Putting the SCART before the (Composite) horse - Torch Triple X - Part 12

One of the major issues of vintage/retro computing as a hobby is that a lot of our beloved equipment is just wearing out. When these machines were originally built I suspect that most people would not have expected them to still be in action some 30 or 40 years later, especially the great hulking CRT monitors that accompanied PCs and workstations. I say 'great hulking' but there were also some small and cute monitors out there, and one of those happens to be a Sony Trinitron based monitor used with the Torch Triple X (yes, I'm still banging on about the Torch).

 

It's a Torch Monitor - Sony Trinitron Tube

My example is still working fine although I have to run it at maximum brightness to see the image clearly. There's not really any 'bloom' that might appear in CRTs this age when I do this, but it is Trinitron (there's a great video on Trinitron and summary of its development here and here.). However, it does make me worry that if this monitor dies, I'll be left with no way to use the Torch. I do have a second compatible monitor which is actually bigger, but this one has, you might say, some issues.

 

First up, it's green. It's not supposed to be but some for reason it's forgotten about the RB bits of RGB. Second of all, it's been in the wars. I opened it up - I don't like opening CRTs - and found what can only be described as a 'bit of a scrape'. An entire corner of the PCB is missing and cables have been soldered in to try and fix the problem. 

 

 

Green. Yep, definitely green.

Still green...

How the heck did that happen!?

This may be a future project but, for now, I'm still worried about what I might do should the cute Torch monitor up and die on me.

Fortunately, the video output of the Torch is RGB. Yay! This means that I should be able to build a SCART cable for it without too many issues which means I should be able to use any SCART equipped TV with the Torch. Yay!

Torch Schematic Extract - Connector is 8 pin DIN

The SCART standard (also known as 'Peritel') originated in France and normally carries RGB signals along with video composite and audio. It can also carry luma/chroma signals too apparently. But that's not what we're concerned with here. The connectors are comically huge compared to current standards like HDMI, and the pins also seemed overly fragile given that they are flat and not rounded like you'd find in a DIN cable, but they were surprisingly robust. For some reason, the standard was hugely popular in Europe but not really anywhere else in the world..

 

 

SCART Type Connector - It's a bit bulky..

 

Anyway, of the 21 pins in the SCART standard connector we obviously need the red, green and blue to connect through from the Torch. We also need a ground which has to also link to the RGB ground pins on the SCART connector. That makes 7 pins in use. But we also need a composite 'sync' signal so that the TV will correctly display the frames of video. Without this, the display would be, at best, a rolling unreadable mess, or at worst, a black screen.

And that's a bit of a pain. You see the Torch doesn't output a composite sync signal. It actually outputs separate HSYNC (horizontal sync) and VSYNC (vertical sync) signals. One or other of these is not enough to correctly drive the SCART video. They need to be combined somehow..

This is an issue that has already been found by others on different systems. A chap called Chris had exactly the same problem with his Atari STE. He solved it by using a CD4077 integrated circuit with an XNOR logic gate to combine the HSYNC and VSYNC signals. His description is here. A lot of what I do is similar to what Chris does - up to a point.

My first problem is that I didn't have any CD4077 ICs. The main purpose of this chip is to provide the XNOR logic gate. I am lazy and impatient (and cheap) so I didn't want to have to order any CD4077s. But, as luck would have it, I did have several CD4011s which have four NAND gates. And, as we all know, what can make an XNOR gate? Anyone? Anyone? Bueller? Bueller?

That's right. As we all know very well, an XNOR gate can be made via the use of five NAND gates. Yay!

To accomplish this actually requires the use of two CD4011 integrated circuits. As mentioned, each chip has four NAND gates so, obviously, it needs two chips to use five gates. These need to be connected like this:

Professional and high quality diagram...

Composite signal emerges from pin 3 of IC2. 

Next problem. The chips need 5v to run but the Torch doesn't output any voltage on the video port. Fortunately, most TVs these days have at least one USB port on the back, so as a quick and dirty hack I decided to use an old USB cable to drive the chips. This would be connected to the prototype board I would be using for the chips, and which I would put at the TV end of the cable. This isn't ideal, but for now, it will do.

Now to the SCART connector. After digging through my box-o-bits I found an old connector from a previous project. It's not great as it was from a commercial (if somewhat cheap) cable and so the pins aren't really intended to be re-used with new wires. But it was still possible to solder wires to them so I went ahead and connected up extension wires from each of the RGB pins, the main ground pin and the composite input pin. This would make it easier to alter the connections if I needed to (Future me:- "It didn't."). I also linked the RGB ground and composite ground to the main ground pin with some of the coated fine copper wire I have.

 

 

I don't think I'll win any beauty awards..

 

All set. And....

Nothing. No signal at all. Just a resolutely black screen on the test TV. Darnit.

I did some more reading and checked that output signal from my CD4011s looked correct (it did). But then I noticed something. The peak to peak voltages of the RGB signals were only a few hundred millivolts. The csync signal I was pumping into the TV was nearly 5 volts. The SCART spec says that the RGB signals should be a maximum of 0.7v and the composite signal around 1 volt. Ooops.

To get down to this I added a voltage divider using a couple of resistors so that csync signal would be a slightly more sensible 1v.

And....

...nothing. Still a black screen.

I did wonder if I needed to apply some voltage to the 'Blanking Signal' on pin 16 but it appears that this is only required if you want the TV to switch to the SCART input automatically so I didn't change anything there.

I went back and checked the signal from the CD4011s and it looked good, correct amplitude and definitely looked like the expected csync signal. Very strange.

With limited time to work on this I started to wonder if I could just generate a composite signal but that's not really that easy when trying to combine the RGB signals. But Amstrad did it with the CPC. Except their composite was monochrome. Hmmm.

So, for now, I have abandoned the SCART plug. I took the csync signal and added another small resistor, then I took the green signal and added the same value resistor as on the csync. Then I joined them together to make a monochrome composite signal.

 

 

Now with phono plug! 

And...

..it works! The picture quality is TERRIBLE but that's more likely down to the rather 'heath-robinson' construction of the circuit. There's wires crossing all over the place and it's probably a small miracle it does anything. There's interference every time it does anything, and the picture is offset to the left, enough to chop off a bit of the screen, but not enough to make it unusable. Fortunately, for my purposes, the offset picture in black and white is more than enough to use the Torch and explore it more fully. 

It works! And it's terrible!

These pics really don't show how bad it is...

But at least it's usable!

The added advantage is that, following careful negotiations, Mrs Crashed has permitted the Torch to enter the dwelling area of Crashed Towers. That means I can play with the hard disk and floppy images without freezing my backside off in the unheated workshop area* (*workshop are may consist of a corner of the garage). And having a bigger display for this makes it much easier too, so expect a long winded, nerdy deep dive into UNIX SysV, specifically the Torch XXX version. 

But why did the SCART cable not work? I'm not sure to be honest. I have a suspicion, now that I've had time to do a bit more digging (i.e. reading the wikipedia article properly), that I needed to connect some voltage to pin 16. Without this, the TV would assume that any signal coming in would be composite and not RGB which would, I think, lead to the black screen I was seeing. 

When I get a bit more time (Ha! Good one!), I'll revisit the SCART cable and have another go so we can get the colour output working properly. If I'm feeling flush I might even order some CD4077s to simplify the circuit. But I am cheap. :)