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General Category => Amstrad CPC hardware => Topic started by: Devilmarkus on 12:00, 01 April 11

Title: Split luminance signal for BW image on CTM monitors
Post by: Devilmarkus on 12:00, 01 April 11
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.
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Bryce on 12:24, 01 April 11
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.
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: arnoldemu on 12:28, 01 April 11
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.

Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Bryce on 12:47, 01 April 11
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.
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Gryzor on 15:08, 01 April 11
...next project? :D
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Bryce on 15:25, 01 April 11
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 :D Loading times have never been longer :D

Bryce.
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: arnoldemu on 15:47, 01 April 11
Quote from: Bryce on 15:25, 01 April 11
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 :D Loading times have never been longer :D

Bryce.
:laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh:
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Devilmarkus on 16:52, 01 April 11
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!!!!!!!!!!!!!!! :D
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Gryzor on 18:37, 01 April 11
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?
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Bryce on 19:14, 01 April 11
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.
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: MaV on 20:39, 01 April 11
Quote from: Bryce@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.

:laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh: :laugh:

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

MaV
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Devilmarkus on 20:40, 01 April 11
Next step is a Windows 7 module you can plug & pray to your CPC.
Performed by Bryce!

*whooopps* I forgot: No reverse engineering... :P
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: DaDMaN on 13:01, 02 April 11
Quote from: Devilmarkus on 12:00, 01 April 11
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...  :o
Title: Re: Split luminance signal for BW image on CTM monitors
Post by: Devilmarkus on 13:06, 02 April 11
Quote from: DaDMaN on 13:01, 02 April 11

Emmmm...  :o

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