768 shades of grey?

[blackevilweredragon]

Do I understand this right?

I hooked up my first ever PC, an IBM Model 25, and it sports an MCGA graphics chip..  It does 256 colors when in 320x200...

But, my model has the monochrome monitor fitted in it (well, greyscale), but internally, it "thinks" it's color..  Like, it will say "this box is blue", but it will seem grey..  And if I slap a color CRT inside it, it will be blue...

So, assuming this monochrome takes the RGB values and combines it to one gun, does that mean I can pull off 768 shades of grey?

Or am I confused about how they combine?  (both CRTs use the SAME exact connector, and monochrome CRT uses the R, G, and B lines..

http://www.spymac.com/upload/2007/01/19/jFxDiftXGK.jpg

(it's the computer on the right)

EDIT:  Oh, and why is it, when it's in Windows, things look grainy, like this:  http://blackevilweredragon.spymac.com/toph.jpg

It's like it's only doing 1-bit color (purely monochrome), but at DOS, it does beautiful shades of grey..   (in Windows, it's in 640x480)

[viletim!]

So, assuming this monochrome takes the RGB values and combines it to one gun, does that mean I can pull off 768 shades of grey?

That's a interesting question! In theory the answer is yes, because each sub-colour is summed to produce a greyscale output. In practice, I don't think the MCGA will have a pallette big enough to let you use all 768 shades.

EDIT: Oh, and why is it, when it's in Windows, things look grainy, like this: http://blackevilweredragon.spymac.com/toph.jpg

It's because MCGA is crap. There's a good reason it didn't last long next to VGA.

[ido8bit]

Mono VGA monitors typically only use the G signal, giving a maximum of 256 shades of gray.  For some specialist applications special adapters are used to sum the RGB signals.

Those PS/2 onboard video (on the pre-486 models) has 256KB of VRAM and can't do any better than 16 colors in 640x480, so any image with more than 16 colors will be dithered and look like shit.  

[blackevilweredragon]

Mono VGA monitors typically only use the G signal, giving a maximum of 256 shades of gray.  For some specialist applications special adapters are used to sum the RGB signals.

Those PS/2 onboard video (on the pre-486 models) has 256KB of VRAM and can't do any better than 16 colors in 640x480, so any image with more than 16 colors will be dithered and look like shit.

well interestingly, this one gets all channels, red, green, and blue...

and i can't even pull off 16 colors in 640x480..  everything i try on it come up with 640x480 2-colors, or 320x200 256 colors (or 4 colors)

EDIT:  Oh, forgot to mention, this is an 8086-2 model...  with 640K RAM..

[RobIvy64]

Um yeah, those 8088 models didn't have 256K of VRAM.

[blackevilweredragon]

Um yeah, those 8088 models didn't have 256K of VRAM.

well, this ones an 8086, not an 8088..   (this model never had the 8-bit varient)

but, interestingly, it doesn't seem to have ANY VRAM at all.. i ran a program on it, and it said i had none..  so must be stealing it from the 640K base RAM it has (this machine has no support for extended memory, as far i know)

[blackevilweredragon]

hmm, interesting question i have now...

i have another monochrome monitor, VGA, but it does take only green...  how do i go about combining the colors with that?

[RobIvy64]

Um yeah, those 8088 models didn't have 256K of VRAM.

well, this ones an 8086, not an 8088..   (this model never had the 8-bit varient)

but, interestingly, it doesn't seem to have ANY VRAM at all.. i ran a program on it, and it said i had none..  so must be stealing it from the 640K base RAM it has (this machine has no support for extended memory, as far i know)

What is the name of the program you ran?

Even if it was using a chunk of the 640K, it should not register it as having "none".

It has to have some amount of VRAM.

[viletim]

MCGA doesn't have a high res 16 colour mode. If it did it there would be no difference between VGA and MCGA.

blackevilweredragon,
Calculate it....

Like with most video signals, each VGA colour signal has a 75 ohm source resistance. So to add three together so they will sum to the total of one (green) signal you should increase the source resistance to 75 * 3.

You'll end up with:

Code Select Expand


      75    150
Red ---RRR---RRR-+
      75    150 |
Grn ---RRR---RRR-+
      75    150 |
Blu ---RRR---RRR-+---//---+
                         |  
                         R 75
                         R
                         R
                         |
                        ---GND

[blackevilweredragon]

Um yeah, those 8088 models didn't have 256K of VRAM.

well, this ones an 8086, not an 8088..   (this model never had the 8-bit varient)

but, interestingly, it doesn't seem to have ANY VRAM at all.. i ran a program on it, and it said i had none..  so must be stealing it from the 640K base RAM it has (this machine has no support for extended memory, as far i know)

What is the name of the program you ran?

Even if it was using a chunk of the 640K, it should not register it as having "none".

It has to have some amount of VRAM.

I don't remember what the program was..  I know it has to have VRAM, but no ones been able to figure it out...

All sources say "N/A"..  it's in an un-directly addressable region or something..  If I had to guess, I'd say either 64K or 128K..  Don't know for sure though..

[blackevilweredragon]

MCGA doesn't have a high res 16 colour mode. If it did it there would be no difference between VGA and MCGA.

blackevilweredragon,
Calculate it....

Like with most video signals, each VGA colour signal has a 75 ohm source resistance. So to add three together so they will sum to the total of one (green) signal you should increase the source resistance to 75 * 3.

You'll end up with:

Code Select Expand


      75    150
Red ---RRR---RRR-+
      75    150 |
Grn ---RRR---RRR-+
      75    150 |
Blu ---RRR---RRR-+---//---+
                         |  
                         R 75
                         R
                         R
                         |
                        ---GND

Ok, thanks, i'll try that...  I'm not used to messing with video signals directly like this, only other components that are TTL and CMOS level..

[RobIvy64]

Didn't you just run it though?

Could you take a quick peek and see what the program is? i'd be interested in running this myself.

[blackevilweredragon]

I've ran this program a year ago, when the computer last worked before it died..

(i only fixed it a couple days ago)

[blackevilweredragon]

hmm, this is interesting..  I just ran a test on it..  notice a photo loaded on it looks great (minus pixelation)..

But when I load the color test (and greyscale test), the colored squares VERY in intensity..

Green is brightest, red is in the middle, and blue is HARDLY seen..  But they ARE there..  Shouldn't they be equal in intensity considering that the values are R=255, G=255, and B=255..

(also what's interesting, is that the text, which is white, comes out BRIGHTER than the squares..)

http://blackevilweredragon.spymac.com/tubetest/1.jpg
http://blackevilweredragon.spymac.com/tubetest/2.jpg
http://blackevilweredragon.spymac.com/tubetest/3.jpg
http://blackevilweredragon.spymac.com/tubetest/4.jpg

[viletim!]

Ah, yes....they are weighted after all (do a search for "RGB greyscale formula"). So that would mean in theory you'd get 16million greys....I think...

[ido8bit]

From memory 8 bit/256 color video modes typically allocate 3 bits to red, 3 bits to green, but only 2 bits to blue.  Most people's eyes are suposedly less sensitive to blue, so back when most PCs had 256KB of VRAM it was an acceptable trade off.  So even with the R, G and B signals summed you've still only got 256 shades of grey.

The only way your going to get better than 8 bit video in a PS/2 25 is to replace the motherboard.  A couple of companies made upgrade motherboards for the PS/2 25 that drop straight in without modifying the case/cabling.  These boards give you a 386 or 486 CPU, VGA or SVGA video and an IDE interface.  No idea where you'd find one though.  

If you're willing to do a LOT of work there are 486/Pentium motherboards with onboard VGA and IO intended for embedded systems that would be small enough to fit and low power enough to run from the original PSU.  

I picked up a tiny 100MHz 5x86 board ages ago intending to stuff it in a Mac Classic for the hell of it, but it's one of those projects I still haven't got around to.  

[viletim!]

ido8bit, you're absolutely right - 8 bits = 256 colours (no matter what's done with the output)...It's too hot today

BTW I've worked on a stick a PC in a compact mac style project. Though I picked an SE because they have real power supplies.

[blackevilweredragon]

im fine with the planar "motherboard", it has right now...  i like the odd sound of the MFM hard drive

But, does having a weighted grayscale monitor make it better?

[ido8bit]

viletim!:  Nice project.  If I ever get around to putting my PC in a Mac together I'm aiming for something that looks completely stock from the outside.  There's just something perverse about what looks like an old Mac Classic running Win98.  I was going to "cheat" on the monitor side by using PCB from a mono VGA monitor rather than reworking the mac's existing video board.  

blackevilweredragon:  I guess having a weighted grey scale monitor has to be better in theory.  The only way to say if it actually makes a difference would be to compare it side by side with a machine displaying the same image on a mono monitor that only uses the green signal.  There is a sense pin on the VGA connector that allows the card to determine whether it is connected to a color or mono monitor.  I vaguely remember seeing at least one PC displaying a grey scale image on a color monitor when the cable was wired incorrectly.  Maybe the signals are summed or otherwise combined in the card itself when a mono monitor it connected?

I seem to recall reading that for specialist applications requiring more than  the usual number of greys the special adapter was required so the computer treated the display as color, then software adjusted the palette accordingly.

[blackevilweredragon]

in this case, the computer has no idea what CRT is attached

[ido8bit]

The model 25 should be able to identify it's monitor.  From memory the model 30 that it is based on could and the internal display connector has enough pins to include the ID pins.  One of the ID pins (pin 12 on a VGA connector) would be tied to ground to indicate a mono monitor.  

[blackevilweredragon]

not this one..  because on this one, if i split the lines to a color VGA monitor, it's still in color..