What exactly is digital anyway?

I apologize for the silly title as I had no other idears. This thread may end up being useless bore but if it makes anyone think and learn then I guess it will suit a purpose afterall!

Anyway, I had a 'interesting' discussion with a fellow about a simple single-pole switch used in everyday homes to turn on your lights. You either turn it on or turn it off. Simple, right?Well, he is arguing that this makes the single-pole switch a digital device because it is on or off (HIGH or LOW) and this is the defintion of digital. I would have leaned toward an electro-mechanical device myself.

I pointed out this would make analog components like a comparator, an op-amp used as a comparator (often not suggested and perhaps redundant?), or a simple transistor when viewed as a switch all digital devices since each can be used as on/off, HIGH or LOW. Unless I am missing some nitpicky definition somewhere, I consider these devices purely analog.

The only wiggle room I can see is if one considers a logic gate; e.g., an AND gate. My old Digital Fundamentals by Floyd starts off explaining logic gates. Art of Electronics starts discussing logic gates at the start of its Digital chapter (Chapter 8?). But it also notes that digital can be a little fuzzy and I am tending to agree. Texas Instruments puts logic under its analog product umbrella. And no, just because TI does this means they are the be-all-than-ends-all on definitions -- but it's still curious.

An AND gate has two distinct outputs: on/off, HIGH/LOW and only those states. Ok, fair enough. But if an AND gate is strictly a digital device, then why not a comparator? Why not the single-pole switch? In all these cases, there are only two states: on/off, HIGH/LOW.

But, in all these cases, the output is a continuous signal albiet in only two states. The signal is discrete -- thinking about a 1-bit ADC

-- but it is not quantized; i.e., represented by a certain number of bits.

I have always thought of digital device (or digital transmission for that matter) as a device that receives or transmits a series or set of

1's and 0's representing data and not simply isolated pulses. A microcontroller or DSP fits the bill but I guess this can get fuzzy too.

I deal with analog and it has been a while since I dug deep into digital much less worry about strict electronic definitions in the matter. If you don't use it, you lose it. Or perhaps I slept in class. Practically all the defnitions I rounded up specifically meantion a series or set of pules that represent data. Not all, but most.

So, I guess after reading all of that nonsense can an electronic philosopher offer their thoughts? Hope I didn't waste bandwidth on a question that does not deal with a schematic!

Reply to
Wayne
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Well, that was a meandering discussion so rather than quote it, I'll meander too.

Pretty much everything at a macro scale (things involving more than a few fundamental particles) is analog in its macro behavior. At the individual particle basis, this isn't so -- there are defined observations and only those may be observed. But at any level where these individual aspects accumulate (mass of some tangible object) or where the measuring system cannot differentiate each possible state but instead only 'sees' most of them the same and reports that (for example, temperature only arises as a macro-scale observable and has no meaning at all with individual particles and measuring devices show you the same value for far and away most of the possible states of the system), it's analog.

Even an electronic digital logic gate is analog. It's just that it spends very little time in the analog amplification of its input and spends most of its time with its output near one rail or the other. But even then, it is analog.

Digital (and I suppose 'analog', too) is more a state of mind. If you want to simplify the overall thing you are considering, then you simply ignore the analog aspects (which, with digital systems, isn't much of the time anyway) and focus on the digital mental model.

So, whether or not a mechanical switch is analog or digital is more a matter of what you are doing with it and what's important to keep in mind when considering the situation.

Jon

Reply to
Jonathan Kirwan

There is nothing that says it can't be both electromechanical and digital. Actually, you might argue that is is a form of comparator that compares the position of the lever with an internally stored trip point. And if it is a well designed switch, it prevents you from teasing it at the arching point by including some positive feedback so that it turns on at one lever angle, and you have to back up a bit toward the direction you came from, before it turns off, again, making it a mechanical Schmitt trigger. Relays have the same property, in that they will drop out at a lower coil voltage than it takes to pull them in.

Comparators have analog inputs and digital outputs. They are, in effect, a one bit analog to digital converter. And almost any analog amplifying device that can be over driven, so that its output has only two stable states can be argued to have a digital output. Digital outputs don't have to be binary, either, only to have quantized values of input or output, or both. Binary is simple the most common because it is the simplest. All analog to digital and digital to analog converters bridge between the quantized and smoothly variable worlds.

Purely analog implies smoothly changing voltage or current choices. It is very hard to hold a comparator output half way on.

If it is discrete, it is quantized. By the way, lots of digital devices can be biased to operate as analog devices. 4000 series CMOS is famous in this regard. Well, perhaps not so famous that you've heard about it. But whole application books have been published on the subject.

So you haven't heard of decatrons?

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They are digital, but have 10 unique output states.

Especially when many micros now contain analog to digital converters and comparator functions as inputs.

Reply to
John Popelish

True, I didn't mean to imply that a device could not be both. But the main issue would be is it a digital device? You mention that one could see it as a comparator. I would not argue otherwise; however, I do not see a single-pole switch or a comparator as a 'digital device'.

Maybe I am splitting hairs, but the device and the nature of the output are two different beasts to me. A comparator is an analog device and the nature is well, digital because a comparator (you mention this later) can essentially be a 1 bit ADC. Mixed signal maybe?

I do not disagree here and in my discussion with the gentlemen I did find the need to clarify between the device itself and the output signal. As to 'quantized' I will try to clarify later as you will eventually make a comment about it.

Well here is my issue. You mentioned that the output of a comparator can be viewed as digital because it is essentially a 1 bit ADC. I certainly do not disagree. But again, this is the output signal. I do not know if it is still common place to use a simple BJT as a switch (if at all really -- I am a young engineer in the industry) but since it can be viewed that way, it to can output a digital signal (as we have described it). In all cases, to me, these are analog devices that can output a digital signal.

Well, let me clarify on what I was meaning by discrete as I was not using the proper terminology. A continuous analog voltage is sampled by an ADC and the output is a quantized representation of that signal and are in the forms of a digital code; 8 bit, 16 bit etc. The output of a comparator is one of two states, (still a contiunous output) and that is what I meant when I said discrete. A physical discrete value 0V or 5V (or whatever the supply happens to be). Bad use of terminolgoy on my part.

The AND gate, the comparator, the op-amp, and the transistor as a switch may have two output states but the output is still continuous and not a quantized representation of an analog signal shown as a digital code. But given that a comparator can be viewed as a 1 bit ADC there is that fuzzy "Welllll......yeah but..." feeling about it.

I hope this is a clarification but I probably did not do a good job.

No I have not. I am a young lad in the business. But, I have more reading to do now. :-)

Exactly.

Reply to
Wayne

Wayne wrote: (snip)

But almost every device in use as a digital unit, has an analog input, if you are careful enough with how you vary its input signal. Purely digital devices are just analog units intended to operate with signals that over drive them, so that you can neglect everything that happens between a 1 and a zero, except, possibly elapsed time.

(snip)

Is is very common.

There is almost nothing else. Name a device that has no analog properties to its input response.

Quantum mechanics is truly digital. Everything else is an analog approximation of digital. The harder you push digital electronics, the more you deal with the approximation, and become an analog designer.

Reply to
John Popelish

"Wayne"

** For a word to have an unambiguous meaning the context of its usage MUST be defined.

The word "digital" might refer to the fingers or toes or the numerals displayed on a mechanical clock.

Essentially, words mean what PEOPLE mean when THEY use them.

Here are a few **different definitions** covering many of the CONTEXTS where one finds the word "digital" being used.

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Pulling a word OUT OF CONTEXT and then trying to define it is fool's errand.

Anyone got a left had screwdriver ......

........ Phil

Reply to
Phil Allison

Well microcontrollers and DSP's are digital devices and do have some analog inputs. TI's TMS3206416 DSP, I believe, do not have any analog inputs: GPIO, Clock and Data lines, etc. And I can see what you are saying with the nature of the signals...reminds me of a joke that digital engineers only need to worry about 1's and 0's while and analog engineer needs to worry about everything else in between. But this still keeps things fuzzy.

So, to keep from getting to far off track (Phil brought up a good point about context) I guess I should say that I am interested in a general defintion of the difference between an analog device and a digital device that is accepted throughout the semiconductor industry; although, I am sure there will always be those 'gotchas.'

This is why I mentioned that most, albeit, not all definitions I come across specifically describe 'digital' as a set or series of 1's and

0's that represent data. Phil found many with his link and I tend to lean in this direction to steer clear of that fuzziness of analog VS digital.

Not to beat a dead horse, but the 1-bit ADC analogy for our comparator can fall into this category because what makes it different from

4-bits, 8-bits so on and so forth? 1-bit is the smallest series of set of data :-).

Well taken. :-)

Then what of a microcontroller even moreso with a DSP? I know there are several DSP's from TI and ADI that have ADC's or comparators that are integrated into the die but all the other inputs are clock/data lines, S/PDIF, EMIF, etc. I don't think a good definition should consider how many analog inputs VS digital inputs; although, you said 'analog response' can you clarify?

Because let's face it, on the general level people do not get down and dirty into discussions like this...maybe it is not worth the time because it is obvious? But if you were to ask a young gun like me, is a comparator an analog or digital device, I would say analog. DSP? Digital. The key issue is 'Why?' What definition is there to go off of?

Digital engineers wouldn't have it! :-)

Reply to
Wayne

"Wayne"

** Well that chucks your silly example of the light switch right out the door.

You still fail to recognise the importance of context - it is not the device that determines that alone, it is also how it is being used.

Essentially, *words* mean what PEOPLE mean when THEY use them !!

If a component maker places something into the "digital" category, it is because he considers that is how it will most likely be used by buyers. A device or circuit is called " digital " if that title fits in with accepted industry practice.

Context is the main issue, but also the need for consistency to avoiding ambiguity and misleading people is just as important.

....... Phil

Reply to
Phil Allison

relayus are both digital and electromechanical.

light switches are doubly digital, as digits ( = fingers ) are used to actuate them. :)

all digital signals are analog signals.

I've seen hex inverters used for the gain stage in preamps...

no, it just has very high gain, with the right input it'll give an in-between output voltage.

one is a certain number, in many applications one bit would be sufficient to represent the output state of the device at any instant.

analog or digital the device is what you make of it,

with a little fidling a lightswitch can sometimes be put in an in-between position where the contacts arc and the lamp only lights partially...

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   Jasen
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Reply to
Jasen Betts

Lots of discussion and hair splitting going on. The switch is an electro-mechanical device and if there was a dimmer on the switch , it would also be an electro-mechanical device. In normal use the switch would have a digital output (one of two states, off/on in normal operation) while the dimmer would have an analog output as it has many possiable voltages as the output.

Reply to
Ralph Mowery

Digital and analog are not descriptions of physical objects; they describe ways of encoding information.

And yes, in that sense a toggle switch is a digital device; its purpose is to have only 2 states and no intermediate states.

We usually don't use the terms "digital" and "analog" until we've gotten to the level of something more complicated than that, though.

Reply to
mc

The first form of long distance electrical communications was digital.

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Reply to
Michael A. Terrell

Ok, let me give you an example where I think you are going with this. Me, stick, hollow log, jungle. I beat on the hollow log with a stick to warn my fellow survivors on Lost Island that a man eating cat is on its way to the shelter. The beats are short and long pulses that represent words (no different than telegraph, just a different medium)...digital transmission in the strictist sense. Ok, fair enough. You would argue that this is a digital device because, in this context, what am I doing with the stick and log...transmitting digital data. Taking the same stick and log I now play music to celebrate our kill of the man eating cat. Digital? I would say analog in this context as the notes I create with the stick from beats and rasps are all continous in nature.

with you. I would argue that in both cases, the device (stick and hollow log) is analog but the implementation of that device is different. In one case you have an analog device implemented in an analog way and in the other case, you have an analog device implemented in a digital way. Much like an AND gate or a comparator etc.

Not to be presumptuous, but it would seem John would agree. We both agree that a comparator and similar components that are analog devices that can be viewed as having digital outputs. This would seem to jive with my stick and log example, no? Take this back to the light switch and I would come to the same conclusion.

I agree that this can make discussions harder than they should be. But I try to clarify when people find what I say confusing.

Then this would make one think of the devices primary function. But this can certainly be fuzzy which is why I started this whole topic to begin with...to see what others opinions are and see if we can iron out a good 'definition.'

Reply to
Wayne

"Wayne"

** Hey, Wayne - you're an idiot.

** Me Tarzan - you Wayne

ROTFL.....

Hey, Wayne - you're an idiot.

( snip excruciating pile of asinine drivel)

** Wayne has missed the point entirely.

Hey, Wayne - you're an idiot.

** Hey, Wayne - you're a f****ng idiot.

A another bloody TROLL.

PISS OFF !!

......... Phil

>
Reply to
Phil Allison

Phil Allison wrote: (snip)

Sigh. You guys are everywhere.

Reply to
Wayne

"Wayne"

** Me Tarzan - you Wayne

ROTFL .....

Hey, Wayne - you're an idiot.

....... Phil

Reply to
Phil Allison

Mr. Popelish could have given you some other facts to support his statements, too. In fact, an "open" switch actually has capacitance between the switch contacts, as well as potential small leakage currents between the switch contacts and from the contacts to the switch case and toggle switch bat. An open relay (an electromechanical switch) has capacitance between contacts, and also contact-to-coil. This capacitance is usually insignificant at DC or line frequencies, but becomes important for certain high frequency switching applications.

Also, a closed switch isn't purely digital -- it has contact resistance which will increase as the switch contact becomes worn, leading to losses in the form of heat and eventual switch failure due to thermal runaway. Exceeding rated switch current can also lead to increased heating of contacts, loss (voltage drop) and accelerated failure. And a switch or relay contact does have a characteristic impedance -- again, not important at line frequencies, but important if using a relay to switch video or other high frequency signals.

Take the time to keep learning. It might be best not to get stuck in the "either digital or analog" trap. Both perspectives are of value in proportion to how well they work for the problem at hand.

Good luck Chris

Reply to
Chris

No, it's not a silly title, and in fact this is something that very few engnineers or engineering students ever really stop to think about

- and as a result, many notions about what is and is not "digital" or "analog," and what the real advantages or disadvantages of the two are, are wrong.

When you get right down to it, the only distinction between "digital" and "analog" that makes any sense at all is to consider these as two different ways of ENCODING INFORMATION. That's it. That is ALL they are. Signals or circuits, per se, are NOT "digital" or "analog." Electricity is electricity, and all signals and circuits behave according to the exact same set of physical laws no matter how we are supposed to interpret those signals or how we design the circuits that carry and manipulate them.

The real meaning of both words is pretty clear when you really consider the words themselves. In an "analog" system, the value of one parameter (voltage, say), varies in a manner *analogous to* the variations of another parameter (sound, perhaps - or more correctly, air pressure, in this case). The two are varying in a like manner, so one is an "analog" of the other. "Analog" does not necessarily mean or imply "linear" or "continuous" - both perfectly good words, but they mean what THEY mean and should be used appropriately. You can have sampled, discontinuous analog systems, and God knows that there are plenty of non-linear analog systems out there are well.

Similarly, "digital" simply means that we are dealing with "digits" - the the information is being conveyed in a manner such that it can be interpreted directly as numeric values, but without the signal being varied in a "like" manner to the original in the sense of the above. The most common "digital" representation is binary, which is a two-state system - in electrical terms, "on" or "off," "high" or "low," etc. - but don't make the mistake of believing that a simple binary representation is all there is to "digital." (Consider, for instance, just what sort of signal comes out of a modem, or what's being transmitted in an 8-VSB or 64-QAM "digital" transmission.) "Digital" also does not necessarily mean "discrete" or "sampled" or so forth, even though most practical digital systems also exhibit these characteristics.

With this distinction in mind, a lot of the supposed advantages or disadvantage of the two systems are seen to be mistaken, or at the least attributable to something other than the two encoding methods. "Analog" signals do NOT provide "infinite" precision or accuracy, or anything close to it. "Digital" system are also not the perfectly, completely-immune-to-noise-and-distortion things that many believe them to be. You start to understand such things as channel capacity in the presence of noise in a new light, and see how these two compare when you actually are doing an apples-to-apples (i.e., similar data rates, etc.) comparison. And you stop saying silly things like "well, the world itself is digital (or analog), therefore..." The world is the world; what we're talking about are two different means of representing a description of it.

Another comparison I've used with students before, to show the distinction between the two, is to use examples which have nothing at all to do with electronics. Let's say you need to record a changing temperature over some period of time, perhaps 24 hours. There are at least two ways you could do this:

- You could rig up a chart recorder; maybe you've got a pencil that's attached to some sort of device that moves according to temperature, and you arrange for a piece of paper to be dragged under the point at a constant rate. This is an "analog" recording, right?

- Another way you could do it is to get yourself a thermometer (exactly how IT works is irrelevant - let's assume, though, that it can deliver accuracy and precision well beyond what you can right down), and simply record the temperature readings in a book every five minutes. You always record the temperature to the same number of significant digits, by the way. What you have HERE is a "digital" recording.

A little thought will show that neither system is perfect, and that both have their limitations; they also both have certain advantages that may make one a better choice that the other, for certain situations. "Better" is ALWAYS a matter of "what works best given the criteria I'm dealing with RIGHT NOW." NEITHER system is "better" than the other in an absolute sense.

Think about it, and see what answers you come up with...

Bob M.

Reply to
Bob Myers

(snip)

Thanks Bob and to everyone else. You have given me a lot to read and hash through (unlike some other people).

Reply to
Wayne

By your definition digital ICs aren't digital.

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Reply to
Michael A. Terrell

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