3 Apr
2024
3 Apr
'24
6:20 p.m.
On Apr 3, 2024, at 1:49 PM, Rick Bensene via cctalk
<cctalk(a)classiccmp.org> wrote:
...
Even with only having to render the digits zero through nine and a decimal point (the
calculator didn't support negative numbers; they were represented using tens
complement form), the display generator also used a batch of diode-transistor gates to
generate the digits. The interesting thing about it is that instead of generating
strokes to create the digits, the machine uses sine/cosine waveforms that are gated by the
character generation logic to draw the digits on the screen. The position of the digits,
like the CDC scopes, is derived by precision resistor DACs, and then a mixer takes over as
the character is drawn using gated segments of the sine and cosine waveforms mixed
together with the position voltage. The result is really beautifully rendered digits
that look almost like they are drawn by a draftsperson who is extremely consistent in the
drawing of each digit. The CRT has yellow-orange phosphor with a moderate persistence, so
when the digits change, they look like they quickly morph from one digit to the next.
The digits are among the nicest looking digits that I've ever seen on a CRT display,
including those on the CDC scopes as well as IBM console displays.
I have, somewhere, a copy of a paper that describes analog circuits for generating
waveforms for digits along the lines you describe. Might have been from MIT, in the
1950s, but right now I can't find it.
The CDC console waveforms start out as step function waveforms, with delta x and/or y of
+/- 1 or 2 units, at 100 ns intervals. Given the bandwidths of the circuits involved they
get rounded off in the generation, and a whole lot more in the DD60 deflection amplifier
signal chain. I've tried to create a SPICE model of that signal path to try to
reproduce what we know actually showed up on the display screen, but haven't had much
luck. Too much of the circuit involves parts with unknown properties, starting with the
transistors, on to the wirewound resistors that apparently show up in various places, and
ending with the deflection plates of the CRTs themselves. Still, a crude IIR filter
mimicking some of the more obvious contributions do produce acceptable character shapes in
my DD60 emulation software.
Speaking of nice looking numeric displays: probably the best ever are the projection
displays made by IEE, in the 1960s I think.
https://www.antiqueradios.com/forums//viewtopic.php?f=12&t=341355 shows a sample. A
few computers from that era used them for the console, the CDC 1604 seems to be an
example.
paul