RGB-VGA Converter -- buy or build?

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Feb 8 16:03:38 2001

----- Original Message -----
From: <liste_at_artware.qc.ca>
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, February 08, 2001 1:20 PM
Subject: Re: RGB-VGA Converter -- buy or build?


>
> On 07-Feb-2001 Tony Duell wrote:
> > The main problem here are the scan rates. If those are compatible, then
> > actually fiddling with the signals (adding/separating sync-on-green, for
> > example) is not too hard. But scan rate conversion can get very
> > complex...
>
> I've yet to find any docs on how to seperate out the sync-on-green. I
> have an HP X term that outputs sync-on-green VGA and no compatible
monitor.
>
Sync-on-Green and VGA, as far as I know, shouldn't appear in the same clause
(except for this type of clause) since it's like saying it's a black shade
of white or a round square. They are VERY different. Until you KNOW,
absolutely, what the frequency requirements are, and that you can't get a
proper replacement monitor at the junkyard, I think your efforts would be
best spent trying to get a schematic of the video adapter circuit. I've
tossed out several very nice large fixed frequency monitors after trying to
give them away, simply because nobody wanted either to come and get them or
pay the shipping costs. I think this is a natural consequence of chosing to
center your hobby around these formerly VERY costly systems, carefully
designed to prevent you from using another vendor's hardware in case it
might cost less. If you find a compatible monitor, it's unlikely it will
cost more than $50, in addition to the cost of shipping.

You can produce a separate, ttl-compatible composite sync easily enough, but
the fact that it's a composite sync (with horizontal and vertical sync
combined in the same signal) means that you'll need to separate the signals
from their composite form and subsequently filter them. It's not as easy as
it might seem, and I've normally gone back into the video adapter circuitry
to look for suitable timing, which, due to the current tendency to integrate
everything into a single LSI, tends to be more difficult these days. In
systems that were designed to be used with sync-on-green monitors, the DAC
normally had the capability to put the negative-going sync pulses right on
the green video signal was common. You should be able to handle that with
software. However, if you don't know the DAC or other circuitry because
it's all stock-numbered, you may be up the proverbial creek, and even more
so if the DAC is internal to the LSI.

Dick


>
> -Philip
>
>
Received on Thu Feb 08 2001 - 16:03:38 GMT