Digital, or analog?

That's a good sig!

"John E. Hadstate" wrote in news:a9k2h.18885 $ snipped-for-privacy@bignews6.bellsouth.net:

True, but only well enough to differentiate a low from a high, and you can also build in error correction.

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.

I find it an interesting paradox that in the old days nobody sold single gate devices. Now simple logic has been reduced to the role of glue, a number of suppliers make tiny one gate devices.

Steve

Yes, in terms of your description of what analog means, I think you were right on target.

Depends on what you mean by signal. (This conversation is becoming downright Clintonesque.)

I read it. I thought it was brilliant.

I started to make that distinction. Then I thought about many examples of analog and digital hydraulic and mechanical circuits and decided to go with the more general concept.

I think we are in violent agreement ;-)

Sure. You may think of a one-shot as a digital device, but it takes a fair amount of analog savvy to build one whose pulse width is independent of duty cycle.

I recall one audio device that used an entire hex inverter chip as a stereo headset amplifier.

I may have a few one-gate RTL packages (six lead, round).

Jerry

As a digital guy, I sure don't consider one-shots digital. By one-shots, I am referring to those devices that use the physical characteristics of components (typically capacitance and resistance) to set a time delay.

The distinction I use in my book (which sadly will probably never be finished because of the time involved writing it) is that an analog circuit depends on the physical properties of the components of the circuit as an integral part of the processing. A digital circuit processes the signal through a logical or numerical manipulation of the signal so that the circuit itself is only incidental to the processing.

Clearly, with that definition, one-shots such as the 74123 belong to the class of analog circuits regardless of whether they were offered as a component within a digital logic family. The only thing digital about these parts is the logic family compatible I/O and the simple logic controls built in to allow the one shot to be operated by and output to the logic family.

Another circuit that used to be commonly used with digital logic that is really an analog part is the delay line.

I'd say that a one-shot is digital in intent and analog in execution. I believe that's true of any physical digital component, just more clearly evident with circuits that intentionally use Rs and Cs for timing. All circuits incorporate Rs and Cs, whether intentionally or not. Memory buses are "digital" too, but modern clock speeds dictate that they be designed like the broadband transmission lines they really are.

Very few dichotomies are entirely clear. For most, it is possible to assign most items to one class or the other although the matter is ambiguous for some near the division. The analog-digital dichotomy puts the fuzzy dividing line over near one edge. We may use current, pressure, voltage, fluid flow and other analogs to represent ones and zeros, but they are all analogs nonetheless. Those ones and zeros exist in pure form only in out minds. Even marks on paper are analogs of a sort. We move the division to match a particular purpose. Assuming that it is fixed hobbles our thoughts.

I began this thread to isolate this discussion from the rancor developing in another one. I'm pleased to note that it seems to have succeeded.

Jerry

Well, THAT would depend on just what your definition of "is" is....:-)

But I think at this point we ARE in violent agreement about pretty much all the real points, and are in danger of starting a task of rearranging the semantic deck chairs, so I think I'll quit until something else comes along.

Thanks!

Bob M.

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.

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. 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).

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.

I knew someone once who built a low power FM transmitter. The final output was a 74S04 TTL gate, cheaper than other

-- glen

(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

(snip)

One shots usually have both digital and analog circuitry. Flip-flops and voltage comparators are both important.

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.

So DRAM is analog?

-- glen

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

Steve

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.

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

Its all done with smoke and capacitors.

-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85.

Michael A. Terrell Central Florida

They only smoke when overclocked. :-)

Steve