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'[OT] Sampling (was Oscilloscope Question'
1997\12\13@075750 by Martin McCormick

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       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

1997\12\14@082016 by Gene Norris

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At 06:52 AM 12/13/97 -0600, you wrote:
>        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.
>
You are absolutely correct.  The machine that I am familar with was the
IBM 701 which I first saw in 1957.  The machine cycles of the IBM mainframes,
up until the late '70, were called "I" for instruction, "E" for execution and
"ER" for execute and regen even though at that time there was no need for
a regeneration cycle.

You do bring back memories, Gene.





Gene Norris
spam_OUTGnorrisTakeThisOuTspamearthlink.net
E. Windsor, NJ USA

1997\12\15@031544 by Eric Smith

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Martin McCormick <.....martinKILLspamspam@spam@DC.CIS.OKSTATE.EDU> wrote:
>         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.

It was "Williams Tube" memory.  It did need to be refreshed, and did not
acchieve tremendous density.  It also was not terribly reliable.  It is
rumored that the first video games were played on these; kinda neat having
the frame buffer and display in the same device.

I wonder how difficult it would be to make such a thing now?  And use a PIC
to drive it, of course!

Cheers,
Eric

1997\12\15@033211 by Scott Newell

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>Martin McCormick <martinspamKILLspamDC.CIS.OKSTATE.EDU> wrote:
>>         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.
>
>It was "Williams Tube" memory.  It did need to be refreshed, and did not
>acchieve tremendous density.  It also was not terribly reliable.  It is
>rumored that the first video games were played on these; kinda neat having
>the frame buffer and display in the same device.


Without a doubt, a cool application.  But how about the use of CRTs in
analog computers as arb function generators?

"The input voltage X1 is applied between the horizontal deflection plates
of a cathode ray tube through a suitable d-c amplifier.  The voltage X0
between the vertical deflection plates is taken to be the output voltage,
this voltage is made to vary as a function  X0=f(X1) of the input voltage
X1 by a feedback arrangement which forces the electron beam to follow the
boundary of an opaque mask placed over the lower portion of the cathode-ray
screen.

The bias voltage tends to force the electron beam away from the mask.  As
soon as the spot on the cathode ray tube screen emerges from behind the
mask, however, a photocell mounted in front of the screen applies an error
voltage across the input terminals of the vertical deflection d-c
amplifier.  This error voltage is phased so that the beam is forced
downward toward the edge of the mask.  The feedback loop comprising the
cathode-ray tube, the photocell, and the vertical deflection amp will,
then, tend to keep the spot on the screen traveling just below the edge of
the mask as the ouput voltage X1 changes."

(from "Electronic Analog Computers", Korn and Korn)


The basic arrangement was good for 1% to 100kHz or so, and a refined
arrangement was expected to show better accuracy.  Less 'clunky' than
wiring up servos and pots, and somewhat more elegant than the diode based
breakpoint schemes.


newell

1997\12\15@120452 by myke predko

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Eric Smith wrote:

>It was "Williams Tube" memory.  It did need to be refreshed, and did not
>acchieve tremendous density.  It also was not terribly reliable.  It is
>rumored that the first video games were played on these; kinda neat having
>the frame buffer and display in the same device.

I think if you look back at the old computer games, you'll see that the
screen buffer is used for both video memory and program memory.  This also
lead to a number of problems of making sure that the program memory was only
accessed during the vertical retrace period.

The ultimate of this was the Sinclair ZX-81, which used the processor to
output the composite video, line by line and character by character and then
did the actual computing during vertical retrace (needless to say, it was
pretty slow).
Lots of interesting stuff from the past!

>I wonder how difficult it would be to make such a thing now?  And use a PIC
>to drive it, of course!

It shouldn't be that hard; a PIC ZX-81?  I think that would be an
interesting project!

myke

"I was well aware that the processes of puberty are often fatal to psychic
power."
- Sir Arthur Conan Doyle

1997\12\16@010245 by Eric Smith

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I wrote:
> It was "Williams Tube" memory.  It did need to be refreshed, and did not
> acchieve tremendous density.  It also was not terribly reliable.  It is
> rumored that the first video games were played on these; kinda neat having
> the frame buffer and display in the same device.

myke predko <.....mykeKILLspamspam.....PASSPORT.CA> replied:
> I think if you look back at the old computer games, you'll see that the
> screen buffer is used for both video memory and program memory.  This also

I'm not sure, but I think you may have missed the point.  I wasn't talking
about the frame buffer being part of main memory, or vice versa.  That is
very common.  There have even been some x86 PCs that do that, but AFAIK only
the Cyrix MediaGX based PCs do it now.

What the original poster and I referred to was a device in which the
computer's main memory literally was the display tube.  The use as a frame
buffer was completely incidental.

I wrote:
> I wonder how difficult it would be to make such a thing now?  And use a PIC
> to drive it, of course!

myke wrote:
> It shouldn't be that hard; a PIC ZX-81?  I think that would be an
> interesting project!

That defeats the purpose.  The question is whether it is possible to make
a Williams tube memory.  Having a RAM frame buffer is way to common to be
of even the slightest interest.

Cheers,
Eric

1997\12\19@132826 by myke predko

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Hi Eric,

>I wrote:
>> It was "Williams Tube" memory.  It did need to be refreshed, and did not
>> acchieve tremendous density.  It also was not terribly reliable.  It is
>> rumored that the first video games were played on these; kinda neat having
>> the frame buffer and display in the same device.
>
>myke predko <EraseMEmykespam_OUTspamTakeThisOuTPASSPORT.CA> replied:
>> I think if you look back at the old computer games, you'll see that the
>> screen buffer is used for both video memory and program memory.  This also
>
>I'm not sure, but I think you may have missed the point.  I wasn't talking
>about the frame buffer being part of main memory, or vice versa.  That is
>very common.  There have even been some x86 PCs that do that, but AFAIK only
>the Cyrix MediaGX based PCs do it now.

I guess I did miss your point - I was reacting to the "first video games"
comment.

How are the values sensed?  I'm thinking of something like an analog storage
scope screen, with a unique segment for each of the "pixels" (for lack of a
better term).

>What the original poster and I referred to was a device in which the
>computer's main memory literally was the display tube.  The use as a frame
>buffer was completely incidental.

As to the original program, finding the largest prime factor of 2 ^ 18 - 1,
I just came up with 73 (the factors are 3 (3x), 7, 18, 93).

myke

"I was well aware that the processes of puberty are often fatal to psychic
power."
- Sir Arthur Conan Doyle

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