10 metres audio cable going into PC = too long?



Yep, well thats balanced operation which as you say will go for miles over telephone copper lines without humm..
Oddly enough in the UK they don't as a rule use screened cable, the twisted balanced pair has very good rejection.
I seemed to think we were talking about domestic unbalanced lines here?.....
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Regular telephone cable does not have a shield on each pair, but does have a shield around the entire bundle of pairs. The above diagram shows the reason!
The effects of shielding is almost useless at 50-60 Hz AC power frequencies, which means that noise immunity would be only the common mode rejection ratio if there was no shield or if a shield is grounded at only one end. Instead the shield is grounded at both ends, which allows any induction to not only induce current into the cable pairs, but also into the shield. The shield has is a larger conductor than the pairs, hence has less resistance and therefore significantly more current flows. That current flow in the shield causes an opposing current to be induced into the signal pairs! And that reduces the amount of noise in the signal pair significantly below what it would be if common mode rejection was the only noise reduction mechanism.

The ground loop part is exactly the same in either case. The example above is just a very convenient way to demonstrate positively that cables *are* grounded at both ends, and that it not only does not necessarily cause ground loop noise, but actually is a way to reduce noise in the signal wires.
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Over here it seems to, well the half a dozen or so I've looked at!. We we're involved in a short term radio broadcast some years ago and the cable co supplied free of charge a few circuits about 3 odd miles to link Two studios together, and apart from a small amount of HF loss..no hum at all or other noise for that matter and all that cable was unshielded....

Balanced working.. ever read up about it or used it in practice?...

Really;-?....
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I'm not sure what you are agreeing with there... that cables do or don't! :-)
Typically of course a customer never sees any part of such a telephone cable. What you see is a "drop wire" run from that cable to your location. That cable will not be shielded.

When done right, it works *extremely* well.

About 40 years of working with it every day in a huge variety of situations.

Yup. I posted this URL in another message, but just in case... here is a very interesting, if somewhat technical, article about measured effects of grounded shielding. It is very interesting in the context of this particular thread.
64.70.157.146/pdf/Bondingcableshields.pdf
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Doesn't matter either way as long as its balanced working and in any case telephone bandwidth isn't that responsive to 'ummmm...
Anyways these days in the UK the copper part isn't that long in new cable co installations, the fibre to copper conversion is done very locally to a subs premises and in the BT system the copper is longer but doesn't humm..

Yep but they don't use shielding on a lot of phone multicore in the UK and it wouldn't matter anyway..

Yes.. Thats got some good points but they don't seem to be very savvy on some matters about EMC and RF and you can pick a few holes in that but yes their correct in screening or shielding earthing at both ends provided that the balance in the sending and receiving ends is what it should be, injecting current into the shield won't affect what's carried in the encased conductors. However in practice the final result is and can be affected by transformer and electronic balanced inputs and how "floating" they are.
I think we could all agree that balanced working isn't really a problem.
Now they mention unbalanced working, but haven't given it much attention.
Now ASCII art permitting are we agreed that the following isn't going to cause too much upset?..
-------------------------------------------------------------- A ________________________________________________________________ M
-------------------------------------------------------------
Poxy ASCII!. Now consider A is an amp input and M is a source microphone
The dotted lines are the shield on a lump of single cored microphone cable. Now the amp is connected A to the centre conductor at the amp end the screen to the earthed side of the amp input, at the other end the microphone has say a phono type connector, and the mic is a dynamic moving coil type with one end connected to the inner shielded conductor of the cable, the other end is connected to the outer shielded conductor, the mic is in a metal case and is connected to the shield of the cable too.
The mic case is not connected to any earth, other than the outer shield of the connecting cable, and lets say thats 10 meters long or 12 yards;) The mic is suspended in free space by a lump of nylon cord and isn't connected to anything else at all...
Now are we agreed that that arrangement will or won't hum?......
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Ahem. That is absolutely false. Telecom engineering necessarily goes to an extreme effort to reduce what is called "power line influence". The reasons should be obvious: telephone and power cables are often run side by side, on the same poles, and in the same crawl spaces, sometimes for miles at a stretch. It is not uncommon to see as much as 40 to 50 volts of power line AC on a telecom cable. That requires an astounding amount of noise immunity to allow a circuit to work.
Consider that the test tone level at a customer premise telephone set is nominally targeted at -9 dBm, and the worst case acceptable Signal-to-Noise ratio is 24 dB, which means that all noise should be at least at -33 dBm, which is about 0.0000005 watts. But a 40 volts hum across a 600 ohm impedance is 2.7 watts, and there is roughly 67 dB difference!
Do you have any idea how many telephone lines actually have a 67 dB SNR?

So? "Very locally" can mean more than a *mile*...
What do you mean by "BT system the copper is longer but doesn't humm.."? They have hum resistance copper??? ;-)

Virtually *all* "multicore" telecom cable is shielded. (Some customer premise cable is not. But you won't find anything within a telephone central office that isn't, and you won't find any outside plant distribution cable that isn't.)
Where are you coming up with these ideas? Have you ever even seen the specs for any of this?

Heh heh, lets see you try picking any holes in it!

You didn't read it, did you? It *does* affect the signal pairs. It reduces the noise on them, significantly.

In practice, what they showed was that it improves noise immunity.
"Floating" makes no difference at all. Longitudinal balance is the most significant factor. Magnetic shielding is ineffective below about 10 kHz, and reverse induction via the shield (by grounding it at both ends) is much more significant for power line frequencies and their harmonics (which commonly exist up to 2 or 3 kHz).

We could all agree that common mode rejection is not always sufficient, and that reverse induction is virtually *always* applied to outside plant communications cables because of that.
Exactly what you mean by "balanced working", I'm not sure.

It is rarely used for critical circuits where induction interference from power lines would be important. (For obvious reasons...)

Nothing you have said suggests it could possibly hum, given that you have not mentioned the presence of any power line related equipment at all. If this thing is located out in the ocean, on a floating barge that has no AC electric power, it won't hum.
On the other hand, if you place a fluorescent light fixture close to it, it might well hum!
Regardless, that is one of the worst possible ways to wire 10 meters of cable to a microphone.
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Yes they do, in fact we've got a broadcast transmitter site which is fed by a bit of BT, (British Telecom, the national Telco), overhead wire for some miles and no hum at all!. And that is on the same pole set as 240 volt mains wiring and I've actually seen 11 kV lines with phone lines near them. Not that advisable owing to the safety factor!.
Yes of course you can get leakage via induction and capacitance into the telecom lines but this does not matter as it will inevitably be induced in both conductors and cancelled out by common mode rejection. Doesn't matter providing the insulation in the line and transformers will stand it to have some kilovolts actually on the line as such...

Can you explain how your measuring or have that configured please?..

Often less than in ntl or telewest installations but longer in BT ones. Ntl care the cableco in the UK but that name is to disappear and their to be called Virgin!...

Nope;!, just a way of putting that, see above,...

In a central office most all of it here is twisted pair. I think some terminology things betwixt the UK and USA are showing up here. All the cable co Telco multicores I've seen, though not all, are unshielded.
What do you define shielding as, just a wrap of aluminium foil with a drain wire or a fully woven copper mesh?..

OK then, part 2 "On the other hand cable shields which are bonded at one end etc". Read that thorough carefully, doesn't make sense. Then take a lump of Andrews 4-50 Heliax and see what a good radiator that is even greater number of wavelengths . They didn't even state if it were open circuit or terminated on a load...
Actually we've had a lot of EMC experience over the years in radio, audio and automotive environments and what's made by far and away the biggest effect is bypassing of transistor junctions at RF frequencies....

Were is this noise coming from then?...

Well think about that, Say we have a cable the inner pairs are wrapped around one of the power lines that you describe, and there are a LOT of volts induced on that wiring. OK now into a transformer there will be galvanic isolation i.e. the ends or centre tap of that transformer isn't connected to anything. Now take a electronically balanced input. At some point that will be connected to say an input IC which will have supply rails etc, and that IC will be coupled through to the output of that line receiving amplifier now don't you think that if there were some matter of kilovolts on said line, then that will break down the transistor junctions ?..

I think you have that wrong. Provided that the rejection is what it should be then whatever is induced on the pairs will cancel out.

What we've been discussing. Take a signal source and connect a transformer thereto and connect that to a pair of wires twisted together and then connect that to another transformer and the out put winding of that to a load. That do?..

Yes..
Why?.

Yes agreed and you wouldn't do that, well not in a pro environment anyway.
Now if say you ground that to the local mains earth at one end, and say 10 meters away at the microphone case end earth that to a driven rod earth, will it or wont it hummmmmmmmmmm?.....
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...
Have you ever verified what the CMRR actually is on such a circuit? The perception that CMR just cancels out everything is naive. Typically common mode rejection is *not* sufficient to provide a functional dial loop on a line with 40+ volts of AC voltage.
It varies, and CMRR may not be enough to deal with significantly less voltage than that.

That *is* the explanation of it? All that I left out was the arithmetic.
The maximum noise acceptable is -33 dBm (with a signal of -9 dBm and a minimum required SNR of 24 dB). The AC voltages seen can be in excess of 40 volts. 40 volts would be +34 dBm. That is 67 dB difference.
Common mode rejection ratios commonly are less than 65 dB on typical cable pairs.
...

Virtually *all* "multicore" cable is shielded. That is *not* individual shields on each pair, but the entire cable is inside a (foil) sheath.
Suggesting it is otherwise is ignorant.

Shielding is shielding, whether it is aluminum foil or copper braid.

I take it from your statements above and the lack of an answer here that you have no experience with specifying or installing telecommunications cabling.

"On the other hand, cable shields which are only bonded at one end cease to provide shielding when their length exceeds one-tenth of the wavelength of the frequencies to be shielded against, so for example a cable 10m long only provides any significant shielding for frequencies below 3MHz. When cable lengths exceed one-quarter of a wavelength, shields which are bonded at one end only can become very efficient RF antennas radiating RF noise and picking up RF from the environment more efficiently than if there was no shield at all. Although the RF noise in pro-audio products is usually caused by digital and switch-mode circuits, it appears as common-mode (CM) noise on all the analogue inputs and outputs too."
So be specific. It makes sense to me. What part would you like explained?

Heliax is, just as they state, a good radiator if it is not bonded properly. It provides good shielding when properly bonded, and can become a very effective antenna at lengths approaching or exceeding 1/4 wavelength when not bonded.
That is true regardless of whether there is a resistive load, or not.
Please review any book on antennas! The statement made describes the physical construction of more than one popular variation of an antenna.

I like chocolate chip cookies myself. But that has nothing to do with the topic we are discussing either, so I haven't brought it up. You probably should stay on topic too?

The paper discusses reduction of power line noise on communications cables. Induction from nearby power wiring is the most common source of such noise, and that is specifically the type of noise which is reduced by allowing current flow through the shield of a cable.

It may or may not, depending on the components. But that is an entirely different discussion. It has *nothing* to do with what we have been talking about, and has nothing at all to do with the paper we are currently discussion.
The point is that "floating" does not affect noise immunity.

That is simply not true. Have you ever *measured* it? It does *not* simply cancel *everything* out.
Do you know what "longitudinal balance" is? That is the characteristic which most determines how much is canceled out by common mode rejection.
It is *never* perfect.

Look up the specs on various transformers. One of those specs will be for longitudinal balance. It is never perfect. Some are *much* better than others. (Then look up such things a bifilar windings, and learn more about what causes better or worse CMRR in any given transformer design! It really is a very interesting topic. The first thing you will note is that by merely specifying "a transformer", you have not necessarily provide high CMRR for your circuit...)
It is also sort of fun to play with if you never have. Set up a hybrid bridge using transformers, and measure the isolation. Then try getting the balance as good as you can. At one single frequency it is possible to get as much as perhaps 70+ dB of isolation from good transformers. But to drop that by 10-20 dB all you have to do is put your hand on any part of the balance circuit! Just getting near will be enough if you actually do get a good balance.
That is all just longitudinal balance...
...
Regarding your coax circuit...

Because the shielding is not effective at powerline frequencies and harmonics.

Your circuit is using a single ended coaxial cable. The return path for the circuit includes the shield. Hence you've just connected the ground differential to the signal circuit. It won't hum if you are 100 miles from the nearest power line...
Your example is nonsense and does not demonstrate anything about noise immunity. It merely provides and example of poor circuit design.
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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 application..

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 drain wire...

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 well.
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 perhaps......

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!...

--
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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 road.
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.
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On Sun, 30 Apr 2006 04:31:07 -0800, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

In most places in the US your POTS line becomes one channel of a digital T1 pretty close to your house. A tip is whether a 56kbs modem works for you. That is exploiting the 64kbs T1 channel hardware. The "last mile" <or less> is usually the only part of POTS that is analog. There may still be a number five office somewhere in the US, switching analog lines but I am not suure where it would be. Everyone is digital at the central office.
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snipped-for-privacy@aol.com wrote:

You haven't described anything different than what I mentioned.
AND make no mistake... "pretty close to your house" is up to 3 *miles* of cable that is an *analog* tail.
The fact that digital switching has finally become nearly ubiquitous is not significant to the fact that POTS is still delivered as an analog line. (And I might note that here in Alaska we were nearly 100% digitial in the CO more than 15 years ago, at a time when the rest of the US was only 33% digital.)
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On Sun, 30 Apr 2006 04:31:07 -0800, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

Yes, it is.

Oh, so the US is technically challenged? Seems unlikely - although maybe in Alaska...................

Are you insane? Ppower line influence is by and large non-existent for digital services. That's the whole *point* of digital - it's *rugged*.

That just shows that you're an asshole (hey, whoulda thunk?). The grunts on the ground doing the actual work are *always* at the cutting edge of technology - they install it!

Actually, you are the one who has so far failed to demonstrate any actual knowledge of current systems.

Tell all you like, the reality is that you *are* guessing - wrongly.

Why am I getting the impression that Alaska really is like Northern Exposure? Are there any normal people up there? Could you put them on the line, please?
--

Stewart Pinkerton | Music is Art - Audio is Engineering

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On Sun, 30 Apr 2006 18:17:39 +0100, Stewart Pinkerton

No it isn't. The line into my house (and everyone else's house) is copper, delivering an analogue voltage to an analogue handset.
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On Sun, 30 Apr 2006 23:56:35 +0100, Laurence Payne

I think he is having trouble understanding the term "end to end".
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On Sun, 30 Apr 2006 23:56:35 +0100, Laurence Payne

OK, if you want to be picky, it's digital up to the point where it leaves the last distribution box.
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On Mon, 01 May 2006 11:22:43 +0100, Stewart Pinkerton

That's not being picky. It's a vital factor. The service available to a user is limited to what will go down an analogue connection. No matter if that connection is only a few inches long, it's still a bottleneck.
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Laurence Payne wrote:

But to continue being picky, the pairs in a length of CAT5 don't know or care if the signal they carry is being called analog or digital, its still just a voltage that varies with time. And the "analog" cable from the phone co distribution box here carries analog voice, and used to carry digital ISDN, and now carries both analog voice and digital ADSL at the same time. So what is it, analog or digital, or maybe it doesn't matter.
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wrote:

We're talking about POTS aren't we? It's analogue.
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Laurence Payne wrote:

Isn't everything when you get to the wire? But the same pair still carries digital information.
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