Split luminance signal for BW image on CTM monitors

[Devilmarkus]

Hello,
I want to know, if it's possible to split the luminance signal from CPC into R,G and B signal to get a proper BW image on my CTM monitor?
Do I need resistors between R,G and B or would a simple connection work?
I want smooth BW images on my CTM monitor thus I could check GT65 / Grayscaled things.

[Bryce]

The simple answer is no, not really.

The complicated answer:
The greyscale image you're looking to make can't really be made from the RGB signals. In a greyscale image R = G = B, whereas the R G and B signal in a colour picture are continuously changing independent of each other. It would take more than a couple of resistors. Strapping R G and B together would make a Lum-like signal, but not the way it should be, for a start the colour signals are usually around 714mV peak to peak and a Lum signal is around 1.8V. So the picture would be pretty dark most of the time. Pure colours, ie: Pure blue would be the exact same value as pure red or green, so 27 different shades wouldn't be produced. Red text on a blue background would be invisible.

The real value needed is the HLS (Hue Luminance Saturation) value, which isn't possible from a simple resistor network.

Bryce.

[arnoldemu]

The CPC/Plus Lum signal *is* made by strapping r,g,b together with some resistors.

CPC: R uses 3.3K, G = 1K, B = 10K.
Plus: R uses 91Ohm, G = 1.2K, B = 330Ohm.

The Plus one is done for compatibility, so 4096 colours goes down to about 256 Luminance values.

EDIT: I doubt the scale is monotomic for either, and it certainly isn't the same as true B&W.

[Bryce]

Yes, it works in that direction of course, Strap RGB outputs together with resistors and you'll get more or less Lum, but that's not what Markus wants to do. He want's his CTM (which only has RGB inputs) to produce a greyscale picture. This would mean strapping the three RGB inputs of the CTM together and feeding them with the LUM signal from the CPC. Other than the colour discrepancies and darkish screen that I mentioned above, electrically it would be very unadvisable.

A small circuit with weighted resistors for each colour, being switched by transistors would be needed to get the correct values.

Bryce.

[Gryzor]

...next project?

[Bryce]

I doubt it. Otherwise my follow-up "reverse projects" could be: Convert a 6128 to 32K RAM, 2Mhz CPC Underclock and of course (as a tribute to Jeff), the HxCassette - Store all your dsk images on analogue cassettes Loading times have never been longer

Bryce.

[arnoldemu]

I doubt it. Otherwise my follow-up "reverse projects" could be: Convert a 6128 to 32K RAM, 2Mhz CPC Underclock and of course (as a tribute to Jeff), the HxCassette - Store all your dsk images on analogue cassettes Loading times have never been longer

Bryce.

[Devilmarkus]

Why should be a BW switch a reverse project?
I am sure it would make sense not just for me...

But HxCassette sounds suitable! JEFFFFFFFFFFFFFFFFFFFFF!!!!!!!!!!!!!!!

[Gryzor]

So wait - how would an HxC tape emulator work? Would it be connected as a floppy (as it is now), load audio data and serve them as disk data?

[Bryce]

Exactly, if you wait long enough.

@Markus: A B&W switch is reverse, because some very smart people have come up with an enhanced system since then called "Coloured TV" It's like B&W just more..... colourful, you should try it. It's amazing.

Bryce.

[MaV]

@Markus: A B&W switch is reverse, because some very smart people have come up with an enhanced system since then called "Coloured TV" It's like B&W just more..... colourful, you should try it. It's amazing.

Oh, come on now, Bryce. This is a retro type of forum. What could be more retro than trying to retro even the CPC? 

MaV

[Devilmarkus]

Next step is a Windows 7 module you can plug & pray to your CPC.
Performed by Bryce!

*whooopps* I forgot: No reverse engineering...

[DaDMaN]

Hello,
I want to know, if it's possible to split the luminance signal from CPC into R,G and B signal to get a proper BW image on my CTM monitor?
Do I need resistors between R,G and B or would a simple connection work?
I want smooth BW images on my CTM monitor thus I could check GT65 / Grayscaled things.

Emmmm... 

[Devilmarkus]

Emmmm... 

Don't be afraid! This has nothing to do with our last conversation.