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Author Topic: Audio over telephone dry-pair  (Read 11346 times)

Chris Clark

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Audio over telephone dry-pair
« on: January 18, 2014, 06:47:58 pm »

I am working on a plan to link a new area into a distributed system (between multiple buildings), but at this time I do not have the ability to bury a dedicated audio line or the budget to purchase a fiber interface (which is how we are getting to other areas currently, borrowing fiber lines from the IT department). I can secure a dry-pair from the telecom department that can be patched from source to destination, which by mine and the telecom supervisor's estimations will be between 1350 to 1500 feet (1350 ft is based on flat-ground point-to-point measurement, not taking into account elevation changes or curvature of the buried lines around obstacles, hence the 1500 "high" estimate).

I know it is theoretically possible to do this, as my college's radio station maintained a few dry-pair audio links, one as a backup between the studio and the transmitter in case the microwave STL went down, and a set from the campus hockey rink press box for broadcasting games. What I don't remember is whether they had any interfacing equipment or whether they patched the balanced audio directly into and out of the lines.

The equipment to be installed will be balanced on both sides so I'm not worried about baluns, but I'm wondering if a line driver (or a booster?) or isolation of some kind is still necessary, especially at this distance? Also would the lightning protectors (as much as they can actually provide...) at each building entrance be adequate do you think? The lines are underground except where in buildings or pedestals so I would presume that would create some inherent protection. The 25/50/100/what-have-you pair shield is also grounded at each point of emergence from the ground (building entrances, pedestals). I know lightning is unpredictable enough it would take out everything anyway if it wanted to, but anything that would help as a just-in-case...

Any other thoughts on this idea? I hope it to only last a year or so as I plan on putting in some major upgrades next year, budget permitting (either more fiber convertors or more hopefully conversion to a networked system based on, say, Cobranet), but they want the sound there this year.

PS, sound quality of the resultant background music/paging doesn't need to be of the utmost quality, the speakers in the area are of the "mushroom" landscaping type and therefore lack high-end anyway. In fact the end system has been in place for a while but the goal is to tie it in so its audio source comes from the main rack and can therefore be controlled centrally and include paging.
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Cailen Waddell

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Re: Audio over telephone dry-pair
« Reply #1 on: January 18, 2014, 07:13:11 pm »

If you don't mind an experiment, try a cat5 audio balun, one with screw terminals since you will only have one pair. It may not work, but it just might...
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Kevin Graf

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Re: Audio over telephone dry-pair
« Reply #2 on: January 18, 2014, 07:15:47 pm »

That brings up memories of 111 (one-elevens) repeater transformers, from 40 years ago.
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Lee Buckalew

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Re: Audio over telephone dry-pair
« Reply #3 on: January 18, 2014, 07:24:29 pm »

If you don't mind an experiment, try a cat5 audio balun, one with screw terminals since you will only have one pair. It may not work, but it just might...

No balun required.  A twisted pair is all that's needed for balanced audio, either analogue or digital.  That said, it would be best to isolate the buildings by using a transformer at one end of each line for an analogue run. 

Lee
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Greg Bellotte

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Re: Audio over telephone dry-pair
« Reply #4 on: January 18, 2014, 11:43:57 pm »

Let 'er rip! I work in remote broadcast audio, and we used to run audio over telco wire at many outdoor events. The Detroit Grand Prix comes to mind, when they used to run through downtown (late 80s?). We used 25 pair telco cable in 500' lengths with amphenol connectors. This was all temporary, and just laid on the ground just outside the race fence. The race circuit was a 2 mile loop, and the TV compound was outside the circuit at one end. The runs around either way came to well over 5000' feet each. A mic with battery powered line amp was placed where needed, and tied into the 25 pair cables, one pair per mic. We would have 20-40 efx mics around any given race track, and maybe some RTS intercom in the wire as well. The other end of the cables would breakout and patch into the console or intercom as needed. We used to wire golf courses like this too until fiber came along...
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TonyWilliams

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Re: Audio over telephone dry-pair
« Reply #5 on: January 19, 2014, 01:12:55 am »


Let 'er rip!

Yup. Assuming the twisted pair is in good condition, it should work fine. We also use these in remote broadcast in older football stadiums to get from the truck to the pressbox. Both RTS and line level audio.

We had something like this:
http://m.markertek.com/product/detail/http%3A%7C%7Cwww.markertek.com%7CMobile%7CConnectors-Adapters%7CAudio-Adapters%7CAudio-Binding-Posts%7CSescom%7CSES-MKP-23.xhtml



- Tony Williams
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Chris Clark

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Re: Audio over telephone dry-pair
« Reply #6 on: January 19, 2014, 02:30:14 am »

Awesome news! I won't even need the XLR to wire conversion honestly, as the equipment at both ends will have euroblocks on it anyway. I think my budget can support an isolation transformer as Lee suggested (just in case). I was just hoping I didn't have to find any special line amplifiers or something, it looks like I don't!

Lee or others, any preference on which side the isolation transformer goes on? I'm personally thinking closer to the source side since the router (a Digitool MX32) is a tad bit more expensive than the amplifier on the other side and in my brain it seems this would provide another step of electrical protection from the outside world, is this a correct thought process or am I just spinning wheels and it really doesn't make a lick of difference?

Thanks guys!
« Last Edit: January 19, 2014, 02:39:00 am by Chris Clark »
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Lee Buckalew

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Re: Audio over telephone dry-pair
« Reply #7 on: January 19, 2014, 09:01:54 am »

Awesome news! I won't even need the XLR to wire conversion honestly, as the equipment at both ends will have euroblocks on it anyway. I think my budget can support an isolation transformer as Lee suggested (just in case). I was just hoping I didn't have to find any special line amplifiers or something, it looks like I don't!

Lee or others, any preference on which side the isolation transformer goes on? I'm personally thinking closer to the source side since the router (a Digitool MX32) is a tad bit more expensive than the amplifier on the other side and in my brain it seems this would provide another step of electrical protection from the outside world, is this a correct thought process or am I just spinning wheels and it really doesn't make a lick of difference?

Thanks guys!

Here's a link to the Jensen application notes page. 
http://www.jensen-transformers.com/apps_sc.html
Depending on the specifics you may want to use something like a DIN mounted solution for either input or output or you may want to build something that has some transient suppression capability (if the lines are run outdoors between buildings for instance) or you may just want a preterminated isolator in a box.  This gives you many options.  Do take note that the impedance wants to be low on the driver side and high on the receiver side.

There are many solutions outside of Jensen as well.

Lee
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Tim Perry

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Re: Audio over telephone dry-pair
« Reply #8 on: January 19, 2014, 09:43:07 am »

Commonly available passive Di boxes will work well for this application.

Have the low Z "facing" the copper line and the high Z to the equipment.


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Lee Buckalew

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Re: Audio over telephone dry-pair
« Reply #9 on: January 19, 2014, 09:56:00 am »

Commonly available passive Di boxes will work well for this application.

Have the low Z "facing" the copper line and the high Z to the equipment.

Maybe, maybe not.  Try it.  See if you can test a model that you plan to use before paying for it and finding a problem.
The potential problem comes from the input impedance of the isolation DI at the receiving end.  You don't want to drive an impedance that is lower than or even equal to your driven impedance or you may well have current limiting issues that create clipping at the driven end.

Lee

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