isw <isw@witzend.com> wrote:
> floyd@apaflo.com (Floyd L. Davidson) wrote:
>> >
>> >The SIGNALS are electrical or optical.
>>
>> Really?  Nothing could be acoustical?  Are you deaf?

I should have put a smiley on that, sorry for missing it.

>> Regardless, that does not address the incorrectness of
>> stating that all signals are analog.  Morse code is not
>> analog.
>
>The carrier or tone that is keyed on and off to send the code starts out

Morse code does not necesarily have either a carrier or
a tone involved, but we can ignore that for this
discussion without changing the validity of our
conclusions.

>at a certain strength at the transmitter and grows weaker in a
>continuous fashion as the receiver moves further and further away, until
>at some point it becomes impossible to understand the *message* it is
>carrying.

Well, lets take exactly that as an example, because it
is a good one.  We could use a tone as the carrier if
you like, and send it down a regular twisted pair cable.
I'm going to describe this for Morse Code signaling, but
I'd like to point out that virtually any FSK modem does
exactly the same thing with exactly the dynamic range
I'm describing here.  Instead of on/off though, it uses
two tones.  Everything else is the same, except the
modem is many times faster than a human can decode Morse
Code.

If we put it on the cable at 0 dBm, we'll likely have an
SNR of roughly 50 dB or so, plus or minus a few.

The message is sent using on/off keying of a tone, so at
the cable head we have a 50 dB range which is used to
determine on vs. off.  If we head down the road several
miles and get to a point where the signal level has
dropped 10 dB (about the maximum that can be used by a
POTS line), we now have a 40 dB SNR range to deal with.
We could go twice that distance again (losing 10 dB of
signal each time) and get to a point where our signal is
-30 dB and we have only a 20 dB SNR.

At 20 dB SNR there is no reason at all that you won't
get perfect copy, with no errors.  Clearly the *signal*
has not changed, even though it has dropped 30 dB in
power.  That is because the symbols used are discrete.

From perhaps -40 dBm to 0 dBm there is *no* *change* *in*
*the* *value* *of* *the* *symbols*!

>That is, the signal is analog. How can it be digital if it can take on
>*any* value?

Obviously it does *not* take on any value.  The value
for Morse code is either on or off.  There is no "on at
-22.4 dB" value, just on.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

isw <isw@witzend.com> wrote:
> floyd@apaflo.com (Floyd L. Davidson) wrote:
>In both of those (as they are actually used in the real world),
>communication is accomplished by the propagation along them of
>electromagnetic fields; never anything else.
>
>Doesn't matter one whit whether you turn the field on and off, or vary
>the amplitude or any other characteristic of it continuously, as a means
>of sending a message from one end to the other, those fields can take on
>*any value* from the maximum level injected into the cable by the

Yes, the electrical fields can take any value.  It is
inherently an analog medium.  But that has no
relationship to the signal which is used to send a
message.

The *signal* does or does not have the ability to take
on various values.  If the signal uses discrete symbols,
it is a digital signal.  If the symbols have a
continuous range of values, it is an analog signal.

This is not an insignificant distinction.  It precisely
the reason that at Bell Labs Claude Shannon studied the
theory of how the two differ.  As a result of his Theory
of Information the telecommunications industry began to
develop the technology required to implement digital
system to replace the existing analog systems.  They did
that based on what Shannon had shown to be theoretically
the most effective for telecommunications.

Digital systems typically trade SNR (which can be very
low) for bandwidth (which will be very high) compared to
using analog signaling.

The inherent noise immunity of a digital system is
great, and because the analog noise in the medium is
*not* directly proportional to the signal value, a
digital signal can be transmitted with zero errors (due
to noise) if the SNR on the analog medium is above a
minimal level.  It happens that that SNR is so low that
a system using analog signals would be unusable at the
same SNR.  (Fiber optic cables are an example, where
they are virtually useless with analog signaling for the
typical long circuit lengths that are provided when
digital signaling is used.)

The actual minimum SNR depends on the type of digital
encoding used.  But some typical values for various
communications purposes are interesting to look at.  A
dialup telephone connection is supposed to have at least
a 24 dB SNR.  That is relatively useful for voice
communications, but a typical dialup modem won't work
very well unless the SNR of the connection is above 30
dB above random noise (because it has been converted to
a bandwidth limited analog signal).

On the other hand a binary polar signal (such as the
RS-232C digital interface to that dialup modem) will
have an error rate of less than 1 in 10^5 with an SNR of
only 9.5 dB.

But that isn't even the most significant benefit!  Noise
is additive on an analog system, but not on a digital
system, which is specifically the difference between
digital and analog that has revolutionized all
telecommunications in the years since Shannon showed
that digital was superior.

What that means is if we use 5 analog channels tandem
linked to get our message from one location to another,
the end to end noise must be added to determine the SNR,
and that total SNR must meet the above criteria for a
higher SNR than is needed by a digital system.

But if 5 digital channels are tandem linked, only the
errors are additive and not the noise.

That is, with analog both the noise and the errors in
the first link are sent to all succeeding links, and
that noise causes errors in each link on analog system.
On a digital system only the errors are inputted to the
succeeding links but not the noise, so noise in the
first link does not cause errors in the succeeding links
as it does with an analog system.

>transmitter down to far below the ambient noise level, depending (for
>example) on the length of cable being used. IOW, those signals are,
>without exception, *analog*.

No, those signals are digital if the symbols they use
are discrete.  The fact that the voltage, for example
can range from 0 to 1 volt has no significance in terms
of the signal *if* that signal uses exactly two symbols,
one of which is represented by any voltage less than 0.4
volts and one of which is represented by any voltage
greater than 0.6 volts.  That describes a digital signal
(which indeed is being sent through an inherently analog
channel).

              encoder     medium     decoder
             +------+               +------+
             |      |               |      |
  input >----+      +---------------+      +----> output
             |      |               |      |
             +------+               +------+

                    |               |
                    |<---Analog --->|
                    |    Channel    |

       |                                        |
       |<--------------- Digital -------------->|
       |                 Channel                |

Typical examples of the above are where the input to the
"encoder" is a DS1 and the "medium" is a twisted pair
cable, or where that input is an OC3 and the "medium" is
a fiber optic cable, or where the encoder is a satellite
modem and the medium is a "bent-pipe" geosynchronous
satellite.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

"Mr.T" <MrT@home> wrote:
>"Floyd L. Davidson" <floyd@apaflo.com> wrote in message
>news:87r6lojir0.fld@apaflo.com...
>> >> Yes, you do have an infinite number of *voltages*
>> >> between 3 and 18,
>> >
>> >Only in theory, in practice nothing is infinite in this universe.
>> >Noise will set the resolution limit.
>>
>> The noise itself has an infinite number of possible
>> values,
>
>OK, prove it.

Eh?  You seem to have misunderstood what was said.  It
has *nothing* to do with resolution.

It has to do with the fact that no matter what level the
noise is, it could be either a little bit more or a
little bit less.  That means whatever value you think
you have resolved, could in fact actually have had two
other possible values.

Which of course means that the number of voltages
between 3 and 18 is indeed infinite, with or without
noise.

Your ability to resolve those values is an entirely
different topic.

>> and therefore even if the signal itself is
>> supposed to be just 1 value, add the noise and there are
>> an infinite number of values.
>
>Only for those who failed mathematics at high school.

Add a random number with an infinite range of possible
values to *anything*, and you have a result with an
infinite range of possible values.  Pretty simple math.

I'm sorry to hear that you didn't do well with math in
highschool.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

In article <87sl63i4mb.fld@apaflo.com>,
 floyd@apaflo.com (Floyd L. Davidson) wrote:

> "Bob Myers" <nospamplease@address.invalid> wrote:
> >"isw" <isw@witzend.com> wrote in message
> >news:isw-00A66F.10310528082007@newsgroups.comcast.net...
> >> In article <87mywcjdq7.fld@apaflo.com>,
> >> floyd@apaflo.com (Floyd L. Davidson) wrote:
> >>
> >>> Most physical channels are inherently analog!  Wire
> >>> cables and fiber optic cables are two examples.  Digital
> >>> signals are commonly sent via either of them.
> >>
> >> I'll probably regret jumping in here, but:
> >>
> >> The *message* may be digital, but the *signals* are most definitely
> >> analog.
> >
> >The SIGNALS are electrical or optical.
> 
> Really?  Nothing could be acoustical?  Are you deaf?
> 
> Regardless, that does not address the incorrectness of
> stating that all signals are analog.  Morse code is not
> analog.

The carrier or tone that is keyed on and off to send the code starts out 
at a certain strength at the transmitter and grows weaker in a 
continuous fashion as the receiver moves further and further away, until 
at some point it becomes impossible to understand the *message* it is 
carrying.

That is, the signal is analog. How can it be digital if it can take on 
*any* value?

Isaac

In article <87wsvfi4rc.fld@apaflo.com>,
 floyd@apaflo.com (Floyd L. Davidson) wrote:

> isw <isw@witzend.com> wrote:
> >In article <87mywcjdq7.fld@apaflo.com>,
> > floyd@apaflo.com (Floyd L. Davidson) wrote:
> >
> >> Most physical channels are inherently analog!  Wire
> >> cables and fiber optic cables are two examples.  Digital
> >> signals are commonly sent via either of them.
> >
> >I'll probably regret jumping in here, but:
> >
> >The *message* may be digital, but the *signals* are most definitely
> >analog.
> 
> That is not correct.  Whether a message is or not
> digital is entirely unrelated to whether the signal used
> to transmit it is analog or digital (and it can indeed
> be either, without regard to the message).

You specifically mentioned "wire cables and fiber optic cables", so lets 
talk about those and ignore other possible transmission media.

In both of those (as they are actually used in the real world), 
communication is accomplished by the propagation along them of 
electromagnetic fields; never anything else.

Doesn't matter one whit whether you turn the field on and off, or vary 
the amplitude or any other characteristic of it continuously, as a means 
of sending a message from one end to the other, those fields can take on 
*any value* from the maximum level injected into the cable by the 
transmitter down to far below the ambient noise level, depending (for 
example) on the length of cable being used. IOW, those signals are, 
without exception, *analog*.

Isaac

"Floyd L. Davidson" <floyd@apaflo.com> wrote in message
news:87r6lojir0.fld@apaflo.com...
> >> Yes, you do have an infinite number of *voltages*
> >> between 3 and 18,
> >
> >Only in theory, in practice nothing is infinite in this universe.
> >Noise will set the resolution limit.
>
> The noise itself has an infinite number of possible
> values,

OK, prove it.

> and therefore even if the signal itself is
> supposed to be just 1 value, add the noise and there are
> an infinite number of values.

Only for those who failed mathematics at high school.

MrT.

"Bob Myers" <nospamplease@address.invalid> wrote:
>"isw" <isw@witzend.com> wrote in message
>news:isw-00A66F.10310528082007@newsgroups.comcast.net...
>> In article <87mywcjdq7.fld@apaflo.com>,
>> floyd@apaflo.com (Floyd L. Davidson) wrote:
>>
>>> Most physical channels are inherently analog!  Wire
>>> cables and fiber optic cables are two examples.  Digital
>>> signals are commonly sent via either of them.
>>
>> I'll probably regret jumping in here, but:
>>
>> The *message* may be digital, but the *signals* are most definitely
>> analog.
>
>The SIGNALS are electrical or optical.

Really?  Nothing could be acoustical?  Are you deaf?

Regardless, that does not address the incorrectness of
stating that all signals are analog.  Morse code is not
analog.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

isw <isw@witzend.com> wrote:
>In article <87mywcjdq7.fld@apaflo.com>,
> floyd@apaflo.com (Floyd L. Davidson) wrote:
>
>> Most physical channels are inherently analog!  Wire
>> cables and fiber optic cables are two examples.  Digital
>> signals are commonly sent via either of them.
>
>I'll probably regret jumping in here, but:
>
>The *message* may be digital, but the *signals* are most definitely
>analog.

That is not correct.  Whether a message is or not
digital is entirely unrelated to whether the signal used
to transmit it is analog or digital (and it can indeed
be either, without regard to the message).

"Message" specifically means a complete set of data
formatted for transmission, and is not related to
analog/digital data signals.

For the term "signal", you can choose from several
definitions (FS-1037C):

    signal:

        1. Detectable transmitted energy that can
           be used to carry information.

        2. A time-dependent variation of a
           characteristic of a physical phenomenon,
           used to convey information.

        3. As applied to electronics, any transmitted
           electrical impulse.

        4. Operationally, a type of message, the text
           of which consists of one or more letters,
           words, characters, signal flags, visual
           displays, or special sounds, with prearranged
           meaning and which is conveyed or transmitted
           by visual, acoustical, or electrical means.

Hence you can see that using "message" and "signal" in
the same sentence is bound to cause confusion in the
context of this particular discussion.  It simply does
not mean what you were thinking of.  When used properly
the terms "signal" and "message" would be something like
this, "Our actions are intended to send Congress a
message, and we wish to signal our intense displeasure
with corruption."

But we've been discussing signals that meet either
the 1st or 2nd definition above, and specifically not
numbers 3 or 4.

In context, the signals are either digital or analog,
and which they are depends mostly on whether the data,
or individual parts of the information (message) that
the signal carries, is digital or analog.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:
>Floyd L. Davidson wrote:
>>glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:
>>>Floyd L. Davidson wrote:
>
>>>Previously when I was in these discussions instead of
>>>digital vs. analog it was modulated (and so in need of
>>>a modem) vs. not modulated.
>
>> Okay...
>
>>>Passing a digital signal
>>>through an analog channel is said to require a modulated
>>>carrier.
>
>> That is not necessarily true.  The problem has nothing
>> to with analog vs digital.  It instead a question of
>> whether DC can be amplified or only AC, and over what
>> bandwidth, on that analog channel.
>(snip)
>
>When DSL started to become popular, there was discussion
>that a DSL modem wasn't a modem because the DSL signal
>was digital.  Everyone (just about) knows that v.90 needs
>a modem because it goes through the voice telephone system.

Everyone who thinks that is wrong.  A v.90 modem is
digital on both sides, and will not pass through a
"voice telephone system".  It requires a *digital*
switching system to work.  V.90 is not a D/A-A/D
protocol, it is a digital level encoding scheme.

(Indeed, a lot equipment originally used for 56Kbps
digital services was often called a "modem" by not just
customers, but also by telecommunications people.  None
of them were technically modems, they were all level
changers, with digital signals in one side and out the
other in a different, but equally digital, format.)

>But DSL does use a modulated carrier, and the box is
>a modem.

But is that because it has a digital side, or is that
because the bandwidth restrictions of the channel cannot
pass the input signal due to the low frequency
components?

It is a bandwidth problem, and it has nothing at all to
do with digital or analog.

In fact, a v.90 modem sends an entirely digital signal
down that very same line.  Of course in the process it
necessarily uses the same bandwidth that on a DSL is
allocated for a normal voice channel, and therefore
while DSL can co-exist separately with POTS the v.90
modem cannot.

>> However, I do think that your last example, discussed
>> below, appears to be a valid example of an analog
>> representation of digital signal, except it is not an
>> electronic representation...
>
>(snip)
>
>> Any time you encode discrete values from a finite set,
>> that is digital.  Period.  Whether it can be passed over
>> an inherently analog channel or not is fairly
>> meaningless.  The voltage, phase, or whatever that is
>> encoded with the information may have only a discrete
>> set of values for the information, but they obviously
>> take on an infinite number of possible values for the
>> characteristic itself.
>
>But why phase modulation, for example?

It has *nothing* do to with whether it is digital or
analog...

>Two reasons
>that I see.  One is that the channel does not have
>the appropriate frequency response, and the other is
>the need for a clock.

Phase modulation does not uniquely conserve bandwidth
(Manchester encoding uses twice the bandwidth of NRZ,
for example) nor is it unique in the ability to recover
a clocking rate from the data.

>Consider 10baseT ethernet.
>Phase modulation allows for transformer coupling that
>is needed to avoid ground loops and ensure a balanced
>signal to avoid EMI problems.

Alternate mark inversion (AMI) provides the same
characteristics.

But while all of this is indeed very interesting
stuff...  it has *nothing* to do with analog vs. digital
or the definitions of either.  I don't see any point to
your discussion.

>Using synchronous
>phase modulation allows for the recovery of the clock
>from the signal, which is also important.

That is one way to do it, but there are others.

>If, for example, the signal was not modulated and one
>decided to send 1000000 zeros in a row, there would be
>no way to recover the clock to know how many zeros

There would be no way *only* if you select an encoding
scheme such as NRZ.  Manchester encoding provides for
easy clock recover, but so do other encoding schemes.

>were sent.  If you can't separate the bits, you are
>losing an important part of a digital signal.

That is not true.  Consider AMI using B8ZS encoding...

>> Hence the voltage on a binary system carries only two
>> values, on and off, but that is the value of the
>> information.  The voltage that is encoded might be +3 to
>> +15 for an on and -3 to -15 for an off.  And when the state
>> switch from on or off to the opposite value, those
>> voltages do not change instantly, and they do cover an
>> infinite number of voltages during that change.
>
>> That is an infinite number of possible voltages, but
>> they have a value of either on or off.
>
>> The information values are what makes it a digital signal.
>
>> But indeed, you can pass that signal through an analog
>> amplifier.  Depending on the characteristics, it may or
>> may not destroy the information.
>
>If the information is destroyed, then the signal
>didn't get through.

Of course.  But it has *nothing* to do with the amplifier
being analog.  There *are* analog amplifiers that will not
destroy it.  (And that is no different for analog data either,
which will also be destroyed if the amplifier does not have
suitable characteristics to pass it.)

>> Obviously the
>> amplifier would need to pass DC voltages unless we
>> encode the information in a way that guarantees some set
>> minimum time interval between state changes (T1 digital
>> carrier systems typically do that, for example).

Don't ignore what has already been made clear!

>>>signal instead of an analog channel with a modulated
>>>signal?  I might believe it for NRZ, but just about
>>>anything else I don't.
>
>> If the digital signal has DC components it can be
>> modulated onto an analog carrier to pass through an AC
>> coupled analog channel.  It could also be re-encoded
>> in a manner that will pass through an AC only channel,
>> and then be transmitted over the same AC coupled analog
>> channel.
>
>> Most physical channels are inherently analog!  Wire
>> cables and fiber optic cables are two examples.  Digital
>> signals are commonly sent via either of them.
>
>Some can pass a digital signal without modulation,
>others can't.

But is has nothing to do with analog vs. digital.  If the
amplifier cannot handle the signal's bandwidth, it makes no
difference if the signal is analog or digital, it will not
"pass" the data from input to output.

You are trying to impute something to digital that is
actually common to analog as well.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com