RGBI to SCART RGB for popular line doublers

[matty72]

Help, need a correctly wired 6128 Scart cable cable to suit popular line doublers (OSSC or Retro Tink 2xSCART)

My issue is that I can't find a Scart cable with appropriate circuitry to convert the RGBI+S (4 bit color) out of a 6128 to RGBS, I.e. using the LUM pin 6 to change intensity of the RGB color so that 2 shades of 8 colors (16 color) results.
I don't believe an active (powered) cable is required for the stated line doublers as they both expect RGB input.

My guess is that the inner circuitry could simply be a voltage divider between LUM pin and each RGB line, I'd like your input.
I'm not aware of Amstrad modifying any of the colors as Commadore did (yellow became brown), please inform me otherwise.
Can anyone point me to a pre-made cable or a decent/correct wiring diagram?

[llopis]

For what it's worth, I'm able to feed the output of a CPC using a regular SCART cable into OSSC and it accepts the signal perfectly fine. I didn't have to convert it to digital RGB plus two bits of luminance.

[pelrun]

RGBI+S (4 bit color)

No, there's no conversion needed, because the CPC monitor output is *not* digital colour at all! R, G and B are full 3-level analogue, and LUM is just the (unused) monochrome video signal for the greenscreen monitor. Since LUM also has the composite sync signals present, it can be used directly for that purpose if your input isn't too fussy. If it is fussy, you'll just need to feed it through a sync stripper.

[Bryce]

Help, need a correctly wired 6128 Scart cable cable to suit popular line doublers (OSSC or Retro Tink 2xSCART)

My issue is that I can't find a Scart cable with appropriate circuitry to convert the RGBI+S (4 bit color) out of a 6128 to RGBS, I.e. using the LUM pin 6 to change intensity of the RGB color so that 2 shades of 8 colors (16 color) results.
I don't believe an active (powered) cable is required for the stated line doublers as they both expect RGB input.

My guess is that the inner circuitry could simply be a voltage divider between LUM pin and each RGB line, I'd like your input.
I'm not aware of Amstrad modifying any of the colors as Commadore did (yellow became brown), please inform me otherwise.
Can anyone point me to a pre-made cable or a decent/correct wiring diagram?

The CPC LUM signal is derived from the R,G and B signals, so tying this signal back to the RGB signals externally would just be providing endless positive feedback (essentially going in circles) and every signal would instantly saturate resulting in a white screen. Even if it did work (by limiting the levels or whatever) you'd end up with some really wierd colours that would be far from what a CPC should look like.
Why would you want to do this? The RGB values already represent exactly what the colour mixture should look like.

Bryce.

[ComSoft6128]

I'm using scart cables from Cool Novelties for a 6128 and a 6128 Plus to the OSSC and they work fine for most uses......BUT (yes there always is one) - some demos and games will NOT display correctly using this setup.

[Bryce]

I'm using scart cables from Cool Novelties for a 6128 and a 6128 Plus to the OSSC and they work fine for most uses......BUT (yes there always is one) - some demos and games will NOT display correctly using this setup.

That's due to the OSSC not being able to change frequencies mid-screen which many demos use for effects. It has nothing to do with the cable.

Bryce.

[ComSoft6128]

I didn't think it was the cable but I should have been clear on that point

[matty72]

Ok, so now I'm puzzled.


My understanding was that the CPC6128, like many other 80s PCs, output CGA color, or more accurately RGBI which TMK is the analog version of CGA (still 4 bit output).  There is no mention in CGA explanations of the "LUM signal is derived from the R,G and B signals" inside or outside of an output device, rather RGBI being discrete signals from a device (4 bit) that a receiving device must decode to form all 16 available colors - there are a few cga2rgb convertors that this, all for the commadore (C128 specifically I think) - I've seen no such attempt for the CPC as a device or in video cable.  Have I totally misuderstood the CPCWiki? If so, exactly how does 16 colors come out of 3 bits (RGB)? Why does LUM pin exist if it's not to output the last bit of CGA/RGBI?

It occurs to me that 3 bit output (ignoring LUM) would look just fine in most cases considering most programs dont use all 16 colors, Demos sure would though.

Thanks for info on OSSC, i thought it'd be quicker.

[pelrun]

To reiterate, the CPC outputs *analog* RGB video, not RGBI. It's also not "3-bit", it's three *level* - low/mid/high. That's why the palette is 27 colours (3 cubed) and not a power of 2.

Anything that claims the CPC is CGA or digital colour is completely erroneous. It shouldn't be referred to as such anywhere in the CPCWiki; if so it needs to be changed.

[robcfg]

As for LUM, it's just the luminance signal for use on monochrome monitors.

[matty72]

Thanks guys, you've set me straight and it's all good news for buying a Scart cable for use with popupular line doublers, I.e. simple analog RGB, no conversion required