CGA/Mono Video Problem

[FishFinger]

I'm having trouble with a CGA/Mono combo card I got recently. The card is an ATI Small Wonder Graphics Solution. It can work in either CGA, or MDA/Hercules modes. It has both TTL and composite output, and you can set the monitor type independently of the mode, so for example it can do Hercules on a CGA or composite monitor - which is what I originally bought it for.

I don't have a working TTL monitor at the moment, so when I first got the card I tested it worked by setting it to CGA mode/CGA display and hooking it up to the TV on my desk with a composite cable. I got a nice steady picture, but since the TV doesn't support NTSC colour all I got was greyscale. Later I made a CGA to Scart adaptor, hooked it up to the same TV, and again got a steady image, this time in colour. So all appeared to be fine (see here).

When I tried it on another TV however, the video does not appear to be stable. The image has ragged verticals towards the top of the frame, as if the TV is having trouble syncing to the input signal correctly. At first I suspected my Scart adaptor, but when I tried with a composite cable I got the same result, so it would appear to be something else. I tried it on all four TVs in the house, and the only one that worked correctly is the 14" on my desk, which I'd tried originally. On any other TV I end up with something like this (with both the Scart adaptor or a composite cable):

Things are much the same in mono mode. With the card set to Mono mode/CGA display (I have no mono display) the three other TVs display a ragged image as above. The 14" on my desk acts slightly differently. It will display hercules graphics perfectly, but will not accept mono text modes at all (it displays a single frame, then blanks as if the input is not present).

 

[per]

When I tried it on another TV however, the video does not appear to be stable. The image has ragged verticals towards the top of the frame, as if the TV is having trouble syncing to the input signal correctly. At first I suspected my Scart adaptor, but when I tried with a composite cable I got the same result, so it would appear to be something else. I tried it on all four TVs in the house, and the only one that worked correctly is the 14" on my desk, which I'd tried originally. On any other TV I end up with something like this (with both the Scart adaptor or a composite cable):

Things are much the same in mono mode. With the card set to Mono mode/CGA display (I have no mono display) the three other TVs display a ragged image as above. The 14" on my desk acts slightly differently. It will display hercules graphics perfectly, but will not accept mono text modes at all (it displays a single frame, then blanks as if the input is not present).

I can tell you that I almost have the exact same problem with all of my CGA-compatible cards (I got 4 of them, and I have tried them all. Instead of just un-sync in the top, it's like that all over the screen). It turns out that some/most TV's in Europe can't handle the sync rates, even they are supposed to be NTSC-compatible.

The TV I got is a Lg-branded LCD widescreen. I've tried it in all screen modes, but same ressult as you have in all of them. Some few times it mannages to sync, but then the colours gets all wrong and mixed-up.

Maybe the only way to sucessfully use the composite out from CGA-based cards is to actually buy an american TV.

 

[FishFinger]

Bit of a bummer if that's the case.

The TV's can accept 60Hz just fine, just not NTSC colour encoding. Maybe the output is a shade over 60Hz and out of spec?

 

[nige the hippy]

I've even had TVs that suffered from that on a TV video source if the source was "too black & white", possibly down to the sync separator confusing the edge of the video signal for sync (???) I don't know how you've got it hooked up, but if you could decrease the luminance part of the video signal it might possibly make a difference.
I might be talking out of my bottom!
(p.s. nice animation!)

 

[FishFinger]

I've used the RGB input pins on the scart socket, so the video and sync are completely separate. I get exactly the same result with a composite cable too, so I'm thinking it's probably a timing problem rather than the signal being unreliable.

 

[Trixter]

The TV's can accept 60Hz just fine, just not NTSC colour encoding. Maybe the output is a shade over 60Hz and out of spec?

That is likely (when I wrote the CGA Compatibility Tester I was astonished to find that my vertical refresh rate was NOT exactly 59.94Hz but rather something like 60.2Hz).

There was a trim pot on early PCs to tweak the output signal, but I'm afraid you don't have that option on your card...

 

[reenigne]

That is likely (when I wrote the CGA Compatibility Tester I was astonished to find that my vertical refresh rate was NOT exactly 59.94Hz but rather something like 60.2Hz).

How strange - it should be closer to 59.92Hz (157500000/11/262/228/4 - exactly 19912 PIT cycles) with the normal BIOS CRTC parameters. How were you measuring it?

I think FishFinger's problem is unlikely to be the sync rate in any case - if it were, the picture would probably roll. I agree with nige - those symptoms look more like incorrect levels. I don't know if any of the TVs have gain or DC offset controls, but tweaking them might help. The signal voltages output by a CGA card are far further from NTSC spec than the timings are.

There was a trim pot on early PCs to tweak the output signal, but I'm afraid you don't have that option on your card...

Also, it only adjusts some of the hue phases so wouldn't help a problem that also appears with a mono output.