VGA monitor driven by a CTC2 chassis
 

My CT100's 15GP22, like most, is unusable. Until we are able to rebuild it I want to be able to restore and run my CT100. As Don proposed at the 2013 ETF presentations, being able to drive a VGA monitor would let us use our CT100s with bad 15GP22s. Here is my attempt to get a 21CT55 CTC2B chassis to drive a VGA monitor. My current design clips onto existing points in the CTC2 chassis with no changes required to the chassis. The CRT R, G, and B grid signals are converted to 1V peak video driving 75 ohms. A horizontal yoke lead and a spring around the vertical output tube provide the sync signals that are used to form the negative going composite sync signal. Most VGA monitors, like my CRT based NEC FE950 do not accept the CTC2 interlaced video. I use a Jamma Boards GBS-8220 to scan convert/up-sample the interlaced RGBS video to progressive scan VGA RGBHV. Here are some pictures.

The CTC2B user controls affect the VGA monitor picture. The user can adjust the brightness, contrast, color level, hue, and fine tuning which all operate as the original sets CRT. The V and H hold do cause the scan converter to break up some of the time and display a No Signal between attempts to display nonsense. Careful adjustment does result in a stable picture. The possibility of driving a VGA monitor using a converter allows a modern monitor with a color gamut similar to the 15GP22 to be used to provide set that operates as the original. Anyone have any suggestions on a monitor that has the same color gamut? Here are some more pictures showing the affect of adjusting the above controls. I have seen claims of 92% NTSC to up to 200% NTSC color gamut, whatever that means.

I have never seen 200% NTSC - don't believe it's possible except as a typo.

Monitors that do better than 95% or so NTSC (or Adobe RGB, which essentially has NTSC green and somewhat less orange / more saturated red) will be so close to the 15GP22 that's it's not worth worrying about. The standard sRGB or HDTV monitor covers about 70% of NTSC, mostly due to the yellower green. In fact, since LCDs don't have the purity problems that a CRT can have, and also have blacker blacks, they will do as well as the 15GP22. especially in the lowlights. The only drawback I can think of is that wide gamut monitors are generally more expensive.

By the way, 70% is measured on the 1931 chromaticity diagram, which does not correspond to uniformly perceptual color space, so you are really losing considerably less than 30% of perceivable NTSC color space in a HDTV; but the differences in greens and cyans should be noticeable.

Very interesting. In the photos with two screens, what is the screen on the left?

Phil Nelson

Probably just some sales hype. Here is a link. It is a pico-projector that uses lasers as the light source.
http://www.thinkgeek.com/product/eb43/

Scroll down and it says: "◦Color Gamut: >200% NTSC"

Oh, sorry, I am using my 21CT55 CTC2B as a stand-in for a CT100 CTC2 video source. So it shows the video and sync lines from the 21CT55 chassis processed by the circuit and scan converter driving the VGA monitor. And the effect on the picture as the 21CT55 user controls are adjusted.

Dave

With lasers, greater than 100% NTSC for sure, but greater than 200% is doubtful.

Plus: you don't WANT so much greater than sRGB/Rec 709 for video sources, or the colors will look too saturated, so the colors should be mixed by the circuitry to simulate Rec 709 primaries;
Plus: the picture from this dim device will be washed out by the least amount of ambient light unless you keep the image very small, so whatever colors it starts out with will be desaturated in actual use.

So: hype? Yes indeed.

So, if I understand correctly, an LCD with close to 100% Adobe RGB is the probably the closest gamut match to the 15GP22, but may be expensive. And since its image size cannot be adjusted it will have to be about a 15" diagonal 4:3 or a 17" 16:9. Seems like most monitors are big these days so it may be tough to find one that meets this.

I had a computer monitor that could do 525i scan, same as NTSC. With inputs for red, green and blue channels, analog. But it was set to underscan, so it couldn't present a normal TV display. Think it had "Digital" for the badge, the old computer company near Boston.

Yeah, the older CGA/EGA/early VGA monitors supported interlaced.

I have a few "TV-RGB" monitors here that can natively do 15.75khz RGB input. It was a popular computer output format before the VGA standard arrived in 1987. The Applecolor RGB (usually paired with the Apple IIgs) and the Commodore 1084 series will show a properly over scanned TV picture (the latter even has composite inputs).