Since the mid-’80s, everything in the pro audio world has been going digital. It has been a relentless progression toward making binary digits the primary thing we manage rather than voltage. Much of this has been driven by the decreasing price of the chips involved in digital signal processing (DSP), which has generated huge leaps in power in mixing and processing. Over the last 10 years, we have seen these capabilities put to work in places that are less obvious, such as in guitar pedals and wireless microphone technology.
Digital Wireless
Strictly speaking, there is no such thing as digital wireless. All wireless systems use a transmitter that generates an analog carrier. This is the frequency that we refer to when we talk about the frequency range the system is in. It is most common to see these in the UHF range, now legally limited to 470–608 megahertz, which is about one-third of the bandwidth that was available before the FCC sold off bandwidth to telecoms for 5G use.
While we also have access to spectrum in the VHF and gigahertz ranges, UHF has the best balance of features for wireless use, which is why we are all fighting over it! It allows for good range at low power and reasonable antenna lengths. Also, unlike 2.4 gigahertz, it is a licensed range, so it is far easier to predict what you will be competing with for spectrum.
The digital part of the equation hinges on the way that we modulate the base carrier signal. Analog wireless includes inexpensive products like Shure BLX but also high-end products by Sennheiser. In this traditional approach, the carrier frequency of the wireless system is modulated by the actual microphone signal, typically over a 50kHz range. That signal has been turned into a voltage “analog” of the air movement that the microphone has picked up. The wider the frequency range the carrier frequency can be modulated, the higher the quality that can be represented, similar to sampling rate in a digital audio file.
Learn more about radio frequencies in our RF 101 – The Basics article!
These frequency variations are demodulated (decoded) at the receiver side, and the resulting signal is the original signal from the mic. That signal is then plugged into a console and routed to a speaker to be converted to something you can actually listen to.
Since nearly all modern wireless uses the same FM-based modulation scheme, if you have any company’s wireless receiver that will tune to the same frequency, then you can listen to the signal being transmitted. Ever wonder where those embarrassing recordings of bad vocalists soloed came from? Lots of them came about just this way.
Digital wireless adds an additional step to the process at each end. Before the analog signal is modulated, it goes through a digital conversion, and that resulting digital signal is what is modulated onto the RF carrier frequency. At the receiver end, the process is reversed, so there is an RF demodulation and then a conversion from digital to analog. This process adds some complexity that nets us some benefits and some potential downsides, as well.
First of all, the audio quality can be higher than analog; but it may not be, because everything is a tradeoff in wireless. Encoding at the equivalent of 24-bit/96kHz is possible, but that takes up a lot more space, energy, and time than a lower-quality option. Higher-end digital wireless systems like Shure Axient allow you to choose a quality level, but higher quality means you will be able to get a lower number of channels working at the same time. In most digital systems, the manufacturer makes that choice for you.
Second, because of the way digital-encoding schemes work, only the same range of product from a manufacturer can decode its signal. Many of these systems also support encryption and single pairing, making it nearly impossible to intercept these digital signals. At least one major pop singer switched to a high-end digital wireless system because someone had stolen their solo mic output at a show and posted it on the web all by its lonesome.
Time and Comfort
However, this potential for higher quality and power comes with a tradeoff. Digital systems take a certain amount of time to do the conversion back and forth to digital, whereas analog wireless is instantaneous. Digital conversion adds somewhere between 3 to 20 milliseconds of delay to the signal, depending on the system. Performance-based systems like Shure ULXD and Sennheiser 6000 series are among the fastest, while systems like the Rode Wireless GO are in the 6-millisecond range. Sennheiser Speechline, which is aimed purely at speech, is up around 19 milliseconds. A delay less than 8 milliseconds is generally considered to be imperceptible, but the closer you are to the source of the audio, the more this can be disturbing to a performer.
Since audio takes about a millisecond to travel a foot, the delay in air from standing in front of a guitar amplifier is also about 3 milliseconds. From a singer to the main PA speakers could easily be 20 to 30 milliseconds, so at some point these latency figures are not relevant… until you introduce in-ear monitoring into a system.
With 3 milliseconds of latency when the speakers are in your ears, that can be downright distracting, since you can hear your voice physically inside of your head along with the delayed signal coming through the wireless system’s conversion. This is why we don’t see any digital in-ear systems currently on the market. If you had a digital IEM system and sang through a digital mic that was going through a digital console, the total latency could easily be above 10 milliseconds, which anyone would find distracting. Some people are more sensitive to these kinds of micro-delays, so if a singer falls into this category, an analog-wireless microphone may be a much better choice because of its negligible latency.
Concurrent Channels and Intermodulation
One of the most compelling reasons that digital wireless has made such an impact in the marketplace is because of the shrinking amount of radio bandwidth we have to operate in. While there are systems in the 2.4-gigahertz range, these are competing with WiFi, which can run at much higher power, making it challenging to get multiple channels of it working dependably.
In the precious UHF range, digital systems can run more channels than their analog equivalents in a small slice of spectrum. This is because running multiple channels of wireless at the same time causes lots of interaction between systems. While digital systems actually generate more intermodulation interference, they spread it around more widely in the frequency range. So, they create more artifacts but at a lower level and more separated in range. This means that in a small spectrum space, like a block the size of a TV channel, a digital system can run 12 or 13 channels, while most analog systems can only manage seven or eight channels. We have more in-depth coverage of intermodulation in our “RF 101” article if you want to do some additional reading.
Shure’s excellent tool Wireless Workbench is free and manages the task of assigning appropriate frequencies to multiple channels of wireless. Below, I have a couple of frequencies showing the maximum number of channels they can coordinate with a high-end digital system and a high-end analog system.
However, digital versus analog is only part of that equation. The other significant feature to look at is the range that a system will tune across. A digital system like the Shure BLX tunes across a 24-megahertz window, whereas the analog Shure ULXS has a 35-megahertz tuning window, making it potentially more flexible and therefore more reliable. Generally, cost goes up with wider tuning windows but not always. Some Sennheiser G4 systems actually can tune across an 88-megahertz window. This is a case where paying attention to the specs can make a real difference.
In short, wireless is definitely a case where you get what you pay for, whether it is digital or analog. Good analog is generally preferable to inexpensive digital, and all big touring and install systems in the future will probably be a mix of analog and digital until there are some additional technological innovations on the digital side of things. Further, if you are using IEMs onstage, anyone sensitive to small delays, like guitar players and vocalists, may still feel more comfortable using analog wireless for latency reasons.
If you need some help determining what wireless system will be best for your personal application, give your Sweetwater Sales Engineer a call at (800) 222-4700, and they can help guide you to the perfect choice for your needs.
