Wireless Microphones - A General Overview

There are obvious advantages of wireless microphone systems, especially if you move around a lot as a performer, but naturally, there are also some tradeoffs.

Wireless microphone prices have dropped dramatically in recent years and this has resulted in their greater use in the Live music and Conference markets, also for amateur musicians and even in pubs for use as Karaoke microphones.

Pro's and Con's of Wireless Microphones

The obvious benefit of wireless (radio) microphones is the lack of restriction of movement and any possibility of getting tangled in microphone cables. It is not just microphones that benefit from wireless technology, guitarists, and even keyboard players can use radio transmitters to feed their signal to the P.A. system.

The potential downsides of wireless microphones are radio interference from other transmitters e.g. taxis and other wireless microphones, loss of signal due to phase cancellation or a weak signal reaching the receiver and power failure - use fresh batteries in the transmitter before each important event.

Wireless microphone transmitters use a compressor circuit to reduce the large dynamic range of the microphone and then an expander circuit in the receiver de-compresses the signal. This ‘compansion’ technology is very good nowadays but it can have some ‘pumping’ side effects on some sound sources. So where absolute fidelity is required it’s best to use a wired microphone.

A dynamic microphone can be used with a wireless system and so can an electret microphone but a condenser microphone needs a power supply between the microphone and the transmitter.

Types of wireless microphone

There are two types of radio microphone, handheld and body pack (belt) transmitters.

Handheld microphones incorporate the transmitter into the body of the microphone and the antenna is situated where the lead would normally go. Controls can be situated externally but are usually inside the body where they wont accidentally get changed.

Body packs are typically the size of a pack of cards and have a lavallier microphone or headset microphone plugged into them. The antenna and microphone input are usually on the top of the pack along with the various controls such as audio gain and frequency selection.

An additional receiver can be used to play back a backing track or ‘talkback’ from a control room to the headphone part of a headset microphone. `some headsets only have a microphone attached, they are used to get the microphone very close to the user so that the possibility of acoustic feedback is reduced.

The first wireless microphones employed the VHF (very high frequency) radio band and these are still typically used in the cheaper and amateur systems, they will provide a good quality output. The latest ones use the UHF band (ultra high frequency) because they provide a wider frequency response and are less prone to radio interference in the UK (and no doubt other countries) as the VHF band becomes more congested.

How they work

The audio signal is modulated by a radio signal and de-modulated at the receiver into an audio signal that is sent to the audio mixer or directly to the amplifier.

The first wireless microphone receivers had one antenna (Non-diversity) but the latest receivers employ a ‘Diversity’ system whereby there are two separate receiver circuits each with their own antenna and a signal strength circuit analyses which receiver has the strongest signal and noiselessly swiches between them accordingly many times a second. This avoids audio dropouts and any ‘shash’ where reception is poor. The antennae are positioned at right angles to one another to ensure that one receiver’s antenna is always optimally aligned to the transmitter’s antenna.

In the UK the cheaper wireless microphones tend to use de-regulated (free) frequencies and only a few switchable frequencies per kit, this is why for professional use there are more frequncies per kit and they are in ‘paid for’ frequency bands that guarantee a greater chance of protection from interference. Each country will have its own system of allocating frequencies and any payments due.

Some receivers have a ‘squelch’ control that can be altered, this is used to prevent a loud ‘shash’ being heard when the radio signal disappears, its threshold should be set to where it mutes the audio when the signal is poor but not so high that it keeps muting the audio when the signal is low but still usable. To set it up you will need an assitant to take the transmitter far enough away from the receiver so that the signal is almost useless and then set the threshold of the squelch to mute the output.

Some systems have this control pre-set internally and not accessible to the user and some systems have a very high frequency control signal that has to be detected by the receiver before it will un-mute the audio.

There are digital wireless microphones available which make better use of the radio bands (more channels can be used than in the analogue domain) but care should be taken where conventional microphones are used as well since there is a processing delay as the audio signal is converted to digital which can cause out of phase artifacts.

Summary

Wireless microphones allow freedom of movement and for the most part can be used without any concerns about quality, though the compansion circuit can cause problems with some sound sources e.g. accordian.

Radio interference is the main concern, where possible use the UHF diversity systems for best audio quality and reduced interference. Receivers are usually mains powered so film and video sound recordists will need to buy or rent a small battery powered receiver. Per microphone they are still expensive but their cost has dropped sufficiently that where just a few are needed their cost is not prohibitive.