The telecom people would find it very amusing to see how
much their customers take for granted. They work very hard
and use many tricks and techniques to keep power-mains hum
out of phone loops. Apparently they are doing a good job if
we think that it is effortless.
As with various of the other statements I have seen in this thread on
various sub-topics, the above seems to me to be an over-simplification.
Interesting to speculate if in this case it is the above statement that is
ambiguous, or the ways in which the terms are actually used by engineers
are ambiguous... Perhaps this supports the argument that people become
engineers because they can't communicate very well... :-)
If you go back to some of the early sources [e.g. 1] then you can find some
that describe what is observed by the receiver/destination as something
like a 'received signal' which may include some 'noise' (and some
distortion or other systematic alterations).
However the sources also routinely refer to 'signal to noise' ratio.
Shannon seems to resolve this by distinguishing between the 'signal' (i.e.
what the source transmitted) and the 'received signal' (i.e. what the
destination actually observed to arrive).
So if we were to use a term like 'received signal' in the above statement
it would essentially become either a tautology or self-referential as the
signal includes the noise. Thus the problem with the statement is that it
is unclear due to the ambiguous use of 'signal'. Hence, as often is the
case with such ambiguous statements, people start arguing about the meaning
when they are simply using different definitions which the ambiguity
FWIW for the above reason, when teaching Information Theory/ Comms/
Instrumentation I tended to use another approach which is common in the
area. This is to say that a 'signal' means that the pattern (or part of the
pattern) *is used to convey information content*.
Thus in the context of communications a 'signal' means that the sender and
destination have to have pre-agreed the coding/modulation system to be
employed, and the meanings of the code symbols or distinguishable patterns.
In the context of a physical scientist making observations - e.g. an
astronomer observing what can be received from a distant radio galaxy - the
'signal means that the observed pattern will be used to obtain information
about the distant source.
The status of 'signal' then stems from the deliberation or requirement that
it conveys information on a defined basis.
In both contexts what distinguishes 'signal' from 'noise' is the
information conveyance the 'signal' provides, and that 'noise' tends to
obscure, or limit, or make uncertain, the information recovery. This then
helps make clear the actual meaning in practice of terms like 'signal to
noise ratio'. (Although there may then be hours of fun for all the family
as they argue about the distinction in this phrase between assuming
'signal' means either the intended/transmitted or the 'received' signal.
 e.g. Shannon
 Probably best at this point not to start worrying about distortion as
being 'signal' or not... ;->
Not to mention that "no hum at all" is only in the perception of
the customer, whereas telco people tend to actually measure it.
Granted though, a telephone installer just uses a very simple
test set that gives a "good/bad" indication, not a specific
number. And that would be the most that a customer would likely
ever see. But when a cable is installed the pairs are very
specifically measured and compared against design
specifications, which were calculated very closely prior to
construction. Nobody wants to invest in new cable plant and end
up with a cable that can't be used...
Actually the language is probably a bit *too* precise for
non-engineers... and it gets worse too, because nobody had
mentioned "distortion" until your article.
First, here are correct technical definitions, from Federal Standard
1037C, for signal, noise, and distortion. (Just be aware that they
don't necessarily mean what one might thing!)
1. Detectable transmitted energy that can be used to
2. A time-dependent variation of a characteristic of
a physical phenomenon, used to convey information.
3. As applied to electronics, any transmitted
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.
Note that it is something that "can be used to carry information",
but there is no requirement that "information" either be present or
The energy used for AC power *is* a signal. In this thread
*all* references to hum (which clearly *does* carry information,
otherwise we would not be able to hear it and distinguish that
it as unique) and to "power line" or "AC" are always correctly
referred to as a "signal", and may or may not be a "noise"
depending on the circumstance.
1. An undesired disturbance within the frequency
band of interest; the summation of unwanted or
disturbing energy introduced into a communications
system from man-made and natural sources.
2. A disturbance that affects a signal and that may
distort the information carried by the signal.
3. Random variations of one or more characteristics
of any entity such as voltage, current, or data.
4. A random signal of known statistical properties of
amplitude, distribution, and spectral density.
5. Loosely, any disturbance tending to interfere with
the normal operation of a device or system.
Each of those definitions carries some baggage, which usually
goes unnoticed until someone gets pedantic about technical terms.
Definition 1, the most precise and restrictive definition,
requires that the disturbance be "introduced", which implies
that it originate external to the circuit itself. That is the
difference between "noise" and "distortion", when the two are
differentiated. Generally though, a distortion is a noise, but
a noise is not necessarily a distortion. (Much as a signal
might be noise, but noise is not necessarily a signal.)
Definition 2 includes the term "distort". Definitions 3 and 4
use the term "random". And definition 5 is the more commonly
used catch all term.
1. In a system or device, any departure of the
output signal waveform from that which should
result from the input signal waveform's being
operated on by the system's specified, i.e.,
ideal, transfer function.
Note: Distortion may result from many mechanisms.
Examples include nonlinearities in the transfer
function of an active device, such as a vacuum
tube, transistor, or operational amplifier.
Distortion may also be caused by a passive
component such as a coaxial cable or optical fiber,
or by inhomogeneities, reflections, etc., in the
2. In start-stop teletypewriter signaling, the shifting
of the significant instants of the signal pulses
from their proper positions relative to the beginning
of the start pulse.
Note: The magnitude of the distortion is expressed in
percent of an ideal unit pulse length.
The significance of the distinction between noise and distortion
might be lost on anyone but a design engineer, or perhaps a
theoretical physicist. At a maintenance and operations level,
it makes no difference.
Ahem, Shannon is an "early source"???? Telecommunications as we
know it today was a hundred years old by the time Shannon began
publishing! And that has only been ~60 years now. I spend many
years working on equipment that was designed before Shannon...
Shannon does not exclude noise from being a signal. He merely
uses the proper terms to distinguish between different signals,
with the realization that we have no interest in the information
carried by some signals... :-)
What is commonly called "Signal to Noise Ratio" is commonly more
correctly called "Signal + Noise to Noise Ratio". In
circumstances where the ratios are greater than, say, 15-20 dB
or so, it is of little importance. Hence in typical
telecommunications voice channels it is rarely considered. On
the other hand in some data circuits and when applied to noise
figures for microwave radio receivers, where the ratios are much
smaller, the fact that the signal is actually Signal + Noise is
Ah, but ignorance on the part of some is not the fault of those
who actually *are* using the term without ambiguity. Some
posters, Don Pearce being the most obvious, have not understood
the term and have been confused, and made efforts at confusing
But that doesn't mean the terms are actually ambiguous.
Note the difference between something that "can" and something
that "is". Also, "information" seems to be misunderstood in
that definition... if you are suggesting that "hum" is a noise
that does not contain information, which is not the case. :-)
That would not fit the typical way the term is used in practice by
people who work in the telecommunications field.
Again, "can" is appropriate, but "will be" is going to cause a
That is too restrictive.
And it might well be the information carried by the noise signal that
makes the information from the desired signal uncertain...
Everyone who has any interest in effective communications should
study what Claude Shannon summarized. It is absolutely
fascinating to read.
Can it contain information?
Distortion can *always* be counteracted by the introduction of
an "error signal" which is opposite to the distortion.
Therefore it would seem that distortion is necessarily a signal
in all cases.
Wow, that brings back memories! The Shannon Day conference/celebration
was quite an interesting event. Now I'm going to have to dig through my old
files for that packet of papers.
I'd tend to say that distortion adds to noise side of the SNR, and
some can be corrected ...but *always* ? Let's say the distortion
is the result of clipping...
[ ...or maybe I've missed your point. ]
Absolutely always. Recall that distortion is a known condition
resulting from the communications channel itself. The output is
known *before* the signal is input. (E.g., clipping is not
arbitrary, and produces a very specific error signal.)
Which of course is something Shannon describes, and uses in
examples, in "A Mathematical Theory of Communication".
Seems I must have missed something.
From my reading of that paper it would seem that it is only the case
in a closed loop, discrete system when the (mythical) perfect observer
and error channel exist to generate said error correction.
With a continuous source Shannon noted: " ... Since, ordinarily, channels
have a certain amount of noise, and therefore a finite capacity, exact
transmission is impossible. "
From the subject line I would expect to be dealing with continuous
source "ordinary" channels.
That describes the theoretical "equivalent" implementation that
Shannon used to illustrate the point.
For a practical example, consider typical implementations of
equalizers to counter amplitude distortion. By measuring the
characteristics of the channel, and one time adjustment can be
made that corrects amplitude distortion. The equalizer
essentially introduces an equal and opposite error to the known
distortion introduced by other parts of the channel, with the
results that amplitude distortion is removed from the equation
(to the degree that the equalizer can actually match the
(I'm not sure what you mean by a "continuous source" channel. I
more or less ignored your odd use of "discrete system" above, but
it suffers the same problem of being ambiguous in this context.
The two words should related to analog vs. digital, but I don't
think that's what you meant.)
Keep in mind that I merely said it "could" be done. I did *not*
say it was practical. Of course in many cases that is exactly
what is commonly done (e.g., with amplitude distortion as
described above), but in others it just is not practical for any
number of reason, one of which would be when enormous bandwidth
is required. For example, it would hardly make sense to reduce
quantization distortion with that method!
Regardless, the point is that distortion is a known change which
can always be predicted from the characteristics of the channel
when a known signal is applied to the input. The difference
between distortion and noise is that noise is external to the
definition of the channel, and cannot be calculated before the
fact. Hence there is no "known error signal" with noise, but
there is with distortion.
"Laurence Payne" wrote in
message news: firstname.lastname@example.org
Oh come on, we're very selective with the fights we pick. Our targets are
always very weak.
Someone already did a big number on downtown Manhattan, but it was not
sufficient to get a new electrical code written.
You don't ("you" being the generic "Usenet user"). "He" on the other
hand does. On many occasions he's quoted back several pages and added
one or two lines to the bottom... the word "trim" doesn't exist in his
vocabulary. One wonders if he uses AOL.
Still, it could be worse. At least it's not top-posted HTML.
The first parts of Shannon's paper deal with the mathematics of
desecrate systems then later deal with what he calls "continuous
You said " *always* " and "absolutely always."
I'll accept that as a hand waving limit of sorts.
I'd contend that your assertion may be true within the limits of channel
characterization for a noise free channel. I'd like to see the math that
extends this to a system with noise; math that proves the distortion
can "absolutely always" be removed.
The fact that it can be done does not make it reasonable to
It means simply that the cost of implementation might well be
Shannon discussed it in terms of a discrete channel *with* noise:
PART II: THE DISCRETE CHANNEL WITH NOISE
11. REPRESENTATION OF A NOISY DISCRETE CHANNEL
We now consider the case where the signal is perturbed by
noise during transmission or at one or the other of the
terminals. This means that the received signal is not
necessarily the same as that sent out by the transmitter. Two
cases may be distinguished. If a particular transmitted
signal always produces the same received signal, i.e., the
received signal is a definite function of the transmitted
signal, then the effect may be called distortion. If this
function has an inverse -- no two transmitted signals
producing the same received signal -- distortion may be
corrected, at least in principle, by merely performing the
inverse functional operation on the received signal.
Well, certainly *I* am not qualified to take the math to a level
that Claude Shannon specifically avoided, saying that it was too
complex and would be a distraction anyway.
"We will not attempt, in the continuous case, to obtain our
results with the greatest generality, or with the extreme
rigor of pure mathematics, since this would involve a great
deal of abstract measure theory and would obscure the main
thread of the analysis."
Shannon, "A Mathematical Theory of Communications"
Part III: Mathematical Preliminaries, 2nd paragraph, P32
However, the case for an analog channel is not really different,
*in principle*! It merely requires that you have an error
channel of either zero noise or infinite bandwidth (and
therefore infinite capacity). In practical situations, just as
Shannon points out that it is impossible to actually have 100%
recovery at the output of an analog channel because channels
have noise and therefore a finite capacity.
And of course you quoted previously that signficance, but did
not note Shannon's comment on it, which is in the paragraph
following what you quoted:
"This, however, evades the real issue. Practically, we are
not interested in exact transmission when we have a
continuous source, but only in transmission to within a
certain tolerance. The question is, can we assign a definite
rate to a continuous source when we require only a certain
fidelity of recovery, measured in a suitable way. Of course,
as the fidelity requirements are increased the rate will
increase. It will be shown that we can, in very general
cases, define such a rate, having the property that it is
possible, by properly encoding the information, to transmit
it over a channel whose capacity is equal to the rate in
question, and satisfy t he fidelity requirements. A channel
of smaller capacity is insufficient.
The exact same is true for an error channel which supplies the
required error correction signal to cancel distortion from the
channel discussed above. To the degree that one desires a
"noise free" output it is equally possible to generate a
"distortion free" output.
Again, it might not be reasonable to construct such a system...
In article , Floyd L. Davidson
Yes but quite some time ago now. FWIW we don't or very rarely have long
lumps of overhead line anymore that carry baseband audio. For voiceband
circuits these days its digital end to end with a A/D and D/A convertor
at each end.
And for phones its going much the same way, well over here at least.
BT have the 21CN nets which are data circuits which you run data or
audio or whatever you like over them..
I asked a couple of cable jointers who were working beside the road the
other day re that one, and it seems that its the exception rather than
the rule these days. There is some cable which has a foil screen around
it, but as to woven braids seems they aren't used anymore..
Well the ones ntl use here according to a friend of mine who works with
their plant day in and day out sez otherwise. Seems only some of the
cable they use has a foil screen but then again they use fibre and co-ax
for distances of any length, seems digital rules;)..
No its not, you have to define what your using it for an in what
Yes except that if we're talking like we were about currents circulating
in the "screen" of a multicore cable, then there is going to be quite a
bit of difference in practice between a heavily woven copper braid and
the light foil wrap where the connection to that is by a fairly thin
Yes we sometimes do, but very rarely these days, it s getting to be a
very digital world over here. Analogue circuits are quite rare nowadays
and BT have been known to have to get guys out of retirement to work on
the few remaining ones!. If you wanted say a speech band 300- 3500 Hz
point to point circuit these days it'd be digital end to end or if you
required a music grade circuit that would definitely be digital copper
would only be for the patch leads to connect the gear.
Even some recording and sound re-inforcement systems use digital leads
from the stage area to the mixer now..
Well they don't define what you are doing with that. Consider say 10
meters of Andrews LDF 4-50 cable connected to a transmitter with the
correct plug, what are they connecting that other end to?. Nothing or a
load partially connected?.
Or do they mean the connection to the shield, referred to the point
where that would normally be connected, is greater than one tenth of
lambda?. If thats what they meant then they didn't describe that very
It seems that they were thinking of say a braided cable like perhaps
RG214 or similar when you "could" take that out as a pigtail
I think its relevant on the subject, but YMMD as they say..
I'll have a look at that again when I get a moment and try some
experiments here too...
Well how far do you want to go with that;?...
What do you do over there are you involved in a Telco?..
The above was only to demonstrate what I meant by balanced working..
As above just for demo..
Well I have tried that and it doesn't hum at least not what I can hear!.
And out mains is quite unclean;!..
Humm... What do you use out there in deepest Alaska, batteries;-?.....
Yes it is poor circuit design, but people do it all the time!...
In article , Richard Crowley
Do they have engineers anymore?, the accountants that run the industry say
they don't need 'em!..
That above example was for a small transmitter that is in a remote location
that is fed by a long overhead copper pair, well two of them for stereo, and
that goes into line trannies and equalisers and it didn't have any discernible
humm on it. However thats about to change, a digital microwave link is to be
installed as soon as, copper is on its way out it seems!...
In article , Floyd L. Davidson
They seem to do things differently over there Floyd, my friend who works
for a Telco here reckons that if they get 80 working pairs out of a
newly installed 100 pair their doing well;!.
All due to employing subbies who sub out and then sub some more;(.
He said they didn't measure things like signal to noise ratios and such
anymore as they don't need to its all going digital anyway and digital
is perceived as "perfect" so no need!........
Most residential POTS service in the UK is fully digital from
end to end?
That certainly is not true in the US, and I've never heard anyone
in the UK say that it was there before either.
I'm not familiar with the terminology. However there are of
course such circuits here too (ISDN, for example), but by far
the majority of POTS service is delivered as an analog line,
after being trunked to a remote unit with digital services.
However, none of that is relevant! Power line influence is, if
anything, *more* of a problem for digital services than it is
for old fashion POTS via an analog line.
I don't have a great deal of confidence in someone who is
getting their information from "cable jointers" alongside the
Lets be blunt: you don't know what you are talking about.
"Woven braid" has *never* been used for telephone cable. And
I'll repeat it just one more time: multipair cable for long runs
is virtually *all* wrapped with a shield, and additionally has
at least one single strand of bare wire running along with the
shield to provide greater conductivity.
I guess I need to tell you that I am *not* guessing.
If you can't cite a valid source... please don't exaggerate what
you do know.
But that doesn't change the way the shielding functions. All it
does is change the effectiveness of that functionality, and
clearly copper braid is much more expensive... to a degree that
the difference is not worth the cost.
I take your lack of a responsive answer as an affirmative one.
I'm finding that to be a little difficult to believe, given the
other statements you've made.
They detailed it precisely enough. The outer conductor is not
connected. It makes virtually *no* difference what you do with
the inner conductor. :-) The point is that depending on the
frequency and the length (not on what it is connected to) it
will (or not) act as a very good antenna.
They mean the length of the cable is longer than 1/10 of a
wavelength, and that there is no connection to the shield, but
there is (to virtually anything you'd like to connect, including
a box of "nothing") to the center conductor. Under some
circumstances, which depend on the length and the frequency, it
will act as an antenna.
That would be one example.
Please review this portion of what I wrote in my last message:
But chocolate chip cookies are more relevant.
The intent is to go as far as is practical, in terms of cost.
Just over four decades in telecommunications.
Proper technology seems to work the best.