800XL and 1084S: Artifacting

[flashjazzcat]

Great pictures, thanks! I'd say the results more or less correlate to my own, however. Note the green and pink areas on the cross-hatching around the magnifying glass in the last pic. Perhaps this is just a fact of life with the 800XL. Are the first three photos from a different machine?

[mimo]

Great pictures, thanks! I'd say the results more or less correlate to my own, however. Note the green and pink areas on the cross-hatching around the magnifying glass in the last pic. Perhaps this is just a fact of life with the 800XL. Are the first three photos from a different machine?

 

They are all the same machine.

The cross hatching was not noticeable to the naked eye, I think I need a polarising filter on my camea

[+bob1200xl]

OK -- let's talk about how the CRT makes a picture.

 

The CRT has two ways to make a bright spot on the screen. There are three identical 'guns' that can be used to activate a single color pixel. (Red, Green, and Blue) The monitor/TV does this by varying the voltage on the gun. It can either feed voltage to the cathode input pin or it can drive the grid input pin of each gun. On most CRTs, the three cathodes are tied together and fed with LUMA, giving you black and white images. (all three guns on together) For color, each grid is fed with the decoded CHROMA signal of that color. So, you can make only make color if you feed the grids, while you can illuminate a pixel with either CHROMA or LUMA. From what I can see, you may have these problems: (anyone else want to jump in here?)

 

You are not turning off CHROMA when you have a black and white display. (what I call 'birdies') Too much CHROMA?

 

Your screen looks like it has low resolution pixels, making it harder to cover the pixel triads. Can't fix that... but if LUMA only works well, you're OK here.

 

The 130XE has a much higher LUMA level than the 800XL. If the monitor is normalizing the display, this mutes CHROMA. Again, too much CHROMA?

 

 

May be too much CHROMA. Nothing should be fed to the grid of the CRT when you are displaying black and white. The LUMA plug feeds the cathodes and the CHROMA plug feeds the grids. Something is wrong (for a 1084) in the CHROMA. Where are you getting CHROMA? If a stock 800XL has the same problem, you probably need to tweak CHROMA.

 

Of course, it may be too little CHROMA and the 1084 is trying to 'compensate' by cranking up the CHROMA gain. Isn't this fun? If I remember, the 130XE outputs lots and lots of CHROMA, so this may be the case. The fact that you fixed the LCD by cutting the LUMA/CHROMA connections makes this likely.

 

The LCD, of course, doesn't feed the CHROMA and LUMA to different control circuits. And, it probably has much more accurate CHROMA decoding circuits. The point is that you are feeding your CHROMA drivers when you should not.

 

Artifacting should only work in COMPOSITE mode where the bandwidth of LUMA is severly restricted. Can you try something that uses artifacting in s-video mode?

 

 

Bob

 

 

 

 

I'll take a picture later (camera is flat), but basically I'm talking about classic Atari artifacting. Chequerboard black and white patterns are either green or pink depending on their location, half colour clock vertical lines have green/yellow tinges, etc. It's the kind of effect games used to make use of.

 

Another, stock 800XL looks exactly the same. This seems to be something peculiar to the XL line. I'd be intrigued to hear from any other 1084S owners and their findings using chroma/luma inputs. This particular modified 800XL has the best, cleanest, sharpest s-video output I've ever seen from an Atari when plugged into a flat panel LCD. Artifacting in that scenario was apparent until I made modifications to the video circuit (i.e. severing connection between luma and chroma, at which point artifacting vanished and video quality attained excellence). I can accept the fact that the 1084S and the 800XL may just not get along well this way, but I'd heard great things about this monitor, hence my slight surprise that the 800XL's picture isn't great.

[Jacques]

Just to mention, I ALSO have artifacting from 800 XL's S-video on LCD monitor via S-VHS, even though LUMA and CHROMA seem to be separated completely.

So there's no rule that it's on CRT only. The same LCD gives me the perfect non-artifacted S-video picture from XE.

[flashjazzcat]

OK -- let's talk about how the CRT makes a picture.

 

The CRT has two ways to make a bright spot on the screen. There are three identical 'guns' that can be used to activate a single color pixel. (Red, Green, and Blue) The monitor/TV does this by varying the voltage on the gun. It can either feed voltage to the cathode input pin or it can drive the grid input pin of each gun. On most CRTs, the three cathodes are tied together and fed with LUMA, giving you black and white images. (all three guns on together) For color, each grid is fed with the decoded CHROMA signal of that color. So, you can make only make color if you feed the grids, while you can illuminate a pixel with either CHROMA or LUMA. From what I can see, you may have these problems: (anyone else want to jump in here?)

 

You are not turning off CHROMA when you have a black and white display. (what I call 'birdies') Too much CHROMA?

 

Your screen looks like it has low resolution pixels, making it harder to cover the pixel triads. Can't fix that... but if LUMA only works well, you're OK here.

 

The 130XE has a much higher LUMA level than the 800XL. If the monitor is normalizing the display, this mutes CHROMA. Again, too much CHROMA?

 

 

May be too much CHROMA. Nothing should be fed to the grid of the CRT when you are displaying black and white. The LUMA plug feeds the cathodes and the CHROMA plug feeds the grids. Something is wrong (for a 1084) in the CHROMA. Where are you getting CHROMA? If a stock 800XL has the same problem, you probably need to tweak CHROMA.

 

Of course, it may be too little CHROMA and the 1084 is trying to 'compensate' by cranking up the CHROMA gain. Isn't this fun? If I remember, the 130XE outputs lots and lots of CHROMA, so this may be the case. The fact that you fixed the LCD by cutting the LUMA/CHROMA connections makes this likely.

 

The LCD, of course, doesn't feed the CHROMA and LUMA to different control circuits. And, it probably has much more accurate CHROMA decoding circuits. The point is that you are feeding your CHROMA drivers when you should not.

 

Artifacting should only work in COMPOSITE mode where the bandwidth of LUMA is severly restricted. Can you try something that uses artifacting in s-video mode?

 

 

Bob

OK - one of the most useful and informative posts yet. If you look at the first post on this topic, you'll see the same 800XL through a modern, quality LCD display. The picture is - to me - perfect. But one thing I remember remarking on at the time I did the mods, was that the display was a bit dark. The "balancing" and compensation of chroma/luma you speak of (or should that be "CHROMA/LUMA") really hits a chord for that reason. Upping the brightness on the CRT really improved the definition on the characters. So, my next job will be to moderate the chroma and boost the luma - just a bit of resistor juggling.

 

Just to mention, I ALSO have artifacting from 800 XL's S-video on LCD monitor via S-VHS, even though LUMA and CHROMA seem to be separated completely.

So there's no rule that it's on CRT only. The same LCD gives me the perfect non-artifacted S-video picture from XE.

There are absolutely no rules with this. None of my XE's can approach the quality of the modded 800XL on a particular LCD set. Another LCD set covers a multitude of sins, and completely hides the vertical banding from the XEs without degrading what is a very sharp and pleasing image. I think Bob's ideas about what the monitor does with unbalanced luma and chroma signals are very informative here.

 

All the above notwithstanding, even the XE with its zero-artifact display cannot present pleasing and sharp 80-column text via s-video. It's kind of readable, but we're really up against the limitations of the technology here. That same XE simultanously pumps out a matchless picture (from VBXE) to the 1084S's analogue RGB in if you flick the channel selector on the monitor. Quite why s-video does not approach the quality of RGB is another discussion in itself. I would venture that for starters, RGB has a separate sync signal, and that the colour carrier signal is basically in the native form used by the monitor's guns.

Edited May 10, 2010 by flashjazzcat

[flashjazzcat]

Thanks Bob: you cracked the problem.

 

I set about raising the resistance on the chroma, first with a second 100 Ohm resistor, then a third. It quickly became apparent that artifacting was getting worse, so I backed it off by removing all the resistors, including the initial 100 Ohm resistor on the luma pick-off. Result: artifacting almost disappeared. So in this particular case, the problem was too little chroma.

 

As a bonus, the picture through the LCD panel has not been degraded in any way.

Edited May 10, 2010 by flashjazzcat

[Roydea6]

Thanks Bob: you cracked the problem.

 

I set about raising the resistance on the chroma, first with a second 100 Ohm resistor, then a third. It quickly became apparent that artifacting was getting worse, so I backed it off by removing all the resistors, including the initial 100 Ohm resistor on the

What? I thought we were looking at the Chroma resistors.

 

luma pick-off. Result: artifacting almost disappeared. So in this particular case, the problem was too little chroma.

[flashjazzcat]

What? I thought we were looking at the Chroma resistors.

 

luma pick-off. Result: artifacting almost disappeared. So in this particular case, the problem was too little chroma.

Well done. I was just checking that someone was awake while I wasn't.

 

It should read CHROMA pick-off. I also fitted a 22uf cap and the artifacting has utterly disappeared. The 80 column text is still spectacle-inducing, but that's s-video for you. It's a 100% improvement on what it was before.

 

 

 

[+bob1200xl]

FJC originally had 'artifacting' on his LCD, also. It really isn't artifacting as in the case of COMPOSITE signals since the CHROMA is already separate from the LUMA. Artifacting occurs because the circuits that separate the color information from the input sees the LUMA component as an encoded color rather than an intensity level.

 

The LCDs just work differently from the CRTs. The CRT processes CHROMA through a separate process than LUMA. The LCD has to fold them together again. It's just 'noise' (unintended information) on the CHROMA input, usually. I doubt that you can get true artifacting thru s-video.

 

Bob

 

 

 

Just to mention, I ALSO have artifacting from 800 XL's S-video on LCD monitor via S-VHS, even though LUMA and CHROMA seem to be separated completely.

So there's no rule that it's on CRT only. The same LCD gives me the perfect non-artifacted S-video picture from XE.

[Shawn Jefferson]

Just to clarify, this 800XL has all 4 legs of the RF box disconnected, and components R56, C54, C55, R67, and R68 removed from the board.

 

I just took the RF modulator out of my 800XL, and I was thinking of removing the rest of the composite video circuit, too, since without the RF, composite video is gone anyway... Are these the components that needs to be removed? R56, C54, C55, R67 and R68 ?

 

PS. This 800XL has had the SuperVideo 2.1 mod done.

[Kjmann]

What? I thought we were looking at the Chroma resistors.

 

luma pick-off. Result: artifacting almost disappeared. So in this particular case, the problem was too little chroma.

Well done. I was just checking that someone was awake while I wasn't.

 

It should read CHROMA pick-off. I also fitted a 22uf cap and the artifacting has utterly disappeared. The 80 column text is still spectacle-inducing, but that's s-video for you. It's a 100% improvement on what it was before.

 

 

 

 

I don't know if you already did this in another way, but I thought I would bring it up anyway.

 

Boosting the voltage to the Q3 Transistor (3904) can make a huge difference as well. It definitely improves the luma signal. The original design of the 600 and 800XL has the transistor only getting 4.5 volts, Which starves it for voltage. To function properly, it should be getting as close to 5 volts as possible. I lifted the lower Leg of Q3 out of the board and ran a line from the positive side of C3 (the vertical Cap to the right of the power jack) to the Lower leg of Q3. This gave Q3 a FULL 5 Volts and improved the Luma a lot. It also improved the Artifacting issues as well.

[+bob1200xl]

Removing the RF modulator doesn't affect COMPOSITE, does it? It doesn't look like it would.

 

Bob

 

 

 

Just to clarify, this 800XL has all 4 legs of the RF box disconnected, and components R56, C54, C55, R67, and R68 removed from the board.

 

I just took the RF modulator out of my 800XL, and I was thinking of removing the rest of the composite video circuit, too, since without the RF, composite video is gone anyway... Are these the components that needs to be removed? R56, C54, C55, R67 and R68 ?

 

PS. This 800XL has had the SuperVideo 2.1 mod done.

[flashjazzcat]

Removing the RF modulator doesn't affect COMPOSITE, does it? It doesn't look like it would.

 

Bob

Just to clarify, this 800XL has all 4 legs of the RF box disconnected, and components R56, C54, C55, R67, and R68 removed from the board.

 

I just took the RF modulator out of my 800XL, and I was thinking of removing the rest of the composite video circuit, too, since without the RF, composite video is gone anyway... Are these the components that needs to be removed? R56, C54, C55, R67 and R68 ?

 

PS. This 800XL has had the SuperVideo 2.1 mod done.

Nope - not on the XL line. The RF mod in the XE generates the composite signal; not so with the XL, which has its own composite circuit. Without great expertise, it may have been overkill to remove R56, C54, C55, R67 and R68 from the board, but judging by the schematic they seem to be at the sharp end of the composite circuit and removing them and disconnecting the RF box brought about breathtaking improvements with the LCD.

 

The story then seemed to be that the chroma was too weak for the 1084S, so removing the 100 ohm resistor from the chroma pick-off and adding a 22uf cap instead sorted it out. It still looks spot-on through the LCD, but I now notice luma "ghosting" through the CRT which is causing degradation to the vertical definition of characters. I'll be unplugging the chroma and looking into amendments to fix this...

 

I don't know if you already did this in another way, but I thought I would bring it up anyway.

 

Boosting the voltage to the Q3 Transistor (3904) can make a huge difference as well. It definitely improves the luma signal. The original design of the 600 and 800XL has the transistor only getting 4.5 volts, Which starves it for voltage. To function properly, it should be getting as close to 5 volts as possible. I lifted the lower Leg of Q3 out of the board and ran a line from the positive side of C3 (the vertical Cap to the right of the power jack) to the Lower leg of Q3. This gave Q3 a FULL 5 Volts and improved the Luma a lot. It also improved the Artifacting issues as well.

 

I think I'll give that a try. Thanks.

Edited May 11, 2010 by flashjazzcat

[Jacques]

 

The story then seemed to be that the chroma was too weak for the 1084S, so removing the 100 ohm resistor from the chroma pick-off and adding a 22uf cap instead sorted it out. It still looks spot-on through the LCD, but I now notice luma "ghosting" through the CRT which is causing degradation to the vertical definition of characters. I'll be unplugging the chroma and looking into amendments to fix this...

 

Could You please give some more precise information which resistor has been removed, in which place you've added a cap and from which place do you take the CHROMA signal to the monitor, now?

[flashjazzcat]

Could You please give some more precise information which resistor has been removed, in which place you've added a cap and from which place do you take the CHROMA signal to the monitor, now?

 

OK.

 

1. Download the UltraVideo XL 1.0 guide and follow the instructions. That brings the 800XL to the condition mine was in before I bought the 1084S (minus the chroma pick-off).
2. For the chroma pick-off (currently not mentioned in the UltraVideo guide), instead of a 100 ohm resistor, use a 22uf capacitor with positive facing the chroma pin on the jack. Connect negative to the underside of the board where the left-hand leg of R68 used to be. This boosts the chroma signal to counteract the 1084's tendency to "compensate" for uneven chroma/luma levels (and so introducing artifacts). It also filters DC current from the chroma signal.
3. Remove L5 and run a wire from where the front leg used to be to the front of C3 (improves power supply to the video circuit and can cure vertical banding).
4. Lift the lower leg of Q3 and run a wire from the raised leg to the front of C3 (improves power supply to luma output transistor).
5. (Optionally) replace U20 (CD4050BE) with an HC4050BE chip (improves character definition).

Better or worse, that's what my 800XL looks like now. It's picture still isn't as good as Preppie's, however, which has better vertical definition on the 80 column characters. Maybe this is down to my mods, or maybe it's the monitor. There is a feint vertical ghosting on the luma, which I'm uncertain how to fix. It's still great through the LCD, though.

Edited May 11, 2010 by flashjazzcat

[Kjmann]

OK.

 

...Lift the lower leg of Q3 and run a wire from the raised leg to the front of C3 (improves power supply to luma output transistor).

 

 

Did this make a difference for you in the same way It did for me? or did you get different results?

 

 

(Optionally) replace U20 (CD4050BE) with an HC4050BE chip (improves character definition).

 

Hmmm. I'll have to order some of those chips and give that one a try.

[flashjazzcat]

OK.

 

...Lift the lower leg of Q3 and run a wire from the raised leg to the front of C3 (improves power supply to luma output transistor).

 

 

Did this make a difference for you in the same way It did for me? or did you get different results?

 

I'd didn't seem to do much in my case, but I figured it wasn't hurting anything and I left it in. I'd still like to get results as good as Preppie's!

[Kjmann]

OK.

 

...Lift the lower leg of Q3 and run a wire from the raised leg to the front of C3 (improves power supply to luma output transistor).

 

 

Did this make a difference for you in the same way It did for me? or did you get different results?

 

I'd didn't seem to do much in my case, but I figured it wasn't hurting anything and I left it in. I'd still like to get results as good as Preppie's!

 

I think I know why you didn't see a lot of difference. Changing R116 in your video mod improves the voltage to Q3. I brings it to about 4.7 volts. Going to a full 5 volts probably made a difference but after your first mod, it's not as noticeable.

 

R116 Does NOT Supply voltage to other things, only Q3.

[flashjazzcat]

I think I know why you didn't see a lot of difference. Changing R116 in your video mod improves the voltage to Q3. I brings it to about 4.7 volts. Going to a full 5 volts probably made a difference but after your first mod, it's not as noticeable.

 

R116 Does NOT Supply voltage to other things, only Q3.

Surely this explains it, then. Perhaps there are extra factors to consider: the CRT's age and calibration, and variances in the tube quality? I notice the 800XL's half-colour clock verticals are of varying width and definition depending on their horizontal location (see "TXT" on the bottom line of my last screenshot).

Edited May 13, 2010 by flashjazzcat

[Kjmann]

I think I know why you didn't see a lot of difference. Changing R116 in your video mod improves the voltage to Q3. I brings it to about 4.7 volts. Going to a full 5 volts probably made a difference but after your first mod, it's not as noticeable.

 

R116 Does NOT Supply voltage to other things, only Q3.

.... Perhaps there are extra factors to consider: the CRT's age and calibration, and variances in the tube quality?

 

I would have to say that would be a big factor. I have 4 1084 monitors and not one of them ever looks the same using it's chroma/Luma inputs.

[flashjazzcat]

I have 4 1084 monitors and not one of them ever looks the same using it's chroma/Luma inputs.

Perhaps I need to get me a second one, in the name of science of course!

[OX.]

OK.

 

• Download the UltraVideo XL 1.0 guide and follow the instructions. That brings the 800XL to the condition mine was in before I bought the 1084S (minus the chroma pick-off).
• For the chroma pick-off (currently not mentioned in the UltraVideo guide), instead of a 100 ohm resistor, use a 22uf capacitor with positive facing the chroma pin on the jack. Connect negative to the underside of the board where the left-hand leg of R68 used to be. This boosts the chroma signal to counteract the 1084's tendency to "compensate" for uneven chroma/luma levels (and so introducing artifacts). It also filters DC current from the chroma signal.
• Remove L5 and run a wire from where the front leg used to be to the front of C3 (improves power supply to the video circuit and can cure vertical banding).
• Lift the lower leg of Q3 and run a wire from the raised leg to the front of C3 (improves power supply to luma output transistor).
• (Optionally) replace U20 (CD4050BE) with an HC4050BE chip (improves character definition).

Better or worse, that's what my 800XL looks like now. It's picture still isn't as good as Preppie's, however, which has better vertical definition on the 80 column characters. Maybe this is down to my mods, or maybe it's the monitor. There is a feint vertical ghosting on the luma, which I'm uncertain how to fix. It's still great through the LCD, though.

 

Hi FlashJazzCat, I recently did your Ultravideo mod being both an Atari 8 bit and electronics newb, I undertook the task without looking into the Supervideo mods or the updated instruction shown in this thread to up the voltage on video lines to 5v, yet still have some faint vertical lines showing on an lcd screen and one thing struck me - in the schematic pictured below from the Supervideo mod it shows ferrite beads leading off both the composite (now redundant) and luma lines to suppress noise, instead of adding a 100ohm resistor or 2.2cap as suggested would it not be advisable to add a ferrite bead to the line instead and maybe this will solve your artifacting issues? Excuse any newb ignorance in advance.

PS. Being a lad from Sunderland I noticed your also a north east native.

Edited February 26, 2015 by OX.

[flashjazzcat]

Sounds good. I'm not knowledgeable regarding electronics and I silently deprecated the UltraVideo mod. If you can improve on it (which wouldn't surprise me), please do, and share your results. And yes: nice to hear from someone in the local area! I used to study and later work in Sunderland and I get down there quite regularly to see friends.

[OX.]

I'm going to supply the full 5v to the video lines (this alone will probably alleviate the faint vertical lines) and add the ferrite bead to the chroma line to reduce any noise as soon as I get the part, I'll follow up with the results.

[kenjennings]

gah! Atari people are such extreme perfectionists. The average C64 owner is happy when the thing spits out six discernible colors.