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juansolo

Investgating PAL 7800 picture issues (was 7800 RGB Musings)

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I'm curious about how many people out there would want one? Mainly because I really struggle with the horrible picture a PAL 7800 puts out when playing 7800 games (it's way better for 2600 stuff) even with either of the commonly available S-Video mods. It's better, but in comparison to everything else I run, it's still plagued by dot crawl and other artifacts. Then it dawned on me why we haven't got one, because the 7800 is two machines in one and blending the two outputs together through one RGB mod is probably a big part of the issue. So my musing is, how many of those wanting the RGB mod would be prepared to sacrifice 2600 backward compatibility to have an RGB 7800? I know I would, but then I also have an RGB 2600 anyhow (might be one of the reasons the 7800 pains me so when I switch to it).

 

Really I just want to keep demand alive for this so clever people like eTim will have some motivation to develop one... (pleeeease).

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I've been interested in an RGB mod for some time, and I've been in touch with Tim Worthington over the years as he's been working on one since 2016. However, I haven't heard any updates since I last checked in with him and I don't believe anyone else is working on such a mod. I actually set aside a 1984 model 7800 for a 7800RGB mod but now I'm starting to think about putting in a UAV board instead, now that I have a composite/S-video transcoder for my OSSC and was very pleased with the results on a UAV-modded 2600. It's certainly better than the LHE board in my other 7800 which was disappointing.

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I think the NTSC 7800 fairs way better than the PAL one does with picture quality and the UAV. It's a marked improvement certainly, but on the PAL consoles I've modded, the 7800 side of things still has a pretty bad picture.

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6 hours ago, juansolo said:

I think the NTSC 7800 fairs way better than the PAL one does with picture quality and the UAV. It's a marked improvement certainly, but on the PAL consoles I've modded, the 7800 side of things still has a pretty bad picture.

Curious if you have any screenshot examples to show how it looks on PAL? 

 

And I assume you have tried Magic Knight's s-video solution? I tend to recall that one works better on PAL than NTSC although when dialed in just right, it can look pretty much on par with the UAV.

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1 hour ago, -^CrossBow^- said:

Curious if you have any screenshot examples to show how it looks on PAL? 

 

And I assume you have tried Magic Knight's s-video solution? I tend to recall that one works better on PAL than NTSC although when dialed in just right, it can look pretty much on par with the UAV.

Yeah I've fitted both to two separate 7800's and the results when dialed in are very similar. They're good, and vastly better than RF, but still weak next to the 2600 output of the 7800, and the RGB 2600 is a huge step beyond that.

 

It's hard to screenshot as it looks fine static, I'd probably be better if I can record a video of what they do. I'll have a go in a bit. It's most noticable on big blocks of colour.

Edited by juansolo

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I see. Well at least from the NTSC side of things, the 7800 games tend to look the best through composite and s-video than the 2600 games do. But it is very difficult to notice with only a few games making it more obvious. Pitfall! is a good example as it still shows some slight jail bars and ghosting on the sides of the trees in composite and in S-video but effects like this are not seen from 7800 games in 7800 mode.

 

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It's the dot crawl on blocks of colour on the 7800 in PAL. Both my machines do it (MK and UAV). It's easiest to see on Ballblazer here. Just imagine it moves all the time. On dark games it's far, far less noticable. But Ballblazer and Scramble look particularly bad for it. Conversely the 2600 side of things doesn't really suffer from dot crawl at all.

P1060424.thumb.JPG.29c0519f52dff611a47887ce55fce8b4.JPG

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Does it appear as diagonal lines usually moving from upper right to lower left that you see?

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How does it compare to the officially released PAL RGB 7800?

 

Mitch

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1 hour ago, juansolo said:

Yup, and they both do it.

Disconnect the TIA colorburst signal when you have a 7800 game in and see if the diagonals go away?

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9 hours ago, -^CrossBow^- said:

Disconnect the TIA colorburst signal when you have a 7800 game in and see if the diagonals go away?

We tried that, it doesn't. I know that used to work with the older mods.

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21 hours ago, Mitch said:

How does it compare to the officially released PAL RGB 7800?

I assume that's a French one? I know when other consoles have been modded for RGB in the French market it's been far from optimal.

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11 hours ago, juansolo said:

We tried that, it doesn't. I know that used to work with the older mods.

I only suggested it because it sounds exactly like the TIA colorburst interference pattern that would occur using the older LHE mod boards. I've not see it with a UAV but I have seen it ever so slightly with the Magic Knight if I had the brightness up beyond a certain spot on the trimmer on the board. 

 

So yeah, was just curious if that pattern went away when disconnecting the TIA color from it. I'm going to have to guess that in the case of the UAV it could just be a PAL thing since the UAV was designed for NTSC signals as far as I know.

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I assume that's a French one? I know when other consoles have been modded for RGB in the French market it's been far from optimal.
The French one is terrible on 7800 games. I have one using on a CRT and there are several distortions on the screen and washed out colors. When you startup the first 3 seconds the screen is also in black and white.

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On 2/22/2020 at 3:37 AM, juansolo said:

I'm curious about how many people out there would want one? Mainly because I really struggle with the horrible picture a PAL 7800 puts out when playing 7800 games (it's way better for 2600 stuff) even with either of the commonly available S-Video mods. It's better, but in comparison to everything else I run, it's still plagued by dot crawl and other artifacts. Then it dawned on me why we haven't got one, because the 7800 is two machines in one and blending the two outputs together through one RGB mod is probably a big part of the issue. So my musing is, how many of those wanting the RGB mod would be prepared to sacrifice 2600 backward compatibility to have an RGB 7800? I know I would, but then I also have an RGB 2600 anyhow (might be one of the reasons the 7800 pains me so when I switch to it).

 

Really I just want to keep demand alive for this so clever people like eTim will have some motivation to develop one... (pleeeease).

I would be interested. Obviously Id prefer the 2600 compatibility to be there but if it wasnt, I'd still buy one. It would be nice to run it thru a OSSC.

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AFAIK the French machine generates RGB from an internal chroma/luma, so it‘s not „native“ RGB either. True RGB would require a mod like Sophia for the 8-bits. 

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Posted (edited)

Bit of an update. So the PAL 7800 dot crawl issue isn't crosstalk from the TIA. I decided to do a bit of an experiment by disconnecting the 2600 side video signal completely by removing 74LS32 IC and the pair of resistors on the colour line. Then I ran the LUM0-3, sync and Colour lines direct from the MARIA pins so I could run the wire out of the way of any circuitry. Dropping in a UAV (as it has all the bits to put it together). So the absolute best possible senario.

 

The results is that it's certainly the sharpest the picture has ever been, but the dot crawl is still there and as strong as it ever was. So the only conclusion I can come to is that it's coming from the PAL MARIA. Now I'm at a loss as all my games are PAL/PAL-60 so swapping it for an NTSC 7800 isn't really an option. I suppose I'll just have to put up with it.

7800-EXP.JPG

Edited by juansolo

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3 hours ago, juansolo said:

The results is that it's certainly the sharpest the picture has ever been, but the dot crawl is still there and as strong as it ever was. So the only conclusion I can come to is that it's coming from the PAL MARIA. Now I'm at a loss as all my games are PAL/PAL-60 so swapping it for an NTSC 7800 isn't really an option. I suppose I'll just have to put up with it.

If that's right, it might be worth trying to add a ground plane on top of Maria, and see if it helps. Easy enough to do with some copper tape and a ground wire.

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I suspect it's already there in the signal as it comes out of the Maria I mean. I doubt it's emmisions from it. It's also super consistent across all the PAL 7800s I've ever seen. At this point I think I can conclude that it's a 'feature'.

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I was thinking emissions from one part of maria to another, but for sure it's more likely leaked or inducted, assuming maria is indeed the source.

 

[edit - last observation. The noise appears to be at about half the 6502 clock rate.]

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Posted (edited)

You may be onto something, I'll make a shield at the weekend and try isolate the maria.

 

FWIW, I've been playing some more and, dot crawl aside, this is easily the best picture I've had from the 7800. The sharpness and colour is better than it's ever been. So I'm going to leave it wired this way regardless.

Edited by juansolo

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It was a longshot, but worth trying. 👍

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