CM14 scart input cable.... Dreamcast & Xbox image rolls uncontrollably

[CraigsBar]

Hi hardware peeps,


I have soldered up a Scart input RGB cable for my CM14. this works fine with a CPC scart lead used as input from any of my CPC's here. So I guess modern RGB signals need some modification for an anitique monitor.


Craig

[Bryce]

Have you tried adjusting V-hold?

Bryce.

[CraigsBar]

Yes, it does not go low enough to slow it down, just enough to see that the scroll is diagonal. interrestingly if I connect 3 sources though a switch box, as I discovered by accident. the CPC image is totally stable with the MUCH SLOWER diagonal rolling DC screen in the back ground. Most odd I guess.


I was hoping the addition of a battery, or resistor might be all that is needed. Let me see if a video helps explain the issue.



https://www.youtube.com/watch?v=Nm0Q_dZFeR4d.

[TotO]

The problem look that you are "mixing" the CPC and the Dreamcast video signal...

[Bryce]

Yes, it does not go low enough to slow it down, just enough to see that the scroll is diagonal. interrestingly if I connect 3 sources though a switch box, as I discovered by accident. the CPC image is totally stable with the MUCH SLOWER diagonal rolling DC screen in the back ground. Most odd I guess.


I was hoping the addition of a battery, or resistor might be all that is needed. Let me see if a video helps explain the issue.



https://www.youtube.com/watch?v=Nm0Q_dZFeR4d.

Wow, cool Demo, really smooth diagonal scrolling!

No adding batteries etc is for SCART connections, the CM14 won't need these. Looks like the Dreamcast is refreshing too fast for the CM14. Not much you can do about that without hacking the Dreamcast.

Bryce.

[chinnyhill10]

The problem look that you are "mixing" the CPC and the Dreamcast video signal...

Yep. How on earth he is managing to do that I have no idea, but the poor old CM14 looks like it has two signals going into it at once.

[gerald]

What pinout do you use for both dreamcast and CPC to the CM ?
The "mixed" version show that the dreamcast only has a little frequency drift compared to the CPC.
The CM14 should be able to sync with it unless the sync signal is too weak.
The CM14 get a dedicated composite sync signal from the CPC, which is in the 0-5V range. I doubt the dreamcast can provide anything else than a composite signal which is weaker (0-2V).

[CraigsBar]

Yep. How on earth he is managing to do that I have no idea, but the poor old CM14 looks like it has two signals going into it at once.

Well OK... I have a scart switch box that has 2 Scart inputs and outputs ONLY RGB, Sync and Audio. so I replaced the Scart plug on the Switchbox with an inline Din socket to plug it directly into the CM14 with 3 switchable scart inputs. I also then built a Plus to Scart cable and put the now spare Scart plug on the end of that, so I can plug the CPC into the switchbox and then run the signal to the CM14. When the plus is connected and turned on it's fine.


when the DC is connected to the second connection on the switchbox. And the switchbox switched to the DC socket then the screen rolls uncontrollably in a diagonal direction.
 
Finally if both the DC and Plus are turned on and the switchbox is switched to the Plus source then the DC background is shown. I guess that the switchbo is only disconnecting certain pins on the input and as such some of the signals leak through.


Oh well nevermind. I guess I still need my 21" crt TV to have the DC correctly displayed in 4x3 resolution. Shame that I really wanted rid of it LOL


Craig

[chinnyhill10]

You need to remove the switch box from the equation to ensure it's not causing the problems.


I use a manual switch box for my old kit. If I have the wrong combination is switched in I get exactly the symptoms you show in that monitor.


Also ensure the Dreamcast is in 50Hz mode.

[TotO]

To solve the problems, better to use a old 14" Sony Trinitron TV with SCART input, to plug all your systems on it.

[CraigsBar]

To solve the problems, better to use a old 14" Sony Trinitron TV with SCART input, to plug all your systems on it.

i tried to find a decent 14 inch TV a while ago. And found none at a reasonable price (Well none with Stereo Sound anyway) I guess I can put up with a the stretch look on the DC on the 32" Samsung Monitor it currently shares with my Mac.

[TotO]

The DC had better to be plugged with a VGA cable!

[CraigsBar]

The DC had better to be plugged with a VGA cable!

The display in question has no vga. Just hdmi scart component and composite.

[pelrun]

I had my dreamcast working with my ctm 644 years ago.  I remember having to put a buffer on the sync lines otherwise they were too low for the monitor to detect them.

[steve]

644 years ago.

 

[Gryzor]

Heheh good one

[CraigsBar]

I had my dreamcast working with my ctm 644 years ago.  I remember having to put a buffer on the sync lines otherwise they were too low for the monitor to detect them.

How is this done? I'd love to get this working.

[Bryce]

Just run it through a buffer IC inside the  computer just before the socket. Or a simple transistor setup.

Bryce.

[pelrun]

Here's the schematic I used:

[Bryce]

I had to do exactly the same on my Spectrum +3.

Bryce.

[MrBlueSky]

Sorry for this digging up, please tell me if the forum rules ask for a new topic instead of using a 4 years old one.
Nevertheless, all the above information helped me a lot in order to find a solution for a stable Dreamcast image on my CTM 644 Monitor. I simply cut the pin 10 of the Dreamcast AV Connector and connected a Sync Stryper to it. The LM1881 of the Sync Stryper is powered through the pin 5 of the AV Connector, and ground goes through pin 1.


I use an official (I guess) Dreamcast RGB cable which only has pins 1, 2, 3, 4, 7, 10, 14, 15, 16 available. The idea was to use pin 10 for a perfect Sync signal.

The result is great compared to previously. No more diagonal screening however the image is still very dark and bluish 
      

The defaults certainly come from the cable. Used with a CPC 1628 or a MegaDrive the colors of the monitor look great. However, on a CRT TV or a flat screen TV, the colors of the Dreamcast are totally normal...

My next try will be to disconnect all the capacitors used in the Dreamcast cable but if you have other suggestions in order to get a non-bluish and a lighter image please let me know.


My cable is slightly different from the picture above with Composit coming from pin 10 of the AV Connector and a capacitor right befor pin 8 of the SCART.

Here is a little video of the curent situation. Thank you in advance for your advices.


https://youtu.be/HJ3UaXbbbDY


[Edit]
The solution was to connect the ground 
If you connect the shielding of your Scart --> Amstrad cable on pin 5 of the female DIN and on pin 21 of the Scart you get a perfect image:

     

TL;DR
To cancel the diagonal scrolling: connect a sync Stryper to the Composite output of the DC
To get a perfect picture: don't forget to connect the shielding of your cable to the ground