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vertical lines on many XE computers


Marius

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Right, this is as good as I can get the 800XL picture. I'm using an ebay cable with a diode fitted, and trusting that it's benefitting the overall result (should really try it with a plain cable too).

 

post-21964-126192080454_thumb.jpg

 

post-21964-126192081381_thumb.jpg

 

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What this is is a selective and adapted version of the Supervideo 2.0 and 2.1 mods. I've changed a couple of resistor and cap values here and there via trial and error. I promise to write it up some time this week. The output is now comparable to and in some ways surpasses that of my stock XEs. I compromised with very slight grain and almost imperceptible vertical banding against sharper characters and reduced shadows and ghosting. The only thing still bugging me (which the XE doesn't suffer from) is artifacting. Anyone know what I need to chop out in order to banish that?

Edited by flashjazzcat
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You'll get artifacts no matter what, if you're using a composite output, I believe. If you switch to S-Video fully then presumably if you remove the interconnection between the luma and chroma circuit, the luma signal won't affect the chroma signal. That's what I'd go after first. Also the modulator will be doing the same thing. According to Sobola's schematics, R67 (75 ohm) and C54 (.001uF) are what form the pathway between chroma and composite in a stock, unmodified 800XL (probably different in an XLF).

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You'll get artifacts no matter what, if you're using a composite output, I believe. If you switch to S-Video fully then presumably if you remove the interconnection between the luma and chroma circuit, the luma signal won't affect the chroma signal. That's what I'd go after first. Also the modulator will be doing the same thing. According to Sobola's schematics, R67 (75 ohm) and C54 (.001uF) are what form the pathway between chroma and composite in a stock, unmodified 800XL (probably different in an XLF).

Thanks for that info - I'll look into the circuit tomorrow. Actually I disconnected the RF box on the 800XL yesterday and the zigzag lines returned. I eventually discovered that this was down to the Supervideo 2.1 mod - specifically step 4. With that resistor removed (and also the cap from step 4 of the original 2.0 mod), the picture is quite superb with the RF disconnected. When I get a chance to test it again (the 800XL is out of action until I finish the 1MB upgrade) I'll check for artifacting. If that is (or can be) eliminated, the picture will be nigh-on perfect. I suspect the composite circuitry still hasn't been completely eradicated on this machine.

Edited by flashjazzcat
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Removing R67 and c54 seems to improve the picture. Hard to tell whether the greenish areas which persist around text are artifacts or just the colour offset (i.e. the green stripe you often see down the left side of the blue playfield). Shadowing and ghosting seem to be the most stubborn problems. The particular XL now has the composite (apparently) and RF completely severed. I may backtrack on a couple of bits of the SuperVideo mod, since that obviously was designed with the RF connection intact. Disconnecting it has drastic effects. Also, many more components (R56, C54 and R67) have now been stripped out.

Edited by flashjazzcat
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If you want to "prove" that the problem is or is not artifacting, try a simple test program:

 

10 GR.8+16

20 FOR I=0 TO 319 STEP 2:PLOT I,0:DRAWTO I,191:NEXT I

30 POKE 709,15:POKE 710,0

40 FOR I=0 TO 500:NEXT I

50 POKE 709,0:POKE 710,15

60 FOR I=0 TO 500:NEXT I

70 GOTO 30

 

If there is any sort of artifacting, this will show it. If the picture is pure monochrome, then ghosting is probably the issue.

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If you want to "prove" that the problem is or is not artifacting, try a simple test program:

 

10 GR.8+16

20 FOR I=0 TO 319 STEP 2:PLOT I,0:DRAWTO I,191:NEXT I

30 POKE 709,15:POKE 710,0

40 FOR I=0 TO 500:NEXT I

50 POKE 709,0:POKE 710,15

60 FOR I=0 TO 500:NEXT I

70 GOTO 30

 

If there is any sort of artifacting, this will show it. If the picture is pure monochrome, then ghosting is probably the issue.

Thanks, I will do that!

 

The current state of the 800XL picture is, I think, exceptionally good:

 

post-21964-126228016304_thumb.jpg

 

post-21964-126228017899_thumb.jpg

 

Hard to remember what parts I've added/changed/removed, but it's not too drastic and I'll quantify the changes in a day or two. Apart from the "apparent" artifacting, the one remaining problem is the noisiness of the image - like an animated graininess. Quite when this was (re)introduced, I'm not sure. I actually refitted L5 just there, since I thought it actually looked better than with the direct 5v feed. The definition of characters on the screen (especially pseudo 80 column) is as good as I've ever witnessed on an 800XL.

 

...NOTE: you can see what looks like artifacting in the bottom right corner of the top photo.

Edited by flashjazzcat
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Maybe not so much "small" as faint. Guess I have a lot more hacking to do on my video circuit icon_smile.gif

Yes - that was the main problem which was making 3-bit 80 column characters unreadable until I sorted out the picture. The verticals of light characters on a darker background looked half the width of the gaps between them. The 80 column text looks really great now, though. I just need to figure out whether the noise is on the luma or chroma and do something about it. It's like watching the screen through a swarm of locusts! icon_confused.gif

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I just need to figure out whether the noise is on the luma or chroma and do something about it. It's like watching the screen through a swarm of locusts! icon_confused.gif

Please let me know if you get that sorted. I get that through the scan doubler on my 130XE even with the RF circuitry removed. It's not terrible but definitely distracting . I am going to hook it up to my CRT TV shortly. I'll let you know how the image looks on it.

 

Stephen Anderson

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I just need to figure out whether the noise is on the luma or chroma and do something about it. It's like watching the screen through a swarm of locusts! icon_confused.gif

Please let me know if you get that sorted. I get that through the scan doubler on my 130XE even with the RF circuitry removed. It's not terrible but definitely distracting . I am going to hook it up to my CRT TV shortly. I'll let you know how the image looks on it.

Yep - that's a job for today once I emerge from this biblical hangover. There's a cap Ben Poehland introduced into the circuit with the SuperVideo 2.0 mod intended to clean up the signal. I removed it the other day (also the resistor introduced in version 2.1 which was to provide negative feedback to the colour amplifier: all it did with the RF disconnected was create vertical banding), but I might put it back in and see if it helps. The pseudo-artifacting effect also needs cleaning up: there's definitely no composite signal now (I tested the cable), so the presence of colour on mono images is puzzling.

 

If I get everything the way I like it, I'll write up the mod properly: it's diverged from SuperVideo in several ways to compensate for the effect of removing the RF circuit. Really, though, when I compare the picture now to that of my first SuperVideo modded 800XL, the improvement is pretty staggering. icon_mrgreen.gif I'd love to see the output through a CRT, if I had one to try. The image is always going to be compromised through a TFT by the upscaling. Even VGA/DVI input from a PC looks poor at the wrong resolution.

Edited by flashjazzcat
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FJC - if it's your color circuit giving you grief, have a look at the one from supervideo XE. The modified 130XE color circuit is very simple and should be highly immune to noise. I intend to build it for my 600XL (as opposed to using the original laid out color circuit) as soon as my resistors arrive in the mail...

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FJC - if it's your color circuit giving you grief, have a look at the one from supervideo XE. The modified 130XE color circuit is very simple and should be highly immune to noise. I intend to build it for my 600XL (as opposed to using the original laid out color circuit) as soon as my resistors arrive in the mail...

The curcuit diagrams for both the XL and XE mods are very illuminating: I'd never looked at them before since in the past I was just following the mods to the letter, give or take the odd part. I've now totally disconnected the luma and chroma by removing the entire bank of components joining them: R56, R67, R68 C54 and C55 (R68 and C55 were the last to go and made no discernible difference to the apparent artifacting, which I found odd).

 

By temporarily disconnecting the chroma pick-off, it was easy to establish that it's the luma carrying the noise. Any noise-reducing modifications I can carry across from the XE mod, I will attempt. I hadn't realized until now that the RF modulator in the XE produces the composite video signal (unlike the XL).

Edited by flashjazzcat
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Breakthrough re noise: The diode in the ebay cable was causing it. It's actually there - IIRC - to reduce the vertical banding. I only realized when I tipped the motherboard back and noticed that the noise got a lot worse - presumably a loose connection in the (hot glued) plug. I retrieved my "clean" cable from the cupboard, and the noise is no more. Moreover - the diode must be shared between the luma and chroma lines, because the clean cable has eradicated what was clearly genuine artifacting. There's about three hours wasted because of a pesky diode!

 

From a distance, this could almost be a VBXE machine:

 

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And finally, we say goodbye to artifacts:

 

post-21964-126237986733_thumb.jpg

 

This is the nearest I've seen to RGB clarity on any Atari I've owned. If I can get an XE up to this standard, I'll be even happier. It's hard to know what could be improved with this picture. There's the faintest suggestion of vertical banding, but only if you go looking for it on a plain screen. I've eschewed the direct 5v line and reinstated L5. Not much of the Supervideo XL mod is left, actually. This is more akin to a stock 800XL with a couple of different resistor values and lots of components removed.

 

As I say, I'll write this up soon for those who want to try it. All I'll say to conclude for now is that if you do this mod (getting rid of composite and RF completely), you need a CLEAN s-video cable. The ebay diode cables will actually introduce signal noise and artifacting.

Edited by flashjazzcat
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Well, I hooked up 3 machines to my Toshiba CRT. A stock NTSC 130XE, super-vid with RF box removed NTSC 130XE, and stock PAL 130XE. Both NTSC units had bands all across the screen, and the PAL unit had the crazy zigzag lines. My TV can display the image, but not the color unfortunately and the size / position is all screwed.

 

No way could I use that machine on the TV like that.

 

Stephen Anderson

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The 130XE is going to be tougher work. The aim is to make it look good with the same "plain" s-video cable I'm using with the 800XL. The 130XE looked bad through this cable before I did the Supervideo mod, and believe me it looks worse afterwards. I never imagined the 800XL would blow the XE away like this; the latter supposedly having corrected at least some of the flaws in the XL circuit:

 

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It would be nice to write both the XL and XE mods together, but obviously the XE needs a lot more work. I should add that the first thing I did was take out the RF modulator. What's not appreciable from the pictures is a ton of noise, too.

 

...removed R202 and R205 temporarily (severing chroma), and sure enough the mono signal is clean. All noise, crap, grain, etc, is on the colour curcuit, so I'll concentrate my attention there.

Edited by flashjazzcat
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Since the XE has a 14 MHz clock, would it be possible to do something like:

 

- have a /2 to give a ~ 7 MHz input

- use the 7 MHz input as the clocking to a 4-bit latch - run the luma lines direct from GTIA to the latch.

- use the outputs from the latch to drive your own luma circuit

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XE is just about sorted:

 

post-21964-126243410709_thumb.jpg

 

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All it needed was one of those little orange-ended diodes (?) in place of the resistor R205. The original resistor value at R202 of 180ohms is preferable to the 75ohm value in the Supervideo mod (gives a cleaner signal).

 

The only thing required now is better definition on vertical lines: they exhibit the withering effect apparent on stock 800XLs. Some attention on the luma circuit tonight should yield the ultimate XE picture to match the ultimate XL display! icon_mrgreen.gif

Edited by flashjazzcat
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