One of the necessary evils of the modern live sound world, wireless microphones are gaining in popularity, freeing performers of leads and providing them with more on-stage freedom of movement. Hugh Robjohns breaks free with AudioTechnica's Liberator VHF Wireless System.

I have always felt that radio microphones fall under the 'necessary evils' category of audio equipment (along with gun microphones and piezo pick-up bugs!), because they are all compromised to various degrees in terms of their audio quality or reliability, but offer practical advantages which usually outweigh such concerns. In the case of radio microphones, it is the freedom of movement that appeals to many users, even at the expense of the reliability normally conferred by fixed leads.

The Audio-Technica Liberator Wireless System is intended for the relatively hospitable and predictable environments of an indoor stage in a club or theatre. It has been designed to meet the specific needs of a singer, performer or guitarist who requires the enhanced freedom of movement (across a relatively small area) that a radio microphone system can provide.

BITS & PIECES

The Liberator VHF Wireless System is available in four different versions to suit a wide variety of applications, but all versions are supplied with a plug-top mains power unit, a receiver, a transmitter pack, and an appropriate microphone or interface cable, all housed in a neat plastic carry-case.

The transmitters are set at the factory to operate on a single fixed frequency in the 'MPT 1345' de-regulated section of the VHF band, so you do not need a licence to use them. Both of the review units happened to be tuned to 173.800MHz, but systems are available set to any standard channel between 173.8 and 239MHz. Each system is supplied with a matched single frequency receiver.

Three of the models are based around a pocket transmitter unit, whilst the fourth is a true self-contained hand-held radio microphone. The pocket transmitter versions are called the Lib-21 G, Lib-21 H, and Lib-21 L, where the letter denotes the supplied microphone type or interface lead. The 21G model is supplied with a special cable to connect between an electric guitar and the transmitter pack; the 21H comes with a head-worn electret microphone; and the 21L has a tie-clip (lavalier) electret microphone. The hand-held radio microphone is designated the Lib-22.

RECEIVING

The receiver unit included with all variations of the Liberator, the ATW-R03, is an extraordinarily lightweight device, weighing a mere 290 grams and measuring a reasonably compact 35 x 110 x210mm (HWD). Unfortunately, being light, it could be easily knocked over or even pulled off its resting place by a heavy output cable.

The R03 is powered from a supplied plug-top mains unit delivering 12V DC. This connects to the receiver through a coaxial socket on its rear panel, and the grounded side of the DC power supply is fully isolated from the mains earth. Unfortunately, there is no retaining clip for the power lead. Although the sockets were fairly tight on the review models, I wonder if they will remain so with heavy use — a retaining clip would have guaranteed protection from loss of power at a critical moment.

Other rear panel facilities include a standard, unbalanced quarter-inch jack socket for the audio output (a jack-to-jack lead is supplied in the kit to connect the receiver to a mixer or amplifier) and a screwdriver operated control to adjust the squelch level. The operating frequency of the receiver is shown on a self-adhesive label stuck to the rear panel above the DC power socket. The front panel accommodates a pair of telescopic aerials, three LEDs indicating power, acceptable RF signal level, and audio overload. There is also a small knob to set the audio output level.

Internally, a good quality, but sparsely populated, circuit board occupies the whole of the case's floor area; the electronic components are a mixture of standard and surface-mount types. The underside of the printed circuit board is fully screened around the critical RF front end, but the case provides no other electromagnetic screening at all.

The unit's specifications are quite respectable with an audio bandwidth of 100Hz to 15kHz, distortion less than 1%, and an 80dB signal to noise ratio. The latter is no doubt largely due to the undocumented use of a simple compander system between the transmitter and receiver.

The two telescopic aerials at the front of the receiver might give the impression that the R03 is a diversity system, but in fact, only the rear aerial is 'active'; the front one serves as its ground plane. In use, the two aerials are normally arranged at 60 to 90 degrees to each other, generally pointing upwards in a V shape. Adjusting the angle and positioning of the aerials may make a small difference to the working range and reliability of the wireless system in some environments, although I found the unit to be fairly uncritical in this respect over the working distances typical of a medium-sized stage.

Although calibrated at the factory, the user can adjust the squelch level through a screwdriver preset on the rear panel (this sets the minimum RF signal level which may be decoded and sent to the audio output). I found no need to adjust either of the review units, but it may be a useful facility if there is a high level of interference at a particular location. However, high squelch settings make the receiver less sensitive and so reduce the working range of the complete system.

As with all radio receiving equipment, best results are obtained if the aerials can 'see' the transmitter, so placing the unit as high as possible and with direct line of sight to the transmitter aerial is a good idea. Ideally, the receiver should be as close as possible to the transmitter, but to avoid overloading the 'front-end', there should be over a metre between the two.

One of the most common problems with radio microphones is fizzing and drop-outs caused by multi-path reflections. The transmitter sends its radio signal in all directions, and consequently, the receiver picks up not only the direct signal from the transmitter, but also reflections from various objects in the immediate environment. Needless to say, these interfere with each other, producing the unpleasant fizzing effects. The best way of avoiding such problems is to keep the receiver away from large metal surfaces (many buildings have a lot of metal in the walls); experiment with its location to find the optimum position. In this age of digital equipment, interference from spurious emissions is a common problem. I found it very wise to keep both the transmitter and receiver well away from anything digital like reverbs or other signal processors, digital recorders, mobile phones, motors, mains leads, and fluorescent or neon lighting.

"... very good and completely reliable."

TRANSMITTING

The pocket transmitter pack is designated the ATW-T21, measuring 25 x 65 x 110mm (HWD), and with a battery inserted, it weighs a minuscule 125 grams. It may be carried in a pocket, or attached to a belt via a spring-metal clip on its rear surface.

The top of the unit houses a 4-pin mini-DIN socket for the microphone or guitar interface lead, a three position switch to turn the unit on, and an associated red LED which shows the condition of the battery. The on/off switch has a central 'standby' position in which the transmitter sends out its normal RF signal, but the audio is muted. The 40cm long 'quarter-wave' wire aerial, which is permanently attached at the top of the transmitter, is intended to dangle freely. Power is supplied by a single 9V PP3 alkaline battery, housed in the base of the unit. This should provide up to 15 hours of continuous use.

Again, the internal construction is sound, with good quality components throughout, and the same combination of normal and surface-mount devices on both sides of the circuit board. There is no electromagnetic screening whatever in the transmitter, so it would be wise to avoid placing it near any equipment which might be susceptible to RF interference.

The factory-set transmission frequency is marked on an adhesive label on the rear panel. In terms of specifications, the T21 produces an RF output of 2mW — easily sufficient to provide solid coverage over a typical stage area. The multi-pin socket allows the connecting plug to be wired to provide either a high or low input impedance (for guitars or microphones respectively); it also supplies a DC power signal for electret microphones.

Under the battery cover, a clip provides a storage place for the included miniature trimming screwdriver, which is required for adjusting the audio input sensitivity of the transmitter (thereby optimising its noise and distortion performance). There are two trimmers, labelled MT and GT (accessible under the battery cover). The operating instructions suggest that the MT control sets the input level for both types of signal; there is no comment at all about the GT control.

MIKING TIES

The Lib-21L review model was supplied with an AT829D4 tie-clip electret microphone, attached, via a one metre lead, to a suitable plug for the transmitter. This microphone appears to have a cardioid polar pattern, which means it must be used with the top facing upwards. Without its supplied foam windshield, the microphone is a little susceptible to wind blasting, but its directional response prevents it from being used upside down, which is the normal cure for this kind of problem. A rather chunky tie-clip bracket is furnished with the microphone.

In fact, by modern professional standards, the AT829D4 can be considered a very chunky microphone indeed, measuring about a centimetre in diameter and 2.5cm long. It seemed very susceptible to mechanical noise pick-up from the lead or microphone body (not terribly surprising given its directional polar pattern). Good results can be achieved, but only with careful attention paid to the mounting of the microphone and the dressing of its lead.

HOLDING HANDS

The hand-held ATW-22 radio microphone is a very serious looking piece of kit, measuring nearly 6cm across at its widest point, and 24cm in length, finished in a satin black paint. The complete transmitter electronics and aerial have been built into the body of the microphone, and a standard AT hypercardioid capsule completes the package behind a substantial wire mesh pop shield.

If the main body of the microphone is unscrewed, the battery compartment is revealed together with a small plastic tweaking tool (like that enclosed in the pocket transmitter). The ATW-22 also uses a single 9V PP3 alkaline battery for its powering with a similar operating life. The microphone's sensitivity is adjusted via a tiny screwdriver pot just below the battery with the supplied tool.

The bottom of the microphone has a three position on/standby/off switch and associated red LED to show the condition of the battery. The unit's transmission frequency is also indicated on a small label here. A mounting bracket for a microphone stand is included as a standard accessory.

I found the microphone to have a very tight pick-up pattern which, when combined with a sensibly low sensitivity, should ensure an absolute minimum of pick-up from foldback wedges or other instruments. The microphone has a decent frequency response for the average singing voice, with plenty of presence and good rejection of popping. The only potential problem I found was a surprisingly high susceptibility to handling noise on the microphone's case, which may be the cause of rather poor results in the hands of some performers.

LIBERATING

The most important piece of advice I could give anyone using a radio microphone system is to always power up the receiver and listen to its output before turning on the transmitter. In this way, you will be made aware of anyone else in the vicinity operating on the same frequency, or of any other sources of interference. This is especially important when using devices such as the Liberator, which are tuned to the de-regulated VHF channels, simply because of the increased likelihood of finding someone else already using a system on the same channel in the same area! These channels operate on a 'first come, first served' basis. Should you discover that your particular channel is already in use, I'm afraid there is nothing you can do, apart from locating the other user and trying to negotiate a time when they will switch their system off so that you may use yours. Powering up your transmitter when there is already another user on your channel is completely pointless, because all that will happen is both you and the existing user will suffer unacceptable interference.

I used the Liberator system components in a variety of locations and, with one exception, always achieved good results, with very usable working distances, complete freedom from multi-path problems and good signal-to-noise performance. The only time I experienced any problems at all was when trying to connect the Liberator receiver to a television studio's sound desk when I had very bad buzzing on the audio output.

Apart from my earlier comments regarding handling noise from both types of microphone assemblies, I found the overall quality of the Liberator system to be very good and completely reliable. Bearing in mind the variety of microphone options and guitar interfaces available, this is a very flexible system indeed, and one that should be high on your short list if you are interested in wireless systems.

See price box.

Audio-Technica Limited, (Contact Details)

Hugh Robjohns is a lecturer at the Centre for Broadcast Skills Training at BBC Wood Norton. The views expressed in this article are the author's own and are not necessarily those of his employer.