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Author Topic: Audio over telephone dry-pair  (Read 12019 times)

Mac Kerr

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Re: Audio over telephone dry-pair
« Reply #20 on: January 19, 2014, 06:11:49 pm »

Because I'm rather limited on where I can order from (setting up a purchasing account sometimes takes an act of congress around here...) one of the companies we have an account with has a Rane LT22 available, seems to be one of the few isolation units I can find through any of them. Would a pair of these (one at each end) be sufficient, do you think?

You really only need one of them at the destination end. The router is providing a balanced output. You want a transformer at the input of the destination to maximize CMRR (Common Mode Rejection Ratio) by providing a perfectly balanced input. The transformer will also provide electrical isolation. Be aware that the maximum level for that transformer is +4, while the Jensen I linked to is greater than +20. If the transformer saturates from too much low frequency level there will be increased distortion and less high frequency response.

HERE is a chapter from Glen Ballou's book relating to transformers, written by Bill Whitlock.

Mac
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Chris Clark

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Re: Audio over telephone dry-pair
« Reply #21 on: January 19, 2014, 07:03:40 pm »

Thank you for the help, all! I will keep looking at what other transformers might be available from our vendors, but the maximum level for the Comnet fiber transmitters we use on the other legs of this system is +6 so it shouldn't be a stretch to keep them all below +4. The playback systems in place from the central rack are also already high-passed at 100Hz as well so that the 70V transformers throughout the system are at less risk of saturating too, so I think that will help as well.

Thanks again!
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Ivan Beaver

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Re: Audio over telephone dry-pair
« Reply #22 on: January 19, 2014, 07:16:23 pm »

Thank you for the help, all! I will keep looking at what other transformers might be available from our vendors, but the maximum level for the Comnet fiber transmitters we use on the other legs of this system is +6 so it shouldn't be a stretch to keep them all below +4. The playback systems in place from the central rack are also already high-passed at 100Hz as well so that the 70V transformers throughout the system are at less risk of saturating too, so I think that will help as well.

Thanks again!
Be sure to understand what the "max" ratings actually mean.

Typically if you read a level on a VU meter (say +4), that means that the peaks are way above that-say 20dB.

But if the max input is +6 on a digital system, then that would be the max-so the "average" that would be read on a typical VU meter may be 20dB or so BELOW that.

I don't know the particulars in this case-just pointing out some things to be aware of that could possibly cause distortion.
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Ivan Beaver
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Joseph D. Macry

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Re: Audio over telephone dry-pair
« Reply #23 on: January 20, 2014, 10:47:27 am »

I have seen this done at football stadiums, using a passive balancer to send analog audio around the stadium (via underground conduit, manhole-to-manhole) to the visitor's side amp rack, about 1400-1600 feet.
The recurring problem, however, was lightning-induced spikes getting into the long copper line and knocking out the DSP on either end. Same thing happened with a power sequencing control line (sends a contact closure); lightning surge killed sequencing relays. They had me replace the audio line with fiber optic after twice replacing DSPs.
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Chris Clark

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Re: Audio over telephone dry-pair
« Reply #24 on: January 20, 2014, 02:08:11 pm »

I agree, I worry about lightning induced issues, even though the vast majority of the infrastructure is underground. I can tell you that, as I mentioned, every point where the trunk lines emerge from the ground has the cable shield tied to ground, as well as gas-tube protection on the pairs where they enter buildings. In the run we'd be using, we're looking at protectors at the exit point of the building housing the source audio and the entrance point of the building housing the amplifier, as well as at least 6 points where the cable shield is tied to ground (based on my knowledge of where the cross-connect points would be for this circuit).

The building housing the source audio is also a building where one of our three PBX hosts are so I would think there's a fairly heavy lightning protection presence at the patch point into the main trunk line. I'm severely hoping this is enough protection for the digitool since to my knowledge we haven't had any losses to the PBX equipment due to lightning and we tend to take a lot of strikes.

Also like I said I'm hoping this will only be in place for a year or two before I have the budget to switch over to fiber (or better yet, I'd love to convert the whole thing to, say, Cobranet, but that's not happening for the foreseeable future).


Ahh, the wonderful dB/dBU/dBm measurements... One of those things I truthfully have never fully comprehended. I understand the logarithmic scale, the fact that dB is inherently a ratio, 3dB is essentially 2x or 1/2x the power, etc... But comparing the different forms (dB, dBu, dBm, dBx :P ) usually gets the better of me.

The Comnet Fiber tx/rx pairs already in use on the other legs are listed as "4.4V Peak to peak (+6dBm)" (I understand the 4Vpp measurement in itself... in that if ran a pure sine wave to it and measured with a DMM I could crank the voltages as high as 4Vpp, or 2V Peak, or roughly 1.4V RMS, etc..), and the Rane LT22 I was planning to get lists the 40Hz max level as +24dBu (also states "Limit 0.5dB"), and the Digitool lists the max output also as +24dBu (adjustable from -24 to +24). I hate to continue burdening you guys, but any insight as to what it all means and how it will interact with each other?
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Hal Bissinger/COMSYSTEC

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Re: Audio over telephone dry-pair
« Reply #25 on: January 20, 2014, 07:11:02 pm »

The building housing the source audio is also a building where one of our three PBX hosts are so I would think there's a fairly heavy lightning protection presence at the patch point into the main trunk line.

Typically the protector clamp voltage for telco lines is in the range of 240-600 volts. Certainly much too high for an audio pair. The thing to do is to add additional protection to that pair at each end that clamps at maybe 25 volts. They should be located at the building entrances and the grounding is important. Hopefully the ground for the existing protectors was done properly back to the service entrance electrical ground.

-Hal
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Lee Buckalew

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Re: Audio over telephone dry-pair
« Reply #26 on: January 20, 2014, 07:48:17 pm »

I have seen this done at football stadiums, using a passive balancer to send analog audio around the stadium (via underground conduit, manhole-to-manhole) to the visitor's side amp rack, about 1400-1600 feet.
The recurring problem, however, was lightning-induced spikes getting into the long copper line and knocking out the DSP on either end. Same thing happened with a power sequencing control line (sends a contact closure); lightning surge killed sequencing relays. They had me replace the audio line with fiber optic after twice replacing DSPs.

Refer back to my original link

http://www.jensen-transformers.com/apps_sc.html

and look at the isolation application for \long runs with lightning protection.
Here's a direct link to the wiring for lightning protection.

http://www.jensen-transformers.com/as/as028.pdf

Lee
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Frank Koenig

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Re: Audio over telephone dry-pair
« Reply #27 on: January 22, 2014, 02:40:08 am »

Chris,

You have received lots of great guidance on the technical aspects of dry pairs. I can only supply a story.

Some years ago (in the '90s) I hung out at an FM station that normally got their signal from the studio to the approximately 2 mile away mountain-top transmitter by an FM radio link (950 MHz, I think). They leased a pair of dry pairs from the local phone company as a backup, but preferred the radio link, which worked most of the time, for its better sound quality.

At the time (maybe still) the whole area suffered from a pair shortage. I had a phone tech confess to me that there were actually fewer pairs than subscribers and that at any given time someone was doing without. Sort of time-division multiplexing paced by service ticket response time.

Anyway, at the station it became clear that if there was no signal on a pair then sooner or later some phone guy hunting around with his buttset would appropriate it for another customer. Running a good loud test tone on the pairs at all times greatly improved the situation. Maybe not a bad thing to keep in mind if you are paying for pairs that sit idle some of the time. Perhaps a recorded message along the lines of "Don't f with this pair. It belongs to...".

--Frank 
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Josh Millward

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Re: Audio over telephone dry-pair
« Reply #28 on: January 22, 2014, 12:05:37 pm »

Running a good loud test tone on the pairs at all times greatly improved the situation. Maybe not a bad thing to keep in mind if you are paying for pairs that sit idle some of the time. Perhaps a recorded message along the lines of "Don't f with this pair. It belongs to...".
Yes, tone/program is your friend!

Running tone or program down the line at all times will let the tech who is hunting know that this pair IS being used and to not mess with it.

If it is silent, they may just up and use it for something else.
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Josh Millward
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Chris Clark

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Re: Audio over telephone dry-pair
« Reply #29 on: January 22, 2014, 01:50:00 pm »

Ha, that's a good idea, although I doubt it is necessary here, nor would I want to blast test-tone in that area all night. As I said our lines are all internal with the exception of the incoming pairs on a separate cable by our telco for DID (direct inward dial) into the PBX. I've already verified that our telecom supervisor has patched and marked out the the pair we'll be using on both the 66 blocks and the wirelists, and during operating hours there will be constant background music on them. (If it seems odd that I know all this it is because I actually went to school for telecom engineering, but chose sound as my career lol)
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Re: Audio over telephone dry-pair
« Reply #29 on: January 22, 2014, 01:50:00 pm »


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