Re: Can twisted wire replace shielded wire?

Delta V = ~0 at about any place on the twisted pairs as a result
of EM noise from the environment, assuming that it's wavelength
is at right angles to the lines and it's wavelengths far larger than
the pair conductor to conductor distance (excluding speed of

How did it get there?

You could add a rectifier and get free energy, right? Just from
the thermal background noise ...

 

I'd say it depends on the application - frequency, impedance and
allowable attenuation.

I personally know of an example of a strip mine that wanted video
cameras at each of their gates, all viewable from a central guard
shack. The gates were about a mile apart and coax had too much loss so
they used twisted pairs for the video. I saw the video at the guard
shack and it was flawless.

Unfortunately I know very little of the characterists of twisted pairs
at higher frequencies so I wouldn't risk it. However at audio and
video frequencies I'd give it a try.

 

Twisted pairs eliminate noise because the amplifier/reciever on both ends
have matching impendence.. that is to say both ends of the cable are
balanced
in audio and video applications it's very rare that both ends will be
balanced, a twisted pair is generally bad choice for wiring audio and video.
In computer networks they work very well because the network cards/devices
all have the exact same impendence, and the noise is canceled out. If the
impendence is NOT the same, then the twisted pair does nothing to sheild
away noise.


a normal coax cable consists of an inner conductor, a dielectric, and an
outer conductor, and a jacket.. these are also known as "transmission"
cables, if you dont' use the right coax cable (ie matching impendence) you
can have a lot of loss... ie 50ohm coax on a 75 ohm system, or 75 ohm coax
on a 50 ohm system
if you had a lot of loss, you probably could have just as well used a normal
Shielded cable and not get the loss.. a shielded cable consists of an
innerconductor, an insulator, and an EM sheild

if your twisted pair video system worked really well, then it was because
both ends were balanced (quite possible) , or you dont' have a lot of EM
noise around,


**********
shu

 

So if you have an impedance mismatch coax would solve
the problems? For audio too?

Amazing indeed.

How so?

Transmission of what, exactly? Compared & contrasted to what
other forms of "cables"?

Unlike a coaxial cable, eh?

She's a hoot <G>.

 

This, of course, relies on the "state of the art" technique
of predictive noise cancellation.

With this, the system detects the impending noise and
generate an exactly matching negative noise signal to cancel
it. That is why matching the impendence is so critical.
Using the wrong impendence will actually cause a mismatch in
the time-domain.

Actually, this use of impendence matching techniques in
electronics has gone of for years. I have what I believe was
designed as an impendence-matching modem on the end of some
fairly long transmission lines across Dartmoor. This can
sense the imminent arrival of a critical email and drop the
system from Showtime before it arrives.

I believe that Zaphod* has/had/will have a pair of
sunglasses that use the same technology.

 

Yes, twisted pairs do work because they are balanced but it is not true
that it is rare that both ends of audio and video lines have the same
impedance. With audio the standard impedance is 600 ohms and with
video it is 75 ohms.

Secondly the principle effect of a mismatched line is not a lot of
loss, but reflections. Reflections on a video line would be seen as
multiple images. I did not see multiple images on the strip mine
monitors I described which means they correctly matched the twisted
pair to 75 ohms.

Thirdly, I'm not claiming that there was a lot of electrical noise
around the strip mine and that is one of the reasons it was a good
choice. I did say that the use of twisted pair depends on the
application and a high level of EMI would be a vote against using it.

I think you've answered yourself quite well.

 

No that is not what eliminates noise and that is not what
"balance" means.

Both ends are _balanced_ _to_ _ground_, and each receiver has a
"differential input" circuit. Which is to say the input is the
voltage *between* the two wires, rather than the voltage between
ground and the wire. Almost all noise is between the wire and
ground and if the two wires are really well balanced there will
be no differential voltage due to noise, and hence no noise
voltage will be seen by the input. That is called "common mode
rejection". (Do a web search on it.)

The "balance" of the two wires is improved vastly by twisting
them. That assures that any electrical field which induces a
voltage into one of them will induce and equal voltage into the
other one. (If the wires are not twisted the one nearest to a
noise source will have a higher voltage.) The higher the
frequency, the more twist required to be effective.

Coax is a single ended circuit, with the receiver looking for a
voltage difference between the center conductor and ground. The
outer shield attempts to prevent noise from being induced into
the center conductor.

Almost all audio uses unshielded twisted pair wiring. Think
about the millions of miles of telephone cables!

Video is a mixed bag, as both twisted pair and coax are used in
different circumstances. Most video that you'll notice is coax.

That isn't true in any way shape or form. (Actually there is a
good bit of variation in the impedance of computer network
devices.) The requirement is that each of the two wires has the
same impedance to ground. It makes relatively little difference
what that impedance is, or what the impedance of the line is
from end to end. In fact twisted pair cable impedances are
often listed, but for some specific set of parameters that in
practice will not be what is experienced.

All of the above is equally true for twisted pair transmission
lines. Note however that there will *not* be "a lot of loss"
from mixing 50 and 75 ohm coax. It will actually be a very
small loss. (There are other factors that are much more
significant, but that gets more complicated than I'm going to
get in this article. Certainly if someone wants to discuss it,
we can and there will probably be a number of people who can
describe it in detail.)

That describes coax.

There are such things as shielded twisted pair transmission
lines. Shielding increases the loss (by significant amounts at
higher frequencies) and is therefore not often used.

Generally it is the lack of noise sources that allow poor
implementations to work. Unfortunately they then fail as soon
as someone installs a noise source in close proximity.

 

actually there are two Main standards for Coax "low loss" 50 Ohm. and 75 Ohm
on a coax the audio can travel down the same line, and have the same
impendance, ie 50 or 75
audio and video impedances themselves are fairly wide and varied
I've seen 75 Ohm 22 Ohm and 50 Ohm video
audio is worse with impedence rarely matching on even the same system
ie.. 18 Ohm's in .. 50 Ohms out. i've not seen any 600 Ohm systems.




while there may be a "Standard" audio impedance

if it's not done right, you'll get a lot of loss.

well ok, but you could have just as well used a shielded (non transmission)
cable rather then a twisted pair, and gotten the same result

*****
shu

 

--
**********
shu

well, they have to have the same impedance and be balanced

ok, i agree with all of that.

heh.
Telephone Aside....
if you're setting up speaker systems, or amplifier systems.. stereo's etc..
it'll be rare that any of it matches up perfectly so that a twisted pair is
the best choice for wiring.

You're Nit picking, or misunderstanding
The network cards and Devices have to have the same impendance, OF course it
is with Regard to the wire and ground what ELSE would it be in regard to?
Nor did I ever say that the Actual impedance along the wire is important, My
whole point is that the Noise cancellation Occurs AT the cards themselves
which all have the exact same impedance (TO GROUND) Regardless of the wire's
impedance (which ideally should be as LOW as possible)

no no no..
.. a "coax" cable has a Dielectric between the outer conductor and the inner
conductor and THEN on top of that there MAY or MAY NOT be a shield,
a normal.. like "rca* type cable has NO dielectric, and No outer conductor.
it just has an Innner conductor. and Insulator (not a dielectric) and a
Shield

a Coax is designed to transmit over long distances, a "rca" type cable is
design to just be shielded and for short use

**********
shu

 

FLD - "Almost all audio uses unshielded twisted pair wiring."

The exception being the use in Homes of good old every day
Speaker Wire (Paired-Wires) which are general laid Flat
(Un-Twisted) works very well just that way in most instances.

Ah! that Reminds Me - {Note to Self }
That I have to write about the Speaker Wire Dipole Antenna.


jm2cw ~ RHF

 

http://www.installer.com/tech/cat5.html

*******
shu

 

"Floyd L. Davidson"

** A pair of parallel wires that are is not twisted is simply a long loop.
Any nearby alternating magnetic filed (ie like AC hum) will inject a small
voltage into that loop. If the wires are spread apart, the loop area
increases and so too does the injected voltage level.

Also, the phase of an injected voltage in a loop reverses if the loop is
rotated through 180 degrees - same as reversing the two connections at the
receiving amp end.

Now, if half a loop is rotated through 180 degrees and the other half is
not, then voltages induced in each half are in antiphase. If equal in
magnitude they will cancel each other.

So, by having many twists in a pair of signal carrying wires one maximises
the chance of cancellation in the general case - ie it *prevents* the
pair of wires acting like a loop receiver.

The two conductors in a balanced microphone cable are twisted quite tightly
for this reason.

** True - but the surprising thing about co-axial cable is that it does
NOT act like a loop.

A co-axial cable suffers ( ideally) no voltage injection from a nearby
alternating magnetic field.

Since the outer conductor shields against external electric fields entering
the cable co-ax is largely immune from EM interference - making it ideal
for many wide band applications, 75 ohm video being one.



.......... Phil

 

The *balance* is in regard to "wire to ground". But the circuit
impedance is between the two wires, not to ground.

They don't all have the exact same impedance to ground. The two
wires on any given device have similar characteristics relative
to ground (resistance and reactance), but between any two
devices those characteristics might be (and commonly are)
significantly different.

And *NO* the wire's impedance is not as low as possible.
Typically a twisted pair has a characteristic impedance of about
100-200 ohms, if it is short. The impedance can be
significantly higher if the cable is longer.

However, most telephone cable is intended to have a
characteristic impedance between 900 and 2000 ohms!

Note that the lowest typical impedance for twisted pair is
higher than that of commonly used coaxial cable, which is
usually less than 100 ohms.

....

You need to do some research. That "shield" which is on "top"
of the outer conductor is *part* of the outer conductor in
almost all cases. (I can't think of one where it isn't.) Low
cost coax is of course single shielded, and the double (or more)
shielded coax is both more expensive and of higher quality as a
transmission line.

Cables that use "RCA" connectors are in fact coax cables. Some
of them are *extremely* cheap, and have an outer conductor that
has very little area. But yes it is there. On the cheapest
ones, if you cut the cable open what you'll find is that the
outer conductor looks just like a multi-stranded wire that has
very little twist.

Also note that *any* insulator is a dielectric... and *any*
"shield" is in fact the outer conductor of a coax cable.

Coax is no more, or less, designed for long distances than it is
for short distances. It has been used for both, though it is
commonly more often seen in short applications than long, but
again that depends on your perception of "short" vs. "long" too!

 

We probably should note that flat untwisted wires are, just like
twisted pair, a *balanced* transmission line. In fact they are
virtually identical! The flat line is twisted pair with zero
twist, that's all.

(Of course in the case of speaker wire, that is not significant
at all. The impedance is very low and the power levels are very
high, hence the SNR is always going to be *very* high, even in
a very hostile environment.)

Zip cord! (Oh, that's what I use for speaker wire anyway...

That is certainly an interesting URL. Unfortunately it is very
poorly stated, though almost 100% correct (and only one
interesting nit pick about something that is not quite
"correct"). I can see where you are being confused by what it
says, for example, because many of the things you've said that
are absolutely incorrect appear to be supported by the way he
presents his information. But what he actually means is not
what you are taking it for. (I won't go into it here, because
it is all in the other two articles I've posted.)

However, as a brain teaser, I'll point one technical error on
that web page. This is not a serious criticism, because this is
*way* over the heads of his intended audience, and I would not
have mentioned this either if I were writing that article. (I
would word it a little differently though... as you will see.)

But when I talked to the computer experts that were
familiar with this noise cancellation phenomenon, I
realized not many except the real gurus realized the
cancellation occurred in the balanced line receiver
and NOT the cable itself!

Again, its NOT the twisted pair cable that gets rid of
noise, its the balanced recievers! IN FACT, the
balanced line twisted pair wires are NOT shielded so
that BOTH wires get equal noise, so the noise (if
equal) will be cancelled out!

In fact, for most but not for all twisted pair cables that is
true. The exception is interesting though. For long telephone
feeder cables, those that are more than a few hundreds of feet
long, a typical cable is laid in sections, for example 3000'
feet long. At each splice box the shield that surrounds the
entire cable (which might have anything from 25 to maybe 600 or
even more twisted cable pairs) is grounded very carefully, and
is bonded to the next section.

That has the effect of canceling noise in each and every pair.
(Not by shielding it either, but by cancellation.) But before I
describe how that works, let me point out that the exact same
effect can be had by grounding all unused pairs at both ends!
(Clearly _that_ is not an effect of shielding!) Of course that
works well only if there are many spare pairs...

So, the question then is just how does that cancel noise in the
twisted pair conductors that make up a multi-pair cable?

Any noise source will generate an electrical field that causes a
voltage and a current to appear on a conductor. If there is
good continuity, the voltage will be low and the current will be
high. With poor continuity the opposite is true. Also the
potential is directly related to the area (i.e., the size) of
the conductor. Hence many pairs tied together, or the outer
shield, will have more potential than a single smaller
conductor. And by grounding the shield (and maybe all spare
pairs too) at both ends we cause the *greatest* amount of
possible current to flow from any given noise source that is
causing induction. Typically each pair that is in use will be
terminated in an impedance somewhere between 100 and 1200 Ohms,
so compared to the directly grounded shield and conductors there
will be much less noise current in the used pairs.

Okay? Now comes the fun part. The large noise current in the
grounded conductors causes an equal and opposite potential to be
induced into each of the other pairs! It therefore cancels some
of the induction directly from the noise source.

Hence, while it is commonly the case that most of the noise is
canceled by common mode rejection at the balanced receiver,
that is not the only noise reduction mechanism for twisted pair
cables.

 

Read what i said again
Ideally you *want* the impedance to be as LOW as possible
i didn't say the Impedance WAS low relative to coax, nor did i imply that...

A twisted pair typically has an impedance of 100-200 Ohms yes
the impedance on longer cables is there Simply because the cables are Longer
this isn't a GOOD thing.. you WANT it to be as LOW as possible

are you channeling Cliff?

no no no no you're ignorant
here
http://www.westpenn-cdt.com/pdfs/T176o177.pdf
this is how Every single Coax cable iv'e seen is made
and RCA cable. does NOT have the dielectric
and the outer conductor is NOT the same as the outer shielding
the Outer shield is braided to spefically lessen outside interference,
it is entirly irrelevent that the outerconductor touches the shielding
the outer conductor however is NOT braided. it is designed to react to the
pulses between the inner conductor and the dielectric (read Capacitor) AS
well as aid in shielding.. YES it can do both.
because of this interaction it allows the Pulses to transmit further with
less impedance then a simple wire
the Speed of the transmission is signficantly less then the speed of light,
and can be calculated out by knowing the dielectric constant of the
dielectric
because of this if the impedances dont' match.. you can get Echos
(reflections) and actually notice them, on a VERY long cable.. like this
mile length cable. i'm doubtfull you would notice relfections because it's
just too damn long.. it require TWO bounces to notice reflected images,...
in a real long cable. some of the signal will be reflected back, . and then
reflected again at the other end, but by that time it's too out of phase,
and too weak to produce much in the way of ghosting. instead it looks like a
simple signal loss. you can however have Very noticable ghosting on shorter
cables if the impedance is mismatched

the Insulator in a SIMPLE rca type cable is no where near the quality of the
dielectric compounds used in coax cables, and hence has little if any
effect, and also the Shielded part of the RCA cable does NOT behave like the
outer conductor in a coax, . the shield is again braided and absorbs signals
if there is an outerconductor it's simply there to help shield, as you want
a Foil type shield and a braided shield, but again it doesnt' beheave anyway
like a coax because of the lack of a decent dielectric
in short.. RCA cables are absolutely not like Coax cables.. you're just
wrong.

coax cables are spefically designed for long range transmissions with
miniumal loss
by long.. i mean Longer then a simple insulated , or simple shielded wire
can transmit

 

Balanced systems are generally better than unbalanced systems in audio
applications, especially if there is any audio frequency potential
difference between the signal grounds of two pieces of connected equipment,
otherwise known as common mode interference. Well balanced systems reject
common mode interference; unbalanced systems consider it as part of the
signal.

Balanced systems necessarily use two conductors floating with respect to
ground and to maintain balance the conductors need to be identical and
symmetrical in their relationship to the world around them - twisting helps
assure that. Unbalanced systems can work with one conductor and the earth as
the return conductor but a second conductor is normally used to assure the
return path has a low resistance.

Shielding is applied to both single conductor, unbalanced wiring and twin
conductor, balanced wiring to reduce the coupling of higher frequency EMI
into the conductors. Systems that are balanced at audio frequencies may
become imbalanced at higher frequencies and input amplifiers of imbalanced
systems may rectify/detect/demodulate induced signals just as the input
amplifier of an unbalanced system. Hence shielded twisted pair for balanced
systems and the shielded single conductor you describe as 'rca' for
unbalanced systems.

These copper or aluminum foil shields are ineffective at power line
frequency where the interference is due to magnetic coupling. Hence the
common practice of running critical audio cabling, even shielded twisted
pair, in steel conduit.

While some may use twisted pair cable in unbalanced systems, one of the
conductors is connected to equipment grounds at both ends, just as the
shield would be, but does not shield as effectively as a complete braid or
foil shield does, and contributes nothing toward common mode rejection.

I used "Twinaxial" cable for a mile long analog video run in the Arctic
where 'ground' is a myth. The common mode interference at 60 Hz was so high
that a clamping amplifier had to be used in addition to a balanced
equalising amplifier and still there was a noticeable hum bar.

On high speed data networks as in video systems, the receiving devices must
have an input impedance close to that of the cable feeding them, in order to
minimise reflections (echoes, ghosts). A match at the sending device also
helps in this regard. Maximum power transfer is not the reason for matching
in these applications; minimal distortion of the pulse waveform is.

Noise rejection could care less about the match between the cable and the
receivers. All that is required for common mode rejection is that the two
receivers match each other in every respect and that the two conductors
match each other.

Sorry, 'rca' cable as you describe it is co-axial but not precisely
co-axial. Any insulator is a dielectric (has a dielectric constant, a ratio
with respect to air). All that co-axial means is that there are two things
sharing a common axis, one enclosing the other. Their cross-sections may be
completely different - the inner could be square, and the outer triangular.
Co-axial cable impedance is determined by the ratio of the diameters and the
dielectric constant of the insulator(s) between them. Precision co-axial
cable has a uniform impedance along its length, i.e., the centering,
diameters and dielectric are uniform. So-called 'rca' cable is imprecise
co-axial cable. Impedance becomes relevant when the cable length becomes
more than a fraction of a wavelength of the signal being transported - the
electrical wavelength at audio frequencies is very large.

Low loss co-axial cable is needed for long distances and may need to be
precise for certain applications. Lossy, imprecise co-axial cable is good
enough for lower frequencies and shorter distances.

Shielding is often used in order to satisfy EMC requirements, i.e.,
potential interference to radio systems in addition to self-protection. Note
that CAT6 shielded cable is used in critical Gigabit Ethernet installations.

Tom

 

Say what?

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

 

What do you think "ohms" means in terms of that coax?

LOL ....

 

http://www.nancysinatra.com/intro.html

HTH

 

You probably have ohms confused with ohms.