RGB SCART - ARGHHH

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GadgetUK164
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RGB SCART - ARGHHH

Post by GadgetUK164 » Sat Jun 27, 2020 6:32 pm

Hi,

I recently purchased an RGB SCART cable from Cool Novelties. I am finding the TV is outputting Composite (displays CVBS instead of RGB up top). Looking at how its wired, it has Composite going through an 82ohm resistor to the sync/comp pin on the SCART hood. RGB lines are attenuated via 150ohm resistors. The blanking pin has the VSYNC signal going directly to it.

Does everything sound correct there - it seems to match more or less what all the guides out there show, but I just don't get RGB. I am assuming the VSYNC signal is OK (since its used in various places internally), and RGB must be OK or Composite wouldn't look correct as it does. This leaves me wondering what on earth is going on. Pretty sure on my other ST I had it ouputting RGB with another cable a few years ago on the same TV, and I know the TV is fine. Any ideas or suggestions before I start scoping things to see if I can understand what's going on?
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Re: RGB SCART - ARGHHH

Post by exxos » Sat Jun 27, 2020 6:34 pm

composite is normally used for sync for RGB to stop the "jail bars". Though the TV shouldn't switch to composite, it should switch to RGB.
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Re: RGB SCART - ARGHHH

Post by GadgetUK164 » Sat Jun 27, 2020 6:37 pm

exxos wrote:
Sat Jun 27, 2020 6:34 pm
composite is normally used for sync for RGB to stop the "jail bars". Though the TV shouldn't switch to composite, it should switch to RGB.
Yeah, that's the problem, it's not switching to RGB - always just shows Composite. I've seen that behaviour before in other cables, and normally you can feed 5v through a resistor to lower it to say 2 to 3v, and normally that switches anything into RGB, provided red, green, blue and sync (composite) are there. I wonder if maybe red,green, or blue aren't getting through - having said that, pretty sure I've seen the TV still switch into RGB if a colour is missing. That just leaves the sync being wierd or something.

EDIT: It's an ST I've repaired btw. I did notice that VSYNC goes straight from GLUE out to the video socket - doesn't it need coupling?
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Re: RGB SCART - ARGHHH

Post by exxos » Sat Jun 27, 2020 7:30 pm

RGB switch might need 12V on the TV, or it doesn't support auto switching. 5V isn't enough. needs to be at least 9V IIRC.
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Re: RGB SCART - ARGHHH

Post by GadgetUK164 » Sat Jun 27, 2020 8:11 pm

exxos wrote:
Sat Jun 27, 2020 7:30 pm
RGB switch might need 12V on the TV, or it doesn't support auto switching. 5V isn't enough. needs to be at least 9V IIRC.
Tried that too! Still doesn't switch into RGB! It has to be something related to the Composite signal I think.

EDIT: Tried my other ST - that's the same! So at least I know it's not something specific to the one ST! I will scope things tomorrow and seee if I can work out what it is. I think its just revenge of the weird ST video output and my TV just not liking it. Sure I had this working in RGB with another cable years ago though =/
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Re: RGB SCART - ARGHHH

Post by stephen_usher » Sat Jun 27, 2020 10:14 pm

How many SCART inputs does the TV have? If multiple then many TVs only accept RGB on one of them, the other(s) will be composite/SVideo only.
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Re: RGB SCART - ARGHHH

Post by exxos » Sun Jun 28, 2020 1:51 am

GadgetUK164 wrote:
Sat Jun 27, 2020 8:11 pm
Tried that too! Still doesn't switch into RGB! It has to be something related to the Composite signal I think.

EDIT: Tried my other ST - that's the same! So at least I know it's not something specific to the one ST! I will scope things tomorrow and seee if I can work out what it is. I think its just revenge of the weird ST video output and my TV just not liking it. Sure I had this working in RGB with another cable years ago though =/
You could tie V&H sync together like in the STF schematics and use that for "composite" ... its only there for the sync anyway. Or you can just not use composite at all and just tie V&H sync to the scart, but image quality will likely suffer a bit that way.

But can you not manually set the input on the TV regardless of it switching to composite ?
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Re: RGB SCART - ARGHHH

Post by terriblefire » Sun Jun 28, 2020 9:59 am

Just incase you guys have missed this (which i doubt)

Code: Select all

Pin 16		
Blanking signal up
RGB-selection voltage up

0–0.4 V → composite
1–3 V → RGB
Spec says....

16 RGB switching (2V = RGB; (0V = Composite Video)

http://fr.meric.free.fr/Articles/articl ... nector.pdf
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Re: RGB SCART - ARGHHH

Post by exxos » Sun Jun 28, 2020 10:17 am

http://martin.hinner.info/vga/scart.html

Audio/RGB switch / 16:9
High (9.5-12V) AVmode
Low (0-2V) TVmode

I've always had to drive at 12v.
Many TV sets have 2 SCART sockets. One is usually loke table 1 and the other like table 2, and pin 20 switchable from composite to S-Video luminance.
The first can switch from a composite input to RGB input. The second can switch from a composite input to an S-Video input, pin 20 being either composite in or luminance in. Usually the second socket outputs a selectable composite signal on pin 19.
Some scart sockets are wired differently. There used to be several scart cables back in the day depending on TV manufacture.
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Re: RGB SCART - ARGHHH

Post by stephen_usher » Sun Jun 28, 2020 1:26 pm

For my Sinclair QL I use 3V, two AAA batteries in a holder to switch. That's been reliable on all the TVs I've used.

@exxos those voltages/specifications are for Pin 8, not Pin 16, and not quite correct.

0V-2V Off
5V-8V 16:9 aspect ratio
9.5V-12V 4:3 aspect ratio
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

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