No Wires, Mate- Part II

In Uncategorizedby tfwm

Last issue we reviewed the basic components and technology of RF wireless microphone systems and established that wireless mics are much like tiny FM radio stations. Now let’s talk about real world applications.

When considering a wireless system product, a few of the things to think about are: construction material and quality (metal, plastic, etc.), true diversity reception, frequency agility (and if agile, the frequency bandwidth), sound quality, RF performance, and manufacturer support.

Component construction and diversity reception are essential factors, and were reviewed in Part I (March 2005 issue).

Frequency agility is highly recommended, particularly in light of our presently very congested radio frequency spectrum. Non-agile systems are termed fixed frequency, and must be serviced or even replaced if the operating frequency is to be changed. However, whether or not a system is frequency agile does not necessarily have anything to do with its sound quality, design quality, or RF performance – just whether it is tunable “on the fly” by the user. With the RF traffic we often encounter these days, tunability might be the most important feature for many users.

As for sound quality, any wireless system, no matter how great the RF link, can only sound as good as the original audio that it is fed. The type of microphone(s) available on a wireless is absolutely critical. The RF link itself also influences sound quality – we mentioned companders in Part I. A great test of any wireless system is to simply put it through those two transducers on the sides of our heads. Let’s make sure that we like the sound of the mics we’re putting in front of our musicians or presenters before we go to the trouble (and cost) to send them through the air as radio signals!

Proper transmitter audio gain is also critical for great sound and is either fixed, or adjustable at the transmitter as a manual control or menu operation. This MUST be adjusted to achieve plenty of usable audio level without over-modulation. If the gain is too hot and causes over-modulation the signal will already be clipped when it leaves the TX. Conversely, if the gain is too low we’ll experience needlessly reduced signal-to-noise ratio and noisy/hissy audio. Some systems offer metering on the TX and/or the RX to aid gain adjustment.

RF performance may be tough for non-engineers to determine from product cut sheets, but real-world testing always tells the truth. Remember, for any wireless to be fairly evaluated it should be operated on a frequency that is clear and open, or a false impression of its performance ability and operating range may result. The number of compatible systems that can be simultaneously operated in the same environment is an indicator of receiver design quality.

Great support is also important for wireless systems, and any reputable radio mic manufacturer (or their rep/dealer) should offer useful support to those requiring assistance in frequency planning or other challenges when selecting, purchasing, and setting up their wireless microphone systems.

Single Channel Operation
Wireless mics are secondary users in the radio spectrum and operate at very small power levels, (typically around 20 – 50 mW, just a fraction of a watt!) usually in the VHF or UHF bands of the radio frequency spectrum. Our wireless mics “share the air” with other sources of radio traffic like TV broadcasts, cell phones, communications services, and a long list of other things that create RF energy. It is critical that any wireless mic we use is set to a carrier frequency that is not already occupied in the local area. If a conflict or interference occurs it is the wireless mic that must adjust (retune or turn off). One large source of potential RF interference to be dodged by wireless mics in the United States is television channels. Fig. 1 shows where the U.S. (NTSC) TV channels line up in the radio frequency spectrum. And since TV channels typically broadcast many thousands or millions of watts of power, guess which signal “wins” if we try to operate a tiny radio mic inside the frequency range of a TV channel? Plus, the TV stations are licensed for operation. By knowing what TV stations are running in our local environment and by referencing a chart like fig.1, we can select frequencies that do not fall in active TV channels. We are currently in the “digital transition period” in the U.S. Nearly all TV stations now broadcast both analog and digital TV channels – effectively doubling the amount of TV traffic with which we have to contend! Think of some of the TV channels in your town and review the chart in fig.1 and you’ll see how the TV channels fall within in the radio spectrum by frequency. Each TV signal in the U.S. is 6 MegaHertz wide (6 million Herz).

Suppose we’d like to operate one wireless system in a worship venue in Orlando, Florida at a frequency of 634.125 MHz. Well, that frequency happens to be right in the middle of TV channel 41/WRBW – a digital TV (DTV) broadcast in that region. Not a good choice! It is quite likely that the wireless will not be usable, or its operating range will be drastically reduced by the overwhelming TV broadcast signal. (Our distance from the TV transmitter site and various other parameters affect our success also). What to do?

If this is a fixed frequency wireless system, it will have to be replaced, or serviced to be set on a new/clear frequency. If it is a frequency agile wireless system we can instantly tune it (both the TX and the RX) to a new frequency with the push of a button or switch, hopefully choosing one that is NOT occupied in the local environment (ch. 42 is open, for instance).

We should have originally planned ahead for a more appropriate operating frequency by using data similar to that in fig 2. The FCC and several wireless manufacturers maintain various forms of this information. The idea, again, is to select a frequency that is NOT already occupied in the location of use (touring systems get real fun!). And if we are using a “frequency agile” system we aren’t selecting a single frequency, but rather we are selecting the frequency range (expressed in MegaHertz). For instance, an agile system can tune through a switching bandwidth that may span across one or several TV channels, or several dozen MHz. Some agile systems are fully tunable in fine steps and some may have channel presets, or both. Many now also have “auto-tune” capabilities that help steer around local RF traffic.

One good way to plan is to map the local active TV traffic and overlay it with the available frequency range(s) of the wireless microphone product being considered. That’s what the blue shading represents in fig 2. Note that these particular frequency ranges/bandwidths are 36 MHz in width, which means they span six television channels each. Range A (518- 554 MHz) is the most congested and would probably be our last choice of range in this instance, Range B is a bit more open, and Range C only has one TV channel active in the area, and that one is low power (LP). At this stage, it is necessary to know something about the wireless system(s) we are considering for use. How many compatible channels can it run in one TV channel, or two or more TV channels? This and other questions must be answered by the manufacturer. For multi-channel systems, it is not uncommon to divide them up between multiple ranges, assuming the ranges are compatible- a few channels in each. Note that while it is not too unusual to get away with operating radio mics in or near analog TV channels, it is MUCH more difficult to do so with DTV digital channels, unless we are operating many miles from the DTV transmission. Completely avoiding both is always better, if possible!

Hopefully this has given you some more information to go on. Next issue we’ll cover mic receiving setup, frequency coordination and some troubleshooting tips to consider when optimizing your wireless systems for worship!