800XL displaying monochrome, colour adjuster gives strange response

[Dr Do]

My PAL 800XL is producing black and white video output, with colour occasionally coming in but only for a second or two each time. I tried to adjust the color setting using the access point at the bottom of the unit. However, this just results in garbage appearing on the screen (as shown in these screenshots). Turning the machine off and on restores the proper display, but still in monochrome.

 

Any ideas how to fix this, please?

Edited May 9, 2021 by Dr Do
Typo attack

[+x=usr(1536)]

Which output are you using - RF or the monitor port?

[Dr Do]

1 minute ago, x=usr(1536) said:

Which output are you using - RF or the monitor port?

It's RF. My 65XE's display is in colour when used in place of the XL.

[Mazzspeed]

As far as I'm aware, chroma isn't connected on the 800XL's monitor port. It's a simple mod to connect chroma however.

[Rybags]

That looks like a crash, typical of a corrupt display list when a program plays up.  That aside, the colour trimmer just changes the spread of the phase offsets and I don't think it ever causes a colour to revert to monochrome (?)

The problem with RF aside from the poorest video quality is that the port itself is made from rubbish mild steel that easily oxidizes and gives poor conductivity, not to mention the physial design isn't much good either.

Whenever possible it's a good idea to use composite video at the least.

[Dr Do]

On 5/10/2021 at 11:49 AM, Rybags said:

That looks like a crash, typical of a corrupt display list when a program plays up.  That aside, the colour trimmer just changes the spread of the phase offsets and I don't think it ever causes a colour to revert to monochrome (?)

The problem with RF aside from the poorest video quality is that the port itself is made from rubbish mild steel that easily oxidizes and gives poor conductivity, not to mention the physial design isn't much good either.

Whenever possible it's a good idea to use composite video at the least.

Thanks - that helps me narrow down what to do next.  I have a monitor to SCART lead but the TV I'm using won't take the signal at all (and I note from @Mazzspeed's comment that a mod might be needed for that anyway), so I will have a look at some alternative options.

 

That makes sense that a crash would cause the display to go haywire like that, but I'm now curious as to (i) why this would occur as a result of adjusting the colour trimmer; (ii) why it would happen when there was no program running (unless there's always something running in the background when the machine is at the READY prompt)?

[R.Cade]

39 minutes ago, Dr Do said:

Thanks - that helps me narrow down what to do next.  I have a monitor to SCART lead but the TV I'm using won't take the signal at all (and I note from @Mazzspeed's comment that a mod might be needed for that anyway), so I will have a look at some alternative options.

 

That makes sense that a crash would cause the display to go haywire like that, but I'm now curious as to (i) why this would occur as a result of adjusting the colour trimmer; (ii) why it would happen when there was no program running (unless there's always something running in the background when the machine is at the READY prompt)?

Maybe you are touching some part of the signal path instead of turning a trimmer? You are using a plastic adjuster and not a metal screwdriver, correct?

 

[Dr Do]

On 5/11/2021 at 12:56 PM, R.Cade said:

Maybe you are touching some part of the signal path instead of turning a trimmer? You are using a plastic adjuster and not a metal screwdriver, correct?

 

Well, how embarrassing; I was actually using a metal screwdriver. Fortunately my machine appears to have survived that abuse.

 

To add to the curiosity: I thought I'd have a go with an older TV that I found in the loft, and lo and behold the colour comes up fine using the RF connection. Perhaps it does a better job of picking up whatever signal my machine is still able to put out? Either way the problem's solved for now, but I'll check out some of these alternative ways of getting video output.

 

Thanks all!

Edited May 13, 2021 by Dr Do

[Kr0tki]

On 5/10/2021 at 12:49 PM, Rybags said:

That aside, the colour trimmer just changes the spread of the phase offsets and I don't think it ever causes a colour to revert to monochrome (?)

It does on PAL. The colourburst signal in odd and even lines is phase-shifted in PAL - it leads the "real" colour subcarrier phase by 45° in odd lines, and lags it by 45° in even lines. The resulting 90° difference is used by the TV to distinguish odd and even lines, and to recreate the original phase of the subcarrier.

 

Adjusting the colour pot on PAL machines unfortunately changes the relative phase shift between "odd" and "even" colourbursts - it's not always 90°. It has a few consequences specific to PAL Ataris:

1. Adjusting the pot changes not only colour hues, but also saturation.

2. The pot can be turned so much that the shift between "odd" and "even" colorburst can become so close to either 0° or 180° that the telly throws his hands up and stops even trying to decode colour signal, because it can no longer distinguish odd and even lines. The resulting image then becomes colourless.

3. When the pot is turned to the extreme, the colourbursts' phase difference gets even larger than 180°, which confuses the telly so much that it "inverts" the colours.

 

I implemented emulation if this behaviour in Atari800 a few years ago. You can reproduce these effects by adjusting the emulated colour pot with Alt+6 and Alt+Shift+6.

 

A way more detailed explanation is in the comments in src/colours_pal.c.

Edited May 16, 2021 by Kr0tki

[+x=usr(1536)]

On 5/13/2021 at 3:22 PM, Dr Do said:

To add to the curiosity: I thought I'd have a go with an older TV that I found in the loft, and lo and behold the colour comes up fine using the RF connection. Perhaps it does a better job of picking up whatever signal my machine is still able to put out?

This is almost certainly the case.  Analogue signal processing on modern LCD TVs is generally crap compared to what CRTs were capable of, especially where analogue RF is concerned.

 

We currently have 2012-, 2013-, and 2016-model Samsung TVs here, and the newer they get the worse they handle RF and composite.

Edited May 16, 2021 by x=usr(1536)