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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. |
10 metres audio cable going into PC = too long?
"Richard Crowley" wrote:
"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. Very true. Not to mention that "no hum at all" is only in the perception of the customer, whereas telco people tend to actually measure it. Granted though, a telephone installer just uses a very simple test set that gives a "good/bad" indication, not a specific number. And that would be the most that a customer would likely ever see. But when a cable is installed the pairs are very specifically measured and compared against design specifications, which were calculated very closely prior to construction. Nobody wants to invest in new cable plant and end up with a cable that can't be used... -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
10 metres audio cable going into PC = too long?
On Thu, 20 Apr 2006 08:45:14 -0400, "Arny Krueger"
wrote: All true. It's called "legacy technology". Unlike Europe, the US missed out on the cleansing benefits of being the site of a world war. Ah. THAT'S why America is picking fights with everyone! Fair enough. Where would you like bombed first? :-) |
10 metres audio cable going into PC = too long?
"Laurence Payne" lpayneNOSPAM@dslDOTpipexDOTcom wrote in
message On Thu, 20 Apr 2006 08:45:14 -0400, "Arny Krueger" wrote: All true. It's called "legacy technology". Unlike Europe, the US missed out on the cleansing benefits of being the site of a world war. Ah. THAT'S why America is picking fights with everyone! Oh come on, we're very selective with the fights we pick. Our targets are always very weak. Fair enough. Where would you like bombed first? :-) Someone already did a big number on downtown Manhattan, but it was not sufficient to get a new electrical code written. |
10 metres audio cable going into PC = too long?
Laurence Payne wrote:
All true. It's called "legacy technology". Unlike Europe, the US missed out on the cleansing benefits of being the site of a world war. Ah. THAT'S why America is picking fights with everyone! Fair enough. Where would you like bombed first? :-) World Trade Center? Oh, wait a minute... :-) -- Glenn Richards Tel: (01453) 845735 Squirrel Solutions http://www.squirrelsolutions.co.uk/ IT consultancy, hardware and software support, broadband installation |
10 metres audio cable going into PC = too long?
Roy L. Fuchs wrote:
His diagrams didn't kill the thread, and no, you do NOT quote everything over and over again, dolt boy. You don't ("you" being the generic "Usenet user"). "He" on the other hand does. On many occasions he's quoted back several pages and added one or two lines to the bottom... the word "trim" doesn't exist in his vocabulary. One wonders if he uses AOL. Still, it could be worse. At least it's not top-posted HTML. -- Glenn Richards Tel: (01453) 845735 Squirrel Solutions http://www.squirrelsolutions.co.uk/ IT consultancy, hardware and software support, broadband installation |
10 metres audio cable going into PC = too long?
On Thu, 27 Apr 2006 20:28:15 +0100, Glenn Richards
Gave us: Laurence Payne wrote: All true. It's called "legacy technology". Unlike Europe, the US missed out on the cleansing benefits of being the site of a world war. Ah. THAT'S why America is picking fights with everyone! Fair enough. Where would you like bombed first? :-) World Trade Center? Oh, wait a minute... :-) Poor taste retard. That's what you are. |
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: 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. Yes but quite some time ago now. FWIW we don't or very rarely have long lumps of overhead line anymore that carry baseband audio. For voiceband circuits these days its digital end to end with a A/D and D/A convertor at each end. And for phones its going much the same way, well over here at least. BT have the 21CN nets which are data circuits which you run data or audio or whatever you like over them.. 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.) I asked a couple of cable jointers who were working beside the road the other day re that one, and it seems that its the exception rather than the rule these days. There is some cable which has a foil screen around it, but as to woven braids seems they aren't used anymore.. 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. Well the ones ntl use here according to a friend of mine who works with their plant day in and day out sez otherwise. Seems only some of the cable they use has a foil screen but then again they use fibre and co-ax for distances of any length, seems digital rules;).. Suggesting it is otherwise is ignorant. No its not, you have to define what your using it for an in what application.. 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. Yes except that if we're talking like we were about currents circulating in the "screen" of a multicore cable, then there is going to be quite a bit of difference in practice between a heavily woven copper braid and the light foil wrap where the connection to that is by a fairly thin drain wire... 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. Yes we sometimes do, but very rarely these days, it s getting to be a very digital world over here. Analogue circuits are quite rare nowadays and BT have been known to have to get guys out of retirement to work on the few remaining ones!. If you wanted say a speech band 300- 3500 Hz point to point circuit these days it'd be digital end to end or if you required a music grade circuit that would definitely be digital copper would only be for the patch leads to connect the gear. Even some recording and sound re-inforcement systems use digital leads from the stage area to the mixer now.. 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? Well they don't define what you are doing with that. Consider say 10 meters of Andrews LDF 4-50 cable connected to a transmitter with the correct plug, what are they connecting that other end to?. Nothing or a load partially connected?. Or do they mean the connection to the shield, referred to the point where that would normally be connected, is greater than one tenth of lambda?. If thats what they meant then they didn't describe that very well. It seems that they were thinking of say a braided cable like perhaps RG214 or similar when you "could" take that out as a pigtail perhaps...... 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? I think its relevant on the subject, but YMMD as they say.. 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. I'll have a look at that again when I get a moment and try some experiments here too... 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. Well how far do you want to go with that;?... 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. What do you do over there are you involved in a Telco?.. 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...) The above was only to demonstrate what I meant by balanced working.. 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... As above just for demo.. ... 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. Well I have tried that and it doesn't hum at least not what I can hear!. And out mains is quite unclean;!.. 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... Humm... What do you use out there in deepest Alaska, batteries;-?..... Your example is nonsense and does not demonstrate anything about noise immunity. It merely provides and example of poor circuit design. Yes it is poor circuit design, but people do it all the time!... -- Tony Sayer |
10 metres audio cable going into PC = too long?
In article , Richard Crowley
writes "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. Do they have engineers anymore?, the accountants that run the industry say they don't need 'em!.. That above example was for a small transmitter that is in a remote location that is fed by a long overhead copper pair, well two of them for stereo, and that goes into line trannies and equalisers and it didn't have any discernible humm on it. However thats about to change, a digital microwave link is to be installed as soon as, copper is on its way out it seems!... -- Tony Sayer |
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