What is in this article?:
Early wireless microphone technology has been based on FM over VHF or UHF. Multiple antennas give diversity reception to avoid dead spots in the radio transmission, but, just like FM radio, the system is prone to interference. To give a good audio dynamic range, the systems used companding — dynamic range compression in the transmitter, with complementary expansion in the receiver. Such a method is not without artifacts, however, and some music styles suffer especially; jazz is one example.
For large events where many microphones were used, frequency planning was once complex because of the needed increase of available slots, and also the hassle of avoiding issues like intermodulation between channels.
Newer digital technology allows a higher audio quality, with systems offering an uncompressed audio path. This potentially delivers a dynamic range in excess of 110dB. But, digital processing is not without its pitfalls. Digital systems can “fall off a cliff” when the received signal drops below a threshold. The analog FM systems degrade more gracefully. In the presence of a low signal or excessive multipath reception, the signal-to-noise ratio decreases, but audio remains.
Digital systems need careful design to avoid the cliff effect. RF propagation on set is plagued with multipath effects as well as obstacles to the line-of-sight transmission. This needs intelligent diversity reception along with error concealment to avoid audible distortions. As a last resort, gentle muting can fade a lost signal.
So, although a digital system can potentially offer clean, uncompressed audio, the data and RF components must incorporate clever processing to avoid what could be some very nasty clicks and dropouts. Many popular error concealment techniques have long latencies, which make them unacceptable for a microphone circuit. Processing should not introduce latency over 10ms in order to avoid problems with fold-back to artists.
Analog systems are more forgiving, but have audio performance drawbacks, like companding and noise. The next generation of wireless systems that use digital technology promise a new level of fidelity close to a wired connection. However, digital systems need sophisticated technologies to operate reliably within the propagation conditions typically found on the set or at the venue.
Although television drama started in the studio much like film (since the 1970s), the trend has been for productions to shoot on location. For interior shots, the presence of a ceiling makes it more difficult to use booms. Rooms have four walls, not the three of a set, and this adds up to problems of deploying booms. Nevertheless, these productions often use a mix of shotgun microphones with personal wireless microphones for the main actors, using a discrete clip mic and a concealed body pack. This is easier with period drama, where wardrobe fashions allow more opportunities to conceal a transmitter.
In small rooms with hard-surface acoustics, a boom microphone may pick up too much of the room’s acoustic and amplify the natural resonances. The short distance of a clip microphone from the mouth helps to capture clearer speech. For drama, wireless microphones offer an alternative where the use of a boom or fishpole is not an option.
Wireless microphones find application across genres from drama to live performance. Whatever the limitations of spectrum, they form an essential part of a sound recordist’s tools.