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Article ED00008: Wireless Microphone Systems

Summary:

Convenience and mobility are the major advantages of wireless microphone systems. But there is a little more to it than simply plugging in and switching on.

Application:

Teachers and students and anyone using or planning to use wireless microphone and instrument systems ... particularly when multiple systems are to be used together.

Details:

Wireless microphones are in fact small FM radio stations. The microphone includes an FM transmitter and this signal is then received by an FM receiver that is usually plugged into a mixer or amplifier. Sounds simple, but like any radio transmission a wireless microphone can be subject to noise, interference or even dropouts of the signal.
 
Regardless of what type you choose or how much it costs there are some basic things you should do to minimise the likelihood of any of these problems.

  • Ensure you keep the distance between transmitter (mic) and receiver as short as practicable
  • Whenever possible avoid any walls or obstructions between the transmitter and receiver
  • Use new batteries in the transmitter for any important occasion. Better quality systems usually give an indication of typical battery life in the operating manual as well as a “low bat” indicator on the transmitter.
  • Make sure the transmitter and receiver are set to the same channel (someone may have changed one). Some systems have automatic channel setup, usually a “Sync” button on the receiver.
  • Test the transmission with the mixer/amp volume turned down. When both the mic/transmitter and receiver are working there is usually an indicator light or meter on the receiver (sometimes labelled signal or RF).
  • Turn on the mic (often there is a mic mute button that is separate from the power switch)
  • Turn up the mixer/amp and walk around the area where you will use the mic to test for any blank spots or droput zones. If there are any, you may need to relocate the receiver (or antenna), upgrade your antenna and/or upgrade to a true diversity system (see below)

Of course this will all be routine after you have done it a couple of times.
 

Wireless System Basics

All wireless mics these days use FM radio transmission and although many are digitally controlled,
most are analogue systems. Digital systems are now becoming available as digital process is now fast enough for live sound to overcome conversion latency or delay in the reception. You may have noticed this effect with a digital TV if you have also had an older analogue TV on the same channel. The digital TV was slightly delayed. Of course most analogue TV stations have now been switched off as part of the Federal Government Digital Dividend of reallocating the wireless spectrum.
 

Diversity ... and why you might want it.

Diversity is a common term in wireless mic systems, but there is often confusion about its meaning. At the simplest level it refers to a system that has two antennas on the receiver to reduce dropouts. Dropouts are mostly caused by phase cancellations of reflecting signals (called multi-path nulls). In theory, by having two antennas separated from each other, there would be less chance that both would dropout simultaneously. In practice, simply having two antennas is not that much better than one.
 
Passive Diversity - Very low cost systems usually only have a single antenna. These are referred to as “non-diversity” systems. Other low cost systems that have two antennas are typically labelled as “diversity” systems. But these are technically defined as “passive diversity”. Effectiveness against dropouts is modest but may be quite acceptable in static situations. However, if you are moving (with your mic/transmitter), or other people or objects are also moving about then dropouts are still likely.
 
True Diversity - These systems are more costly but substantially eliminate dropouts due to multi-paths. These systems actually use two separate receivers internally with an antenna for each. The receivers monitor the radio signal strength and seamlessly switch the audio to obtain the best signal at any point in time.
 

System configurations

Wireless microphones generally come in the same variety as wired mics, except that the mic must contain or be attached to the transmitter.
 
Hand held - the most common type, where the mic and transmitter are a single unit. The better quality systems include popular good quality “capsules” such as Shure SM58, Sennheiser E835, AKG D5 and so on.
 
Lapel or Lavalier - clip on mics that plug in to a separate body-pack transmitter.
 
Headset - mount on the head or over the ears and the capsule is reasonably close to the mouth. These are best perfoming for hands free operation and are also available in miniature, skin toned forms for theatre productions. Plug in to a separate body-pack transmitter.
 

Choosing your first system - Important considerations

Something often overlooked when purchasing your first wireless system is what you may want to do in the future. A single system is quite straight forward... say a true diversity, modest cost, hand held system. But what if you might want to have more wireless mics in future, perhaps for a band, jazz choir or theatre performance?
 
This is when you will need to know what frequency bands and channels your systems are using and, more importnantly, how many simultaneous non-interfering channels can your system use. For this reason it is highly recommended you consider this BEFORE you purchase that very first system.
 
Lower cost systems often have up to 16 preset channels available, but perhaps only 4 of these systems may work together. As the quality (and costs) increase, the number of simultaneous channels increases... a measure of their greater radio tuning stability and tighter bandwith tolerance of both transmitters and receivers.

VERY IMPORTANT
Many systems have a wide selection of frequency channels and banks and at first glance it would appear that you can combine a large number of systems, each with it's own frequency. However, the chosen frequencies not only need to be spaced apart far enough, they must also be carefully calculated to avoid creating harmonics which can cause other frequencies to have problems such as dropouts and static. These undesirable effects are known as “intermodulation.” You will find that manufacturers that offer multiple banks have done the hard work by setting preset channels in each bank that will work together without interference. When running multiple systems it is essential that all are using preset channels in the SAME bank.
 
The more advanced systems also have a wider range of accessories available for combining and boosting antennas for the receivers and providing convenient power and mounting (usually rack mounts). It should also be remembered that the quoted maximum number of simultaneous systems in manufacturers’ literature assumes ideal operating conditions. In practice this number may be lower. Hence it is always recommended that you err on the higher side.
 
And finally, when choosing multiple systems it pays to stick with a particular brand and series (e.g. Shure PGX, BLX, SLX, ULX, Senneiser EW100, EW300, EW500 or AKG PWM or WMS series).

See also:

The Digital Divident - Preparing for 2015

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