Really? I'm not aware of any large scale generating facilities that generate single phase, since you can always extract single phase from 3-phase.
No doubt 3-phase delivery is a problem in areas where they run just a single phase. But using the original 3-phase rather than reconstructing 3-phase from single phase has got to be more efficient. It also seems more reliable, since the converter is a piece of rotating equipment (basically a motor/generator set, IINM). Yuck. I bet the energy conversion efficiency is less than 70%.
It would be extremely expensive to retrofit large areas of legacy 1-phase distribution with 3-phase today. However, I'll bet that the main substation gets 3-phase feeds, which it parcels out as single phase in some load-balanced arrangement from each of the 3 phases.
In fact, the rotary converter is a sort of motor and generator combined, with just one rotor and one set of windings. A large 3-phase motor with no load on it can be used, or so I'm told. Voltage is induced into the windings that don't have voltage applied to them. But I'd be really surprised if they aren't using some kind of solid-state converter nowadays, instead of rotary converters.
When the power company wants you to cough up $35,000 or so for running 3 phase from where it is, up to your building, you've got to be talking pretty heavy use to cost-justify the new lines.
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?......
You clearly wouldn't know evidence if it bit you on the backside if you believe a pdf carries more weight than an actual sample.
And of course the stuff in the pdf has no bearing on the actual issue, which is that a ground loop necessitates a loop in the ground. One would have thought that even a limited skill in reading would have made that clear.
And you still haven't explained why you think it is a good idea to connect the screen to one side of the capsule in a microphone - you certainly didn't think it was a stupid thing to do when you posted it
- just bitched about the "fact" (sic) that a microphone hums when you grab hold of it.
All through this thread you have revealed that you don't understand what is going on, you post diagrams that contradict your position, you believe a single connection constitutes a loop, you think that hum is signal, you introduce common mode DC - and I still haven't fathomed what that had to do with anything.
Now please, go away and reflect on all of these things, forget the "theory" you have learned and find out how the real world actually works.
On Sun, 23 Apr 2006 11:37:05 GMT, snipped-for-privacy@pearce.uk.com (Don Pearce) Gave us:
Noise IS a signal. If you knew what the word signal meant, you would know that. ANY perturbation of a circuit is signal. You need to learn that.
UNwanted signals get injected into circuitry all the time. With audio circuit, we hear the result. That doesn't change the FACT that it is still, nonetheless a signal.
Please grow the f*ck up. GTFU
You're a goddamned idiot.
I retract that... you're a goddamned retard.
Fuck off. YOU go back and read what he said, now that you know what constitutes a signal.
No - the wanted stuff is the signal - the rest is interference. Ever heard of signal to noise ratio? You would call it signal to signal ratio. Now that makes much more sense, doesn't it?
No, noise gets injected into audio circuits when you don't know what you're doing - like when you screw up the connections in a microphone so it hums when you grab it. Have a listen to my MP3 and you will hear just how wrong you are.
GTFU? Did you put that bit in so you could read it as well?
A retard who, unlike you, can wire up a microphone so it doesn't hum when you grab it. Seems to say it all, really.
I have explained that to you ad nauseam.
Now even from here, I can see you turning red - that vein on your forehead is looking none too healthy, and all those burgers and fries have probably raised your blood pressure to a dangerous level. I wouldn't like give yourself a heart attack over this. Just take that break, and go and think about it.
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.
I haven't read the paper. I'm not interested in what it has to say - it will either be right or wrong, but that is not the issue. The issue is that you are wrong.
LIsten Floyd, sonny. I can see how desperate you are to shift the argument away from you and onto somebody else, preferably somebody who stands behind the pages of a book and can't be drawn in personally, but your cowardice doesn't impress me at all. Stand up and be a man. I stepped up to the plate and delivered an actual example of why I'm right; all you have provided is a sheepish admission that when you wire up a mic, it hums. That really isn't too impressive, you know.
Uhmmm, Don... *I* am not the topic of discussion. Neither are you.
The topic of discussion is a bit of technical theory. You demonstrated quite well at the beginning of this thread that you do not understand it. Since then you've shown that you have no intention of learning either.
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?.....
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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