21st October 2003 - Radio Microphones - the alternative to wire

In a well-attended talk on 21 October, Lee Stone gave a thorough account of the uses, and problems, of radio microphone systems.

As well as their obvious uses in live performance, they are also widely used in location sound recording for film, and for documentaries and electronic news gathering (ENG).

For film use, there is usually a "sound cart", with a permanent installation of antennae, receivers, mixer and recorder. Documentary recordists require greater flexibility, and can do away with cables altogether by also using a radio link to send their mix to the camera for recording. Camcorders are now appearing with slots for radio receivers, removing the need for straps and cables. Alas, each camera manufacturer has their own slot and connector design!

In live performance, the number of channels in use continues to increase. It is not uncommon to use 40 in a West End musical. In concerts, not only are there wireless vocal and instrument microphones, but performers may have personal in-ear monitor mixes, each requiring another channel, or two for stereo.

Although radio microphones have become ubiquitous, the underlying technology has not changed for quite some time. Most professional systems operate in the UHF band, using spectrum assigned for television channels, but away from areas served by transmitters on the same frequencies.

Using frequency modulation, with an RF bandwidth of 200kHz, gives a dynamic range of about 60dB. The much higher dynamic range demanded by users is achieved by the use of companding: compressing the input audio signal, and expanding the output. The limited rise and fall times cause distortion, but in good systems this can be as low as 0.2%. The lack of any standard for compander operation prevents the interoperation of transmitters and receivers from different manufacturers.

The operating range of radio systems is rarely specified, as it is so dependent on the terrain. A system that may be reliable over hundreds of metres in an open field may only work over tens of metres on a busy street and become even worse indoors.

Kishore Patel, the MD of Audio Limited, played a tape of a microphone user walking away from a receiver, down stairs, across a car park and into the street. This demonstrated the failure modes: we heard slow fades, in which the noise floor slowly lifts, "splats", which are faster fades, and, eventually, the squelch circuitry activating when the noise floor became too high.

There are many factors affecting the propagation of the radio signal. The most obvious one, attenuation due to distance, is rarely a problem in practice.

Shadowing, caused by the performer's body absorbing the radiated signal, can cause a 20dB difference in field strength between the front and back of the performer.

Antennae are designed to work in free space. Placing one close to another object, such as a person, reduces its efficiency and distorts its radiation pattern. Belt-packs are particularly afflicted by this, so it is fortunate that regulations allow them to transmit at 50mW, rather than the 10mW limit for hand-held transmitters.

The trickiest problem is multipath, which is the radio equivalent of comb-filtering. Where destructive interference occurs near the carrier frequency, the received signal strength drops. This effect can vary very suddenly as the position of the transmitter or receiver changes, causing the "splats" heard earlier. Fortunately, this can be overcome by using diversity systems. These use multiple radio receivers, and take the audio signal from the one which currently has the highest signal strength.

In practice, using two receivers with antennae more than half a wavelength apart can improve the fade margin by up to 10dB, allowing a twofold increase in range. Positioning the receivers is done by trial and error, performing walk tests until the whole area is covered without any simultaneous dropouts on both channels.

Intermodulation distortion is another bugbear. This is caused by signals from multiple transmitters mixing in a nonlinear device, causing sum and difference and higher-order products. If any of these distortion products land near a channel that's in use, horrible buzzing can result. The radio circuitry in the receivers becomes significantly nonlinear when overloaded, so intermodulation distortion tends to occur when a transmitter comes too close to a receiver. Interestingly, the distortion can also occur when transmitters come too close together, as interference from one can overload the transmission circuitry of the other. Sticking to a "minimum safe distance" can avoid this problem, but makes duets difficult.

Intermodulation distortion is managed by predicting the frequencies at which distortion products will first occur, and not using the corresponding channels. This leads to rather poor spectral efficiency, with typically only eight audio channels, 200kHz wide, being usable within an 8MHz TV channel. Manufacturers provide tables of frequencies which will work well together.

This approach would clearly become impractical for larger number of channels. Therefore, when multiple TV channels are used, high quality filtering is used to separate the 8MHz channels, so that only intermodulation within one TV channel need be considered.

Lee closed by considering the potential of digital radio microphone systems. He identified latency and spectrum efficiency as key issues. This led to a lively debate with the audience on possible solutions.

Paul Troughton