Using MiSTer Multisystem With Amstrad CM14 Monitor

[PenroseHouse]

Hello,

I bought a new old stock Amstrad CM14 monitor (an RGB monitor) on a certain auction site, plus two cables:

-a cable you plug into the MiSTer's VGA, has UK/US SCART at the other end, supposedly converts the VGA output to RGB (from the auction site)

-a cable you connect to the monitor's cable, so you can plug UK/US SCART (designed for plugging a real ZX Spectrum into the monitor, bought on a certain UK cable site)

...so I'm able to connect the MiSTer's VGA to the monitor, but I get weird effects, mainly that the image is split down the middle, the left-side is on the right and the right-side is on the left, plus the side you see on the right is really dark. There's also some corruption at the top.

(tell me if I should post details about where I bought the cables)

Does anyone know what settings I should change in MiSTer.ini

I have these settings to get the above-described image:
forced_scandoubler=0
ypbpr=0
composite_sync=1
vga_scaler=0
though I have tried lots of other settings.

Note that I'm using the MiSTer board in the UK MiSTer Multisystem, but if anyone doesn't know how to set it up with that but set it up without the Multisystem, I'm happy to buy the parts and set up a regular MiSTer. So really grateful for advice either way. And maybe it doesn't make any difference.

I'll try to figure out how to add a photo.

Thank you!

Aiden

[mapf]

Two images next to each other on a 15KHz monitor can be caused by a 31KHz signal. Please check in the OSD if the "Scandoubler FX" option is set to something other than "None".

Also, a short googling gave no clear answer if the CM14 is able to display anything besides 50Hz. Please try any core that allows switching to PAL output (e.g. the C64) and test using a 50Hz signal.

[PenroseHouse]

It works! THANK YOU, mapf.

One problem remains. The colors get corrupted when the border is white, yellow or green on the ZX Spectrum core, and similar problem on the C64, haven't tried others. Anything white turns pinky purple, it's very ugly.

This is a significant problem as I'm developing games for these machines. If so, the Multisystem board has a physical toggle switch for cync on green/purple, but I have the problem in either setting, it maybe just toggles the problem.

Any ini or menu settings might stabilise it? I'm thinking it's trying to auto resolve some issue that doesn't actually exist.

Thanks!

Possibly to do with cync on green/purple?

[bazza_12]

not related to your question.. but i'd love to see a picture of your set up with the monitor.. will bring back a few memories, cheers

[mapf]

Possibly to do with cync on green/purple?

In my opinion, you should not use sync on green for the CM14. But I could be wrong. There is a similar problem reported here: viewtopic.php?t=5032

The user managed to get rid of the tint by setting

Code: Select all

vga_sog=1              ; 1 - enable sync on green (needs analog I/O board v6.0 or newer).

And switching the SoG switch to "OFF". You could try the 4 variations of the two settings and see if one works.

While you are at it, you could update mister.ini. In your post you had an obsolete setting. Please set

Code: Select all

composite_sync=1
;ypbpr=0               ; set to 1 for YPbPr on VGA output. (obsolete. see vga_mode)
vga_mode=rgb           ; supported modes: rgb, ypbpr, svideo, cvbs. rgb is default.

to make sure that a RGB signal is generated. As RGB is default, I do not think this will help with your problem, though.

Finally: Some monitors do not like the voltage levels of the MiSTer analog board output. For one of my Sony TVs, I need a resistor in line with the sync pin to get proper red colors. The Multisystem might have settings for the SCART output, please check the manual.

[PenroseHouse]

bazza, thank you for your reply, I will certainly post a photo of my setup, just waiting for a new keyboard to arrive, which should work with it really well.

mapf, THANK YOU for such a detailed replay, I will try this this afternoon and report back.

[PenroseHouse]

mapf, I have made the changes as you suggest. With the SOG switch on, it's very green, with SOG switch off it's... half as purple as before, but whites are still purple.

"Finally: Some monitors do not like the voltage levels of the MiSTer analog board output. For one of my Sony TVs, I need a resistor in line with the sync pin to get proper red colors. The Multisystem might have settings for the SCART output, please check the manual."

Bear in mind, if I hit F12 for the menu to come up, the white of the ZX Spectrum screen corrects itself, the purple tinge goes, we have a nice grayish white (it's more grey than white even on the original hardware). The menu itself is a perfect black and white.

Does this mean it isn't worth playing with the voltage levels? I wonder if it's risky to up the voltages.

I've checked the link above, there isn't anything that you haven't described in your post.

Anyone have any ideas?

[PenroseHouse]

PS, ZX Spectrum core, in the F12 menu, if I switch on Narrow Border, again the problem corrects itself - it just doesn't like having a white (or apparently yellow or green) border.

[PenroseHouse]

Sorry for third post in a row, but ... although the narrow border corrects the problem on the main ZX Spectrum screen (though have that big black border), as soon as a game starts loading it all goes greenish!

[mapf]

mapf, I have made the changes as you suggest. With the SOG switch on, it's very green, with SOG switch off it's... half as purple as before, but whites are still purple.
Anyone have any ideas?

Did you try all four combinations of the .ini setting and the switch? Then I have no more idea. You could try downloading the 240p test suite rom for SNES or Sega Mega Drive and test if any of the standard test screens trigger the wrong colors.

[PenroseHouse]

I tried all four, thank you for the suggestion. I just get either the colours wrong some of the time eg when the border is certain colours, or the exact opposite ie when it's NOT those certain colours. So useless for any game where the borders change colours at times, which is most games.

I shall try the tests you suggest, presumably they offer some kind of fix should the tests identify a particular issue.

REALLY grateful for your help. It's looking like this isn't going to happen though!

I'll have to either buy another monitor, or buy a real Spectrum/C64 - or try to get the monitor to work with my Windows PC and use it for something else, but no one seems to know how to do that either! Maybe it can't be done.

Aiden

[PenroseHouse]

Update!

Someone far more clued up on hardware than me has apparently figured out what's causing the whites to turn purple on my Amstrad CM14 monitor when used with my MiSTer Multisystem. Though that doesn't give me a solution, hence this post. I've tested his theory and it seems to be true:

---

you can bet your bottom dollar that what we're seeing is a black level clamping problem.

Summary: at the start of each line, there's supposed to be a short period (not normally visible on the screen) which is black. The monitor uses that as its reference, and, basically, shows all colours on the screen relative to that. So if that period contains red, the whole picture will end up looking cyan. If it contains blue, the picture will look yellow - basically the opposite of whatever is in that "blanking period".

So if the MiSTer is delivering a signal which isn't quite standard, and isn't giving enough blanking time after each horizontal sync pulse, the picture colours will depend on the border colour. This, incidentally, is also the reason why the picture from a ZX81 looks so dark on many TVs: its video output is bright white during the blanking interval so the TV uses that as its black reference. So then the white parts of the screen (which is most of it) look black, and the black parts (the text) look even blacker, and you can only see anything by cranking the brightness right up.

Is there anything like a "horizontal shift" or "position" adjustment in the settings? That would probably adjust the video timing relative to the sync pulses and, if adjusted too far "left", would start the video too soon after the sync pulse, leading to the black level clamping problem you're experiencing. If such a setting exists (I'm clutching at straws!) then adjusting it might well help.

---

I've tested this theory by switching on the large black border on the ZX Spectrum core. The whites largely become very pale pink rather than dark pinky purple.

This is not a perfect solution though. It's still a bit pink. AND there's some weird things, on the C64/Vic the black border is only top/bottom, and seems to get switched off at certain times in certain games.

I'd rather not have a full black border on the Speccy if I can just have a black line down the left side or whatever. Plus, probably most cores don't have the option of a thick black border.

Any way to force a black border on ALL CORES? Or perform the horz shift or whatever as discussed in the paragraph I posted above? The hardware filters that I see in the settings?

Thanks in advance!

Aiden "Aiden Smith" Smith

[breiztiger]

Hi

I have then same « white purple » with my cm14 and analog board with vga to scart and scart to cm14 cable

[PenroseHouse]

You too, breiztiger, huh. Well at least in both cases we know it's not our monitor.

I've tried with two different pieces of hardware converting VGA>RGB.

BTW, how do you thank? It doesn't seem to be in the FAQ, anyone tell me?

[GreyAreaUK]

BTW, how do you thank? It doesn't seem to be in the FAQ, anyone tell me?

5OuxMH6.jpeg (8.36 KiB) Viewed 8075 times

[reno]

Hi,

I'm in similar situation as I just got a CM14 and I was hoping to use it with my MiSTer MultiSystem.
I'll definitely get a SCART-CM14 adapter (from Coolnovelties right ?) because I can always use it with my real hardware (Amigas, regular Amstrad CPC).

Regarding the MiSTer, did you get anywhere ?
I can see there are both cheap (£5) VGA/SCART cables on eBay, and much more expensive (around £30) VGA to SCART converter (as I understand this converts the sync signal and maybe adjusts voltages ?)

In theory, do you really need these converters with the MiSTer, or is the DB15 output configurable enough to output what SCART needs without conversion ?
And, what about the built-in SCART output in the MMS ? Could it make a difference ?

[PenroseHouse]

Hi, I tried two, I think one from coolnovelities and one from eBay, in fact maybe two from eBay.

I definitely tried at least one method without first, before I bought what I bought.

Let me know if you make it work without, and I'll try it again.

[reno]

Hi, I tried two, I think one from coolnovelities and one from eBay, in fact maybe two from eBay.

I definitely tried at least one method without first, before I bought what I bought.

Let me know if you make it work without, and I'll try it again.

Sorry I don't understand, you tried two what and without what ?

[PenroseHouse]

"I'll definitely get a SCART-CM14 adapter" - I've tried two different adapters, so far as I recall. Maybe two with SCART and one with VGA.

Without, I refer to:

"In theory, do you really need these converters with the MiSTer, or is the DB15 output configurable enough to output what SCART needs without conversion ?"

My understanding is that the CM14 uses a very unusual standard and so you definitely need some kind of converstion, though I'm hardly a hardware guy.

[akeley]

You should ask about it on this forum: https://www.cpcwiki.eu/forum/

There are some people there who know CPC hardware inside out.

[PenroseHouse]

Thanks for that, Akeley. I'll hold off as I know I'll end up spending money that I don't have at the moment, but I'll make a note to do so when I have some funds!