next generation Clear Com problem

[ship]

So I got a call over the weekend about a 500' fiber snake with power and 4 channel Clear Com line that was malfunctioning. Asked about the termination boxes - if they were wired right and or if there might be an isolation problem within them. My answer nope, not the boxes plugged into, yet when they bypassed the snake system the new style Clear Com dual channel combining into one line system worked fine - had to be somewhere in the snake or boxes.

First the patch boxes FOH and Dimmer end have air between and four channels splitting off from the cord and going to their outputs - not from the boxes - too much air gaps and space between the four lines.

Other note was that our more normal as it were old style two channel Clear Com worked with the 500' snake, just something about the channel combining system working with a digital grade snake (about the best you can get), at 500' in how they did it.

Once figured out part one of what the trouble was, part two instantly solved it on a tour that's been mentioned recently on the forum.

Just a question of patching something in another place it turned out - this after the symtoms and trouble was figured out in why.

1) What was the cause of this bleed between channels especially at the front of house?
2) What given new Clear Com dual channel combining into one was the solution?

 

[soundman]

I am having a hard time deciphering your post.

I also had a hard time coming up with the best way to tech our two channel belt packs, it could be possible that coms and I do not belong together.

It sounds like the TWC was patched at the wrong location. Depending on what packs you want on A and B determines where the twc 70x unit should go. If its FOH that needs both combined I would run A and B separate till it got to FOH and then combine them there based on my gut feeling. Perhaps to keep them physically insulated or cut down on the loss of voltages? As for how it works, it sends one line of com the normal way and the 'added' line gets sent along with the 30 volt DC system. I can't tell if the system is all fiber or if com lines were ran down traditional cable but it seems that would not be the issue.

 

[waynehoskins]

I might guess that the digital snake and DC power don't play nicely together, but I haven't tried it.

May have something to do as well with putting two (or more) unbalanced signals (RTS-style) on a digital snake pair. Since they don't represent a differential signal, the differential A/D and D/A may get confused. If they frequency-division multiplex (so that channel 3 is on, say, a 40k subcarrier on the channel 1 wire) above the Nyquist limit of the digital system, that will screw it up royally too.

Putting balancing transformers (after appropriate demuxing, if any) and isolating power on each channel of intercom at each end might do the trick. Or my personal favorite, just run copper if you can. A copper pair is dumb, it doesn't care what's on it. Once you start digitizing or otherwise multiplexing stuff, then it matters tremendously what's on it.

Just some thoughts from a guy who's never worked with either digital snake or new CC four-channel intercom system, let alone together.

 

[avare]

It has now been a week since a post on this thread. If I wanted to give an example of terrible problem description and cure, the first post is it. What equipment specifically? How was the equipment connected? Where were the components? What was the problem?

 

[Chris15]

To stimulate thought, Ship was talking about digital GRADE core, not digital snake. There is a very pertinent difference between the two... Digital Grade cable means that when it's manufactured, the manufacturer actually cares about the impedance and will try and keep it at 110 oms give or take a smudge. What that means in audio land is that you can run AES3 over it and it stands a reasonably good chance of arriving there properly and not havuing reflection issues blah blah blah. Running analog audio on digital cable is absolutely fine.

On the specifics... I'll have to think and do some research on that one...

 

[ship]

Two problems with the new Clear Com system. Not our house brand - the show brought it with them and plugged it into our snake so we had no experience with it working or not before.


The new Clear Com system it seems is sort of like Dimmer Duplexing in how it works from what I gather. A/B channels down one three wire line as a base of info.

On the 500' four channel snake was only the single Clear Com line as patched running thru one of the four channels - nothing else. Loomed in with the snake was a fiber optic snake and a 500' 8/5 type SO cable with three phases of 20A breakered power running thru it max. Obviously not all power was needed at FOH so it was not a lot of current running thru it given the gauge and insulation.

Sub-answer, just enough current to bleed into the single channel line at 500' given both A/B lines were traveling thru the same line. When the tour bypassed the snake with their clear com, it was not loomed in with the FOH snake thus no interference.

Main answer, why are we tying together two channels of Clear Com so they can travel down a 500' snake as one channel inside a four channel snake? Once the splitter was placed at Front of house and the tour went back to the standard one channel per circuit, the entire problem of bleed and noise was completely solved.

A - at 500' and with power running parallel to the com line, one does not want to be duplexing the com-line. This especially if you have a bunch of channels available, why duplex them? Worked like a charm.

 

[Chris15]

And that kids is why the guidelines say you don't run power and signal parallel for a long distance. Cross at right angles where absolutely necessary as the fields have minimal effect when perpendicular. Perhaps the entertainment industry can learn from out communications brothers and observe a preferred separation of 300mm between communications style cabling and power. Note that as current draw in the power cable increases, so too should the distance...

Now some maths to explain it...
Any current carrying conductor will induce a magnetic field around it. This is proportional to I and sin theta, where I is the current in a single conductor and theta is the angle between the current carrying conductor and the measurement conductor or coil. Now when you add together a pair of conductors with currents in opposite directions, as an active and neutral are, an interference pattern is generated that will cancel in some places and increase in amplitude in others. The space between the conductors will tend to have cancellation and this will also cause a force pulling those two conductors together, likely a negligible force for the currents any of us are talking about. F = k l I1 I2 / d, where F is the force in Newtons, k is a constant of, from memory, 2x10^-7, l is the length of parallel conductor, in metres, (you can also work it out per metre) I1 and I2 are the currents in the wires, in amperes, and d is the distance between them, in metres. I'd imagine that with much uglier maths and a different constant, this formula translates to imperial units.

So the general thing in to reduce interference such as what caused the problem on the gig Ship described, try and increase distance separation to power and reduce length running together. More current in the power -> need to be further away.

Now you could of course use an ethernet based sort of intercom and run that with your ethernet on the fibre, or use one of the digital matrix styles with specific fibre interfaces run that over the fibre and avoid all these problems entirely...

 

[soundman]

Thats great in theory but find me a tour where the data lines are not taped in with the soca runs. Its bad practice im sure but it still works.

 

[Chris15]

Because something works, does that make it right?

In that case, it's very much about distance.

I can however go one up and show you a cable manufactured with AES or analog audio or any number of other variants and power all inside the same outer jacket...

Now I'm thinking more, I also happen to wondere whether there might have been some nasty crosstalk happening between the two unblanced channels of audio in that core, they are twisted together for 150 metres or whatever 500' is in real numbers...

 

[ship]

Never had a problem before with DMX and Clear Com as say a 8-pair or 16-pair snake taped in before on a 400' run before the Com duplexing system, or the 500' now fiber optic snakes got loomed into them. Com in running single channel two of them in the same snake.... no problems.

Well put but more specific to the technology advance as it were in general - power tied in with it not needed in this situation and more for places with only a single com-line available that might need the techonlogy.

Yes, and well standing point about parallel power but in this case less perhaps a case of if at 500' the line loss power or not near it was sufficient to counteract the duplexed com line. Not needed was the final deciscion in running two parallel lines than only combining them at the power supply side and that solved the problem. Only one run available, sure especially if under 500', you can get two com lines into one cable. After that, in doubt not as much because of the parallel cable but the line loss itself I'm thinking.