VioletGiraffe wrote on 2021-05-08, 15:52:

The problem with old monitors, as I understand it, is that those monitors accept a ~15 kHz video signal, while VGA is 31.5 kHz. So, should the adapter like the one linked above convert the frequency to 15 kHz for S-Video or composite out?

That's what these converters are made for, I suppose. At least 640x480 resolutions @60Hz should be viewable on your old video monitor.
However, the devil is a squirrel. It really depends on the situation, I think. I've been using one myself in the past and it worked fine so far.

https://www.youtube.com/watch?v=VZMme7Edx6s
https://www.youtube.com/watch?v=1xXTF4et1sE

^These ran in 320x200 @70Hz, I believe.

PS: It's also possible to re-program VGA for 15KHz. The arcade people did this in the past with DOS-based utilities,
so they could run the DOS ports of MAME/MESS inside an arcade cab.

That's a really bad way to do this. A better way would be using a GBS-Control, Extron Emotia or Retrotink5x to downscale the 31kHz RGB to 15kHz RGB

maxtherabbit wrote on 2021-05-08, 16:40:

That's a really bad way to do this. A better way would be using a GBS-Control, Extron Emotia or Retrotink5x to downscale the 31kHz RGB to 15kHz RGB

That is probably the best solution, yeah. Another option is to output 240p natively from your graphics by adding custom resolutions/mod lines and use an RGB to s-video transcoder.

Those cheap converters do work and support a lot of resolutions and refresh rates (or at least some of them do, there are several different chips in use). They don't try to achieve best image quality though, they only output 50hz 576i or 60Hz 480i and do full frame rate conversion and filtering etc. and have at least 1 frame input latency due to it. Resulting image is fairly blurry with these things, even when you look past the composhit/s-video aspect.

The devices listed in maxtherbbit's post will all do a much better job, though at significantly higher cost.

I have one of these and they are ok to get a signal out, but suck in many ways.
VGA->Composite is much better compared to the other variant, Composite->VGA. The latter one will give you all kinds of colorful, noisy and crappy image, so stay away from those. Anything around 20€ is complete trash. SNES, C64, NES, old hardware like this sure will not be usable with those. Wii is ok however.

VGA->Composite does work, but usually requires adjustments because the picture will be very much off-center. Those are not saved when you remove the power so have fun doing it again and again. And don't expect to be able to read text at 800x600. 640x480 is barely possible to read when you know what you're looking for.
And even worse you're going to set it up to work fine at one resolution but then you go to another (eg. by dropping from windows back into DOS) and then it'll be completely messed up again because the thing only has global settings and not per-resolution.
Here's a pic i took some time ago at 640x480 with exactly the same converter as seen above, after careful adjustments:

Go figure if this quality is ok for you, or not.

Even with s-video, on a 15khz monitor you can't expect it too look as sharp as VGA. That only reason I would use something like that is for emulation to get a more authentic look. But then again, the lack of 240p support on these cheaper devices makes that pretty difficult too.

Maybe the easiest way is to just use a rasberry pi with an 3.5mm to composite adapter. The DAC on the pi is actually quite good and you can get some very pleasing results.

fgenesis wrote on 2021-05-08, 17:23:

VGA->Composite is much better compared to the other variant, Composite->VGA. The latter one will give you all kinds of colorful, noisy and crappy image, so stay away from those. Anything around 20€ is complete trash. SNES, C64, NES, old hardware like this sure will not be usable with those. Wii is ok however.

The problem is the way in which the colour information is transmitted.
And the noise that's introduced on the way.
The underlying monochrome video+blanking/sync signal isn't too shabby.
VBS also is a monochrome display's equivalent to analogue RGB.
I think we shoukd keep that in mind when we're talking about Composh*t. 😉

Edit: Monochrome security cameras of better quality used to be operating with 800 lines.
And they also used VBS over BNC connectors.
When it comes to image quality, monochrome single CRT monitors were top notch.
I really recommend playing 3D Monster Maze (ZX81) on such a setup.

Edit: Took a quivk photo of my Robot 400 Slow Scan TV Converter from the late 70s.
It uses BNC for monochrome video i/o.
A few decades before, the SO-239 (UHF) connector was commonly found on such equipment. 😀

Thank you all very much for the replies and lots of useful advice!

I was expecting the image to not be perfectly crisp, even with S-Video, but thank you so much fgenesis for the screen photo! That is not as clear as I was hoping for, I guess that settles the matter. The only computers I have for which this resolution would be OK - the C64 and maybe Amiga - are the ones that don't need a converter in the first place.

mothergoose729 wrote on 2021-05-08, 17:15:

maxtherabbit wrote on 2021-05-08, 16:40:

That's a really bad way to do this. A better way would be using a GBS-Control, Extron Emotia or Retrotink5x to downscale the 31kHz RGB to 15kHz RGB

That is probably the best solution, yeah. Another option is to output 240p natively from your graphics by adding custom resolutions/mod lines and use an RGB to s-video transcoder.

No need for a transcoder, the 1084 supports analogue RGB natively.

There is a DOS TSR called VGATV that can force supported VGA cards into outputting 15kHz RGBHV. It works pretty well if you have a supported card. I use it on a S3 ViRGE, ET4000, and Trident 8900 and it works great.

Does anyone know if I can expect a significantly crisper image (than shown on a photo by fgenesis above) on a 1084s from a PC using Retrotink or a similar expensive converter, or is it down to the limitations of the monitor itself?

maxtherabbit wrote on 2021-05-08, 19:25:

mothergoose729 wrote on 2021-05-08, 17:15:

maxtherabbit wrote on 2021-05-08, 16:40:

That's a really bad way to do this. A better way would be using a GBS-Control, Extron Emotia or Retrotink5x to downscale the 31kHz RGB to 15kHz RGB

That is probably the best solution, yeah. Another option is to output 240p natively from your graphics by adding custom resolutions/mod lines and use an RGB to s-video transcoder.

No need for a transcoder, the 1084 supports analogue RGB natively.

There is a DOS TSR called VGATV that can force supported VGA cards into outputting 15kHz RGBHV. It works pretty well if you have a supported card. I use it on a S3 ViRGE, ET4000, and Trident 8900 and it works great.

If your display supports RGB you don't need a transcoder, but you would still need one to output another format like s-video or composite.

VioletGiraffe wrote on 2021-05-08, 19:31:

Does anyone know if I can expect a significantly crisper image (than shown on a photo by fgenesis above) on a 1084s from a PC using Retrotink or a similar expensive converter, or is it down to the limitations of the monitor itself?

A lot of things can contribute to the output. Text is never going to be much fun to read on composite no matter how ideal the rest of your setup is. The signal is just not good enough for text. You want s-video at a minimum, RGB being the most ideal.

There are devices to upscale 15khz to 31khz for older computers and CGA/EGA computers. That might make more sense depending on what it is you are trying to do.

I did mean S-video. Is there a reasonably priced VGA to 15 kHz RGB converter out there?
If I understand correctly, upscaling is for connecting old computers to new monitors, but I have the opposite problem - I want to connect 31 kHz VGA output to an old 15 kHz monitor (the 1084s).

VioletGiraffe wrote on 2021-05-08, 19:42:

I did mean S-video. Is there a reasonably priced VGA to 15 kHz RGB converter out there?
If I understand correctly, upscaling is for connecting old computers to new monitors, but I have the opposite problem - I want to connect 31 kHz VGA output to an old 15 kHz monitor (the 1084s).

maxtherabbit mentioned a few products that might work. These things are typically used with computers for emulating old gaming consoles, at least that is where I have been exposed to it the most. Why do you want to use this monitor with a PC?

I want to use this monitor because it's the only CRT monitor I have, I certainly don't want to get a second one, but I do like using old computers with a CRT. And this is the monitor that doesn't need any scan doubling crutches with really old computers.

mothergoose729 wrote on 2021-05-08, 19:34:

Text is never going to be much fun to read on composite no matter how ideal the rest of your setup is. The signal is just not good enough for text. You want s-video at a minimum, RGB being the most ideal.

I think the same,with rhe exception of the monochrome signal part (luma).
Without the color information, the video signal gains much more clarity.
Professional computers and the Amiga had a clean monochrome output via cinch connector.
Together with a green monitor, the output was very fine and flicker-free.

I know I am not being much help , but there are a few good reasons why this could be hard and expensive. You would want to check the capabilities of any downscaler you choose carefully depending on how you plan to use the computer. In windows the minimum resolution is 640x480 for just about everything. In order to preserve the detail properly you would want to interlace the content, which is what your cheap scaler does. A lot of these down scalers are used to convert 480p to 240p by just removing half the line, which works great for emulation because there are only 240 lines of detail anyway, but that would be pretty terrible for windows text and graphics.

In DOS, most games run at 70hz and 400p. The good news is that DOS games actually only have about 200 lines of detail and are great candidates for downscaling, but that 70hz thing can definitely be a problem. There are apparently VGA cards that support 15khz natively and that might work, but I am worried about how the 70hz->60hz conversion would work on external scalers. Make sure those features are documented before you buy something expensive.

^I don't think you're not a lot of help. What you said goes along well with my experience, too.
Normal CCTV grade CRT TVs for customers are (were) not on par with real VGA or RGBHV monitors.

That's why DVD was such a hit originally,-well except for it's inability to store Videotext pages; S-VHS supported that-:
DVD technology could display the full PAL/NTSC resolutions, so it exceeded what a customer's TV could display, anyway.

VioletGiraffe wrote on 2021-05-08, 20:13:

I want to use this monitor because it's the only CRT monitor I have, I certainly don't want to get a second one, but I do like using old computers with a CRT. And this is the monitor that doesn't need any scan doubling crutches with really old computers.

What kind of VGA card do you have? Give VGATV a shot.

http://mirrors.arcadecontrols.com/VGATV/pwp.n … bo.pt/pscoelho/

maxtherabbit wrote on 2021-05-08, 19:25:

mothergoose729 wrote on 2021-05-08, 17:15:

maxtherabbit wrote on 2021-05-08, 16:40:

That's a really bad way to do this. A better way would be using a GBS-Control, Extron Emotia or Retrotink5x to downscale the 31kHz RGB to 15kHz RGB

That is probably the best solution, yeah. Another option is to output 240p natively from your graphics by adding custom resolutions/mod lines and use an RGB to s-video transcoder.

No need for a transcoder, the 1084 supports analogue RGB natively.

There is a DOS TSR called VGATV that can force supported VGA cards into outputting 15kHz RGBHV. It works pretty well if you have a supported card. I use it on a S3 ViRGE, ET4000, and Trident 8900 and it works great.

This reminds me of an episode on The 8-Bit Guy's channel.

https://www.youtube.com/watch?v=NkpSBK3g-gA

Going the TSR and VGA/RGBHV-RGB route gives a nice picture, apparently.

He also mentions RGBHV to Composite,
which works for monochrome output only (VBS),
of course, because extra circuitry is required to encode in PAL/NTSC.

Edit: I've attached some pictures, hope that's okay.

Edit: These aren't my schematics, just show them to give an impression how simple a cable could be.
I'd also add some little resistors to the RGB lines (just in case, maybe some caps too).
The mixing/filtering cap thingy for the horizontal/vertical combiner is nice, though.
Gives some galvanic insulation as a side effect.