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#81
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10 metres audio cable going into PC = too long?
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#82
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10 metres audio cable going into PC = too long?
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#83
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10 metres audio cable going into PC = too long?
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#84
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10 metres audio cable going into PC = too long?
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#85
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10 metres audio cable going into PC = too long?
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#86
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10 metres audio cable going into PC = too long?
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#88
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10 metres audio cable going into PC = too long?
On Sat, 22 Apr 2006 05:20:46 GMT, Roy L. Fuchs
wrote: On Fri, 21 Apr 2006 17:18:33 GMT, (Don Pearce) Gave us: A single connection is not a loop - it is a connection. Do you even know what a loop is? I'm serious - you appear to have no concept. Tell us again how a person that is NOT connected to anything else at can touch a microphone and cause "hum" then. Then tell us how many SIGNAL PATHS there are while said person is causing said noise. Don't duck this one, Chucko. You cannot avoid the righteousness of the Floyd. OK, I've had it with you pair of incompetents, and this is going to be my last word on the subject. I have made a recording and put it on my web site http://81.174.169.10/ It is of me speaking into the most insensitive microphone I have - a high impedance ribbon. Halfway through I take hold of the microphone body. I am sure you will be amazed to hear that the result is absolutely no hum whatever. Do you have any idea why this surprise result should come about? No? Well, I'll tell you. It is because I know what I am doing, I understand ground loops and I don't allow them in my system. That, as far as I am concerned, is it. Have a nice day, the pair of you, and remember - if you don't know the words, you can always hum. d -- Pearce Consulting http://www.pearce.uk.com |
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#89
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
Jim Lesurf wrote:
In article , PalindrĂ¢-»me wrote: Jim Lesurf wrote: To be able to give a complete answer we would need to know the output impedance values for your stereo and TV. The output impedance (resistance) will tend to combine with the cable capacitance to make an RC low-pass filter. This may or may not matter, but to estimate the effect we'd need to relevant values. A quick bit of mental arithmetic, with the sort of distances and cables mentioned by the OP, gives me an estimate of around 5k output impedance, or greater, before it should give a noticeable effect (OK, a bit lower if you can hear and are concerned about getting 10kHz stuff). Since I am reading (and replying) on an 'audio' newsgroup, my assumption is that people may be concerned up to around 20kHz, and want to be aware of any changes that might be considered worthy of note in the context of using good audio equipment in a domestic situation. If I assume a cable capacitance of around 100pF/m, then a 5kOhm source would, I think, give a roll-off of the order of 1dB in the 15-20kHz region. (Assuming I managed to push the right buttons on my guess-box. :-) ) On this newsgroup I'd normally expect people to regard that as being large enough to be worth at least being aware of it. I haven't come across a line out with a higher impedance than 5k, so I don't think it is going to be a problem. You are probably correct. Alas, consumer equipment sometimes dissapoints out expectations - particularly when the makers have failed to provide the relevant data for the users. ;- Even so, this can be compensated for by a tweak of the tone controls on the PC. Once one becomes aware of the problem, and knows what adjustment to make to correct for it appropriately. Hence my initial comment. :-) My hearing isn't what it was - so I compensate for that by setting the controls to give me what I imagine I would be hearing if I was 18 years young again. Pure heresy, I am sure, but I can no longer make out the fine detail and nuances whilst listening in a concert hall - so, at home, I tweak the settings so that I can. Heaven knows what it sounds like to anyone else, but, living alone, that isn't a problem. Compensating for the odd few dB loss over 10kHz due to cable characteristics gets rather swamped by the compensation needed due to not always having worn earplugs when I should have... But then, I am writing on the "electrical" group - where I suspect many, like me, are rather grateful our hearing has deteriorated to the point we don't notice the noise from lopts, static inverters, brushgear, etc, as much as we once did. The workplace sure has got quieter over the years.. -- Sue |
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#90
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
In article , Floyd L. Davidson
writes tony sayer wrote: In article , Floyd L. Davidson writes (Don Pearce) wrote: The diagram demonstrated exactly what it needed to demonstrate - no more and no less. To get a ground loop, you need to make a loop from the ground. To make a loop, you need the ground from one piece of kit to another by two separate paths. This can be a problem in two ways. Either, if the loop is physically large it can intercept magnetic hum fields and generate a casement flowing around the loop, or if other equipment is using the circuit, those currents flowing through the main ground path will do the job for you. Virtually *every* outside plant telephone cable is wired up exactly like that. There is a ground at both ends of each and every section (3000 or 6000 feet), and the shields from each coupled section are bonded to the other and to ground. A three mile long section of cable might look just like this: 6000' 6000' 6000' -----------o----------o----------- signal pair -----------o----------o----------- +==//==+ +==//==+ +==//==+ shield | | | | | | | +-+-+ +-+-+ | | | | | ----- ----- ----- ----- --- --- --- --- - - - - 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. 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! 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.... 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. Balanced working.. ever read up about it or used it in practice?... I seemed to think we were talking about domestic unbalanced lines here?..... 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. Really;-?.... -- Tony Sayer |
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#91
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
And regardless of that... an easy proof that what you say is
not correct is known to almost anyone who has ever tried cabling microphone systems. Just hook up a microphone through several feet (10m would do!) of shielded cable and connect the shield at both ends... No other connections. No ground. No AC either. No loop. But just see what a nice ground loop that makes as soon as someone picks up the microphone! Bzzzzzzzzzzzzzz. Whatever is going on, on your planet?.. This doesn't happen here!.... -- Tony Sayer |
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#92
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
On Sat, 22 Apr 2006 11:24:56 +0100, tony sayer
wrote: And regardless of that... an easy proof that what you say is not correct is known to almost anyone who has ever tried cabling microphone systems. Just hook up a microphone through several feet (10m would do!) of shielded cable and connect the shield at both ends... No other connections. No ground. No AC either. No loop. But just see what a nice ground loop that makes as soon as someone picks up the microphone! Bzzzzzzzzzzzzzz. Whatever is going on, on your planet?.. This doesn't happen here!.... They have both passed Magna Cum Laude in advanced incompetence. I hope they aren't allowed sharp objects - you may have heard that a knife will cut a few millimetres off your fingers every time you pick it up. d -- Pearce Consulting http://www.pearce.uk.com |
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#93
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
tony sayer wrote:
In article , Floyd L. Davidson writes tony sayer wrote: In article , Floyd L. Davidson writes Virtually *every* outside plant telephone cable is wired up exactly like that. There is a ground at both ends of each and every section (3000 or 6000 feet), and the shields from each coupled section are bonded to the other and to ground. A three mile long section of cable might look just like this: 6000' 6000' 6000' -----------o----------o----------- signal pair -----------o----------o----------- +==//==+ +==//==+ +==//==+ shield | | | | | | | +-+-+ +-+-+ | | | | | ----- ----- ----- ----- --- --- --- --- - - - - 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. 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! Over here it seems to, well the half a dozen or so I've looked at!. We 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. 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.... When done right, it works *extremely* well. 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. Balanced working.. ever read up about it or used it in practice?... About 40 years of working with it every day in a huge variety of situations. I seemed to think we were talking about domestic unbalanced lines here?..... 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. Really;-?.... 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 -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#94
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
(Don Pearce) wrote:
On Sat, 22 Apr 2006 11:24:56 +0100, tony sayer wrote: And regardless of that... an easy proof that what you say is not correct is known to almost anyone who has ever tried cabling microphone systems. Just hook up a microphone through several feet (10m would do!) of shielded cable and connect the shield at both ends... No other connections. No ground. No AC either. No loop. But just see what a nice ground loop that makes as soon as someone picks up the microphone! Bzzzzzzzzzzzzzz. Whatever is going on, on your planet?.. This doesn't happen here!.... It *does* happen, under the circumstances described. If all you do is extend the shield to surround the microphone element, and do *not* connect to it, then you get no noise. Try hooking that shield to the microphone element and see what you get! They have both passed Magna Cum Laude in advanced incompetence. I hope they aren't allowed sharp objects - you may have heard that a knife will cut a few millimetres off your fingers every time you pick it up. For a fellow who has posted exactly *zero* evidence that he understands anything at all about electricity, that is a very telling bit of ad hominem. It says you are so insecure that gratuitous insults are the closest thing you have to an argument... -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#95
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
On Sat, 22 Apr 2006 02:49:48 -0800, (Floyd L.
Davidson) wrote: (Don Pearce) wrote: On Sat, 22 Apr 2006 11:24:56 +0100, tony sayer wrote: And regardless of that... an easy proof that what you say is not correct is known to almost anyone who has ever tried cabling microphone systems. Just hook up a microphone through several feet (10m would do!) of shielded cable and connect the shield at both ends... No other connections. No ground. No AC either. No loop. But just see what a nice ground loop that makes as soon as someone picks up the microphone! Bzzzzzzzzzzzzzz. Whatever is going on, on your planet?.. This doesn't happen here!.... It *does* happen, under the circumstances described. If all you do is extend the shield to surround the microphone element, and do *not* connect to it, then you get no noise. Try hooking that shield to the microphone element and see what you get! OK, now I'm actually laughing. They have both passed Magna Cum Laude in advanced incompetence. I hope they aren't allowed sharp objects - you may have heard that a knife will cut a few millimetres off your fingers every time you pick it up. For a fellow who has posted exactly *zero* evidence that he understands anything at all about electricity, that is a very telling bit of ad hominem. It says you are so insecure that gratuitous insults are the closest thing you have to an argument... So you haven't listened to that piece of evidence I posted on my web site? Thought you might want to avoid that. d -- Pearce Consulting http://www.pearce.uk.com |
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#96
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
(Don Pearce) wrote:
On Sat, 22 Apr 2006 02:49:48 -0800, (Floyd L. Davidson) wrote: (Don Pearce) wrote: On Sat, 22 Apr 2006 11:24:56 +0100, tony sayer wrote: And regardless of that... an easy proof that what you say is not correct is known to almost anyone who has ever tried cabling microphone systems. Just hook up a microphone through several feet (10m would do!) of shielded cable and connect the shield at both ends... No other connections. No ground. No AC either. No loop. But just see what a nice ground loop that makes as soon as someone picks up the microphone! Bzzzzzzzzzzzzzz. Whatever is going on, on your planet?.. This doesn't happen here!.... It *does* happen, under the circumstances described. If all you do is extend the shield to surround the microphone element, and do *not* connect to it, then you get no noise. Try hooking that shield to the microphone element and see what you get! OK, now I'm actually laughing. They have both passed Magna Cum Laude in advanced incompetence. I hope they aren't allowed sharp objects - you may have heard that a knife will cut a few millimetres off your fingers every time you pick it up. For a fellow who has posted exactly *zero* evidence that he understands anything at all about electricity, that is a very telling bit of ad hominem. It says you are so insecure that gratuitous insults are the closest thing you have to an argument... So you haven't listened to that piece of evidence I posted on my web site? Thought you might want to avoid that. Just as your supposed "diagram" didn't show what a ground loop is, your "evidence" isn't evidence. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#97
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
On Sat, 22 Apr 2006 04:22:39 -0800, (Floyd L.
Davidson) wrote: (Don Pearce) wrote: On Sat, 22 Apr 2006 02:49:48 -0800, (Floyd L. Davidson) wrote: (Don Pearce) wrote: On Sat, 22 Apr 2006 11:24:56 +0100, tony sayer wrote: And regardless of that... an easy proof that what you say is not correct is known to almost anyone who has ever tried cabling microphone systems. Just hook up a microphone through several feet (10m would do!) of shielded cable and connect the shield at both ends... No other connections. No ground. No AC either. No loop. But just see what a nice ground loop that makes as soon as someone picks up the microphone! Bzzzzzzzzzzzzzz. Whatever is going on, on your planet?.. This doesn't happen here!.... It *does* happen, under the circumstances described. If all you do is extend the shield to surround the microphone element, and do *not* connect to it, then you get no noise. Try hooking that shield to the microphone element and see what you get! OK, now I'm actually laughing. They have both passed Magna Cum Laude in advanced incompetence. I hope they aren't allowed sharp objects - you may have heard that a knife will cut a few millimetres off your fingers every time you pick it up. For a fellow who has posted exactly *zero* evidence that he understands anything at all about electricity, that is a very telling bit of ad hominem. It says you are so insecure that gratuitous insults are the closest thing you have to an argument... So you haven't listened to that piece of evidence I posted on my web site? Thought you might want to avoid that. Just as your supposed "diagram" didn't show what a ground loop is, your "evidence" isn't evidence. Your posting was essentially an assertion that if you connect up a microphone wrongly, it doesn't work properly. Now maybe you don't understand why you shouldn't connect a microphone the way you suggest, but it is a fact. My web site evidence showed - and yes it did show - that when you know what you are doing and connect everything up properly, there is no hum when you touch a microphone body. d -- Pearce Consulting http://www.pearce.uk.com |
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#98
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
Virtually all power is generated as 3-phase...
Nope. 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. What you actually get merely depends on what the transformer arrangement is. Single phase residential power is nothing other than one phase from a 3 phase distribution system. All that is required to have 3 phase power is *more wires*! http://www.kayind.com/basics/why.htm 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%. Apparently not. If you don't believe me, ask PG&E (Pacific Greed and Extortion, as they're known around here). They don't run all 3 wires as part of their normal power distribution. True for Detroit Edison, as well. 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. I know this thing exists, because I saw the crews working on the damn thing in front of my shop when it malfunctioned and the press stopped working. http://www.homemetalshopclub.org/pro...nv/phconv.html |
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#99
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
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%. 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. |
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#100
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
"Karl Uppiano" wrote in message
news:b1x2g.2652$BO2.21@trnddc02 Virtually all power is generated as 3-phase... Nope. 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. What you actually get merely depends on what the transformer arrangement is. Single phase residential power is nothing other than one phase from a 3 phase distribution system. All that is required to have 3 phase power is *more wires*! http://www.kayind.com/basics/why.htm 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. 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. |
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#101
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
(Don Pearce) wrote:
Just as your supposed "diagram" didn't show what a ground loop is, your "evidence" isn't evidence. Your posting was essentially an assertion that if you connect up a microphone wrongly, it doesn't work properly. Now maybe you don't understand why you shouldn't connect a microphone the way you suggest, but it is a fact. My web site evidence showed - and yes it did show - that when you know what you are doing and connect everything up properly, there is no hum when you touch a microphone body. You haven't shown anything. And I notice that, even though this URL has been posted twice before, you don't have a word to say about it. 64.70.157.146/pdf/Bondingcableshields.pdf *That* is evidence. And it clearly debunks virtually everthing you've had to say in this thread. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#102
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
In article , Floyd L. Davidson
writes tony sayer wrote: In article , Floyd L. Davidson writes tony sayer wrote: In article , Floyd L. Davidson writes Virtually *every* outside plant telephone cable is wired up exactly like that. There is a ground at both ends of each and every section (3000 or 6000 feet), and the shields from each coupled section are bonded to the other and to ground. A three mile long section of cable might look just like this: 6000' 6000' 6000' -----------o----------o----------- signal pair -----------o----------o----------- +==//==+ +==//==+ +==//==+ shield | | | | | | | +-+-+ +-+-+ | | | | | ----- ----- ----- ----- --- --- --- --- - - - - 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. 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! Over here it seems to, well the half a dozen or so I've looked at!. We I'm not sure what you are agreeing with there... that cables do or don't! :-) 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.. 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. Yep but they don't use shielding on a lot of phone multicore in the UK and it wouldn't matter anyway.. 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.... When done right, it works *extremely* well. 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. Balanced working.. ever read up about it or used it in practice?... About 40 years of working with it every day in a huge variety of situations. I seemed to think we were talking about domestic unbalanced lines here?..... 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. Really;-?.... 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 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 andisn't connected to anything else at all... Now are we agreed that that arrangement will or won't hum?...... -- Tony Sayer |
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#103
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
On Sun, 23 Apr 2006 03:13:33 -0800, (Floyd L.
Davidson) wrote: (Don Pearce) wrote: Just as your supposed "diagram" didn't show what a ground loop is, your "evidence" isn't evidence. Your posting was essentially an assertion that if you connect up a microphone wrongly, it doesn't work properly. Now maybe you don't understand why you shouldn't connect a microphone the way you suggest, but it is a fact. My web site evidence showed - and yes it did show - that when you know what you are doing and connect everything up properly, there is no hum when you touch a microphone body. You haven't shown anything. And I notice that, even though this URL has been posted twice before, you don't have a word to say about it. 64.70.157.146/pdf/Bondingcableshields.pdf *That* is evidence. And it clearly debunks virtually everthing you've had to say in this thread. 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. OK? d -- Pearce Consulting http://www.pearce.uk.com |
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#104
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10 metres audio cable going into PC = too long?
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#105
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10 metres audio cable going into PC = too long?
On Sun, 23 Apr 2006 12:07:41 GMT, Roy L. Fuchs
wrote: On Sun, 23 Apr 2006 11:37:05 GMT, (Don Pearce) Gave us: 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. 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. 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? 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. 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. Now please, go away and reflect on all of these things, Please grow the **** up. GTFU GTFU? Did you put that bit in so you could read it as well? forget the "theory" You're a goddamned idiot. you have learned and find out how the real world actually works. I retract that... you're a goddamned retard. 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. OK? **** off. YOU go back and read what he said, now that you know what constitutes a signal. 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. d -- Pearce Consulting http://www.pearce.uk.com |
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#106
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10 metres audio cable going into PC = too long?
tony sayer wrote:
In article , Floyd L. Davidson writes 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! Over here it seems to, well the half a dozen or so I've looked at!. We I'm not sure what you are agreeing with there... that cables do or don't! :-) Doesn't matter either way as long as its balanced working and in any case telephone bandwidth isn't that responsive to 'ummmm... 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? 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.. 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??? ;-) 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. Yep but they don't use shielding on a lot of phone multicore in the UK and it wouldn't matter anyway.. 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? 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 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 Heh heh, lets see you try picking any holes in it! 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. You didn't read it, did you? It *does* affect the signal pairs. It reduces the noise on them, significantly. However in practice the final result is and can be affected by transformer and electronic balanced inputs and how "floating" they are. 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). I think we could all agree that balanced working isn't really a problem. 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. Now they mention unbalanced working, but haven't given it much attention. It is rarely used for critical circuits where induction interference from power lines would be important. (For obvious reasons...) 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 andisn't connected to anything else at all... Now are we agreed that that arrangement will or won't hum?...... 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. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#107
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10 metres audio cable going into PC = too long?
(Don Pearce) wrote:
On Sun, 23 Apr 2006 03:13:33 -0800, (Floyd L. Davidson) wrote: And I notice that, even though this URL has been posted twice before, you don't have a word to say about it. 64.70.157.146/pdf/Bondingcableshields.pdf .... 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. OK? In other words Don, you can't understand what the paper says. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#108
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10 metres audio cable going into PC = too long?
On Sun, 23 Apr 2006 05:38:52 -0800, (Floyd L.
Davidson) wrote: (Don Pearce) wrote: On Sun, 23 Apr 2006 03:13:33 -0800, (Floyd L. Davidson) wrote: And I notice that, even though this URL has been posted twice before, you don't have a word to say about it. 64.70.157.146/pdf/Bondingcableshields.pdf ... 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. OK? In other words Don, you can't understand what the paper says. 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. d -- Pearce Consulting http://www.pearce.uk.com |
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#109
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10 metres audio cable going into PC = too long?
(Don Pearce) wrote:
On Sun, 23 Apr 2006 05:38:52 -0800, (Floyd L. Davidson) wrote: (Don Pearce) wrote: On Sun, 23 Apr 2006 03:13:33 -0800, (Floyd L. Davidson) wrote: And I notice that, even though this URL has been posted twice before, you don't have a word to say about it. 64.70.157.146/pdf/Bondingcableshields.pdf ... 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. OK? In other words Don, you can't understand what the paper says. 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. Giggle snort. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#110
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10 metres audio cable going into PC = too long?
On Sun, 23 Apr 2006 06:29:06 -0800, (Floyd L.
Davidson) wrote: (Don Pearce) wrote: On Sun, 23 Apr 2006 05:38:52 -0800, (Floyd L. Davidson) wrote: (Don Pearce) wrote: On Sun, 23 Apr 2006 03:13:33 -0800, (Floyd L. Davidson) wrote: And I notice that, even though this URL has been posted twice before, you don't have a word to say about it. 64.70.157.146/pdf/Bondingcableshields.pdf ... 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. OK? In other words Don, you can't understand what the paper says. 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. Giggle snort. 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. d -- Pearce Consulting http://www.pearce.uk.com |
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#111
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10 metres audio cable going into PC = too long?
(Don Pearce) wrote:
On Sun, 23 Apr 2006 06:29:06 -0800, (Floyd L. Davidson) wrote: 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. Giggle snort. 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. Don't give up your job flipping burgers... -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#112
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10 metres audio cable going into PC = too long?
On Sun, 23 Apr 2006 07:00:38 -0800, (Floyd L.
Davidson) wrote: (Don Pearce) wrote: On Sun, 23 Apr 2006 06:29:06 -0800, (Floyd L. Davidson) wrote: 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. Giggle snort. 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. Don't give up your job flipping burgers... The reverse is so clearly the case that this barely merits an answer. d -- Pearce Consulting http://www.pearce.uk.com |
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10 metres audio cable going into PC = too long?
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#114
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10 metres audio cable going into PC = too long?
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#115
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10 metres audio cable going into PC = too long?
On Sun, 23 Apr 2006 15:07:55 GMT, (Don Pearce)
Gave us: The reverse is so clearly the case that this barely merits an answer. You're an idiot. |
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#116
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10 metres audio cable going into PC = too long?
On Sun, 23 Apr 2006 19:07:22 GMT, Roy L. Fuchs
wrote: On Sun, 23 Apr 2006 15:07:55 GMT, (Don Pearce) Gave us: The reverse is so clearly the case that this barely merits an answer. You're an idiot. Oh dear! It really has got to you that I could prove my point so easily while you have nothing. d -- Pearce Consulting http://www.pearce.uk.com |
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#117
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10 metres audio cable going into PC = too long?
In article , Floyd L. Davidson
writes tony sayer wrote: In article , Floyd L. Davidson writes 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! Over here it seems to, well the half a dozen or so I've looked at!. We I'm not sure what you are agreeing with there... that cables do or don't! :-) Doesn't matter either way as long as its balanced working and in any case telephone bandwidth isn't that responsive to 'ummmm... 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. 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... 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! Can you explain how your measuring or have that configured please?.. Do you have any idea how many telephone lines actually have a 67 dB SNR? 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.. So? "Very locally" can mean more than a *mile*... 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!... What do you mean by "BT system the copper is longer but doesn't humm.."? They have hum resistance copper??? ;-) Nope;!, just a way of putting that, see above,... 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. Yep but they don't use shielding on a lot of phone multicore in the UK and it wouldn't matter anyway.. 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.) 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?.. Where are you coming up with these ideas? Have you ever even seen the specs for any of this? 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 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 Heh heh, lets see you try picking any holes in it! 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.... 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. You didn't read it, did you? It *does* affect the signal pairs. It reduces the noise on them, significantly. Were is this noise coming from then?... However in practice the final result is and can be affected by transformer and electronic balanced inputs and how "floating" they are. In practice, what they showed was that it improves noise immunity. "Floating" makes no difference at all. 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 ?.. 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). 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. I think we could all agree that balanced working isn't really a problem. 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. 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?.. Now they mention unbalanced working, but haven't given it much attention. It is rarely used for critical circuits where induction interference from power lines would be important. (For obvious reasons...) Yes.. 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 andisn't connected to anything else at all... Now are we agreed that that arrangement will or won't hum?...... 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! Why?. Regardless, that is one of the worst possible ways to wire 10 meters of cable to a microphone. 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?..... -- Tony Sayer |
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#118
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10 metres audio cable going into PC = too long?
tony sayer wrote:
In article , Floyd L. Davidson writes tony sayer wrote: Doesn't matter either way as long as its balanced working and in any case telephone bandwidth isn't that responsive to 'ummmm... 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. .... 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... 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. 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! Can you explain how your measuring or have that configured please?.. 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. .... Yep but they don't use shielding on a lot of phone multicore in the UK and it wouldn't matter anyway.. 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.) 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. 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. What do you define shielding as, just a wrap of aluminium foil with a drain wire or a fully woven copper mesh?.. Shielding is shielding, whether it is aluminum foil or copper braid. Where are you coming up with these ideas? Have you ever even seen the specs for any of this? 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. 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 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 Heh heh, lets see you try picking any holes in it! 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. "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? 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... 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. 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.... 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? 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. You didn't read it, did you? It *does* affect the signal pairs. It reduces the noise on them, significantly. Were is this noise coming from then?... 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. However in practice the final result is and can be affected by transformer and electronic balanced inputs and how "floating" they are. In practice, what they showed was that it improves noise immunity. "Floating" makes no difference at all. 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 ?.. 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. 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). 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. 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. I think we could all agree that balanced working isn't really a problem. 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. 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?.. 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... On the other hand, if you place a fluorescent light fixture close to it, it might well hum! Why?. Because the shielding is not effective at powerline frequencies and harmonics. Regardless, that is one of the worst possible ways to wire 10 meters of cable to a microphone. 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?..... 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. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
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#119
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
"tony sayer" wrote ...
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!. The telecom people would find it very amusing to see how much their customers take for granted. They work very hard and use many tricks and techniques to keep power-mains hum out of phone loops. Apparently they are doing a good job if we think that it is effortless. |
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#120
Posted to alt.engineering.electrical,uk.rec.audio,rec.audio.tech
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10 metres audio cable going into PC = too long?
In article , Roy L. Fuchs
wrote: On Sun, 23 Apr 2006 12:24:51 GMT, (Don Pearce) Gave us: 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? Even with s/n ratio, in an engineering analysis BOTH the signal AND the noise are signals. As with various of the other statements I have seen in this thread on various sub-topics, the above seems to me to be an over-simplification. Interesting to speculate if in this case it is the above statement that is ambiguous, or the ways in which the terms are actually used by engineers are ambiguous... Perhaps this supports the argument that people become engineers because they can't communicate very well... :-) If you go back to some of the early sources [e.g. 1] then you can find some that describe what is observed by the receiver/destination as something like a 'received signal' which may include some 'noise' (and some distortion or other systematic alterations).[2] However the sources also routinely refer to 'signal to noise' ratio. Shannon seems to resolve this by distinguishing between the 'signal' (i.e. what the source transmitted) and the 'received signal' (i.e. what the destination actually observed to arrive). So if we were to use a term like 'received signal' in the above statement it would essentially become either a tautology or self-referential as the signal includes the noise. Thus the problem with the statement is that it is unclear due to the ambiguous use of 'signal'. Hence, as often is the case with such ambiguous statements, people start arguing about the meaning when they are simply using different definitions which the ambiguity allows. :-) FWIW for the above reason, when teaching Information Theory/ Comms/ Instrumentation I tended to use another approach which is common in the area. This is to say that a 'signal' means that the pattern (or part of the pattern) *is used to convey information content*. Thus in the context of communications a 'signal' means that the sender and destination have to have pre-agreed the coding/modulation system to be employed, and the meanings of the code symbols or distinguishable patterns. In the context of a physical scientist making observations - e.g. an astronomer observing what can be received from a distant radio galaxy - the 'signal means that the observed pattern will be used to obtain information about the distant source. The status of 'signal' then stems from the deliberation or requirement that it conveys information on a defined basis. In both contexts what distinguishes 'signal' from 'noise' is the information conveyance the 'signal' provides, and that 'noise' tends to obscure, or limit, or make uncertain, the information recovery. This then helps make clear the actual meaning in practice of terms like 'signal to noise ratio'. (Although there may then be hours of fun for all the family as they argue about the distinction in this phrase between assuming 'signal' means either the intended/transmitted or the 'received' signal. :-) ) Slainte, Jim [1] e.g. Shannon http://cm.bell-labs.com/cm/ms/what/s...day/paper.html [2] Probably best at this point not to start worrying about distortion as being 'signal' or not... ;- -- Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html |
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