'm on... I know there are some great ICs out there to do this already, but this is more of a learning exercise...

Has anyone ever seen or made the propeller convert a VGA signal into a PAL RCA output? Is it even possible? It seems like it would be.

Not really for any particular project, but just out of interest...

Comments

  • LeonLeon Posts: 7,620
    edited 2011-02-21 08:55
    You need far more performance and memory than the Propeller offers for that!
  • TubularTubular Posts: 4,717
    edited 2011-02-21 11:05
    You can look at a converter like the AD725. Its datasheet goes through some of the theory and blocks required to perform the conversion.

    There are applications where this might be useful - for instance getting a large number of grey scales, or a particular colour space.
  • ericballericball Posts: 774
    edited 2011-02-21 12:37
    Actually, I'm not so sure it's impossible.

    The first step is ADC. Delta-sigma trades off spacial resolution for sample resolution. With a 31.5MHz horizontal frequency and an 80MHz system clock that's 2540 samples per line. Thus for 400 pixels per line (320 net) you'd get 6 levels per pixel. For 8 levels per pixel you'd only get 317 pixels per line (net 254). This actually isn't bad given the Prop's composite output abilities. Sync would be simple.

    Next the 333 RGB would need to be transformed into the Propeller hue+luma format. The easiest way is probably via a simple 512 entry lookup table.

    The final step is scan rate conversion. For NTSC it probably would be possible to capture two lines and output one since the VGA horizontal frequency is about double and the vertical frequency is the same. But for PAL you'd need to capture a full frame. Ideally you'd capture all 480 lines, but with only 32K of RAM you could only handle 68 pixels per line. Capturing 240 lines you could handle 136 active pixels per line. That's 170 pixels per line or 15 levels per pixel (although that would make the lookup table too large).
  • CumQuaTCumQuaT Posts: 156
    edited 2011-02-21 14:32
    Definitely something to think about! Thanks ericball!

    Tubular, I know there are chips like that. I mentioned them in my post. I was just curious about the propellers abilities :)
  • TubularTubular Posts: 4,717
    edited 2011-02-21 14:48
    Oops. Re-reading your message, I see you're after an external VGA input, rather than the props VGA output. I really must not reply to messages before breakfast and/or coffee.
  • CumQuaTCumQuaT Posts: 156
    edited 2011-02-21 18:25
    Hahaha all good, Tubular. Happens to the best of us! (And the rest of us)
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