 |
Search Sci.Electronics.Basics |
|
|
|
|
|
|
 |
|
|
Sci.Electronics.Basics -> Questions about equivalents of audio/video and digital/analog.
There are 304 messages in this thread.
You are currently looking at messages 300 to 304.
|
Author: iswDate: 01:43 29-08-07
|
|
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
|
|
|
|
Author: Floyd L. DavidsonDate: 01:48 29-08-07
|
|
"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
|
|
|
|
Author: Floyd L. DavidsonDate: 05:43 29-08-07
|
|
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
|
|
|
|
Author: Floyd L. DavidsonDate: 05:59 29-08-07
|
|
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
|
|
|
|
|
|
|
|
|
Contact | Electronic Portal
|
|
|
