Why 15 colours?

Y'know, other stuff, Sinclair related.
equinox
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Why 15 colours?

Post by equinox » Tue Jan 22, 2019 1:08 am

I'm certain this must have been discussed before, but I did spend a little time searching for "15 colours" "16 colours" "two blacks" and I couldn't dig it up. So, if this is a sucky question, spank me and redirect me. -- I was just wondering why the Spectrum got 15 and not 16 colours. Obviously there are 16 colour "slots" for binary reasons but the two blacks are the same. Why isn't PAPER 0: BRIGHT 1 a sort of dark grey?

I would really like to imagine that this is because they realised "this thing has got the colour clash from hell, and most of the games coming out right now [1982] are Space Invaders and Galaxians clones, so let's give them a fighting chance and let them use 14 colours for aliens without having the aliens travelling in mysterious square grey 'space bubbles'." -- But at the time they designed the thing, they probably thought it would be used for business and not games (COUGH OOPS COUGH!) so that's a bollocks line of argument. I just wanted to talk about grey aliens (Fox Mulder style).

What's the real reason?
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AndyC
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Re: Why 15 colours?

Post by AndyC » Tue Jan 22, 2019 2:48 am

The BRIGHT bit controls analogue circuitry that adjusts the resistance in the colour generation circuitry, which in turn has the effect of altering the brightness of the red, green and blue signals. Since these are all off when producing black, the net effect is simply that black and bright black are the same colour.
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equinox
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Re: Why 15 colours?

Post by equinox » Tue Jan 22, 2019 2:56 am

No that's sh*t. Someone please give a proper explanation that involves Galaxians.

QBASIC also had 16 colours and it had a light black that was grey. You are wrong and a bad human being.

Should I put a smiley in here :mrgreen: how about this terrifying monster. GRONF GRONF !
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equinox
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Re: Why 15 colours?

Post by equinox » Tue Jan 22, 2019 3:02 am

HEY! Could we say it's a bit like the dim colours (DIM should be a keyword) are 0x880000, 0x008800, etc. and the BRIGHT has the effect of *doubling* the colour? So 88 becomes FF but you can't turn a pure zero into a -- yeah? YEAH? YEAH? Did I understand something for once?
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Joefish
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Re: Why 15 colours?

Post by Joefish » Tue Jan 22, 2019 8:53 am

It means the BRIGHT bit acts like a physical dimmer switch. If R, G, or B is switched ON then it gets dimmed. If none of them are ON, then the dimmer has no effect. You can't 'fade down' a 0 volt signal.
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Ralf
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Re: Why 15 colours?

Post by Ralf » Tue Jan 22, 2019 9:56 am

Actually I've heard that Spectrum connected to some chosen monitors would produce grey when "bright black" is used.

Also I believe some emulators make this distinction, although it looks terribly as everybody assumed that brigh and dark black are the same. If I remember correctly that was a problem with SpecEmu.
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Ast A. Moore
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Re: Why 15 colours?

Post by Ast A. Moore » Tue Jan 22, 2019 11:38 am

Each color output is connected to the brightness output via an AND gate. If a color signal is high (1) and brightness is also high (1), the output is high (1). If a color signal is low (0), then it doesn’t matter what the brightness signal level is:

R ∧ B = 1
R ∧ B = 0

Another way of thinking about it is in purely analog terms. Imagine the red signal is at 5V and the brightness signal is also at 5V. There’s a diode and a resistor between the brightness signal and a color signal, so no current will flow through the diode, thus the signal will remain at 5V. If the brightness signal is at 0V, its end becomes negative in relation to the color signal, it’ll sink some current from the color signal and the output voltage will drop to, say, 2.5V—half the normal signal, producing a lower-intensity (dimmer) output.

Now, if both signals are at 0V, there will be nothing to sink from the color signal, and the output will be 0V. If the color signal is at 0V and the brightness signal is at 5V, the diode will prevent it from affecting the output, so it will still remain at 0V.

Don’t know if this clears things up or muddies them even more for you. :D
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equinox
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Re: Why 15 colours?

Post by equinox » Tue Jan 22, 2019 12:09 pm

Mmmm. Sorta. I know that zero AND anything is zero, and so on. Unfortunately don't know anything about electronics (I'll get there some day), only abstract logic gates in the sky. Thanks for everyone's responses. They were... ILLUMINATING...
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Ralf
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Re: Why 15 colours?

Post by Ralf » Tue Jan 22, 2019 12:50 pm

Don’t know if this clears things up or muddies them even more for you.
The second one for me unfortunately :D
There are software guys and hardware guys. I am a software guy. You should always talk to software guy like hardware didn't exist at all ;)

Ill try my own explanation.

Bright white in Spectrum is (255,255,255) in RGB colour system. Dark white is lets say (200,200,200)
Bright red in Spectrum is (255,0,0) in RGB colour system. Dark red is (200,0,0).

So you replace 255 to 200 when you go from bright to dark. But black is already (0,0,0). So there is nothing to replace and
dark and bright black would be the same ;)
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Re: Why 15 colours?

Post by druellan » Tue Jan 22, 2019 1:24 pm

Actually I've heard that Spectrum connected to some chosen monitors would produce grey when "bright black" is used.
I remember the bright attribute for black was visible on my Timex Sinclair 2068, that was pretty odd on some games like Astro Blaster. Perhaps someone can explain if that can be related to the NTSC or the ULA on the Timex.
I'm not sure if there is an emulator that can simulate that, since most of them with Timex emulation fail to display the particular Timex palette.
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