Converting monochrome video signal to VGA monitor?

Curt @ Atari Museum curt at atarimuseum.com
Wed Jan 9 21:48:06 CST 2008


Hi Mark,

   Its very easy to make a simple cable to connect an ST to a VGA 
monitor, but it only comes up in Monochrome, the key is to use a 
multisync monitor.   

The info is here:

http://www.atari-forum.com/wiki/index.php/Atari_ST_to_VGA_adaptor


I was messing around with some LM1881N's to hook an Atari 800 direct 
from its raw RGB outputs from the motherboard to VGA and had limited 
success.

Curt


Mark wrote:
> Hi,
>
> I recently received a Commodore A2024 monitor. This is a 15" monochrome/
> grey-scale monitor which has a special converter board inside, that
> converts the 15kHz video output by an Amiga computer to 31kHz. It also
> supports a "tiled" mode, whereby Amiga video frames are combined to
> produce a 1024x800 or 1024x1024 pixel image.
>
> Unfortunately the monitor was damaged in transit and the tube is
> cracked. However the converter board should be usable. I want to make an
> adaptor to connect the converter board to a standard VGA/multi-sync
> monitor.
>
> Has anyone on the list connected an old workstation (or maybe an Atari
> ST?) with monochrome output to a modern monitor? Those should be
> similar.
>
> There should be no problem with the horizontal and vertical scan rates.
> I need to look at the board circuitry, but I'm pretty sure the
> horizontal and vertical sync signals will be at TTL levels, which is
> fine for connection to a VGA monitor.
>
> The monochrome video signal is output on a phono/RCA socket. Inside the
> monitor, a coaxial cable goes from that to the PCB at the end of the
> tube.
>
> I need to determine the video signal level. Apparently VGA uses 0.7V
> p-p, so I might need to add a resistor in-line to reduce the voltage if
> it's any higher. Unfortunately I don't have access to an oscilloscope.
> Could I just arrange for a full-screen white image to be shown, measure
> the voltage using a normal DC multimeter, and multiply up the value
> shown to account for the proportion of each scanline corresponding to
> sync/flyback?
>
> Once that issue is solved, I should be able to connect the video signal
> to the green signal pin of a VGA monitor and get a green picture. In
> order to get a grey-scale picture, can I simply connect the video signal
> via resistors to each of the R, G and B pins on the monitor connector?
> If so, what resistor values would be needed? If not, I guess I could use
> or build a 1-to-3 splitter/amplifier circuit, then connect the 3 outputs
> of that to the R, G and B lines.
>
>
> -- M
>
>
>
>
>   



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