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Atari ST Display Basics

Posted: Thu Aug 24, 2017 2:15 pm
by IngoQ
Basics and History

When the ST was designed, there were two contrary ideas, how this machine should be used. On one hand, there was the Apple Macintosh, with its clear and sharp but monochrome displays. On the other hand, there was the more casual use, like games and graphics, that Atari knew the upcoming Amiga would offer.

So the idea was to basically have two different video modes: One with a high resolution but no colors for "serious" business use, and another with less resolution and more colors. When the first machine was introduced in 1985, it came out with three display modes:
  • 640 x 400 pixels, monochrome
  • 640 × 200 pixels, 4 colours
  • 320 × 200 pixels, 16 colours
All STs are equipped with a monitor jack, which looks like follows:
din13f.png (9.12 KiB) Viewed 6044 times
It's a female 13-pin DIN connector with the following pinout:
  1. Audio out
  2. not used on ST, on STE: Composite Video (if RF-modulator is present), Composite Sync (no modulator)
  3. General Purpose Output (External Clock select on STE)
  4. Monochrome Detect (External Clock in, when on STE and 3. is grounded)
  5. Audio In
  6. Green
  7. Red
  8. 12V
  9. Horizontal Sync
  10. Blue
  11. Monochrome Video
  12. Vertical Sync
  13. Ground
As long as the ST is using medium or low resolution, it will output a RGB-signal that is compatible to either PAL or NTSC standard, which means it will have 15.75 kHz horizontal sync frequency and 50 or 60 Hz vertical sync frequency, depending on being it NTSC or PAL version.
Since these are regular TV standards, you could connect the ST to a TV that provides analog RGB inputs, for example via a SCART cable like this one: ... itel_cable. At least this was the intention at that time. The Atari SC1224 was another option, it used a cheap and small TV CRT and was often sold in bundle with the computer.

The high resolution with its 640 x 400 pixels went beyond TV specs and needed a special display at that time, which Atari provided in form of the SM124. In order to not damage the connected display with a wrong signal, there is a pin on the connector, that tells the system at bootup, what display is connected. If pin 4 is grounded, high res is used, otherwise it defaults to low res. It would not need RGB signals, only a single monochrome video signal. But it needed a higher horizontal sync frequency of 35.7 kHz and 71.2Hz vertically. The resulting image quality made this a very popular combo which was sold as a bundle as well.

This lead to the situation, that if you wanted to use all resolutions on your ST, you would need two displays, and had to unplug and plug, if you switched. NEC was the first (I believe) to introduce a so called Multisync-Monitor, which was capable of not only having a fixed frequency, but syncing to a bandwidth of input frequencies, hence the name. With a monitor like this, you could use all three resolutions, provided you had a cable, that allowed the switching of high res.

Conveniently exxos has a adapter in his store, that solves the latter:

So if you find a old Multisync-capable monitor, that supports 15.75-35.7 kHz vertical frequency, you are ready to go.

In order to operate your ST on a more modern display, you will have to solve the following issues:
  • The ST outputs analog signals, modern displays (TVs and Monitors) often require digital signals
  • Switching between high res and low/med res if not using two displays
  • the display handling 15.75-35.7 kHz horizontal sync frequency, since most displays only start at 31kHz, which would be a VGA signal.
If you have a display with VGA inputs, for example an older TFT you can at least use the high resolution with a cable like this: ... momo_cable. The high-res signal is more or less comatible to a VGA signal, so this will work as well.

There are still Multisync-capable TFTs out there and there are some lists floating around, that try to give recommendations. We try to compile a list of our own, depending on the informations we find. This TFTs could be used with the adapter mentioned above as well.

The last option is Scaler/Signal converter that provides the analog inputs, internally digitizes and upscales them in order to output them digitally to a modern TV/TFT with HDMI inputs. There are different choices out there, and we will post some experiences here as well.

Re: Atari ST Display Basics

Posted: Thu Aug 24, 2017 3:25 pm
by Maeke
You forgot to say that in some countries which didn't use PAL nor NTSC, but SECAM/MESECAM, there was no ATARI STm or STFm, only ST and STF, and the STE didn't have a modulator, simply because secam wasn't compatible with pal or ntsc, so the modulator of the st line wasn't compatible with secam/mesecam.
This concerned France, Russia and a part of africa (but i don't know if there were a lot of ataris in russia and africa at the time).
So basically, all french Atari st/stf/ste were without modulators.

On the photo, my french ste's motherboard.

Re: Atari ST Display Basics

Posted: Thu Aug 24, 2017 3:51 pm
by IngoQ
Nice of you to say, that I only forgot to mention it ;) The truth is: I didn't know that :-P

But I left out the modulator entirely, mostly because it has no relevance any more.

But for sake of completeness:

In countries where either PAL or NTSC was used as TV standard, Atari offered some of their models with a TV modulator. These machines had either the "M" in its name (like in 1040STFM) or were STE models. In countries where SECAM/MESECAM was used (like in France) there were no machines with modulators available, so that a french STE has no RF modulator. This HF module would firstly combine the RGB signals to a single colour composite video signal and then modulate on a adjustable HF frequency suitable for the HF input of TVs of that time.

The idea was to connect the HF output with the analog antenna jack of the TV, adjust the TVs reception frequency and have the STs video signal displayed. Needless to say, this solution suffered from really bad video quality. But it was a way to get started, if you could not afford a real monitor, and your TV had no SCART input.

Since today modern TVs rarely have a analog RF input (if any RF input at all, it is for DVB-T) , this solution is obsolete and due to the bad quality any other way of getting a video signal out is preferable.

Which leads to another way of outputting video: The composite video. When a modulator is present a byproduct of the modulation process is a composite video signal. It combines all three color signals plus the two sync signals to one signal. As you can see in the table above pin 2 of the video connector will provide that signal, if there is a modulator equipped.

You could use a cable like this ... site_cable and connect it to a RCA composite video signal input (often a yellow plug). Many modern TVs still have these as a "last resort", so to say. But be aware that the video quality will be a lot worse than using separate RGB and sync signals. And of course this will only be possible with TV compatible signals, so no high res.

Re: Atari ST Display Basics

Posted: Thu Aug 24, 2017 5:37 pm
by Maeke
no, as i said no secam/mesecam market's Atari ste had a modulator, just look on my photo, it's absence is obvious.
There actually is another reason why french stes didn't have modulators, they were not needed in france, since scart was initially a french norm named peritel launched back in 1980, and mandatory on all big enough tvs.
Besides, you're wrong some modern tvs do actually have rf inputs, at least in france, for dvb-t.

Re: Atari ST Display Basics

Posted: Thu Aug 24, 2017 10:41 pm
by DrF
Since today modern TVs have no RF input any more,
What I found on the very few that do have RF inputs still is that the tuner is really, really, really bad you can't seem to get it fine enough to get what's considered a good picture. My TV for some stupid reason tunes in odd number increasements so you cant get on just right. I had a very hard time getting my Atari 800 and Sega MegaDrive even close and had to settle for what I could get.

Re: Atari ST Display Basics

Posted: Thu Aug 24, 2017 11:57 pm
by Maeke
Well, that's because most countries now use only dvb-t, the old rf tv is dead, it's just not analogic signals anymore, and the ataris aren't compatible dvb-t.

Re: Atari ST Display Basics

Posted: Fri Aug 25, 2017 8:55 am
by IngoQ
EDIT: Updated my post to make it clearer, that SECAM/MSECAM models had no modulator, and that there are in fact still some TVs with RF inputs.

Re: Atari ST Display Basics

Posted: Fri Aug 25, 2017 9:14 am
by Maeke
Well, not yet accurate enough, the french st/stf/ste didn't have a modulator for the reasons i gave, but there were never secam/mesecam models, french st/stf/ste were in fact pal models without modulators.
Why would they build secam/mesecam models if they didn't plan to put a modulator inside from the start?
You can verify this by looking the schematics, only pal and ntsc are on them.

Re: Atari ST Display Basics

Posted: Fri Aug 25, 2017 9:27 am
by IngoQ
EDIT: Updated post again to reflect the situation in France more accurately.

@Maeke: Satisfied now? ;)

Re: Atari ST Display Basics

Posted: Fri Aug 25, 2017 9:31 am
by Maeke
IngoQ wrote:
Fri Aug 25, 2017 9:27 am
EDIT: Updated post again to reflect the situation in France more accurately.

@Maeke: Satisfied now? ;)
If you state the truth, of course.