An Overview of Digital Wireless Microphone Systems
The digital age has reached deep into our personal and professional lives. More and more often, we use GPS instead of atlases, share contacts on our phones instead of writing down numbers, and start research projects at home with Google instead of at the library's card catalog. Audio is no exception, and there's no turning back.
What started out 70 years ago as the patenting of pulse-code modulation by ITT's Alec Reeves was helped along by the invention of the transistor in 1947. That made the sampling of audio frequencies across the full human auditory bandwidth possible. But it wasn't until the late 1970s that audio manufacturers introduced the first digital recording systems.
Among the digital audio devices that have hit the market in the past 30 years, professional wireless microphone and guitar systems arrived more recently. Shure introduced its first digital wireless system, the entry-level PGX Digital, in 2011. Since then, the Shure line has expanded to include three additional frequency-agile digital systems that, depending on the model, are capable of handling up to 47 compatible systems in one 6 MHz television channel.
In this post, we'll cover the basics: the clear advantages, potential trade-offs and what you need to consider in choosing the digital wireless system that's right for you.
Why Digital Wireless?
Superior Sound Quality
Digital wireless systems offer highly transparent audio quality. This is largely due to the lack of a "compander," a circuit used in all analog wireless microphone systems to minimize noise and maximize dynamic range. (Compander is a contraction of the words compressor and expander.) The audio signal is compressed in the transmitter to accommodate the limited dynamic range of FM radio and then expanded in the receiver. This process, while relatively imperceptible in most good analog systems, can still lead to audible artifacts (like "pumping" and "breathing") that make a wireless microphone sound different from its wired equivalent. Since the transmission of a digital audio signal doesn't require companding, the received signal retains the exact characteristics of the original audio input.
A digital wireless system can also achieve a flat frequency response across the entire audible range (20 Hz to 20,000 Hz), which yields the truest possible sound transmission. The microphone element, not the "wireless" components, more closely defines the frequency response of the system. Digital wireless systems convert analog audio to a digital signal that modulates a radio carrier in discrete steps (think ones and zeroes). The digital audio signal arrives at the receiver unaffected by the radio link. Any RF noise that may be present below a certain threshold doesn't affect the audio quality. The receiver simply ignores anything that isn't a zero or a one. Everything else is discarded. Only the digital signal is sent on for amplification.
Longer Battery Life
In general, digital wireless microphone systems have 30–40% longer battery life than equivalent analog systems. Here's a Shure example: the digital ULX-D® transmitters run up to 11 hours on two AA alkaline batteries and more than 12 hours with the Shure SB900 Lithium-ion Rechargeable Battery.
Better Spectral Efficiency
While this isn't necessarily true of all digital wireless systems, the modulation type chosen by the manufacturer can potentially lead to much higher channel counts in reduced clear spectrum.
The deviation of a digital wireless signal is more predictable than that of a frequency-modulated analog signal, allowing tighter channel-to-channel frequency spacing. This feature is particularly important in light of the continued crowding of the UHF television band where many wireless microphones operate. Depending on the manufacturer and model, digital systems can often deliver nearly twice the channels in the same slice of spectrum as their analog cousins.
Things to Consider
Budget
You can expect to pay about 10–20% more for a digital wireless system. Remember, though: digital systems may offer features not possible with analog systems, including smart battery technology, extended runtimes, more on-air frequencies, encryption, and interference avoidance.
Latency
Latency is the amount of time it takes for a signal to arrive at the output after entering the input of a digital device. In analog equipment where audio signals travel at the speed of light, latency is not a factor; however, in digital equipment, the incoming analog audio signal needs to be converted to a digital signal. The signal is then processed and converted back to analog. While large latency values could potentially be problematic, most high-quality digital systems produce less than 5 milliseconds (that is five thousandths of a second) of latency, generally acceptable to most listeners.
Understanding the UHF Band
Here in the United States, all professional digital wireless systems operate in the UHF band, with many entry-level systems in the 900 MHz band or the 2.4 GHz band. Here's what you need to know:
- All spectrum is shared by multiple users and multiple types of devices. Frequency bands are heavily used by other types of unlicensed devices. Wireless microphones are not entitled to any protection from the interference caused by them.
- All types of RF transmission (analog, digital, or spread spectrum) can suffer interference. The effect can be a reduction in range or fidelity even if the interfering signal does not fully "break in."
- The UHF band is subject to future rule changes that could affect wireless microphones and other devices. The FCC has the right to alter how RF spectrum is deployed, and these alterations can change the amount and location of bandwidth available for wireless mic operation.
There are advantages and disadvantages in the microphone-friendly blocks of the RF spectrum. One involves traffic from other devices in the band, and another is how the radio spectrum is assigned country by country.
470–698 MHz
Most wireless microphone and IEM systems in the US operate between 470–698 MHz in what is known as the UHF (Ultra High Frequency) band or TV band. Because the main sources of interference are local TV stations with fixed locations and known RF signal levels, wireless microphone manufacturers are able to predict the number of systems capable of operating in a specific location. The UHF band is home to both licensed and unlicensed wireless microphone operations.
900 MHz
This unlicensed frequency band is shared with other consumer devices, but it tends to offer greater penetration through obstacles and can diffract around and over obstacles like buildings or trees, compared to higher unlicensed ranges like 2.4 GHz. However, there is limited amount of bandwidth in the spectrum, making it impractical for professional users who need to get many channels of wireless on the air.
There are only a few countries where this band is legal for wireless microphone use (the US among them, but no European countries). Other devices in this band: cordless phones, baby monitors, hobbyist and two-way radios.
1.92–1.93 GHz (DECT)
DECT is an acronym for Digital Enhanced Cordless Telecommunications. In the United States, the DECT band is 1.92 to 1.93 GHz. The frequency band differs by country. It isn't a worldwide standard.
Since DECT was developed for telecommunications, wireless systems may compete with cordless phones in this frequency band. DECT Technology employs a TDMA (Time Division Multiple Access) standard. This makes operation achievable without having to coordinate frequencies as the technology accomplishes this on its own. Saturation of the spectrum by multiple devices is a potential performance tradeoff.
2.4 GHz
This band is available on an unlicensed basis throughout much of the world. It offers a wider frequency range than the 900 MHz band, but systems may experience more interference from other wireless technologies like WiFi and Bluetooth.
In order for wireless mics to be successful at 2.4 GHz, they need to be "smart," or able to change frequencies on the fly to avoid interference. 2.4 GHz devices work in a larger frequency band and can potentially offer more channels and greater potential density than lower frequency devices. Shorter wavelengths make line-of-sight setup between transmitters and receivers important for the most reliable operation.
Choosing a System
If you're shopping for a digital wireless system, there are, of course, lots of considerations just like there are in analog ones. First of all, there's the reputation of the manufacturer. All the major pro audio manufacturers offer digital wireless systems. Choose one that's known for quality, performance, reliability and customer service.
Here are some questions that will help you narrow the field:
How many compatible systems do you need?
A school using a system in a cafetorium, a portable church needing a handful of systems, or a solo artist can often be served by the most affordable options.
Where will you be using it?
If the digital system will be used in an auditorium, a mid-sized venue like a house of worship with a praise band, or a rental rig for touring artists, you'll be looking for a higher number of compatible channels and a fuller feature set.
Inside or outside the US?
A 2.4 GHz system will work anywhere. All other wireless microphone spectrum is allocated differently around the world, so precise selection of frequency bands is important. Frequency coordination is a requirement no matter where you operate your wireless gear. That includes adhering to the local laws.
What options and accessories are required?
The basic components are a handheld (mic) or bodypack transmitter and a receiver. Options, depending on your application, include lavalier, headset and clip-on instrument mics, guitar cables, and guitar pedal receivers/tuners. Higher-end systems include networking protocols such as Dante™ and advanced rechargeable battery technology.
How much do you have to spend?
Shure systems can run from around $500 for an entry level PGX-D to three times that for the Shure ULX-D.
Final Thought
Analog audio has been part of the sonic landscape for over a century, while digital, in just thirty years or so, has become an increasingly dominant force in the industry. Which is better? questions are bound to drive conversations for years to come, as users will likely deploy both technologies in an attempt to meet the varying needs of their productions. These decisions are shaped by three critical parameters of wireless microphone system performance: signal reliability; spectral efficiency; and audio quality.
Analog and digital wireless systems have many differences, but both technologies must always address these interdependent design tradeoffs.
