Maker Pro
Maker Pro

Digital, or analog?

R

Ray Andraka

Jan 1, 1970
0
Jerry said:
I wasn't thinking of the memory chips themselves, with charges on
capacitors standing in for ones and zeros, but rather the analog nature
of the interconnections, with their transmission delays, characteristic
impedances, and matched terminations. A flip-flop' set-up and hold times
arise from the underlying analog nature of its components. Even the
pulse width of the one-shot is quantized in trinary a way: long enough,
OK, and too short. Otherwise, they wouldn't be useful. (The memory
chip's charges are similarly quantized: in the zero range, trouble, and
in the one range.) I don't see the distinction between a digital and an
analog circuit element as fundamentally more meaningful than the
distinction between a digital and an analog wire.

Jerry

Yes, I agree that board designers need to take into account the analog
nature of the signal transmission, and I don't think any current media
for digital logic is wholly digital. That said, I guess I am looking at
it from an algorithmic point of view while you are looking at it from a
physical point of view. To me, digital basically means that your
circuit is using numeric methods rather than physical properties to
obtain the desired result.

Granted, you will have to take into account analog effects in doing a
complete digital implementation, but not for the digital algorithm
itself to work. The distinction is that for a digital circuit any
analog behavior is not germane to the algorithm, rather it exists (often
as a hinderance) as an artifact of the implementation. Contrast that
with an analog circuit where those device physical properties are a
necessary ingredient to a functioning design because you are exploiting
those properties to perform the signal processing.
 
G

glen herrmannsfeldt

Jan 1, 1970
0
Jerry said:
It seems to me important to agree on criteria for deciding whether a
particular circuit or signal is digital or analog
One criterion is intended use; there seems to be general agreement about
that, so I don't address it here. Another criterion is the nature if the
signal or circuit itself, without reference to intentions. That is the
topic of this short essay.

I knew someone once who built a low power FM transmitter.
The final output was a 74S04 TTL gate, cheaper than other
100MHz transistors, and works fine in analog mode.

-- glen
 
G

glen herrmannsfeldt

Jan 1, 1970
0
Jerry Avins wrote:

(snip on open and closed relay contacts)
You could say the same about a door, but degree matters, especially to
the obese.

This reminds me of a discussion about how switch (and relay)
contacts usually have some oxide that the electrons have to
tunnel through. There really isn't metal to metal contact.
(Maybe only one atomic layer of oxide, though.)

-- glen
 
G

glen herrmannsfeldt

Jan 1, 1970
0
Ray Andraka wrote:

(snip)
Ah, but Jerry, I think perhaps you missed the distinction. The one-shot
depends on the properties of the R's and C's to set up the timing
parameter. In essence, the one shot is comparing a decaying analog
voltage against a reference analog voltage and outputting a digital
signal to indicate whether or not the reference threshold has been
crossed. There is clearly an analog component to this circuit that
would not work the same if the values of the components were changed.

One shots usually have both digital and analog circuitry.
Flip-flops and voltage comparators are both important.
Memory, and other digital circuits that incorporate capacitors and
resistors do not depend on the values of those components to set the
behavior of the circuit. The digital realization could be moved to
another logic foundation (say hydraulics) and it would perform the same
function (albiet, maybe not as fast). The analog circuit however
depends on the characteristics of the components, so moving it to
another technology generally means either finding a component with
equivalent characteristics or modeling the behavior of the replaced
component with something else that mimics the physical characteristics
of the component.

In real life it gets worse. Besides DRAM, digital systems have used
analog circuitry for a long time. Pass transistors for one example.
There are stories of Cray using long winding PC board paths to delay
signals enough to arrive at the right time. Intel processors
traditionally (at least the 8080 and 8086) have used dynamic logic,
such that they won't run below some clock rate.
See the fundamental difference is the digital is
performing a numerical or logical manipulation of the signal where the
analog is modifying the signal by subjecting it to physical properties
of the materials, which generally is not quantized.
Digital, by design quantizes the analog nature of the underlying
circuit to represent numbers, but fundamentally you are doing numerical
operations rather than relying on the physical properties of a component
to do the processing (decay of a voltage, for example).

So DRAM is analog?

-- glen
 
S

Steve Underwood

Jan 1, 1970
0
glen said:
Jerry Avins wrote:

(snip on open and closed relay contacts)



This reminds me of a discussion about how switch (and relay)
contacts usually have some oxide that the electrons have to
tunnel through. There really isn't metal to metal contact.
(Maybe only one atomic layer of oxide, though.)

-- glen
I've had corroded switch contacts make a pretty good copper oxide
rectifier. :)

Steve
 
H

Howard Eisenhauer

Jan 1, 1970
0
I knew someone once who built a low power FM transmitter.
The final output was a 74S04 TTL gate, cheaper than other
100MHz transistors, and works fine in analog mode.

-- glen

I once used a TTL NOR gate as a mixer- worked very well.

Worked so well in fact that not only did I get A+B & A-B outputs I
also got A, B, 2A+B, 2A-B, A+2B, A-2B, 2A+2B, 2A-2B & lottsa various
other stuff :).

H.
 
J

Jerry Avins

Jan 1, 1970
0
Ray said:
Yes, I agree that board designers need to take into account the analog
nature of the signal transmission, and I don't think any current media
for digital logic is wholly digital. That said, I guess I am looking at
it from an algorithmic point of view while you are looking at it from a
physical point of view. To me, digital basically means that your
circuit is using numeric methods rather than physical properties to
obtain the desired result.

Granted, you will have to take into account analog effects in doing a
complete digital implementation, but not for the digital algorithm
itself to work. The distinction is that for a digital circuit any
analog behavior is not germane to the algorithm, rather it exists (often
as a hinderance) as an artifact of the implementation. Contrast that
with an analog circuit where those device physical properties are a
necessary ingredient to a functioning design because you are exploiting
those properties to perform the signal processing.

Calling circuits, signals, and systems "digital" is certainly useful
(and I'll keep doing it), but (I think you agree) it describes their use
and design, not their physical nature. I think keeping that distinction
helps me to maintain insight and to remember that technology isn't magic.

A lady brought an old photo of her deceased husband to a photography
studio. She wanted it enlarged and was told that it was no problem.
(Good scanners are cheap nowadays.) She asked if the hat could be
removed, and was told that it would be more difficult, hence expensive.
(Digital photography has made this work wonderfully easy, but only by
comparison.) The photographer then asked what color her husband's hair
had been and which side it had been parted on. "What kind of question is
that?" she asked. "When you take the hat off, you'll know."

Jerry
 
R

Ray Andraka

Jan 1, 1970
0
They only smoke when overclocked. :)

Naw, it's just you only see the smoke when you do something that lets
the magic smoke out of the package. It's sort of like a genie, once you
let the smoke out, the magic is gone and the part no longer functions.
 
R

Ron N.

Jan 1, 1970
0
Jerry said:
It seems to me important to agree on criteria for deciding whether a
particular circuit or signal is digital or analog

At the quantum level, everything might be digital; but above that
level everything looks more analog. However many digital engineers
will only treat anything as analog when dragged kicking and
screaming.

Otherwise, if there exist a useful model of a signal or circuit
which can treat everything as a bunch of "1"'s or "0"'s, and still
predict the systems behavior correctly (within the applications
reliability targets, and when the system is operated within design
contraints), then any of the actual behavior outside of the digital
models is ignored.

So the difference between digital and analog is conscious
ignorance. If the ignorance succeeds, then the circuit or
signal is digital. If there is a failure of this ignorance,
or the system is being operated near the edge of operating
constraints or reliability goals, then analog models are brought
back into the picture to see if they can do better.


IMHO. YMMV.
 
J

Jerry Avins

Jan 1, 1970
0
Ron N. wrote:

...
So the difference between digital and analog is conscious
ignorance. If the ignorance succeeds, then the circuit or
signal is digital. If there is a failure of this ignorance,
or the system is being operated near the edge of operating
constraints or reliability goals, then analog models are brought
back into the picture to see if they can do better.

That seems very cogent to me. Thanks.

Jerry
 
Z

Zak

Jan 1, 1970
0
Steve said:
If there was a choice you'd use a proper op-amp. You only use these
gates because they are leftovers in a multi-gate package. 74xx devices
were used quite often in semi-analogue roles.


Did anyone ever analyse or even draw a schematic of the 'keychain
whistle' devices? When you'd whithle, it would beep back at you to
indicate where it was.

ISTR it used a CD4000 or similar as only active device, and a single
piezo as both mic and beeper.

It would go quite some time on a battery as well. Clever design.



Thomas
 
Top