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Here is a proof of principle. It works and I think it is a bit better than Composite already, but my screenshots are not really convincing when it comes to quality. Better tuning of components and shielding will probably improve it. I am not so sure my old Acer AL1731 monitor is any good at either composite nor S-video, so it would be interesting to test on another monitor.

Thanks to my cellular phone camera the reference Composite screenshot looks almost better... You are maybe able to see a "snake" pattern on the left border of the composite image which is not on the S-video image. The difference is much clearer in real because the pattern is flickering, on both the left and right border.

This is a Dragon 32, main board MKII issue 5 ("Siemens").

Wonderful rat's nest prototyping! I take the S-video Luma signal from the video output of the main board (pin 6 and 5 on the connector going to the power board).

For the S-video Chroma signal, a buffer stage seemed to be needed. I made a simple emitter follower (~unity gain, low output impedance):

I have bent pin 13 of the LM1889 so that it doesn't touch the socket, thus the colour signal is not fed to the normal video output any longer. From pin 13 I have a 56 pF capacitor (C1) going to the base of a C1740 transistor, which is biased by a 56 kOhm resistor (R1) to 5V, where also the collector is connected (tapped from pin 14 on the LM1889 on the photo). On the emitter, a 220 Ohm resistor (R2) goes to ground while a 0.1 uF capacitor (C2) in series with a 56 Ohm resistor (R3) goes to the S-video Chroma input. The components are a bit random, just inspired by similar circuits and what I had lying around. The transistor is a general purpose NPN, you can use for instance a 2N2222 or 2N3904 instead.

Schematic: This is what my circuit looks like:

The jagged border between colours in the composite image is clearly visible here: They are gone on the S-video image but there is some other noise (I think there is a Moire pattern caused by the camera making it look worse than it is):

Oooh. Looking very promising!

I like this. Thanks for sharing, Tormod!

I'll take some piccies and post later this evening......

I'm working on an RGB interface for the Acorn Atom (which also uses the 6847), I've got that pretty much working so I tried putting one of the boards in a Dragon, as I had one with a socketed 6847 and have that working pretty well. It decodes the colour and latches the pixels s that you get nice sharp text & graphics.

Only disadvantage is that most Dragons and CoCos have the 6847 soldered direct to the motherboard

But I like the simple svideo solution of a transistor and a couple of resistors and caps

Cheers.

Phill.

Cool. I'll be back with a second episode on this, probably next year.

I would like to buy stuff from Pollin and replace the Dragon PSU with the VOLTEK SPEC7188B, see: viewtopic.php?f=8&t=4967&start=10#p12294
So i need a other way to output to TFT... This S-Video Output seems to be a good way. A enhancement...

tormod wrote:I have bent pin 13 of the LM1889 so that it doesn't touch the socket, thus the colour signal is not fed to the normal video output any longer. From pin 13 I have a 56 pF capacitor (C1) going to the base of a C1740 transistor, which is biased by a 56 kOhm resistor (R1) to 5V, where also the collector is connected (tapped from pin 14 on the LM1889 on the photo). On the emitter, a 220 Ohm resistor (R2) goes to ground while a 0.1 uF capacitor (C2) in series with a 56 Ohm resistor (R3) goes to the S-video Chroma input. The components are a bit random, just inspired by similar circuits and what I had lying around. The transistor is a general purpose NPN, you can use for instance a 2N2222 or 2N3904 instead.

Any news here? Are these component values well matched?

Is this a permanent solution?

Any update on this or plans for to sell a small pcb we can fit ourselves?? I'd love to upgrade my D32, even the composite out has a slight but ever so annoying 'ghosting' on it.

Chris.

Excellent!. Looks great

I will definitely be coming back to this, but as I said 2 (!) years ago, next year... Too many interesting things came up in between. However with the smaller fonts on the FUZIX graphics mode console, this has become more important again

Note also that "Zippster" on the CoCo list is working on a 6847->VGA solution. It might be a bit invasive though, requiring a socketed 6847.

I was thinking of making a PCB, possibly with connectors so that it can fit to the motherboard/power board flat cable, have an S-video socket, and also its own power regulators to reduce noise and interferences. For designing such a kit to share, it would be nice to know what is the sweet spot regarding invasiveness and installation difficulty (and complexity and cost, I guess). Would many people strongly prefer a solderless option? Or just a PCB and DIY? And personally I'd avoid any non-reversible modifications of the original hardware (other than the warranty seals ).