Probably not a plasma screen in the 70s. It looks like a bunch of individual characters. They are probably vacuum fluorescent like what was on calculators and the Speak & Spell educational toy from that period.
Looks like it is plasma but with characters, not a pure bitmap display.
It is said that the display was an unusual plasma display and not a vacuum fluorescent display, which were much more frequent in the desktop calculators of that time, but it must have been an alphanumeric display with segments, perhaps with 16 segments per character.
These were monochrome orange vector displays. They didn't use segments.
Moreover, I doubt that the cost includes the one thousand high voltage transistors that would have been needed to drive the display.
As I have already written, such a display would have been too expensive for a desktop computer with an 8-bit CPU.
In the early seventies, as an alternative to the red LED displays and green or blue vacuum fluorescent displays, there were the orange Panaplex plasma displays with segments for numeric (7 segments) or alphanumeric (14 or 16 segments) characters.
I believe that the Q1 computer must have had such a Panaplex display, which was cheap enough.
Vector displays are possible only where there is a drawing spot that can be moved in any programmable direction, i.e. only on CRTs or mechanical plotters. They are not possible with plasma displays.
I used PLATO at UIUC in the late 80s briefly, because they were still around, but everything had been WYSE-50 or Sun already (or contemporaries) and the PLATO terminals weren't some monstrosities....just looked 20 years old.
If any transistor in omitted, than that row or column must have a fixed voltage, which means that the corresponding column or row of pixels would no longer be controllable and they would display the same pixels as whatever other column or row is selected at a given instant.
One thousand transistors do not need a huge space (they would fit easily on a PCB of a size compatible with a PLATO terminal, based on the photo from Wikipedia), but they were quite expensive. Suitable high-voltage integrated circuits have appeared later. In 1972, one could have used Nixie decoders like 74141 for the negative voltage, replacing 512 transistors with 64 IC packages, but 512 discrete transistors would still have been needed for the positive voltage.
For segmented alphanumeric displays you need only 14 or 16 transistors for the segments plus one transistor per character.
You can build a system to control a large binary system with simple relays and gates and a comparatively few transistors.
Given that they did much more work with relays back then, in absence of a better idea I would pin my bets on that.
They were subsidized to some extent, but they were nonetheless cranked out by the hundreds.
> Vector displays are possible only where there is a drawing spot that can be moved in any programmable direction, i.e. only on CRTs or mechanical plotters. They are not possible with plasma displays.
I'm sorry, you are simply wrong here. The Plato display did have vector graphics.
Your own Wikipedia citation disagrees with you:
"The display was a 512×512 bitmap, with both character and vector plotting done by hardwired logic. It included fast vector line drawing capability, and ran at 1260 baud, rendering 60 lines or 180 characters per second."
Sounds more like it has Bresenham's algorithm implemented in hardware. It would still plot individual pixels on a 512x512 grid, which is different from how an actual vector display draws lines.
In any case, the PLATO display absolutely did not use "segments", as the OP claimed that it must have. He also claimed that bit-mapped plasma displays "didn't exist at the time".
PLATO used this type of display because it was running over very slow dialup modems, and vectors, while having their limitations, are much more compact than rasters.
Example: suppose you want to draw a line on the 512x512 PLATO display. With a raster method, you have to send 512x512x(number of brightness levels) down the pipe. With a vector display, you only have to send DRAW(X1,Y1,X2,Y2) or (equivalently) DRAW(X,Y,length, angle) 5 numbers versus hundreds of thousands.
The PLATO display had direct hardware support for line-drawing commands. That made it a vector display.
This might not matter if you have a high resolution, multiple brightness levels, and a CPU/GPU powerful enough to do antialiasing, but for 512x512x1bpp the pixels making up each line will be very visible as "stairsteps".
You are thinking of digital vector GRAPHICS displays with hardware line-drawing support. Analog vector monitors and digital vector graphics raster displays are two different things. Vector monitors can draw nonpixellated curved lines as well as straight lines. Vector monitors can draw lines at any angle without staircasing. If a screen has pixels, it is not a vector monitor.
The 1970s Asteroids arcade game had an analog vector monitor without pixels. Ditto the PDP-1 Spacewar and PDP-11 Lunar Lander, both of which I saw in the 70s. Ditto the Star Wars arcade game. Some vector monitors had rapid-fade phosphors and had to be constantly redrawn, others were bistable storage tubes that retained an image for several minutes without being redrawn. The Plato terminal did not have an analog vector monitor.
https://en.m.wikipedia.org/wiki/Vector_monitor
https://en.m.wikipedia.org/wiki/Star_Wars_(1983_video_game)
https://en.m.wikipedia.org/wiki/Battlezone_(1980_video_game)
https://en.m.wikipedia.org/wiki/Direct-view_bistable_storage...
In the shot of the display frame by itself without the tinted plastic in front of it, it almost looks like a miniature arrival/departure board at a train station.
The display in action on a Q1: https://www.1000bit.it/lista/q/q1europe/q1microlite.png
Close-up shot in a Q1: https://www.peel.dk/Q1/img/SN615-05.jpg
Brochure: https://archive.org/details/TNM_Self-Scan_II_panels_for_data...
There are videos of the display being demonstrated on YouTube. Search for Burroughs Self-Scan.