Spectrum Computing

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?

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.

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 how about this terrifying monster. GRONF GRONF !

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?

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.

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.

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.

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...

Don’t know if this clears things up or muddies them even more for you.

The second one for me unfortunately
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

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.

Cos 15 colours is all you need girlfriend!

Could they have chosen orange, or brown, instead of magenta?

And should they have?!

R-Tape wrote: ↑Tue Jan 22, 2019 3:25 pm Could they have chosen orange, or brown, instead of magenta?

And should they have?!

No and no.

R-Tape wrote: ↑Tue Jan 22, 2019 3:25 pm Could they have chosen orange, or brown, instead of magenta?

And should they have?!

All the available browns had been used up by the C64.

You could do something like, make the BRIGHT bit leave red bright and only darken the green and blue channels, which would give you different colours where green or blue were involved (e.g. yellow would darken to orange, white to pink). But just like you don't get dark black, so you then wouldn't get dark red, either. So you'd only have 14 colours.

Ralf wrote: ↑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.

Again, because it's not really a digital circuit. Zero isn't actually quite zero, it's just close enough to mostly be indistinguishable. Some hardware configurations will show the difference (though not usually as pronounced as some emulators seemed to suggest). YMMV etc

Bear in mind also that there are different computers with different video hardware all called ZX Spectrum.
The references to bright black being grey that I've seen are in relation to using an RGB monitor with the Spectrum 128. That machine* puts separate RGB and Bright signals out the RGB port, and how a monitor deals with that is down to the monitor's circuitry.

*The English spec +2 is supposed to be the same, but as far as I remember they all have the "optional" components fitted making the output analogue RGB.

The real answer is....

Due to the limitations of the design.

Sinclair in it’s promotional brochure for the original 16K and 48K (rubber key) machines only described it as having “full colour - 8 colours each for foreground, background and border, plus flashing and brightness intensity control”. Even in the table which compared it to other competitors machines (BBC A, VIC 20, ATARI 400, TI 99/4A) , they list the ZX Spectrum as having 8 colours... [There is however a separate row for a tick for BRIGHT or equivalent].

“The eight colours available are black, blue, red, magenta, green, cyan, yellow and white”.

Why these colours? Simple. Television uses additive colours (unlike print, which uses subtractive colours). In a colour CRT there are three electron guns which fire electrons at the coloured phosphors on the inside of the screen. There are three types of phosphors. One produces red light. One produces green light and the remaining one produces blue light. Each electron gun fires at one type of phosphor, hence the guns are called red, green and blue.

So that’s the first three colours. With all the guns off, you get black (well actually dark grey, as most people don’t watch in complete darkness). So we are now up to four colours.

Now if you switch on two electron guns, two lots of phosphors light up. So the combinations are:
Red and green - the colours add together (as far as the human eye is concerned) and show as yellow.
Red and blue - shows as magenta.
Green and blue - shows as cyan.

Turn on all three guns, and red, green and blue colours add together, producing white. So that’s a total of eight colours:



As explained above, the BRIGHT control increases the signal strength going to the electron guns, hence increasing the intensity. But if all the guns are already switched off (for black), there is no signal to boost... (although in some instances, there may be a small signal, as in practice it’s possible to keep all three electron guns slightly switched on).

To produce different colours would have needed more complex circuitry, but Sinclair’s aim was simply to produce a computer that could produce a high resolution colour tv picture at the lowest possible price. So a more complex circuit was out of the question. And the hardware designers were limited in the time they had to develop the machine anyway. As Sinclair wanted it ready to sell as soon as possible...

Mark

equinox wrote: ↑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 how about this terrifying monster. GRONF GRONF !

Ahh, sorry

The real reason is that aliens nicked the sixteenth colour...

Mark