Out, Ye Demons!

Posted from: Austin, TX
Next tour stop: Grand Prairie, TX

Every show uses them. At some point or another, every show has problems with them. And, a lot of the time, fixing those problems comes down to lots of random trial and error with no real idea of what’s causing the problem. What are they? Party line intercom systems, most commonly ClearCom systems. For some reason, these seem to be regarded as more voodoo-like than just about any area of live sound except for wireless. And it needn’t be. If you really read up and understand how these systems work, the things that can cause problems with them will actually turn out to be very few and easy to narrow down.

Now, anybody running these systems would be well advised to [gasp] read the manuals for all the gear, no matter how much they think, "It’s such a simple thing, I don’t need to read the manual. I plug it in, it works." If you actually read the manuals, you’ll understand what makes them tick (or, more importantly, stop ticking) much better. At the end of this article, I’ll include links to a few of the manuals you ought to look at, as well as a detailed e-book published by Telex that will teach you everything you could ever want to know (and more) about intercom systems, both party line and matrix, wired and wireless, of all popular brands. But, since my job here is to make your life easier, I’m going to give you a crash course in the most common issues that users of ClearCom systems run into. A quick and dirty guide to exorcising ClearCom demons, if you will.

ClearCom Demon #1: Termination

There is one key word when it comes to making ClearCom systems happy, more than any other thing, and that is termination. If a ClearCom system isn’t properly terminated, all sorts of unhappiness can occur. The two biggest symptoms of an unterminated line are beastly feedback squeals the moment you turn the microphone on, and erratic behavior of the call light (most often, instead of turning off immediately when you release the call button, it will latch on for about two seconds and then fade out). Double/multiple termination is usually indicated by really low levels, or by the null circuit not working (which means that you will be a lot louder in your own headset than usual, and, especially with the telephone handsets that we sound engineers tend to use, will produce the same aforementioned beastly feedback).

A quick double-check to make sure there is proper termination on your system is to break out your trusty multimeter, and use it to measure the resistance across pins 1 and 3 of each channel of your system. If everything is on the up and up, you should get a reading of about 4000 Ohms. If it’s reading down around 2000 Ohms, your system is double-terminated, in which case you need to go through to all the main stations/power supplies in the system and make sure one, and only one, has its termination switch turned on. Excessively high readings, on the other hand, indicate lack of proper termination, in which case you should check the termination switch and, if it is on, you probably have a bad terminating resistor in the power supply.

For those who tour and tie into house systems, this is the reason you’ll have issues tying directly into a house ClearCom system. Common wisdom is that it is dangerous to connect two powered ClearCom systems together. Guess what? That is totally, utterly, entirely wrong. What causes the problem is both stations being terminated, NOT both stations being powered. In fact, it is actually noted in some of the ClearCom documentation that if you have a particularly large system (or have lots of add-on devices, such as some of the Production Intercom products), that you may have to use multiple power supplies on one channel. More about tying into house systems below, but for now, let’s go on.

ClearCom Demon #2: Pin 1 to Chassis

Another big cause of buzz in ClearCom systems is pin 1 in an XLR connector being tied to the shell. This makes it super-easy for a ground loop to happen. Pin 1 to shell is never good anywhere in audio, but it tends to show up problems even more readily in ClearCom than it does in standard balanced audio. This could be anywhere in the system–a patch panel in the house, in a rack, a cable anywhere in the system.

In the past, I’ve spent a long work call troubleshooting a comm system that was buzzing like crazy, only to eventually find out that it was a handful of Y’s with Pin 1 to shell connections that was causing a ground loop when the shell touched the metal on a house patch panel up in the spot booth. Swapping out Y’s and cables doesn’t help when they all have the problem! And no, I won’t tell you the name of the shop that sent us out with a box full of incorrectly
wired Y’s; I’ll just leave you with the warning to check them before you go out and use them. Repeat after me, "Pin 1 should never, ever, ever be connected to the chassis." (The one exception to this is when solving a very specific problem [which I won't get into at this time, since it's unrelated to what we're talking about right now], in which case the cable/adapter should be specially made for the situation and profusely labeled to indicate it’s altered and non-standard condition.)

Pin 1/chassis problems can also come into play in ClearCom main stations and power supplies, something that again can be easily checked by reading the resistance between pin 1 and the chassis ground. You see, there’s a 10 Ohm resistor in the station, and if it goes bad, producing a reading about ten times that, it needs to be replaced, or you’ll get buzz. This test will also tell you if you’ve got a problem elsewhere in the wiring, since if you read less than 10 Ohms, there’s that short we were just talking about.

ClearCom Demon #3: Tying in to House Systems

This is the thing that, more than anything else, brings fear into the mind of touring sound engineers when the subject of intercom comes up. On tour, we always try to avoid having to run cable up to the spot lights, since that’s a lot of cable and a lot of time, usually with more than one person involved, and the venue almost always has multiple cable runs up there. Now, as I mentioned before, if you can unterminate your touring main station for that channel (it would be entirely unfair for you to ask the house to do it to theirs, so the onus is on you to do it yourself, in my opinion), you can tie in directly, even leaving both power supplies turned on (you’ll often run into resistance on the latter part, since it’s become such a common misconception that you can’t have multiple power supplies, so do your part in helping to spread the correct information!).

Or, if unterminating isn’t possible, another common solution is to use the venue’s dry lines to run up to the spots, and use only your ClearCom system. The big problem with this, however, is that these lines often run through patch bays. Power through patch bays is never a great idea. If you must, however, make absolutely, positively sure that the lines are patched BEFORE you power up the base station, so that you’re not pushing a TRS connector through a jack with power on it, or vice versa.

The trouble with both of these options is that, since house sound systems are often powered separately from the touring system, especially if you’re doing arena tours like the one I’m currently on (less so with theatres, in my experience), is that you’ll often get ground loops between the two systems that cause lots of buzz. For this reason, I have come to love ClearCom’s MT1 isolator circuit. This is a small circuit board that you can buy (you supply your own housing for it) to put in line between two independently powered and terminated ClearCom systems. It will completely isolate the two systems, but pass both audio and (here’s the super cool part) call signals between the two systems. They’ll operate seamlessly as if they were one system, but without any worries about ground loops. I tour with one permanently installed into the rack that my main station is in, so that I have a dedicated isolated output on my lighting channel for house tie-ins always ready. Any tour really should, it just saves so many headaches.

I’ve played venues where the house guy warned me that every tour that’s been through there recently had issues where the system was absolutely unusable when they tied the road com into house com. I asked him to humor me and let me try it through the MT1. Surprise, it worked perfectly! This little gadget, more than any other, is a real lifesaver.

Update, 3/25/05: One thing to note is that, according to ClearCom’s datasheet for the MT1, both systems must be UNterminated when using it, because it provides termination for the channels (and thus will cause double termination if the channels are terminated on their own). Why they’d do this, I can’t say, because it seems to just be a colossal inconvenience, but who am I to say? Oddly, however, I use the one I’ve got out on tour right now all the time with both systems terminated, and have NO problems at all; the ONLY time I’ve ever had a problem was on a house system that wasn’t terminated. Which may mean that either my MT1 was modified to remove the termination, or that ClearCom has changed this and not updated the datasheet. Or my system is just weird. I’ll take mine apart and see if I can figure out what the deal is between shows tomorrow, and in the meantime, if anybody else knows if there have been any changes (or why ClearCom put termination in the MT1), let me know. And thanks to Kevin McCoy for pointing this out!

Update, 8/9/05: See instructions for bypassing/removing the termination here.

Suggested Reading

Epanorama’s Intercom Technology Page

Telex’s Handbook of Intercom Systems Engineering (4, 976 kB PDF)

ClearCom’s Party-Line Installation Manual (322 kB PDF)

You should also check out the rest of ClearCom’s manuals for the specific gear you use, but the installation manual will give you a good overview to start from.

Updated, 2/19/05: For completeness sake, here is a link to manuals for Production Intercom’s ClearCom compatible systems.

I hope this is of assistance to some of you out there. I know learning these bits and pieces sure have helped me a lot over the last couple of years, and I’m glad to be able to share what I’ve picked up with everybody else.


P.S.-You may have noticed that the majority of the tips I post here at OFTR are sound related, with the remaining few generally being electrics. The simple reason for this is that these are the fields I work in most of the time. That said, I’m always willing to share tips of interest to other departments, too. So, if any of you work in carpentry, projection, props, or wardrobe, and have tips to share, e-mail them to me and I’ll gladly publish them on the site (with full credit to you, of course)!

Update, 2/10/07: I’ve just update all the ClearCom and RTS/Telex links in here that went dead when those companies re-organized their websites. Hopefully they’ll stick with this site layout for a while, argh. If these links break again, please e-mail me, and I’ll see about getting permission to just host mirrors of the PDFs here at OFTR.

8 Responses to “Out, Ye Demons!”

  1. Mike Benonis says:

    We have a Production Intercom base station and beltpack set, and when we press the call button, it stays on for about two seconds and fades out. You’re saying this behavior is incorrect, and due to termination issues? Any idea where the termination switch is on the PI base station?

  2. Andy says:

    That’s a good question. I haven’t worked with much PI gear, other than the Blazon, but looking at the manuals and other specs on their site it seems that there is no external switch; rather, they are internally terminated in the master station (it does note the line termination impedance in the specs at the end of the MSM-1 manual, for example, and the ad copy for the master station that comes with the EconoCom system says that it includes the line termination network). I’d drop them an e-mail at info@beltpack.com and ask them to be sure. It very well could be something inside the station that went bad or came loose. And do post back if you get it sorted out!


    P.S.-I’ve updated the article to include a link to PI’s manuals, as well.

  3. Kevin McCoy says:

    … but doesn’t the MT1 provide termination, so if you tie into a house system and it’s terminated, then at least that side of the comm system is double terminated? I only ask this for informational purposes, since I use the MT1 all the time without problems. :)

  4. Andy says:

    Hmmm…now I’m puzzled. You’re right–according to ClearCom’s info, the MT1 does provide termination and both systems should be left unterminated if connected to one. Yet not only have I never had a problem with symptoms of double-termination when using the MT1 with two terminated systems, but the only time I did have a problem was when the house system I tied into wasn’t terminated. I wonder if the MT1 I have was modified to eliminate the termination.

    And, while I’m at it, why would ClearCom do something like that, and then advertise it as a boon to touring shows, when according to them using it requires changing a setting on both the touring system and the house system that must be restored when the touring show leaves. That’s just stupid.

    Anyway, I’m going to go and edit this article to reflect the correct info on the MT1, according to ClearCom, unless somebody can confirm that perhaps the circuit’s been changed since that datasheet was written. Thanks for pointing that out, Kevin!


  5. Andy says:

    See the update with instructions on how to remove/bypass the termination here: http://www.onefromtheroad.com/index.php?p=64

  6. David Elmer says:

    The link in this article to ClearCom’s MT1 is incorrect…the following is the correct URL: http://www.clearcom.com/Products/party-line/placcessories.html

  7. Andy says:

    Thanks for the correction, David…seems ClearCom ever-so-slightly changed their site’s directory structure since I originally posted this. Will correct the link momentarily!

  8. Andy says:

    Leave it to ClearCom to not only change their URLs again, but require a login to view manuals, AND have a bug with the registration system. I’m gonna see about posting the linked-to manuals locally to fix this crap once and for all. Shouldn’t be a copyright issue, since it’s clearly fair-use. Stay tuned…

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