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I was looking at information on early computers and noticed some had what appear to be circular CRT displays, such as in the following images.


Gerald J Sussman at PDP-6 and 340 Display from MIT Csail


People using PDP-6 from University of Western Australia Computer History Gallery

Was there any reason these displays were circular? Wouldn't a rectangular display be easier to draw to?

Early system that had circular displays like this included the PDP-1, PDP-6, and CDC Cyber 72. In particular, the one used in the PDP-1 and PDP-6 seems to be called a DEC 340, but I can't find any info on it.

While searching for information on these circular displays, I also came across some info that stated early radar displays were adapted from oscilloscopes and eventually developed into their more stereotypical circular shape. I also note that the Whirlwind 1 had a circular display. Could these be related to the origins of the circular displays on the systems I mentioned above?

  • I believe that those displays were not used as terminals, but purely for graphics (note the Teletype terminals in both photos). They would be vector scan displays, not raster scan displays. – sawdust Aug 9 '14 at 6:32
  • I cannot understand the requirement of the questioner. Is this opinion related or is he looking for facts? If yes, what is the expected answer from this question? – Prasanna Aug 9 '14 at 6:35
  • @Prasanna I don't think this is an opinion-based or subjective question. At least not per se. Answers should of course include references and citations. – slhck Aug 9 '14 at 6:48
  • @Prasanna I may have worded my question badly, especially with the inclusion of me asking if a rectangular display is easier to draw to. I am not necessarily looking for an opinion. I want to know if there was a particular reason that these computer displays were circular, rather than the more common rectangular or square displays. However, there seems to be a lack of information on early circular displays, so there may not exist any hard facts about why they are circular. In this case, I would look for someone knowledgeable to provide some speculative insight into why these are circular. – L'aube argenture Aug 9 '14 at 6:50
  • A circular CRT screen means better beam control and less geometric distortion with less cost. – sawdust Aug 9 '14 at 7:41
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Round is the natural form for a CRT--hence the word, "tube". This is very apparent when looking at early (1920's and 30's) television CRTs.

In order to arrange the electrons on to the screen, electrostatic deflection was used (the little electrode plates seen below) to "steer" the electrons. This technology was limited; making a square would of required screens even smaller than the measly few inches they were at the time. (The corners of a square are furthest away from the center)

enter image description here

Later, larger CRTs used electromagnetic deflection. Instead of plates, a "yoke" of electromagnetic coils control the electrons.

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    "In order to arrange the dots on to the screen" -- Vector scan CRT does not draw "dots. It draws a vector or line segment at a time. Televisions use raster scan CRTs. – sawdust Aug 10 '14 at 23:27
  • @sawdust The first photo appears to show text in the round display. I don't think this would of been done using vectors. Nonetheless, I changed "dots" to "electrons", which applies to all CRTs. – Jason Aug 11 '14 at 5:46
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    A vector scan CRT can draw text, but done using line segments (i.e. vectors) rather than dots (as in a raster scan CRT). " I don't think this would of [sic] been done..." -- Then you would be wrong. – sawdust Aug 25 '14 at 7:01
  • Yep, in the olden days there were a number of CRT displays that literally "drew" the characters. – Daniel R Hicks Aug 25 '14 at 21:49
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The displays are described here:

There was also a high quality DEC 340 CRT display unit with a round, ten-inch screen which could draw simple vector graphics, and an accompanying light pen could be used to draw on the display screen.

As for why they were circular, I would expect that it was because that was the natural form factor for early cathode ray tubes. It was, for example, typical for older oscilloscopes to have circular screens.

enter image description here

The programming manual for the 340 display is available here (PDF). Despite the round appearance, the manual states that the programmable region is square:

Points may be plotted on a 9-3/8" square raster centered on the face of the display tube.

  • "Points may be plotted on a 9-3/8" square raster" -- the use of the word "raster" is misleading here, since it is a vector display. That manual goes on to explain that "the electron beam does not scan the face of the tube; its position is determined by the contents of the x and y registers of the display." – sawdust Aug 9 '14 at 7:36
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While the other answers are correct, the simple answer is that a conical CRT is easier to produce and it resists the surrounding atmospheric pressure better.

  • Early TV picture tubes were also circular - the manufacturers just masked off the top and bottom. When color TV was developed, history repeated - the first color TV CRTs were circular, again with a mask to produce a horizontal top and bottom edge (but with curved sides). Google for pictures of the RCA 21CT55, "21" being the diameter of the tube face in inches. – Jamie Hanrahan Oct 27 '18 at 18:00

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