An Overview of Digital Wireless Microphone Systems - Shure USA
An Overview of Digital Wireless Microphone Systems - Shure USA
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 . That made the sampling of audio frequencies across the full human auditory bandwidth possible. But it wasn't until the late s 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 . 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
Six reasons why you should switch your wireless microphone from ...
Of course, audible sound is analogue, but that doesn’t mean you and your audience should have to forego the many benefits of digital wireless microphones. In some environments, it may even be crucial to finally make the switch from analogue to digital. Here are the top six reasons for making the change.
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(1) The audio will sound better and cleaner.
As you may have heard, digital systems do not require a compander (a combined compressor/expander). In analogue wireless microphones, a compander compresses the audio signal, lifting it above the noise floor of the RF transmission, and expands it again in the receiver to recreate the original audio signal. Unfortunately, this noise reduction is not a noiseless process in itself, and you may hear your compander pumping, breathing or hissing. A compander-less digital system will not only massively improve the audio quality for your audience, but will also help you to hear your performance more clearly through your monitoring system.
(2) With features like intelligent switching diversity, digital wireless can work some magic for you.
A phenomenon called “fading” is the natural enemy of any wireless microphone transmission. This means that the microphone signal can be strong in one spot, but then extremely weak in another, where reflected signals cancel out the direct signal between the wireless microphone and its receiver. When you’re really pushing your boundaries, a good digital microphone system like Sennheiser’s Evolution Wireless Digital can work some magic to help you stay “on air”: intelligent switching diversity, an RF channel equalizer, error correction and even error concealment will keep your signal intact and your confidence high.
(3) The frequency manager at a festival will welcome your digital gear with open arms.
The wireless spectrum is a shared and a shrinking resource and very densely populated – not just by microphones but by many other production tools, too. At a festival or venue, the frequency manager is responsible for handling all frequency matters. When you show up with an analogue microphone system, it’ll add to the frequency manager’s workload, because they need to calculate “intermodulation products”. These are basically useless emissions that occur when multiple wireless microphones are used on stage. Did you know that 32 channels of analogue wireless can create 16,000 possible intermodulation products or noise sources? When, for example, available frequencies at a larger event are really scarce, the frequency manager may not be able to fit your analogue equipment in, and your energetic stage show might find itself tied down by a cabled microphone.
A good digital wireless microphone like Sennheiser’s Evolution Wireless Digital, however, makes the job of the frequency manager easier. It will emit its carrier frequency only – and no disturbing intermodulation products that occupy additional spectrum. Transmission frequencies can simply be placed next to each other at regular intervals, whereas with analogue wireless, the frequency manager must work around all the intermodulation frequencies that the microphones produce. Therefore, digital wireless frees up spectrum for additional audio links or other wireless production equipment – and this space is just what the frequency manager needs most.
(4) Setting up a digital system is as easy as using an app.
If you double up as the sound engineer of your band, you’ll know it can be painful to get the audio going. With digital systems like Sennheiser’s Evolution Wireless Digital, an intuitive app guides you through a few easy steps and you’re ready to gig in a snap – on professional TV-UHF frequencies. But beware: Your band members’ awe of your seemingly magical abilities may diminish ever so slightly if you show them how easy this actually is…
(5) You can simply select a free channel without the worry of interfering with other performers.
“But if a frequency is free, why should I worry about disturbing other bands’ gear?” some may ask. The reason for worrying is called intermodulation products. Very simply put: If there is more than a single analogue wireless microphone on stage, the wireless microphones will emit several RF frequencies – not just the ones that you selected for transmitting your audio. The same happens if a wireless microphone gets too close to a receiver – like an in-ear monitoring pack, for example.
These additional and essentially useless frequencies are called intermodulation products. They can interfere with other wireless microphones, or in-ears for that matter, and they eat up spectrum space. With analogue microphones, and unfortunately also many digital microphones, you will always have the issue of intermodulation products.
However, some advanced digital systems like the Sennheiser Evolution Wireless Digital don’t emit intermodulation products. This makes it very simply when it comes to your band’s next gig: You or the venue technician can simply select a free channel and you don’t need to worry about anything. No intermodulation products, no interference – and you have saved valuable spectrum space, too.
(6) Too soft, too loud, clipping? Not with Sennheiser’s Evolution Wireless Digital.
Gain is probably one of the most underestimated issues you may face when setting up a wireless microphone. In setting the gain, you determine how sensitive your mic is. If you scream into the mic and the gain is too high, you will sound horrible and the audio will clip. If you have a soft voice and the gain is too low, nobody will hear you amidst your powerful backline… The good news is that Sennheiser’s Evolution Wireless Digital has an input dynamic range of a full 134 dB, even surpassing the dynamic range of most microphone capsules, so you can pick up everything from a soft whisper to a plane engine without having to adjust the transmitter gain.
Making the switch to a good digital wireless microphone can help improve your performance in many ways. Opt for a microphone system that works on reliable UHF frequencies if you want to take your music to the stage. If you like, you can learn more about Evolution Wireless Digital here and the Smart Assist app here.
You can also visit our website for an interactive version of the six reasons why.
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