I remember reading somewhere that a few of the early tube-type
computers of the 1950's used CRT's as ram devices.  The method for storing
and reading the display involved bouncing electrons off the phosphor as
described for storage tube monitors.  I believe that the devices had to be
refreshed just like dynamic RAM every so often to keep them from fading.

        This topic also reminds me of a project I started on around 1980
that I never really carried to any useful conclusion.  I wanted to build
a circuit to let a person who is blind analyze a wave form.  Those of us
with no usable vision have no use for scopes at all, but we sure appreciate
what they do.  What I did was to use a CD4016 or CD4066 FET switch chip
as a means to create a stroboscope for gating an input signal.  The gating
circuit consisted of a variable frequency oscillator feeding a 74-121 one-shot
which strobed the FET switch.  If one connected an audio amplifier to the
output of the CD4016, the pulses could be heard as ticks or a buzz if the
strobing was at a high enough rate.  If the test signal was a square wave,
and the gating frequency was at or about the same frequency as the test signal,
then one could hear a distinct change depending upon whether the strobe
happened to sample the square wave on the positive or negative half of the
cycle.  If the signal was a single-ended output such as from a logic device,
then the buzz would appear and disappear as the sample caught the 5-volt
portion or the 0-volt portion of the wave.  Sine waves would present a
buzz that gently varied from a peak to a null and then to a peak again.

        The 74-121 one-shot had to be configured to produce a pulse that
was long enough to be heard as a pop, but not so long as to ruin resolution.

        What I could have done to turn this in to something was to use the
sampler circuit to charge a capacitor and then use that voltage via an op
amp to drive a voltage controlled oscillator to make a sort of graphical
representation of voltage level and polarity, but I got involved in other
things like computers.

        Nowadays, this idea is made a bit easier.  I dare say that somebody
has almost done all the work.  The P.C. sound cards can sample at 44.1 KHZ
rates and one can do anything to the raw data after they are stored so this
is much more manageable than my old circuit would have been.  I assume that
sound cards have Nyquist filtering built in so that and the aliasing question
probably means that one would have to be very creative to handle frequencies
above 44 KHZ, but the neat thing is that sound cards are everywhere and
relatively cheap.

        I apologize for the length of this message, but I enjoy reading
similar postings from others.  There might even be some PIC projects come
from some of the ideas.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group