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Issues getting composite CGA graphics on my 1084S-P2

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First post, by boggit

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The other day, I purchased a Commodore 1084s-P2 monitor, hoping to be able to use it with my C64 as well as with my CGA-based IBM 5160/XT (original IBM CGA card, original 5160 motherboard).

Unfortunately, the screen I got seems to be one of the later, cost-reduced variants that lacks the option for digital RGB, only offering the analogue RGB that the Amiga would use:

1084S-baksida.jpeg
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* I managed to get excellent picture on the screen when connecting my Commodore 64 via S-Video. I'm using a converter that goes from the port on the C64 and diverges into three RCA connectors. Sound was definitely decent as well, though only coming out of the left speaker (using a converter that connects to the C64 port and then splits up into three RCA connectors, with only one for (mono)sound).

* However, when hooking up the CGA-equipped XT system to the monitor via composite cable -> PAL/NTSC converter -> composite cable (to the bottom left of the four RCA jacks in the rightmost part of the picture above), I could only get either somewhat shabby monochrome/greyscale picture or extremely bad sorta-color mode rather than the fully decent composite CGA color mode I get when going composite on my LCD (through a hdmi converter).

See below:

Mode CO80 in DOS, Color wheel on front of screen turned to color:
iFb8FZY.jpeg

Mode CO80 in DOS, Color wheel on front of screen turned to no color:
XU6sjM0.jpeg

Mode BW80 in DOS, Color wheel on front of screen turned to color:
BOSqvWY.jpeg

Mode BW80 in DOS, Color wheel on front of screen turned to no color:
BxAtkqr.jpeg

Questions:

* What can I do get good artifact CGA colors on this screen?

* Am I correct in thinking that I should adjust Capacitator C1, as per this information?

* If so, and this might be a stupid question, but can I adjust C1 while the system is running, or would that be potentially dangerous?

Reply 1 of 7, by Tiido

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You need NTSC compatibility (not just 60Hz) for the chroma artifact colors, and this is a PAL monitor which doesn't understand NTSC color encoding.

The capactior can be adjusted while system runs but it won't get you PAL, it only finetunes the NTSC color subcarrier frequency in sub 0.1% range.

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Reply 2 of 7, by boggit

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Tiido wrote on 2024-10-18, 02:53:

You need NTSC compatibility (not just 60Hz) for the chroma artifact colors, and this is a PAL monitor which doesn't understand NTSC color encoding.

The capactior can be adjusted while system runs but it won't get you PAL, it only finetunes the NTSC color subcarrier frequency in sub 0.1% range.

boggit wrote on 2024-10-17, 11:49:

hooking up the CGA-equipped XT system to the monitor via composite cable -> PAL/NTSC converter -> composite cable

😀

Basically, I am running the same setup as this guy. Not the same exact monitor, but the same exact PAL/NTSC converter.

Tiido wrote on 2024-10-18, 02:53:

The capactior can be adjusted while system runs

Excellent! Thanks!

Reply 3 of 7, by Tiido

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Then there's a chance of success, although the NTSC>PAL conversion is a lossy process and the result may still not be quite good in the end...

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Reply 4 of 7, by boggit

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Tiido wrote on 2024-10-18, 07:39:

Then there's a chance of success, although the NTSC>PAL conversion is a lossy process and the result may still not be quite good in the end...

Great! Then I will try adjusting the capacitator with a screwdriver, with the system running, hoping I won't get electrocuted.

Reply 5 of 7, by Tiido

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Only place that is dangerous is inside the PSU itself but there's no need to go inside that and especially with power on.

But you can cause damage to the motherboard etc. if that screwdriver were to slip and fall in somewhere, so some care is needed. Perhaps the pointy bits on backsides of various cards can scrape your fingers etc. if you're not careful but certainly not gonna kill 🤣

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mida sa loed ? nagunii aru ei saa 😜

Reply 6 of 7, by VileR

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Just to make sure... have you tried any other modes, i.e. 40-column text or one of the graphics modes?

Even a true NSTC color monitor/TV will have trouble decoding colors in 80-column text mode. That's down to a CGA design quirk, which ends horizontal blanking too early with respect to the color burst (other modes don't suffer from this; here's a more detailed explanation).

The results are very similar to what you're getting on your monitor, so the NTSC→PAL converter is likely just having similar trouble with the decoding stage. If this is the case (=other modes look better), there are software workarounds you could try, but adjusting the C1 capacitor won't do much.

(As per IBM's docs, color 80-column text is 'supported' only over digital RGB - that's why they let you optionally start your CGA in 40-column mode, using motherboard switches 5/6).

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Reply 7 of 7, by boggit

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VileR wrote on 2024-10-22, 22:51:
Just to make sure... have you tried any other modes, i.e. 40-column text or one of the graphics modes? […]
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Just to make sure... have you tried any other modes, i.e. 40-column text or one of the graphics modes?

Even a true NSTC color monitor/TV will have trouble decoding colors in 80-column text mode. That's down to a CGA design quirk, which ends horizontal blanking too early with respect to the color burst (other modes don't suffer from this; here's a more detailed explanation).

The results are very similar to what you're getting on your monitor, so the NTSC→PAL converter is likely just having similar trouble with the decoding stage. If this is the case (=other modes look better), there are software workarounds you could try, but adjusting the C1 capacitor won't do much.

(As per IBM's docs, color 80-column text is 'supported' only over digital RGB - that's why they let you optionally start your CGA in 40-column mode, using motherboard switches 5/6).

Good to know, but yeah, I tried using CO40 as well, without improvement.

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