Thursday, January 6, 2011

The Basics of Mic'ing a Cast II

Your next problem is establishing a good signal path.

The RF (radio frequency) environment is getting increasingly cluttered. Unfortunately, many of the things found in a working theater, from electric motors, to florescent lights, to the dimmers of the stage lighting system, make radio-frequency noise. So, to a lesser extent, do laptops, cell phones, and electronic gear like keyboards and signal processors.

Add to this hash, the very small group of frequencies we are legally allowed to use.

Back before the DTV change-over, analog television stations were not permitted on adjacent bands within the same service area. This meant, essentially, that every other TV channel (within a particular service zone) was unoccupied. That's where wireless microphones went. Wireless microphones are low-power, unlicensed, analog transmitters.

The Digital Dividend changed this. The existing broadcast channels were packed in tighter towards the bottom end of the broadcast spectrum, and the upper end (above 698 MHz) was sold off to new users. Also, in several designated service areas, additional frequencies were carved out for new emergency management and public service use.

This means the number of vacant television channels is much smaller. In addition, that remaining "White Space" may now be used for new services (streaming video and the like) offered by Comcast and others. Fortunately, after getting a visit from The Mouse (the Disney Corporation) as well as the NFL, and of course Broadway, the FCC relented and is asking these new users to ensure they don't interfere with pre-existing theatrical systems.

Which isn't really much of a protection, even for major established theaters.

What can you do? Well, for one, check your frequencies. If you have gear in the 700 MHz band, get rid of it. As of June 12 of 2010, you will be breaking Federal Law to use them. You ever see one of those W.W.II movies where the Resistance cell is on their little radio, trying to get word of the new secret weapon back to the Allies, while outside in the streets a nondescript van with a big antenna on top creeps closer and closer? Well, the FCC has those. (Or, rather, the broadcasters who spent a pretty chunk of change in the big sell-off of the spectrum has those.) They will find you.

Second, check your available bands. Shure and Sennheiser both maintain current charts of frequency usage in your area. Sennheiser will print you out a customized chart -- Shure has a piece of software, the Wireless Workbench, which will include that data when it calculates a new set of compatible frequencies.

In addition, if you can, scan the environment you will be operating in. Several modern wireless microphones will perform a scan of the local RF environment, and report back on clear frequencies/interference sources. One or two of these (like the latest Shures) will even connect to a program you can run on your laptop.

Another low-end solution is an all-band scanner. I use a Uniden Bearcat, which covers all of the wireless band, and has helped me to spot intermittent interference sources (like a local pirate radio station).

Done yet? Not quite. Besides primary frequency interference, radio gear is susceptible to combined frequencies. When two radio signals of roughly equal strength enter the air, they heterodyne to create a new frequency. Every new frequency added to the environment adds new combinations, increasing the number of false signals geometrically.

In short, although you might have four wireless microphones that work without interference when tested alone, when you switch them all on at the same time one of them might suddenly start showing interference and drop-outs.

Fortunately, you don't have to do the math yourself. Shure's Wireless Workbench software will calculate possible cross-interference, as well as automatically generate a fresh set of compatible frequencies.

You need to site the receivers so they have a clear view of the talent. Salt water can trim 90 dB off your radio signal, and an actor is largely a bag of salt water. Concrete and steel can also reduce the signal strength. The antenna of most wireless packs radiate most strongly in the horizontal plane, and siting the receiving antenna too high can also lower signal strength.

All modern systems are diversity. The reasoning is this; at any point, a single radio path can be either blocked, or worse, combine with it's own out-of-phase reflection (like the ghost image on an old analog TV). When this happens, signal strength drops, perhaps below detection threshold. Diversity receivers have two antenna, and switch between them depending on which has the strongest signal. To make this work best, though, you want the antenna to be as close as possible to 90' out of phase with each other. Setting both antenna to 45', "rabbit ear" style, is a good compromise to all these different positioning needs.

Next -- actually hanging the mics on the actors!

Continued in:
The Basics of Mic'ing a Cast III

No comments:

Post a Comment