VGA to CGA

[Mattpsu03]

I have a Happs Vision Pro 19" VGA/SVGA monitor. Is it possible to convert this to a CGA monitor?  How difficult would this be to accomplish?

[MonMotha]

If it's possible at all, it's probably not simple.

An actual investigation would require a model number and schematic or service manual.

[grantspain]

just buy a cga to vga convertor,very cheap and easy to install

[Mattpsu03]

I'm not looking to upconvert a CGA signal to a VGA display. I was trying to see if there was a way to make my VGA monitor capable of displaying a CGA signal to have arcade perfect pixels. I currently have it converted to VGA and it is ok, but older games look so much better on my other monitor, which is a tri-sync, capable of dropping down to CGA. 

But yeah, it would probably be too much of a hassle if it's even possible. I was thinking maybe just a chassis swap or something. I guess I don't know enough about how arcade CRTs work to make a valid assumption.

[dmckean]

It's not just a scan issue, VGA monitors also have a higher dot pitch. It will never look as good as a real arcade monitor.

[Tithis]

Not as far as I know. Best I think you could do is set up custom resolutions so instead of doing an interpolated or nearest neighbor scale it's just a straight interger scale. Not as good as real CGA, but it's the best you'll probably get.

[MonMotha]

Some VGA monitors have finer dot pitch tubes, and some don't.  Even some capable of SVGA have dot pitches comparable to traditional TVs/arcade monitors.  VGA benefits from the smoothing/blurring effect of a coarse dot pitch at larger sizes just like lower resolutions do.

However, if your monitor doesn't support 15kHz scanrates, adding that capability is not likely to be feasible.  Even if you were willing to completely swap the boardset, you'd likely find that the yoke is not what most 15k-capable designs will want to use.

[SailorSat]

You can run native resolutions IF you double the vertical refresh (i.e. 120Hz instead of 60Hz).

However...

15kHz native


31kHz on a "low dot pitch" screen (120Hz)


31kHz on a "fine dot pitch" screen (120Hz)

[Mattpsu03]

Thank you, this is all very informative.  My monitor's dot pitch is .26   I'm not sure where that stands as far as being too far apart from a CGA arcade monitor.  It's a Happs Vision pro 19" (made by Tovis) and these are the specs:

Signal Input
*Video Input: Analog, Positive Signal (0.7V p-p)
*Horizontal Sync: TTL Level, Positive or Negative Pulse
*Horizontal Scan: 28KHz ~ 70KHz
*Vertical Input: TTL Level, Positive or Negative Pulse
*Vertical Scan: 40Hz ~ 160Hz

I'm trying to figure out the best way to display classic vertical games via MAME other than being stuck with using 640x480 VGA mode and using hardware stretch...

Thank you for all your help!

[MonMotha]

Not all monitors support 120Hz refresh, though this one does.  It'll mess with the decay rate/flicker that some people expect out of fully native graphics, but then phosphor decay rates vary somewhat between tubes, anyway.

If you wish to use this monitor, you will need to either double-scan (draw each scanline twice), which will not give you the native scanline appearance you expect, or double-refresh (draw each frame twice) which is what is described above.  The latter is probably less objectionable and can be done while otherwise preserving all aspects of the video timing (you literally just halve every time period in the video timing).  Since this monitor goes up to 70kHz horizontal scanrate, you can also do the same for mid-res games (~384 lines progressive, normally ~24-25kHz scanrate) if you have any that you want to run.

Any games you want to run that natively run at 480i you can potentially just run at 480p.  You might not even notice that one, and if you do, it probably won't be a bad thing (no interlace "sparkle").  Most classics are ~240 lines progressive, though.

All that said, your .26 dot pitch is fairly fine.  It's more like a computer monitor than a television or classic arcade monitor.  You might find the scanlines too pronounced to be aesthetically pleasing.

[Mattpsu03]

Thank you everyone. This was very helpful and informative!

[tiobolinha]

The latest CRT_Emudriver has already a preset for 70 khz PC monitor which would be very useful for you. Look at the GroovyMame forum.

Look up for CRT_Emudriver and "insert black frame" options on that forum and you should find all the software and information you need to get this done.

I saw some people in the forum accomplish VERY good native resolution results with monitors like yours so I would give it a try. Plus you have the possibility of running medium and high resolution games all in progressive mode which is a huge plus over CGA / CRT TV monitors.

Please let us know how this goes and update some pics.

Good luck