Using your CTM monitor with other devices

[darkhalf]

This hasn't really been clearly documented anywhere, so I found some bits and pieces and managed to connect my CTM640 to my RGB PI module over the weekend.

Details and references:

VGA uses split Horizontal and Vertical sync signals. The best way I found to combine them into a single sync signal was via a transistor, as detailed here:
https://www.retrorgb.com/building-a-passive-sync-combiner.html

The CTM also utilises a luminance signal. Aronldemu nailed the resistor values here, combining VGA RGB outputs via resistors into a single signal 
https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/split-luminance-signal-for-bw-image-on-ctm-monitors/msg21571/#msg21571

Final circuit diagram below:

[darkhalf]

Pics attached of final results

[zhulien]

I used it with my Amstrad TV receiver.  Works but not that great.

[McArti0]

Pin 6 of the color CRT is not used. resistors 1k, 3.3k and 10k can be reduced.

[Bryce]

Pin 6 of the color CRT is not used. resistors 1k, 3.3k and 10k can be reduced.

He's done that to allow monochrome signals to be connected. It would also mean the circuit works with a GT64/65 too.

Bryce.

[McArti0]

He's done that to allow monochrome signals to be connected. It would also mean the circuit works with a GT64/65 too.

Then it has the wrong impedance without an amplifying transistor like in CPC.

[eto]

The CTM also utilises a luminance signal. Aronldemu nailed the resistor values here, combining VGA RGB outputs via resistors into a single signal 
https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/split-luminance-signal-for-bw-image-on-ctm-monitors/msg21571/#msg21571

The CTM does not need the luminance signal, only the GT64/GT65 and the MM12 would use this. So that part of your circuit would be "optional" for those that will use it with a green screen. 

This hasn't really been clearly documented anywhere,

Would you consider documenting this on the Wiki itself? If it stays here in the forum it might get buried over time. 


Btw: Do you have more pictures of it in action? I would be interested how good the quality is for different use cases. 

[McArti0]

https://www.cpcwiki.eu/index.php/Service_Manuals

Monitor diagrams are in the service manuals.  You can clearly see there that the color monitor does not use pin6. Lum. However, the green monitor treats RGB signals with 100ohm resistors to ground

[darkhalf]

The CTM does not need the luminance signal, only the GT64/GT65 and the MM12 would use this.

Okay I didn't go as far as digging into the service manuals. Only saw the post about the Luminance signal and wired those lines. Good to know. I've tried it on the GT64 and it works, but very dark on the main menu. When the C64 emulation runs on there the GT64 has a sync issue (was fine on the CTM640)

Then it has the wrong impedance without an amplifying transistor like in CPC.

Okay that sounds like the reason for this. How would you suggest this should be implemented? @McArti0 

Would you consider documenting this on the Wiki itself? If it stays here in the forum it might get buried over time. 

Yeah I can. Let me know a suitable place and can upload it

Btw: Do you have more pictures of it in action? I would be interested how good the quality is for different use cases.

Sure. What do you want to see? 

[McArti0]

Then it has the wrong impedance without an amplifying transistor like in CPC.

Okay that sounds like the reason for this. How would you suggest this should be implemented? @McArti0

https://www.cpcwiki.eu/index.php/Schematics

[darkhalf]

Thanks. I've done version 2 based from this (not tested).

Have omitted the sync pull down to luminance line however, as the displayed worked with the resistors in the current circuit. Adding the 5V and transistor to this cable is a bit of a pain (can use a 5V socket and power directly from the GT itself)

[d_kef]

You might want to remove the ground wire from luminance (pin 6) though and connect it to... you know... ground (pin 5). 

d_kef

[McArti0]

@darkhalf 
 You don't have sync connected yet. Besides, VGA has signal levels of 0.7v or 1.1v which will be another problem.

[darkhalf]

VGA has signal levels of 0.7v or 1.1v which will be another problem.

The primary problem space for this cable was the RGB Pi. So the transistor for sync works fine with the signals from that. As for the sync out of the transistor, I'm not sure if that is required for the luminance (via 680 ohm) for this application. We know you have it shown on the Amstrad CPC464 schematic. I've added to this version anyway. Values may need adjustment

I've confirmed as per the images for the RGB Pi output that it works without needing sync pulldown to luminance on the GT64

[McArti0]

That's right. GT64 receives synchronization separately. I slightly forgot that the cable is only for Amstrad monitors.  
You could check how it works With an interlaced signal? 312/313 lines? GT65 with Amstrad works terribly.  Is CTM 644 better?