Question: Why is frequency coordination of a wireless system important?
Answer: Using a wireless microphone system – and, in particular, multiple wireless systems – may not be as easy as just choosing one at a music store or from an online source. Yes, that should probably be the place to start in order to find a model with the features that you are looking for, but don’t whip out that credit card just yet. There are other considerations that must be taken into account. Of these considerations, frequency selection is one of the most important. Once you have ascertained what model you want, the next consideration is how many systems you will be using at the same time. If you want or need just one system, more than likely you won’t run into problems, although you will still need to make sure there are clear frequencies available for that one system. What do we mean by clear frequencies? Simply put, a clear frequency is one that is not in use by something else. The most common thing to look for is broadcast TV stations in your area. Many wireless microphone systems operate in the same spectrum as broadcast TV stations, so it is important to stay away from those frequencies. For example, TV channel 28 broadcasts on a frequency range of 554 MHz through 560 MHz, so you will need to stay away from those frequencies if there is a TV 28 operating in your area. Also, other wireless systems in adjacent rooms could cause issues.
Now, let’s complicate things just a bit by adding one or more wireless systems into the setup. Along with finding clear frequencies you must also consider system compatibility. In other words, you need systems and frequencies that play well together. First, when operating two or more systems at the same time, they must be set to different frequencies. It is important to know that attempting to use two transmitters with one receiver will result in severe interference rendering the system unusable. Also note, that it depends upon how the system is designed as to how close together frequencies can be from each other when using multiple systems.
When using multiple systems there are frequency interactions that must be taken into account to first, set the proper frequency, and second, determine how many systems may be used at the same time. The first issue to consider is intermodulation distortion (IMD). Basically, IMD can be thought of as a “ghost signal” that is created by two or more signals that pass through a non-linear device. There is a lot of math that goes into determining the ghost signals produced by wireless system transmitters, but the good news is that these frequencies are predictable. There are many programs available to determine what ghost signals will be produced.
There are other things that can cause interference in a wireless system, such as frequencies generated by the local oscillator within the receiver itself. Also, digital interference from other non-wireless devices can be an issue. There are really two general kinds of digital interference: that which is due to the “clock” oscillators in computers, peripherals and other digital devices, and that which is caused by the processing in a digital device. The first kind is just another form of direct interference that is caused by leakage of radio signals at the digital clock frequency or some multiple. The interference will generally take the form of a steady tone, usually fairly low in level, in the receiver audio.
The second kind of digital interference typically takes the form of a raspy or buzzing whine or tone, often with occasional changes in sound character. Frequently there is a distinct cadence or rhythm present. In some cases, digital interference can result in a high noise level on the audio, including hissing sounds and frying noises. There might not be any detectable tone involved.
This kind of interference is caused by the operation of the internal processor (or digital signal processor) in the device. The interference is not necessarily at a multiple of the clock frequency; instead it can be related to the rate at which instructions are being processed. The level and character of the interference can vary greatly with the type and amount of processing occurring, and the interference can disappear entirely if the digital device is not actively processing.
Digital interference is usually relatively easy to identify. The signals involved are not strong and rarely affect wireless systems at distances greater than about 6 feet (2 m). For more information please see Resolving Interference Problems on the Audio-Technica website.
If you have further questions, feel free to contact the Audio Solutions Department for assistance.