Balanced mic lines down CAT 5

[Stuart91]

At the moment I'm working with some folks who are trying to do a discreet audio installation in a small hall. The building work has just been completed and, regrettably, the AV side of things is pretty much an afterthought. Routes for cable runs are limited, but there is plenty of CAT 5 infrastructure in place.

 

What I'm looking for is a relatively economical way to run four or ideally eight balanced mic lines down a CAT 5 line. Their budget will not stretch to a digital desk (and their volunteer operators would struggle with it) so we'll just be feeding the inputs of an analog board at the back of the room. I've seen some products that take two lines down CAT 5, but as far as I can see that's just an adaptor that physically maps the XLR pins to the pairs in the CAT 5. Unfortunately there's not quite enough CAT 5 lines there to do this - need to get a minimum of 4 channels per line to make it work.

 

Can anyone recommend a reasonably cheap solution to this?

[Dmills]

Are the lines foil screened?

 

If you have UTP cabling then you can do 4 curcuits but phantom power will probably not work (so you may have to inject phantom at the stage end rather then from thye desk), just use the 4 pairs for the signals and a local earth at the stage end.

 

If the cables are screened then you can do the same thing but can also support phantom by using the screen as the return for all the pin ones.

 

Regards, Dan.

[mervaka]

you won't get any more than four balanced cables down one cat5 "on the cheap". any further solution would be to multiplex those wires. an analogue solution would be to use FDM, but no off the shelf solution exists for that (that I know of). the cheapest way to do this digitally that I can find would be to use the brikworm system.

[peternewman]

I've used the pair per channel method successfully in the past, or signals were line level, so we didn't have phantom to consider. Telephone lines will also travel quite happily down it, although IIRC it would need to be a master to only have one pair. I'd imagine that if phantom is going to be a problem, it would be cheaper to buy some external phantom PSUs than go for a more complex system. One of our guys made up some very neat little breakout boxes from the little cases Canford sell that are the same as Techpro beltpacks. Two Ethercons in parallel (for loop out) and then in our case four POs, with the colour coded overlays, but obviously you could use XLRs to suit. It was all very neat for just adding random extra tielines to places.

 

One other related warning, at my old school, someone had tried a similar trick, presumably as a cost saving measure, so each tieline only had two cores, worse still in our case they were wired unbalanced. Anyway the issue that caused us to discover this was that of course Techpro and most other comms won't work properly without the full pin compliment, so you might want some adaptors made up fully wired, and some in the economy mode.

[bruce]

I'd be a little concerned about putting mic-level signals down those lines - even if we didn't have the phantom problem. Not sure if that concern is a real one or just paranoia thought.

 

I've certainly done this plenty of times for line-level signals - works fine. I guess you could have a 8-way mic preamp at the stage end (which boosts the levels, and gets round the phantom problem) and use a couple of the CAT5 lines to get these back to the mixer.

 

But even using the cheapest preamps, that would still be more expensive, and less flexible, than buying a budget multicore.

 

As an aside, in my "box of useful things", I have a couple of XLR-CAT5 adaptors. It's just a standard single-size electrical backbox with a faceplate which has cutouts for 4 XLRs

(a bit like this, but 4 holes)

http://cpc.farnell.com/productimages/farnell/standard/42404791.jpg

It's got 3 XLRs and 1 Ethercon. One end has 2M, 1F, the other 2F1M.

[Jivemaster]

As a cheap option I'd simply look at pulling in enough cat5 cable to have one pair per channel. But with the need to do fixed terminations and then have jumper leads at each end it may not be the cheapest option.

 

Seriously price a suitable length of proper balanced individually screened overall sheathed audio multicore, and work out how you can install it yourself. Done right first time may end up cheaper in the long run.

[stu00c]

As people have said, there isn't a cheap way of getting 8 channels down a single Cat5 cable. There are, however simple ways using audio networking protocols such as EtherSound or Cobranet. There are products using both protocols that are simple network 'on-ramps' and 'off-ramps' without processing etc.

 

One example of products you could use are the ES8mic and ES8out from Digigram. One of these at either end of your piece of network string and problem solved. Not cheap :( , but the quality will be great :D .

[fatfrog]

Another similar product from a company called violet audio

 

http://www.thomann.de/ie/violet_audio_tst3.htm

 

and

 

http://www.thomann.de/ie/violet_audio_tsr12.htm

 

Using this system you could use 3x TST3's giving you 12CH of audio along 3 lines of cat 5

 

Hope this helps,

Alan

[Stuart91]

Seriously price a suitable length of proper balanced individually screened overall sheathed audio multicore, and work out how you can install it yourself. Done right first time may end up cheaper in the long run.

 

I agree wholeheartedly that running straightforward analog lines would be much less hassle overall. However the opportunity to do that has passed, as the plasterboard was on the walls before we got involved. Builders are keen to get their part of the job completed and would not take kindly to having to remove any of it.

 

Digigram stuff would blow their budget completely, alas. The Violet audio stuff looks neat, but I'm not sure if it's doing anything that I couldn't wire up myself for a lot less money. (Does it actually encode the audio?)

 

One possibility is that we add an 8 channel preamp in the amp rack, supply phantom from there, and that would (in theory) get me four line level signals to each CAT 5 line. So 8/4 would take up three lines, which I think we could manage. The big drawback is that it's another box that people need to understand. Having two places to adjust the gain has the potential to cause lots of confusion.

[mervaka]

it does encode the audio, in a digital multiplex. you wont fit any more than 4 balanced signals down 8 wires without multiplexing in some shape or form.

 

is it out of the question to just bite the bullet and look for those routes for more cable runs? I think besides your idea of the preamps feeding unbalanced lines, it remains doubtful you'll get past this one without either multiplexing your signals, or by adding more cable runs. I'd present them with the decision between more runs or more money.

[Dmills]

With suitable transformers I can fit 4 (as main pairs) + 3 as 'phantom' (in the old school telecoms sense) making 7 more or less balanced circuits down a 4 pair cable. Crosstalk will not be wonderful, as the CMRR determines the separation of the circuits.

 

I cannot think of a way to get 8 without things getting hairy.

 

Regards, Dan.

[fatfrog]

From what I can see the violet audio stuff just does what you could do yourself within a few hours with a soldering iron and a few bits.

 

Alan

[mervaka]

I'll take my previous statement back then, phantom circuits can shoehorn in some extra channels as mentioned above :)

[mervaka]

ok I'm gonna dig this thread back up rather than start a new one, but feel free to split it off if you prefer..

 

going back to the idea of phantom circuits, am I looking for 1:1 ratio transformers, and do I need to take cable impedances into account etc?

[ramdram]

Phantom circuits, crikey that takes me back, ** laughs out loud **. You can even have super phantom circuits.

 

The PO knew they were used but put up with it. The point being we got more "lines" than the physical pairs for all sorts of stuff.

 

Seeing as the idea goes back to the late 1800s it is interesting the practice has been revived, for interest:

 

http://iptv.tmcnet.com/topics/iptv/article...l2-300-mbps.htm

 

for more tech understanding:

 

http://en.wikipedia.org/wiki/Phantom_circuit

 

and:

 

http://www.kadiak.org/tel/17-3.htm

 

and:

 

http://oldphoneguy.com/repeat_coils.htm

 

In other words the technology is very well known...hopefully you will find the repeating coils without too much bother.