Most audio connections in live sound applications and use cables to carry audio signals - analogue, digital or networked - from sound sources to stage boxes, from stage boxes to mixers and from mixers to power amplifiers. However, one category of sound source is often connected through radio waves: microphones. Although a cable is the most secure way to connect a microphone, with the highest audio quality, there’s a practical reason to use radio: freedom of movement of the performer. This also goes for worn musical instruments, such as guitars.
Over time, frequency ranges in the UHF (Ultra High Frequency) band from 470 MHz to 823 MHz have been used by wireless microphone system manufacturers, made possible by legislation in countries to allocate certain frequency ranges for entertainment use. With the advance of cellular data, these frequency ranges have been - and are being - re-allocated in favour of television and mobile data networks, reducing the ranges available for entertainment infrastructure. The reason is simply that the economic importance of television and mobile phones has grown to be far more significant than entertainment infrastructure. The result is that the availability and bandwidth available for UHF wireless microphone systems is limited year by year, reducing channel counts or, worse, possibly making systems with a fixed frequency range obsolete.
An alternative to this challenge is the use of so-called Industry, Scientific and Medical (ISM) frequency bands. These bands are designated and guaranteed for use by most of the planet’s governments. Their use for consumer services include 2.4 GHz and 5 GHz WiFi and Bluetooth wireless networks, plus 1.8 GHz DECT wireless local telephony networks. Globally, these applications are used so extensively that it would lead to worldwide chaos if they were cancelled. Therefore the long-term availability of these frequency bands is almost certain.
The bandwidth - and with it the channel count - that ISM bands can support is not high, but neither is it particularly small; in practise around 14 channels at 2.4 GHz WiFi and around 56 channels at 1.8 GHz DECT. The free air connectivity range at these high frequencies is limited to a maximum of about 100 metres, with lower in-house ranges. This is good for WiFi and DECT because every home can have its own wireless network without disturbing other homes, meaning many networks can use the same frequency simultaneously. For microphone systems, of course, this is a constraint, limiting the maximum distance between transmitter and receiver. In addition, the GHz range is vulnerable to blocking by objects such as walls, ceilings and - relevant for entertainment – guitars, drum kits and the performers themselves. The higher the frequency, the higher the likelihood of signal blocking, which is the reason why 5 GHz is sometimes less suited for use.
The main issue, however, is interference, caused by reflections of the high frequency radio signals by walls and other metal objects. For this reason, ‘diversity’ techniques have to be applied, for example by using two carrier frequencies per connection and two antennas for reception. If interference occurs at one frequency or antenna, the system can switch to the other.
The existence of WiFi networks in the same location is, of course, another challenge for 2.4GHz systems. However it’s not as severe as you might think. By selecting carrier frequencies in between those used by WiFi networks - by using directional antennas and making sure that no Wireless Access Points are located in performance area between the microphone transmitters and receivers - the connection is virtually resistant to WiFi traffic. We tested this at live pop concerts with over 1000 concert goers, all in the ‘active social media’ age group, and confirmed that 2.4GHz wireless microphones can easily co-exist even with massive amounts of Facebook selfies.