What exactly is digital anyway?

Discussion in 'Electronic Basics' started by Wayne, May 7, 2006.

1. WayneGuest

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!

2. Jonathan KirwanGuest

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
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

3. John PopelishGuest

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.
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?
http://mypage.bluewin.ch/sagnell/id14.htm
They are digital, but have 10 unique output states.
Especially when many micros now contain analog to digital converters
and comparator functions as inputs.

4. WayneGuest

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
Exactly.

5. John PopelishGuest

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.

6. Phil AllisonGuest

"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.

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

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

......... Phil

7. WayneGuest

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
'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!

8. Phil AllisonGuest

"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

9. Jasen BettsGuest

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...

10. Ralph MoweryGuest

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.

11. mcGuest

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.

12. Michael A. TerrellGuest

The first form of long distance electrical communications was
digital.

--
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

13. WayneGuest

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.'

14. Phil AllisonGuest

"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 fucking idiot.

A another bloody TROLL.

PISS OFF !!

.......... Phil

15. WayneGuest

Phil Allison wrote:
(snip)

Sigh. You guys are everywhere.

16. Phil AllisonGuest

"Wayne"

** Me Tarzan - you Wayne

ROTFL .....

Hey, Wayne - you're an idiot.

........ Phil

17. ChrisGuest

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

18. Bob MyersGuest

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"
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.

Bob M.

19. WayneGuest

Bob Myers wrote:

(snip)

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

20. Michael A. TerrellGuest

By your definition digital ICs aren't digital.

--
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