Author Topic: Connecting another computer to a GT65 CPC monochrome monitor ?  (Read 1671 times)

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Offline gotcha

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Hello,

I want to connect a Philips/Schneider VG5000 (https://en.wikipedia.org/wiki/Philips_VG5000) to one of my unused CPC monitors: the GT65. Looking at the service manual, we can see that the CPC generates multiple signals : separated R G B, separated sync and something called 'luminance' that is a actually a composite signal combining R, G, B and sync.

I think that this 'luminance' signal could itself be sufficient to display an image, but amstrad requires this extra separated 'synchro' signal even in the case of a monochrome monitor like the GT65 (this is confirmed the service manual information).

From what I understood, with other computers (outputing on SCART plugs for example), the so called 'video/synchro' signal is usually similar to the Amstrad 'luminance" signal (combining luminance information and sync), meaning that non CPC computers may not deliver this standalone sync signal.

I'm then wondering how the Amstrac CPC monitors and in particular the GT65 could be used with other computers. It seems to have been done often, at least for the color CTM644: http://www.cpcwiki.eu/index.php/Monitor_compatible).

Thanks!
« Last Edit: 13:04, 27 September 18 by gotcha »

Offline Bryce

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #1 on: 10:30, 27 September 18 »
The LUM input on a GT65 can be more or less used like a composite input. Amstrad used the seperate Sync signal mainly for the colour monitors but also used it to enhance the sharpness on the monochrome monitors although the sync signal is also superimposed onto the LUM signal already. Try connecting the composite out of the other computer just to LUM and GND. It should give you an ok picture.

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Offline gotcha

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #2 on: 11:45, 27 September 18 »
Try connecting the composite out of the other computer just to LUM and GND. It should give you an ok picture.
I just tried. I can see the VG5000 boot screen but there is a synchronization issue since the image is totally unstable. I tried to play with the monitor setting, but it doesn't change much: I couldn't get a stable image.
My guess is that the separated sync signal maybe actually needed.
« Last Edit: 11:46, 27 September 18 by gotcha »

Offline gotcha

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #3 on: 11:56, 27 September 18 »
Here is what I get:


Offline Bryce

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #4 on: 12:43, 27 September 18 »
Then the integrated sync pulse on the VG Video signal isn't defined enough to trigger the monitor. You could try connecting the video to the sync pin of the monitor too and see if it improves.

Bryce.
 

Offline gotcha

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #5 on: 13:03, 27 September 18 »
I did connect the video signal in addition to the sync input of the monitor and it indeed works much better.  ;D
I get a stable image this way and I get black-on-green boot screen that is consistant with the VG5000 black-on-light-blue boot screen. Thanks !

Side question: do you know if we can easily invert the luminance information (getting green-on-black instead of black-on-green) ? Since the luminance and sync are mixed in the video signal, the conversion may not be trivial.
« Last Edit: 13:21, 27 September 18 by gotcha »

Offline Bryce

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #6 on: 13:17, 27 September 18 »
Hmmm, the problem with doing that is that you would have to strip the integrated sync signal out of the video signal, otherwise the inverted sync signal would cancel out the independent sync signal. It's not impossible, there's even a chip made specifically to do this (the LM1881 from Texas instruments) but it would require some circuitry and a power source.

Alternatively, you could remove resistor 3307 (or make it switchable) inside the VG5000 (this is where the Sync gets added to the video signal), invert the video and then add the sync back in.

So it's not impossible, just complicated, more than just some simple connections.

Bryce.

Offline Dagger

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #7 on: 15:35, 27 September 18 »
The monitors can be connected to other computers as I have a cable so that I can connect the Amiga to it. A few years ago I had an Amiga 2000 connected. Forgot about the lead until I saw this thread. Will have to dig it out now.
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Offline Bryce

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #8 on: 16:03, 27 September 18 »
Yes, but that's most likely an RGB cable to connect to a colour monitor. The discussion here is about feeding a monochrome GT65 with a composite video signal (and possibly inverting it). Of course, you could connect the composite output of an Amiga to a GT65 too as described above, but you loose the colours.


Bryce.

Offline VincentGR

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #9 on: 16:32, 27 September 18 »
I was trying to connect my Pi0 with no success, I'll must try again.
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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #10 on: 09:27, 30 September 18 »
I remember I had my Amiga 500 on the GT 64 at some point in the early 1990s, because I wanted something eye friendly for word processing (I also had a 1084 though... who wants to play games in green). And that was an RGB cable btw, not composite.But I remember it required soldering a couple resistors.

Offline Bryce

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #11 on: 12:56, 30 September 18 »
That cable was probably just making the same mix that the CPC does internally by mixing the 3 colour signals (via resistors) and adding sync.

Bryce.

Offline Scarlettkitten

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Re: Connecting another computer to a GT65 CPC monochrome monitor ?
« Reply #12 on: 18:07, 30 September 18 »
This is interesting, I'd also like to feed my GT65 with a composite signal at some point.
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Offline gotcha

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I spent some time looking at the video signal of a 6128 connected to the GT65 monitor.

My idea would be to reproduce a composite signal with the same levels, when connecting another computers.
For example, my VG5000 computer gives an image that I find too bright on the GT65, and I assume the composite signal is not exactly what the GT65 expects.
When the 6128 is connected to the GT65, I get the following levels

Composite signal ('Luma' DIN output):
  • Sync area between 80mv and 840mv
  • Luminance area between 840mv and 1.6v
Synchro signal ('Sync' DIN output)
  • Between 0 and 4.8v
I now know my target for the VG5000 - GT65 connection :)

Offline Bryce

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They are pretty close to the Composite standard. The separate sync signal is TTL so 0V/4.8V is to be expected. The LUM signal is relatively high at 1.6V, so if your VG5000 is even higher then you need to stick a resistor or a capacitor in there to bring it down a notch or two.

Bryce.

Offline gotcha

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They are pretty close to the Composite standard. The separate sync signal is TTL so 0V/4.8V is to be expected. The LUM signal is relatively high at 1.6V, so if your VG5000 is even higher then you need to stick a resistor or a capacitor in there to bring it down a notch or two.
I may indeed need a capacitor because as I remember, when I looked at the Luma output of the VG5000 (when non connected to any monitor), it was ranging from 2.5v, to 3.6v. In the same situation, the CPC generates a signal that starts from 0v.  So I guess the output of the VG5000 needs to be shifted.

I'll do more experimentations as soon as I receive the DIN connectors I ordered for properly linking the VG5000 and the GT65.

Offline VincentGR

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I'm curious, did you get it working?
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