CGA anyone?
cbmeeks
Posts: 634
in Propeller 1
I know this has been mentioned before but I don't know if anything else came out of it.
Anyway, I was wondering if anyone has done anything recently with CGA graphics and the propeller. I wouldn't think so but you never know.
The reason I ask is that my retro computer is starting to take shape. I'm going to use the propeller as the video processor and connecting it to my CGA monitor would be awesome. Yes, green screen NTSC is awesome too but we've already done that. :-)
Anyway, I was wondering if anyone has done anything recently with CGA graphics and the propeller. I wouldn't think so but you never know.
The reason I ask is that my retro computer is starting to take shape. I'm going to use the propeller as the video processor and connecting it to my CGA monitor would be awesome. Yes, green screen NTSC is awesome too but we've already done that. :-)
Comments
My idea of "green screen" predates CGA and for sure did not include NTSC or PAL.
But, I also have a Commodore 1084S monitor which is switchable from CGA (RGBi) and NTSC composite (oh, and S-Video).
Oh and no graphics. And only uppercase text.
I think Tubular is referring to the IBM 5151 green monitor that sold with original IBM PCs. Well, they cost (a lot) extra, but they were sold for use with those computers. They were text only (or block "graphics") with the IBM monitor adapter card, but had a full character set and at least a couple of brightness levels (might've been three or four) for highlighting text. You could get pretty decent graphics with a Hercules graphics card (or the later ATI clone) and the 5151 assuming the software supported it. The text on that monitor was so intensely green that after several hours of use, everything but the screen took on a pink cast.
I also found the documentation for the early ATI board. Would you believe they called it a 'BUG BOARD' (!!). Here's a photo of the opening page where they talk about "emulation" of CGA. Its proof that even if you call a product something silly at the start, you can still go onto greater things and become a leader in your industry.
And I've thrown in a photo of the test rig workhorse here that has what I think might be an MDA display. Its been going solid for 30+ years of continual use. In BASIC it can't select a graphics screen mode, but looks like two intensities of color, as well as underline (it doesn't have that ATI bug board in it)
but as computer could only output 0 or 1 for the pixel but if computer was capable of analog shades, monitor may have displayed those shades.
So it is very close to a b/w video monitor but with green phosphor.
There was a vector game console, that with plastic overlay got you regions of "colors".
Also, did you know that IBM knew not everyone would be able to afford the 5151 so that's why the CGA cards also had an NTSC output as well. So that people could use their cheaper color TV's if they wanted. Smart move on IBM's part. And, recently, some clever folks figured out how to get over 1,000 colors from that NTSC port on real CGA cards using NTSC artifacts. Incredible!
Finally, that's why the original speed was 4.77 MHz which is a multiple of the NTSC color burst. Well, it's 1/3 of 14.31818 MHz which is 4x NTSC color burst.
Yes, I phrased that badly. Of course NTSC has been around for yonks. It's just that I don't recall ever seeing a green screen that used NTSC in the pre PC days.
Computers did not have graphics cards. Monitors were fed with ASCII data over a serial line. All the rastering and character generation was done in the monitor.
One of my favorite ever jobs years ago was writing graphics code for those huge round vector graphic radar displays.
Now those did have graphics cards. They took display lists from memory and generated the required X,Y voltage ramps to draw the vectors.
Elektor produced a circuit for generating vector graphics MANY years ago. I used it to generate flite sim output (just a landing strip and hangar), and it worked very well. I think Elektors idea was to generate shapes on a 'scope driving x and y directly. Used a couple of 13600's if I remember correctly.
Dave
Those old radar displays were beautiful things. They were huge cathode ray tubes, 24" or so. And round, of course. With vector graphics one could draw maps, aircraft plots, text labels and so on in fine detail that would have required a 4K raster display. A raster display was impossible at the time as the RAM just was not available.
The circuitry that generated the vectors was amazing to. A custom processor that took drawing instructions, move, draw etc from display lists in memory shared with the Lucus 16 processor. That was good for drawing long lines for maps and stuff but making text that way would have required too many instructions for all the fiddly little strokes required for a character set. It would also be too slow. The solution was a specialized character generation hardware, built in ECL on a couple of big PCBs with a big aluminium heat sink panel clamped to the chips. It ran hot!
Those screen were monochrome, green or yellow. But there were colour screens under development.
A quiz question: How do you make a colour display in a CRT when you only have one electron gun?
a third color phosphor deposited in a second ring circumscribing said first ring and said central portion
http://patentimages.storage.googleapis.com/pages/US4095144-1.png
but I would guess rapid color changes was not possible,
That's not how the displays we had worked. We had no problem with rapid colour changes.
Radar tubes had long persistence phosphors. Which was a good thing because the refresh rate was pretty slow. A long persistence gets rid of flickering. It also means you can draw a lot more stuff, the memory is in the phosphor!
Still it was plenty fast enough for real-time pan and zoom etc. I would love to have seen an asteroids game made for those displays!
Persistent phosphors glow plenty long enough. With good phosphor, one only needs a 20hz or so total scan rate, 20hz per color, 60hz all together.
And because it's vectors, that means all vectors drawn in that time.
With a static charge memory CRT, just get it all done as quick as can be done. The display holds the image.
Anyway. The displays we had were called Penetron Displays. Despite the patent linked to above bearing the same name they did not have phosphor dots or rings of different colours. They were much simpler.
https://en.wikipedia.org/wiki/Penetron#Penetron
Basically the screen has two phosphor layers of different colours on it.
So, for a some anode voltage the electrons only make it to the first phosphor layer they meet and it lights up with it's colour. For some higher anode voltage they blast through that first layer and make it to the second. Which perhaps has a higher electron/light efficiency and it lights up with it's colour. For voltages in between they both light up about the same and you get a colour mix.
All you had to do was modulate the Anode voltage to get the colours. Doing that at 30KV or whatever is a bit tricky. In that case there was a switching transistor sitting up at 20 odd KV that could switch on and off a further few KV. It was electrically isolated and driven via a fiber optic cable from the graphics driver logic. I thought it was pretty neat.
Best have a dark viewing environment though.
Yes, white CRTs are very hard to find now. And they aren't exactly white either. Most have a blue tint, some orange or pink.
Old monochrome TV sets can still be found here in the wild.
Man, that HV switching! Guess you want a slooooow horizontal scan rate.
Seems like shifting the raster with a slight bias would be plagued with purity problems. Likely to make old style convergence adjustments look like a cake walk...
I have one old monochrome graphics screen. IMHO, an NTSC TV driver does output the CGA frequencies. All you should have to do is generate the sync pulses and run the WAITVID in VGA mode to put put the 16 colors needed.
Without a screen to test... tough to do.
There is another old PC hack out there waiting too. Nobody seems to have done interlace with the CGA monitor and hardware.... Edit: was done, original IBM hardware was botched.
Anyway, you could get 400 lines on that old display and hopefully there is enough video RAM on the card to buffer pixels... Edit: I also wonder about really fast pixel clock rates. Might be able to get nice color on that old screen by flogging the inputs fast... Run 1280 pixels in the active zone and see if it doesn't munge that into grey scales...
You need 4 color pins, two sync pins, and 4 volts or so for the pulses..
Maybe a Prop could drive it directly. May need a level shifter too.
There were sort of "convergence problems". When you crank up the anode voltage to change the colour the electrons are moving faster. That means they respond less to the deflector plate voltages. Your picture gets bigger. So, you had to modulate the deflector drive to compensate.
A radar sends out a pulse and then listens to the echo. In the case of this radar with it's 256 mile range the echo signal has a duration of 2.6 milliseconds.
That echo signal could be displayed on a scope with the brightness controlled by the signal level. Thus highlighting where any reflecting objects are. This is equivalent to a video scan line.
The radar rotates and sends out pulses at the Pulse Repetition Frequency (PRF). So you have PRF of these scan lines per second. You cold display them one under another, raster scan fashion like a TV. Better idea of course is to scan the line from the center of your screen to the edge and rotate that line around the screen in sync with the physical radar rotation. Now you have your highlighted target spots on screen in polar coordinates.
The long persistence phosphor is a bonus here. It means the spots stay lit up after the scan has moved around, which is pretty slow. One revolution in a second or so.
The displays I worked with could display such a raw "video" signal from the radar. But you have a lot of time between scan lines. If the PRF were 50Hz that's 47.4ms of free time. During that free time the vector display generator could use the electron beam to draw maps, range rings, target labels and other information.
All brilliant stuff. I would not say it's a "mess". Simple, elegant, reliable.
Note: That 50Hz is only an example. I did not know what the actual PRF of our radar was. That was information was a NATO classified secret.
Quiz question:
How did they rotate that radar video scan line around the screen in early radars? Remember there were no computers or electronics much to generate fancy signals X-Y deflection signals.
And, I agree with all of that. A vector / radar display, or ones like the air traffic controllers used to use, is indeed a mixed mode device. I got to tour PDX airport back in the day, and it was kind of amazing the old gear they used and how effective it actually was.
Years later I was interviewed for a job at the Marconi Radar Company. One of the questions the head of the displays division asked me was: What shape wave forms do you need to apply to the X-Y deflection of a CRT to create a rotating scan line? We then spent a good long time discussing all the circuity they used to actually do it at that time. I must have given a reasonable answer, I got the job. Funny how these little things in life link together.