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Digital, or analog?

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.

*SIGNALS* A digital signal consists of a sequence of a fixed number of discrete states, with no intermediate states are allowed. Outside the realm of quantum mechanics, a continuous signal is not digital. It can be made digital by quantizing it, and different quantizers will convert the same analog signal into different quantized signals. As far as I know, digital signals exist only as abstractions and in computer circuits. I would be delighted to learn of exceptions.

*CIRCUITS* A circuit intended for a digital application has a fixed number of discrete input and output states. The useful states are limited to those common to both input and output. For simplicity of design, the number of states is usually chosen to be 2, although other arrangements are possible and some have been utilized. States are represented at outputs as voltage or current ranges, and recognized at inputs in the same way. The thresholds need not be the same at input or output. For example, the specification for the 74LS logic family requires a high-level (1) to equal or exceed 2 volts and a low level (0) to be no more than .8 volts at the input, and guarantees that a 1 will be at least 2.4 volts and a 0 no more than .5 volts at the output. That specification makes the devices well suited for digital use, but it defines them as analog devices by defining voltages which are, as far as states go, ambiguous.

A CMOS CD4011B makes that point better. It is a quad 2-input NAND gate. With a 15-volt supply, a 1 is 11 volts or greater, and a 0 is 4 volts or less. The output swings between .05 and 14.95 volts when lightly loaded, and can sink or source more than 5 ma at 4 and 11 volts. The /intended/ use of a NAND gate makes it digital devices, but these are inherently analog by construction. Their inherent analog nature seen by connecting a 1 megohm resistor from output to the inputs tied together. You will have an analog amplifier with 23dB voltage gain and much higher power gain. With capacitive coupling, the four gates can be connected as two H bridges that will deliver 18 milliwatts per channel into 1500 ohms. To be truly digital, a device must have discrete states, and be incapable of exhibiting any other state.

That situation is approximated by cross coupling a pair of the gates to make a set-reset flip-flop. Now there are only two stable states, but the in-between states still exist as transient states passed through when the device is in transition. Rise and fall times greater than zero, and the well known but often ignored metastable state attest to that.

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As far as I know, digital circuits exist only as abstractions and on schematics. I would be delighted to learn of exceptions.

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
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Reply to
Jerry Avins
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Jerry, that's good work on a rational foundation for the terms.

I think, however, that the significant quibbling comes in only on boundary cases where the distinction gets blurred, and it's just always going to be tough to avoid semantic arguments popping up there. For example, even with "digital" devices, by your definitions, at both ends of a circuit board trace, the trace may still need carefully designed termination and interference isolation which are often best treated as "analog" phenomena.

Although your definitions generally hold, there's still plenty of room for argument and confusion at the boundaries. Because of that, I don't think the problem of whether certain things are really "digital" or "analog" is going to be solved with world-class unanimously-approved definitions.

Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions.

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Reply to
Eric Jacobsen

...

I remember an old-time engineer once telling me "There is no such thing as digital". We merely interpret certain behaviors of analog circuits as transitions, noise, or metastability, etc. and then attempt to ignore those for our convenience, or at our own peril.

Of course, he was talking about the real world. We are free to make abstract models, but should not confuse a model with some engineering reality, especially if a circuit is working near its limits of behaving similar to our first-order models.

So I would agree with you that digital circuits exist only as abstractions.

However, there is also no such thing as analog. It is merely a model where we assume that some continuous curve or function is associated with our measurements of what are actually numbers discrete quantum events (the measurements are usually far too course to notice the error).

If the quantum events are below some noise floor and you are already ignoring this noise floor, then a continuous model might be the computationally or cognitively more efficient tool of abstraction.

So, as to the question about whether some circuit or signal is digital or analog, I would say the answer depends on which model best serves your specific purpose or question regarding the given circuit or signal.

For some things DSP engineers do, the answer might well be both.

IMHO. YMMV.

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rhn A.T nicholson d.0.t C-o-M
Reply to
Ron N.

electo-mechanical relays

'nuff said ;)

Reply to
Richard Owlett

Yes, our world is analog, isn't it? I agree that the digital world is an abstraction, therefore not real.

But what is our world or what is real? (keeping the technological perspective rather than philosophical). Isn't it because our sensors integrate incoming signals so that they appear continuous? (I prefer continuous rather than analog, as opposed to digital). Because our world, the real world, is what we perceive using our natural sensors. But this is the macroscopic world, in the microscopic one (as you mentioned quantum mechanics) it could be the other way around. The continuous signals might be the abstraction.

Adrian

Reply to
Adrian Spilca

Better say more. Make before break, or break before make? We're looking at transitions here. Even with SPST, contacts bounce

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

The first time I tried to make a toggle flip flop out of relays (in junior high, I think), it went into metastable oscillation, dependent on the power supply voltage.

Another DPDT relay (there were scavenged discards) had one bad (randomly high resistance) contact.

Try again. :-)

IMHO. YMMV.

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rhn A.T nicholson d.0.t C-o-M
Reply to
Ron N.

The contacts have *ONLY* 2 states.

No matter how much bounce, the contacts are *EITHER* "open" or "closed"

ain't no other option!

Now as to a "system", can you make it emulate 'analog'? YEPP.

Does that make relay analog? NOPE.

Disagree? well, zip is now 658xx so "SHOW ME" ;)

Reply to
Richard Owlett

Said by somebody who's never put a VOM across a batch of old rusted relays (scavenged from dead pinball machine parts found in a outdoor scrap heap I think).

Your abstraction does work a bit better with new ones.

IMHO. YMMV.

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rhn A.T nicholson d.0.t C-o-M
Reply to
Ron N.

Arcing contacts? Zero risetime?

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

BULL! irregardless of "contact resistance" they are *either* open/closed

Reply to
Richard Owlett

BAHH see reply to Ron

Reply to
Richard Owlett

So what resistance value, and for what duration, would you call closed?

Do you think that all "open" relays have zero resistance?

IMHO. YMMV.

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rhn A.T nicholson d.0.t C-o-M
Reply to
Ron N.

I don't think it is possible to say enough to convince some folks.

I'd say a light switch is another excellent example of something clearly digital. That can be compared with a large variable resistor (a "light dimmer") which is analog.

With a switch, the light is either on or off. With the dimmer, the light can be set half way between any two other points.

One is discrete, the other is continuous. Digital, and analog.

--
Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com
Reply to
Floyd L. Davidson

"closed" is CLOSED "open" is OPEN

NO *OPEN* relays have INFINITE resistance actually i'm not cheating fair as i knew what you meant

My previous experience was in industry that wished to "KNOW" within 1 ms when a contact closed with 10's to 100's of ms bounce

Reply to
Richard Owlett

That's what I get for looking at the wrong pin of a DPDT relay. :^)

In my case, it was what voltage, frequency and duration of an oscillating relay pair would close the next (rusty) relay downstream. There was also the amplitude of whacking the side of the breadboard. Analog.

Reply to
Ron N.

You could say the same about a door, but degree matters, especially to the obese.

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

CAREFUL I out weigh you at least 3 to 2, even if decade younger ;)

Reply to
Richard Owlett

I can't remember where, but I seem to recall some ultra high-speed photography of the formation of an arc. It didn't look like it was happening instantly.

Or does Richard count that first electron at "closed"?

IMHO. YMMV.

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rhn A.T nicholson d.0.t C-o-M
Reply to
Ron N.

Circuits and signals themselves are neither digital nor analog, even though we unfortunately do tend to classify them as such.

The terms "digital" and "analog" properly refer only to two different means of encoding information, in this context on to an electrical signal. The signal itself remains just electricity no matter what we call the encoding, and all signals and circuits obey the same basic physical laws, etc.. The true distinction between these two arises SOLELY in how we interpret the signal (or similarly, how it was intended to be interpreted).

In an "analog" encoding, some parameter of the electrical signal (generally, either voltage or current, but there are other possibilities) is manipulated such that it varies in a manner directly related to the variations seen in the original information source - for instance, a voltage which is caused to vary in the same manner as a sound wave, in the case of an "analog audio signal." And hence the name itself - the voltage is varying ANALOGOUSLY to the original, hence "analog" encoding.

Similarly, in a "digital" information encoding or transmission system, various states of the transmitted signal correspond to numeric values - or more generally, symbols - and must be interpreted accordingly. Again, the name says it all - we're not sending something that directly represents another thing, but instead are sending symbols or "digits."

Neither term NECESSARILY implies a lot of what are commonly thought of as the distinguishing features of either; for instance, "analog" does not necessarily imply that the system is either continuous or linear, even though many common analog systems are both. Similarly, "digital" does not necessarily imply a discrete or sampled representation, and certainly is not limited to a straight binary encoding - although again the vast majority of "digital" systems exhibit these characteristics.

From this perspective, arguments as to whether the world itself, or basic natural phenomena, etc., are "digital" or "analog" are meaningless; the world is what the world is, and these terms only refer to methods for encoding information which describe some real, original thing.

We still refer to circuits themselves as "digital" or "analog" primarily because of the optimization of that class of circuits for dealing with that sort of information, but again the ciruits themselves ALL behave according to the same laws of physics. And there are certainly types of circuits which don't fall into either category - power systems being the most obvious example. (The transmission of power does not involve the transmission of information, so to speak of power engineering as dealing with either "analog" or "digital" is just silly.)

Bob M.

Reply to
Bob Myers

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