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Re: Can twisted wire replace shielded wire?

Discussion in 'Electronic Basics' started by Cliff, Nov 24, 2005.

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  1. Cliff

    Cliff Guest

    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 ...
     
  2. skeptic

    skeptic Guest

    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.
     
  3. shu

    shu Guest

    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
     
  4. Cliff

    Cliff Guest

    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>.
     
  5. 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.
     
  6. skeptic

    skeptic Guest

    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.
     
  7. 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.
     
  8. shu

    shu Guest


    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
     
  9. shu

    shu Guest

    --
    **********
    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
     
  10. RHF

    RHF Guest

    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 :eek:}
    That I have to write about the Speaker Wire Dipole Antenna.


    jm2cw ~ RHF
     
  11. shu

    shu Guest

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

    *******
    shu
     
  12. Phil Allison

    Phil Allison Guest

    "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
     
  13. 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!
     
  14. 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.
     
  15. shu

    shu Guest

    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
     
  16. Tom Holden

    Tom Holden Guest

    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
     
  17. David

    David Guest

  18. Cliff

    Cliff Guest

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

    LOL ....
     
  19. Cliff

    Cliff Guest

  20. Cliff

    Cliff Guest

    You probably have ohms confused with ohms.
     
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