RF101 – The Basics of Wireless Mics and In-ear Systems

Wireless microphone systems are actually like small radio stations. But unlike a radio station, where the transmitter is a fixed tower and your car radio is mobile, wireless mic transmitters (contained in the microphone or bodypack) are mobile and the receivers are stationary. Wireless IEM (in-ear monitor) systems use the same concepts, but the transmitter is stationary and the receiver roams around in the performer’s pocket. As we talk through these concepts, we may reference wireless “mics,” but these concepts apply equally to both sides (mics and in-ear monitors) of RF (radio frequency) systems.
FM Modulation
The bulk of wireless systems produced today use a system called FM Modulation. If you are a synth geek, then yes, this is the same technique that was used by Yamaha in a host of synths from the DX7 on, and one that is still being used in products like the Korg Volca FM.
The wireless system uses a fixed frequency to transmit and receive data. The frequency (or pitch) of this carrier is the number (measured in Hz) you will often see on the front panel of a wireless system.
Figure 1: A Sennheiser ew100 wireless system with a carrier frequency of 518.200MHz.
This carrier is not a simple sine wave unless the wireless is transmitting silence. We modulate the carrier signal to encode the audio information onto it, which causes the pitch to vary by up to 50kHz above and below the carrier frequency. These variations represent the audio signal that we are sending through the ether, and they are demodulated (decoded) by the receiver, and the useable audio is extracted at the other side of the RF link.
The challenge is that a wireless receiver can only demodulate one signal at a time, and it will default to whatever signal is the highest amplitude. As an analogy, imagine having a conversation with a friend in a loud social environment. You can only concentrate on one conversation at a time (no matter what you claim to your significant other), so if you are talking to your friend and a loud disagreement breaks out, you may not turn to face it, but you are probably paying attention to that conversation.
While that momentary lapse of attention is annoying in a conversation, it is disastrous in a wireless system during a live performance. Suddenly something else, often a loud burst of static, is coming through on your channel. This is the point where everyone in the room turns and stares at the person running the console, even though it may not be their fault.
Basically, wireless systems are pass-fail systems, which is why paying attention to the frequencies you are operating on, if you are operating multiple mics (a process called frequency coordination), is critical when dealing with more than one channel of wireless at a time.
Wireless Spectrum and the FCC
In case you haven’t noticed, cell phones and other wireless data devices (such as Wi-Fi, Bluetooth, etc.) are multiplying rapidly in the 21st century. As our reliance on those technologies increases, so does the need for bandwidth for those devices, hence the reallocation of frequency spectrum for them. (Note: To oversimplify, more space for wireless telephones means less space for wireless mics.) Millions of words have been written on the impact of the FCC (Federal Communications Commission) reallocation of UHF (Ultra High Frequency) spectrum to be used for commercial purposes. You can read about the specifics in The FCC’s Wireless Spectrum Auction: Why You Should Care and The FCC’s Spectrum Auction and Your 600MHz Wireless Mics — Time’s Up!, but the key point to understand is that we are trying to operate more wireless systems in about one-third the space we had 10 years ago. Simply stated, it is getting crowded in here.
Icebergs and Intermod
When choosing frequencies for wireless, we have to consider the big unmovable objects like TV stations. These high-power transmissions have had to move out of the 600MHz–700MHz range, so they are crowded closer together, and they are extremely high in output. While microphones and IEMs are typically in the 10mW–50mW range (a milliwatt is 1/1,000 of a watt), a TV station can be up to 1,000 kilowatts. Guess who wins if you are competing with a signal that is literally a million times more powerful than your wireless mic? They do. It is a good idea to use a source like the Sennheiser Frequency Finder page to avoid tuning your wireless to a channel that a powerful TV station is operating on, because it’s not going to change its frequency!
Figure 2: It would be wise to avoid channels 31 and 36 here in Fort Wayne.
If TV stations were the only source of trouble, our life would actually be pretty simple. The complicating factor is the way that wireless units interact with one another; this is called intermodulation (IM). Two wireless units will create an almost infinite number of additional sideband frequencies, but most of them are at levels far too low to impact us. Of these, only half (odd-order harmonics) are near the frequencies of the units we are using.
The loudest and closest are called IM3s (intermodulation with high third-order harmonics), and they are the ones we are most concerned about. Two wireless units create two IM3 artifacts, but three wireless units create nine IM3 artifacts. When two of them land on the same frequency (which is very common when you evenly space the frequencies), the amplitude increases, making them more disruptive. Simply put, adding a channel of wireless almost doubles the complexity of the process every time.
Further, when transmitters get close to each other (think about four IEM transmitters in a rack), the amplitude of those IM artifacts increases drastically. You can quickly see how calculating multiple channels of RF can be very complicated.
Figure 3: RF creates its own competition.
Reasonable Channel Counts
This means that if you can find an open TV channel to tune all your frequencies within, eight channels will be about the most you can expect to reasonably function within that space in traditional analog wireless. Having more concurrent channels will require them to be spread across different ranges of the spectrum, and they will still need to be intelligently configured with a software calculation tool like the Shure Wireless Workbench to function reliably.
Analog or Digital?
To be completely accurate, there is no such thing as digital wireless. All wireless uses RF circuitry that transmits an analog carrier signal. Digital units are actually using a more complex modulation scheme to encode digital signal onto that analog carrier. They take up a bit more spectrum to get better audio quality, but unlike traditional all-analog wireless, they can be spaced evenly because of the way they generate intermod artifacts.
In units working in the 2.4GHz range, this doesn’t help all that much, but in units that operate in the UHF range, like Shure ULX-D or Sennheiser 6000 series, it makes it easier to pack a lot of channels into a smaller space. Fourteen channels in an empty TV channel space is a typical maximum.
Of course, in the near future it is unlikely that we are going to go completely digital. The more complex modulation scheme of a digital system means that there is latency in these systems, with the best systems today running at around 3ms. This is fine for microphones and bodypacks, but it introduces too much delay for in-ear wireless.
Auto Scan
Auto scan, the system where you push a button and all your wireless systems are automatically configured, is the best thing in the world! It makes all your problems go away, right?
Well, not exactly. It is pretty easy to understand what a typical auto scan does and how it helps you. When you instigate an auto scan, you want to have the transmitter for the unit off. The system is primarily aimed at working around the icebergs in your world.
Manufacturers break their preset frequencies into banks, and auto scan will typically suggest a frequency in a bank that has the highest number of clear frequencies. This means that auto scan is almost foolproof when it comes to a single piece of wireless.
But as we discussed before, adding one more channel of wireless nearly doubles the complexity of your frequency coordination. Additionally, auto scan does not give a wireless awareness of what your other wireless units are doing unless you have them networked together. A high-end system like the Shure QLX will allow you to do a scan on a single connected wireless, and it will then deploy frequencies to all the connected units.
If you are not that lucky, or have a mix of different wireless, then you are going to be stuck manually selecting frequencies. The reason that a manufacturer will create their units with banks of presets is they have pre-calculated these so that they do not exhibit destructive IM behaviors. This means that the most effective process is to do a scan on one unit, and then choose frequencies within that same bank for all the wireless in your system.
Figure 4: A Shure QLXD set to Group 1, Preset 1.
After you do this, the first thing you should do before you turn on any of the transmitters is to look at the front panels of the wireless and make sure that you do not have any substantial RF level on the meters. A little bit of activity is normal as the receiver is searching, but if you see high levels, you should change the frequency to another one inside the bank before turning on the transmitters. Remember that in wireless, the loudest signal always wins.
If you have a mix of different manufacturers’ wireless units that are tuned in the same range (for instance, Sennheiser A range and Shure SLX H5 range), you are best off doing a scan on one of the units and then manually programming frequencies from that unit’s banks into the other wireless. Almost all modern, frequency-agile wireless tune in 25kHz increments, and math is math, so an intermod-free frequency applies to all wireless.
If you have units that fall in different ranges (like Sennheiser A1 and Shure UHF-R H4), they will still interact with each other, so you need to use a software tool like Shure’s Wireless Workbench or Sennheiser’s Wireless Systems Manager (WSM) to ensure that they do not interfere with each other.
As you can see, there are lots of issues to consider when putting a wireless system together, so give your Sweetwater Sales Engineer a call at (800) 222-4700, and they can walk you through the best options for your application.
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