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Video scan convertor.

M

MV75

Experienced Member
Joined
May 9, 2010
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114
With the wonderful projects being done by those that have more electronic ability than myself, and I love to then build them, has the community done a scan convertor that will allow me to take output from a hercules / cga / ega card and display it on a "vga" (15 pins), screen? Thanks.
 
please post back on your experiences with that jamma board. (some side by side monitor display photos would be ideal)
a device like that could be a huge space saver for my workbench.
 
The first issue with these converters is that they expect the RGB signals to be analog, as they would coming from an arcade board. EGA and CGA signals are digital. EGA does 6-bit RGB in addition to RGBI w/ brown of CGA.

The second issue with this converter is that it is designed for 14.5kHz - 16.5kHz, 23.5kHz - 25.5kHz & 30.5kHz - 32.5kHz input. EGA uses 21kHz and MDA/Hercules 18kHz. The scan rate may be incompatible.

The third issue with this converter is that it upconverts everything into 640x480. CGA graphics are either 320x200 or 640x200, EGA graphics also allow for 640x350. Upconversion by hardware or software when the upconverted resolution is not a multiple of the original resolution is ugly. CRT monitors do aspect ratio correction by increasing the thickness the scan lines.
 
Supposing for a moment that one were do come out with a kit to do better EGA->VGA conversion, what would a tolerable price point be? (Note: EGA->VGA would be the only conversion performed).
 
Chuck(G) said:
Supposing for a moment that one were do come out with a kit to do better EGA->VGA conversion, what would a tolerable price point be? (Note: EGA->VGA would be the only conversion performed).
Click to expand...

I'd go $25 or so, assuming that my Extron converter doesn't work out (haven't had a chance to test it yet, sadly).

Personally, I'd be willing to go up to $100, but I'd be looking for more of a universal scan convertor, i.e. something that I could use to do CGA/EGA-to-VGA conversions as well as 15.7khz RGB to VGA (mainly because it would be sweeeeeeeeeeeeeettttt to have an external scan converter that would not only work with my PCs, but also with my Amiga's and other true RGB inputs (JAMMA arcades, console arcades, etc)
 
Now you see the problem. You have to grab a frame of EGA data, then re-format for VGA use. The commercial converters use commodity chips to do this, but can give less-than spectacular results. To do this as a custom product might well entail a substantial bit of horsepower. I haven't penciled the design out yet, however.

Would sucking in a single scanline of EGA video and then spitting it out twice (i.e. "scan doubler") be sufficient?
 
I'm confused about something, but I claim my lack of knowledge on video technology. VGA can display the resolutions of CGA and EGA. Is it more complicated than just building a D2A to convert the CGA/EGA digital outs to VGA 0.0-0.7, and using the existing sync signals?
 
Yes, it's more complicated.

While the field rates of all three (CGA, EGA and VGA) are approximately the same (60 Hz, give or take), each line is scanned at very different rates. CGA, for example, has a horizontal line scan rate of about 15.750 KHz (262.5 lines). An EGA (depending on setting) has a horizontal scan rate of about 21KHz, giving 350 lines) and VGA has a horizontal scan rate of 31.5KHz, giving about 525 lines.

So, to convert CGA to VGA, ignoring the D-A aspect for now, you'd need to suck up a line of horizontal data (at 15.750KHz), then output it twice at 31.5KHz--you need some memory. D-A conversion is less problematic, since CGA at best can show only 16 colors. You could probably get decent results with a simple R-2R "ladder" network.

You get the idea, I'm sure.
 
Great Hierophant said:
The first issue with these converters is that they expect the RGB signals to be analog, as they would coming from an arcade board. EGA and CGA signals are digital. EGA does 6-bit RGB in addition to RGBI w/ brown of CGA.

The second issue with this converter is that it is designed for 14.5kHz - 16.5kHz, 23.5kHz - 25.5kHz & 30.5kHz - 32.5kHz input. EGA uses 21kHz and MDA/Hercules 18kHz. The scan rate may be incompatible.

The third issue with this converter is that it upconverts everything into 640x480. CGA graphics are either 320x200 or 640x200, EGA graphics also allow for 640x350. Upconversion by hardware or software when the upconverted resolution is not a multiple of the original resolution is ugly. CRT monitors do aspect ratio correction by increasing the thickness the scan lines.
Click to expand...

Oh crud, that's right, it does call for analogue input on that convertor.

Also on functionality, I'm with maverick. I'd want something that converts hercules / cga / ega physically and electrically (scan rates) to vga. Because let's face it, it's getting harder and harder to find and keep original monitors. The hardware cards are lasting much longer.
 
Those won't handle the MDA/Hercules scanrates though. They're just normal 15KHz CRTs with no colour capability.
 
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