Wireless Microphones! What you need to Know

THE MOST OVERUSED JARGON EXPLAINED...​

Wireless microphones are still a relatively new technology and have adopted certain terminologies from other areas of the audio industry to describe how they work. Unfortunately, these terminologies do not always translate smoothly. For example, the word ‘channel’ – while relatively simple to understand outside of wireless – has three different meanings in a wireless context, and understandably, this can be confusing.

‘What group and channel is that channel operating on?’ or ‘how many channels can you get in a TV channel?’ are both completely valid questions, and those used to working with radio microphones and In-Ear Monitoring systems will understand them perfectly. But, what about the rest of us? Well, here’s an explanation…

WHAT ON EARTH IS A CHANNEL ANYWAY?

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In simple terms, a channel is a means of directing something toward a particular end or object. Or, in other words, a means of getting something from one point to another (perhaps through a specified route or medium). It describes both an ‘end-to-end’ system – like a wireless microphone and a receiver – and also the route between the two items. (i.e., the range of frequencies that the end-to-end system uses to transmit data.)

The following are examples of the many wireless ‘channels’ in context:

1. ‘A CHANNEL OF WIRELESS’

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A combination of 1 wireless transmitter and 1 receiver is equal to a single channel of wireless. Likewise, 10 transmitters and 10 receivers would equal 10 channels in total. Things start to get confusing, however, when introducing physical receiver units that can receive more than 1 channel simultaneously. Such units are typically dual or quad channel receivers. For example, 4 transmitters and 1 quad receiver gives you 4 channels of wireless. Or, 8 transmitters, 1 quad receiver, and 2 dual receivers gives you 8 channels.

The principle is identical for In-Ear Monitoring systems, but this time, the transmitters are the stationary item, and the receivers are worn on the body. In this case, transmitters are available as either single channel or dual channel. So, 10 dual channel transmitters and 20 bodypack receivers equates to 20 channels of wireless.

2. TV CHANNELS

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Most radio microphones use UHF (Ultra High Frequency) radio spectrum. Explaining why we use this spectrum is beyond the scope of this document, but the part we’re interested in is

bands IV and V – covering 470-854 MHz. To better explain who or what uses which chunk of this spectrum, it was subdivided into 8MHz wide blocks ready to be allocated to different users. It’s like slicing up an entire French stick ready for diners to spread their preferred pâté or cheese on. Mmmm cheese…..

The primary user of this spectrum was TV broadcast, and so each 8 MHz slice of spectrum was given a TV Channel number and was initially used for a single analogue transmission. For example, BBC1 in London may have been broadcast on TV Channel 24 (494-502 MHz). With modern Digital Television, TV Channel 24 (494-502 MHz) may now carry more than just BBC1.

3. GROUPS AND CHANNELS

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Radio mics shouldn’t be tuned randomly to arbitrary frequencies, as it can result in interference between your own radio mics – not an ideal situation. For this reason, Shure pre-program Groups and Channels. A Channel in a Group will refer to a spot, or fixed, frequency. For example, Group 1, Channel 1 could be 606.700 MHz. Group 1 Channel 2 could be 607.175 MHz.

Now here’s the clever bit; ignoring any possibility of external interference, all the Channels in a given Group are known to be compatible with each other. Unless you’re using any coordination software, always stick to using different Channels in the same Group

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LET’S BRING IT ALL TOGETHER

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So this is where it can get complicated. Typically, Group 1 always covers the entire tuning bandwidth of the radio mic system. This isn’t always desirable, though, as you may only be licensed to operate radio microphones in a specific TV Channel.

Some groups are limited to a specific TV Channel only. For example, Groups 6, 7 & 8 may cover one TV Channel only. In this instance, by tuning separate radio mic systems onto the Channels (spot frequencies) in Group 6, you could get 17 Channels (of wireless microphones) operating in (TV) Channel 38.

Simple, right?! – In hindsight, giving one word three meanings only decipherable in context was a poor choice, but I hope this guide has helped to explain it a little… If not, ‘channel’ your questions in our direction.

Selecting Frequencies

​ Wireless microphones are significantly different from other audio equipment in that wireless receivers and transmitters are radio devices that must operate on specific frequencies. If operating frequencies are poorly chosen, harmful interference is likely. Interference will seriously affect the operation of a wireless system and can even make it unusable. There are certain rules that must be observed if interference is to be avoided. The most important ones are: 

• Each transmitter requires a separate receiver on the same frequency.

• Two wireless systems in the same location cannot use the same frequency.
  

For multiple wireless system operation, wireless frequencies need to be separated so they do not cause adjacent channel interference with one another. That is typically any frequency that is greater than 250 KHz between channels but depends on the actual wireless model being used and it’s capability. For multiple wireless system operation, each wireless system must be on an unoccupied frequency and the frequency must be clear of a third order intermodulation harmonics. A third order harmonic is created when two wireless transmitters are operated in close proximity to one another and they create a third RF signal that falls on a third wireless system frequency causing interference. Use the internal receiver scan function to find compatible frequencies if the scan feature is incorporated in your model of wireless.

• Wireless systems cannot share frequencies with TV stations; they must not operate on frequencies that are the same as those used by local TV stations.
  

• There is a maximum number of wireless systems that can be operated in one location, and this number depends upon the specific model or type of wireless equipment in use and the local RF environment (operating TV stations, etc.).

Just because a wireless system works perfectly in one area or in one particular situation does not mean that it will necessarily work equally well in another area or in another situation.

When you buy or rent a wireless system, someone has to select an operating frequency (for fixed-frequency systems) or an operating frequency range (for frequency-agile systems). This might be you, the dealer or rental company, or the wireless manufacturer. If the choice is made at random, there is a significant chance that there will be interference, especially with fixed-frequency systems. Thus, simply buying or renting the system that happens to be in stock creates an unnecessary risk. It is very important that you, or the dealer, or both, make an active effort to avoid frequency conflicts.  The easiest approach is to simply buy or rent from a dealer who is trained and equipped to perform frequency selection. There are various ways of accomplishing this, depending upon the particular situation. Always ask for this service; do not assume that it has been provided. To do this job correctly, the dealer will need to know two things, and you will usually need to provide this information. They are: 

• The location or area where the wireless system will be used. Alternately, the channel numbers of the local TV stations can be provided. This allows the dealer to avoid TV frequencies when selecting frequencies.

• The frequency, model and type of any other wireless microphone systems that will be in use at the same location. This allows the dealer to avoid using the same frequencies, frequencies that are too close together, or frequencies which interact. It also allows the dealer to determine if the maximum possible number of wireless systems will be exceeded.

TUNING BANDWIDTH / FREQUENCY RANGE

Frequency Range, or Tuning Bandwidth refers to the range of frequencies a radio mic system can tune to. It can be a small range covering 24 MHz / 3 TV channels (Remember that 3 TV channels @ 8 MHz each = 24 MHz) or even up to 64 MHz / 8 TV channels.

​ FREQUENCY FINDERS

Shure will help you find the best tuning bandwidth for their mics here:

wireless-frequency-finder

Sennheiser has a frequency finder that lists actual frequencies ... useful for any brand of wireless.

Choose your city and then choose Show: Usable channels

USA Frequency Finder

REAL VS VIRTUAL CHANNEL NUMBERS​

What's the Difference Between Real and Virtual Channel Numbers? The virtual channel number is the number you see that is displayed on the television screen.

The real channel number is the actual channel frequency in use by a the TV station to broadcast the TV signal.

The frequency spectrum is broken down into 3 bands. VHF Low band. Real channels 2-6 VHF High band. Real channels 7-13 UHF band. Real channels 14-51. Note: Even though the TV may display a channel number above 51 this is the virtual (not real) channel of the TV station and not the real broadcast channel frequency. Currently no full powered TV stations in the U.S. broadcasting above channel 51. ​

WHAT THE.....!?!

U.S TV stations are not the problem.  It's the frequencies they transmit on. We have to stay away.  I found this site that will list the stations in your area and on the end column is listed the frequencies they transmit on. ​ U.S. Digital HDTV Over The Air Signal Coverage Map