VGA via SCART

[fallenangle]

Too complex to go into details but I recently discovered it was possible to make a VGA to RGB SCART lead by splicing the two types together with the addition of an apparently simple circuit. The circuit combines the separate VGA H and V sync output to give the composite sync RGB SCART uses.

It's this sync combiner circuit I'm interested in. There are two versions I've found:-

http://www.nexusuk.org/projects/vga2scart/circuit

http://gamesx.com/wiki/doku.php?id=av:dreamcast_480i-480p_scart_cable

The latter is derived from a larger DIY Dreamcast VGA Box circuit also featured there and found at several other places online too.

Has anyone here made a cable like this using either of the above designs or some other. There is a suggestion in one place that you can even get away with simply connecting the VGA H and V sync outputs together, grounding them with a resistor (no value given) and use that for the composite sync.

Sounds too easy to me but I know nothing about this really so I'm looking for practical advice.

[gunrock]

The first cable is for connecting PC's up to the TV (i.e for PVR type applications) and you have to configure the card (if it allows you to) to output an interlaced signal. Some PC video cards (or their drivers, at least), don't allow that.

The guy who built that cable, did so to get his MythTV PVR to output to the TV in 720x576 interlaced (which is full PAL resolution).

The other is to do with the dreamcast, and thus we already know that it can output an interlaced signal.

[Papercut]

Old Atari ST RGB Scart cables used to just twist the H and V sync wires together - sync combining not such a big deal.

As said above, from a PC this would work only with an originated interlaced signal.

For the DC cable there, the switch enables/disables VGA (ie. progressive) mode, which is why it works. It is quite a weird solution too - you could just use composite video in interlaced RGB mode, which gives composite sync as is - no need to combine sync signals for that. The difference here is that the GamesX guy must have something with a RGB Scart socket that accepts a progressive signal, hence the need for this strange cable.

In the dim and distant 480p CRT past, some Loewe sets would accept 480p via RGB scart in this way.

[fallenangle]

Thanks, I already understood all that what I'm interested in is if the simpler one will work instead of the other with a DC.

The second design is derived from, as I said, a larger combined DIY DC VGA box and RGB SCART converter circuit. The author appears to have made it to allow him to connect the VGA output of the DC to a RGB SCART to YUV transcoder connected (obviously) to the Component inputs of his prog scan capable TV.

As I understand it, in fact I can't think there'd be any point if this wasn't the case, this set up passes the DC's VGA 480p output unmodified except for the sync signal effectively allowing the DC to display at 480p but via Component.

In other words the sync combiner circuit plus RGB to YUV transcoder turns it into a VGA to YUV transcoder. VGA to YUV transcoders like the Key Digital and Audio Authority ones are, at least, three times the price of of a RGB to YUV transcoder so would be an ideal solution for me.

The point is, like author of the second (itself modified) circuit, I have a RGB to YUV transcoder which I've been using with my DC and been very happy with generally but it is only giving me 480i. My idea is to use it with one of these sync combiner circuits/modifed cables and an off the eBay shelf DC VGA Box to get 480p.

I can't think why this shouldn't work with either of the sync combiner circuits, after all they're just doing the same thing. But they are different; indeed one requires a 74LS86 logic gate IC, two transistors and is powered by 5v (from VGA pin 9). The other simpler one seems to just use a single transistor to do the same job.

So will the simpler one work in the set up I've proposed? Any advice/help/comments appreciated.

[Papercut]

As I understand it, in fact I can't think there'd be any point if this wasn't the case, this set up passes the DC's VGA 480p output unmodified except for the sync signal effectively allowing the DC to display at 480p but via Component.

As I say, some older CRTs could do progressive over RGB Scart. Was never standard though.

So will the simpler one work in the set up I've proposed? Any advice/help/comments appreciated.

Should work, if it worked for those people, only real concern would be a picky component transcoder not liking a slightly odd composite sync. I'd try the simpler one first, and see how you go...

[gunrock]

I guess the stupid man's question here, is what equipment are you trying to hook up to that will only accept component input and not VGA? A JVC CRT with component in?

[fallenangle]

Ay, that's exactly it. And if you know the JVC HV-32P37 TV you'll understand why I want to use the Component inputs rather the RGB SCART for gaming. For those that don't - via RGB SCART the TV defaults to composite when sent a 60Hz signal.

All my other consoles are connected via official Component leads it is just the DC that wasn't.

My preferred option was a very nicely priced Key Digital VGA-Component transcoder I found on eBay last year combining it with a DC VGA Box I was going to buy later. But the damned thing went missing in transit. Since then I've not found a sensibly priced replacement so I gave up looking and bought a good quality but surprisingly cheap RGB SCART-Component one instead.

Apart from a slight magenta tint I've managed to pretty much correct the PQ I get with the DC via this is excellent. No nasty artifacts or interference and when picture settings are adjusted to match I'd defy anybody to tell the difference between RGB SCART direct and via the RGB - Component transcoder. Soul Calibur looks particularly good.

But it still irks me I'm stuck with 480i when the DC can output 480p and my TV can display 480p. When I found the details of the VGA/SCART cable mod/sync combiner circuit and read that the guy was using it with a RGB SCART to YUV transcoder I was gobsmacked at the coincidence

Discovering the sync combiner circuit was relatively simple and potentially buildable even with my cruddy soldering skills it all seems too good to be true. That's why I thought it sensible to ask around to see if anyone else has built such a cable.

[fallenangle]

On a related question:

I've been examining and partially dismantling some of my DC RGB SCART cable collection with a view to converting one to VGA. The variations in DC plug internal design is one problem but it's the other differences though which I hope somebody here might explain.

Most of the SCART plugs seem to have 3 identical capacitors fitted, one for each colour channel but the values in each lead are different. The official Sega DC one has an additional very low value fitted too but I can't work out what to. (BTW the soldering and design of the official cable SCART plugs makes all my others look like they were made by amateurs).

With one cable the capacitors are all 100uF 10v, another 220uF 10v and the official one 220uF 6.3v. - so what determines the value and what is the optimum?

Same applies to the resistor connecting SCART pins 8 and 16. The official one is 270 Ohms, another is 100 Ohms and third 75 Ohms (the usual quoted value to use). So why the variation in these DC cables? IS it important?

Final question - what is the best way to strip the insulation from the wires in a SCART cable? My wire stripper simply won't handle such fine wires. Is there a purpose designed tool you can buy?

[Brats]

In the dim and distant 480p CRT past, some Loewe sets would accept 480p via RGB scart in this way.

That's not quite right. The special trick of the Loewe CRT sets were that they could accept a 480i YUV signal via SCART. This was useful in that you could have an imported DVD player or other device that could only output YUV and still get it to work with the Loewe.

They could accept a progressive VGA image, but only with the VGA add on board and via a standard VGA D-Sub cable. It was slightly flawed though as it was only in a couple of resolutions (one of which was non-standard) and in 4:3 ratio only.

AFAIK there is no set that could accept a progressive input via SCART if it were possible.

[fallenangle]

I think it was on the AVForums a couple of years ago that I came across one guy who was asking about a particular Sony CRT on sale via eBay (a German seller and why the buyer wanted info as the spec was all in German) that did do Component via SCART. Whether it was progressive capable I can't remember.

[Baseley09]

I think it was on the AVForums a couple of years ago that I came across one guy who was asking about a particular Sony CRT on sale via eBay (a German seller and why the buyer wanted info as the spec was all in German) that did do Component via SCART. Whether it was progressive capable I can't remember.

Could be the one that accepts 1080i over component over scart for Sonys HDVS format. There was one on ebay about a year ago tempting me! I'm interested to see how this lead turns out as like yourself Dreamcast is beyond gash on my TV.

[fallenangle]

Been practicing my soldering for this project. Whilst I don't think I'll have any problems with building the circuit or wiring up the SCART plug end of the cable I have misgivings about the DC plug end.

To start with I'm lucky to have a spare DC RGB SCART with a plug that a). has all the contacts and b). has solderable tongues from those contacts internally. All my others are either like the official one ie. only have the essential RGB SCART contacts meaning they can't be used for VGA or internally have simple contact pads near enough impossible for a DIYer to solder to.

But even with the usable plug I have the distance between the tongues for each pin is small (3 - 4 mm) and soldering cleanly is, with my eyesight and tools, a bit of a nightmare.

I'm not quite sure how to get round this. Perhaps rather than modding a DC RGB SCART cable as I wanted it would be better to buy a retail DC VGA lead, adding the sync combiner circuit and replacing the 15 D-Sub plug with a SCART.

Thoughts?

BTW I found the ideal tool to strip thin SCART cable internal wires: a pair or medical grade curved dentist's scissors I inherited from a relative. But any very sharp, thin bladed scissors would do the same job.