At some point in time, for what ever the reason, you will need to expand your wireless installation from a microphone or two to, say, eight or ten. Whether this growth comes as a planned, permanent expansion, or whether your church is producing a large-scale musical or an Easter drama event, there are steps you can take that will make the process as painless as possible.
Of course, put simply, the idea of a wireless system is to provide the performance of a wired microphone or instrument with the convenience and flexibility of a radio link between a transmitter — either a body pack into which a microphone or musical instrument can be plugged, or a hand-held microphone unit — and a receiver that will generally be connected to your mixing console and/or PA system. The receiver and the transmitter both must be operating on the same frequency. If any two transmitters are operating on the same frequency, severe interference will result and the wireless system will be unusable. You can “use” a handheld and a body-pack transmitter with one receiver, but only one at a time.
Choosing Your Frequencies
With one or two units, as you may discovered, you can plug the system in, cross
your fingers and it will probably work. The only real potential problems have
to do with interference caused by frequencies already in use in your area, typically
for TV stations.
There may also be interference caused by other wireless systems operating in adjacent areas (the youth service in the basement, the church across the street etc). These problems, however, are relatively simple to overcome, simply by choosing vacant frequencies. It will help immensely if the wireless systems you choose are “frequency agile”, as is the case with most modern units. This means that you can select from a variety of frequencies rather than being limited to a single preset frequency. Also, make certain that the transmitters are ten feet or more away from the receivers to avoid overloading the system.
The
Power of Three
Problems really start to emerge when you cross the threshold from two wireless
units to three, and they get worse as you add more devices. The fundamental
problem is that of Third Order Intermodulation. When you get more than two wireless
units together, they interact to create additional frequencies. As the number
of systems increases, intermodulation problems increase exponentially. This
is why it is vital to carry out careful frequency planning before you start.
This problem applies to all manufacturers, and it applies whether all your units
are made by the same company or by more than one.
Many wireless system manufacturers provide tables of frequencies that can be used together without intermodulation problems, and it is worth consulting your instruction manual, equipment dealer, or visiting your system manufacturer’s web site to see what tools are available to help you. The Audio-Technica (A-T) web site, for example, includes a frequency selection system entitled “Checking Frequency Compatibility” that will enable you to find usable frequencies whether you are using AT systems or those from other manufacturers. See http://www.audio-technica.com/ using/wireless/ for this and other useful tips on wireless operation.
Planning for the Future
It can be helpful if you can have all your systems operating in the same frequency
band (VHF, UHF etc), because you may be able to interchange antennas, antenna
distribution systems, and other accessories. However in a large complex system
with several local TV channels in the area, you may simply not be able to achieve
this. While you do not have to necessarily stick with the same manufacturer,
or the same series from a particular manufacturer, to manage frequencies successfully,
it is a fact that if you choose all your systems from the same manufacturer’s
series, the system can, to a greater or lesser extent, aid with frequency compatibility
issues. Manufacturers may give you a list of coordinated frequency groups that
have been calculated to work together. If you are obliged to go outside a group,
then you need to use a calculator, such as the one referred to earlier, to find
frequencies that will work together successfully. This kind of planning can
be made a lot easier with the most modern systems, which, when linked together,
can scan for usable frequencies. However, only a few, including A-T’s
4000 and 5000 Series, can actually choose frequencies that also bear in mind
the requirements for avoiding third-order intermodulation problems.
Keep a list of the existing frequencies you are using so they can be quickly referenced for frequency planning as the system is expanded. Knowing your frequencies can also be a great help when something unexpected happens. For example, you might have two systems in use that work fine together, and then one day the band has a new guitarist and she brings her own wireless system with her, switches on and everything goes haywire.
Having some means of finding a set of workable frequencies — and a frequency agile system — will be extremely helpful here. However, it is worth noting that it is not necessarily a good idea to choose a system entirely based on the number of frequencies it can operate on. There needs to be sufficient space between the available frequencies to get a signal through, and subdividing that space into a large number of individual frequencies sounds impressive but may not really help, as it’s the number of usable channels that matters. It’s like having a car with a thousand gears: it sounds very impressive but it’s really more trouble than it’s worth.
In addition to having the tools to deal with frequency allocation issues, you may wish to delegate someone to manage your frequency allocation, and make this known to your staff, so that anyone new coming into the building with their own wireless can be sent to your sound engineer or frequency coordinator to get their system appropriately integrated into the house system.
It’s one thing to suddenly add, for example, rented systems for a pageant or other special event, but it is another to add wireless systems as part of an overall plan over a period of time. Here frequency planning and the allocation of bands can be particularly helpful, enabling you to add systems over time and know that they will work together in several different configurations as required. Where can you put it? Once you have a number of wireless systems, you will eventually find that becomes difficult to line up the receivers on top of the mixing console while keeping the setup neat and tidy. You may want to rackmount the receivers in a different room, for example, and use remote antennas with antenna distribution systems. Typically four receivers can be served by two antennas using such systems, thus cleaning up the installation. But it is very important to use the right type of cable and accessories when doing this. You cannot use regular TV coax cable or cable-TV splitters for example: they exhibit a great deal of loss, which can reduce your usable range dramatically and produce other problems. A cable-TV splitter might cost $5, but a proper wireless system antenna splitter may cost $400. Many splitters can also be cascaded (within the same frequency band) so that you still only need two antennas even if you double the number of systems. The correct antennas and accessories together form a complete system that will maintain the maximum performance from your setup.
Bearing these considerations in mind will hopefully help you to expand your wireless system painlessly and effectively.
Bob Green is Product Manager, Wireless & Circuitry Products
for Audio-Technica U.S., Inc.
