Wednesday, August 22, 2012

RF Issues

Wireless mic issues have come up on the job and in questions from other people a bunch this week so it seems like it's high time for another post on the subject.

Last week in the theatre, my esteemed assistant Chachi was having a ton of trouble that he initially attributed to bad elements. While there were in fact a couple of those, you can tell the difference by the sound. A bad element will snap and crackle. When it's RF you get fuzz, fluctuating volume levels and the dreaded drop out.

Let's start with the fuzz. That's usually an issue with another mic being on a frequency that's too close. The radio circuitry in wireless mics is pretty good these days and the rejection of unwanted signal is better than it ever has been. But depending on the environment and a few other things like digital television, you can sometimes still have frequency issues. The easy fix is to use the coordination software provided by the manufacturers on their websites. Even if you're not able to get on the net, just by putting all your mics in the same group and each one on a different channel you can usually get pretty close.  Coordinating different makes and models is slightly more difficult but if you do your homework you can get the job done.

The problem on this show was that the homework had already been done and we were still getting drop outs. The more time went on the worse things got. We finally decided to pin it on a phenomenon called front end overload. That's when there is enough off frequency (not on your channel) RF enegry floating around to saturate the receiver and cause it to loose sync with the transmitter. As smart phones become more prevalent, all that broadband and wifi are clogging up the airwaves in the venue. Even though it's not in the same band as our mics, it can still cause problems. It doesn't help that when you request people to turn off their devices the simply silence them and put them back in their pockets.

So what's the solution? Well, operators like to have the receivers close at hand so they can check on information like transmitter power, battery life and whatnot. But the greater the distance from the performers the more problems can arise. Using an antenna combiner and paddle or helical antenna can greatly improve performance but at a cost, usually several thousand dollars for eight mics. The poor man's solution is to park the receivers near the stage and send all the inputs to the mix over a snake. We considered using a web cam to allow the operator to still get some of the information from them while he's mixing.

Two way radios can affect performance as well and for the same reasons. Because of their higher transmitting power they can even affect more robust equipment. I had a walkie talkie put an amp rack into protect when someone keyed down too close at a festival once.

One last thing that you'll hear sometimes is a mic that fades away and then comes back in super loud. That's not always because the operator is slamming the faders and then remembering the mute button or something like that. Some models will automatically raise the gain, trying to compensate for a weak signal. If the weak signal suddenly comes back, the automatic gain circuit (AGC) takes some time to react. It's not uncommon to get a huge burst of audio, causing feedback and other show marring issues. If you have mics that do this they need to always be parked on the stage.

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