AKG Acoustics Understanding Multichannel Wireless Performance with WMS 4000 Technology

AKG WMS MULTICHANNEL TECHNOLOGY

HOW MULTICHANNEL TECHNOLOGY WORKS

FREQUENCY MANAGEMENT, INTERFERENCE, AND PRACTICAL REMEDIES

Interference due to intermodulation can occur as soon as a radio-frequency circuit consisting of semiconductors or ferrites – like that of a WMS receiver – handles several RF signals at different frequencies. The number of disturbing frequencies (intermodulation products) increa- ses exponentially wherever several radio links (frequencies) are used simultaneously. This laws of physics have the biggest impact when several radio microphones are used at the same time. The innumerable new frequencies genera- ted by the combination, addition and subtrac- tion of the desired frequencies cause additional interference. Expert management of the fre- quencies of all radio sources designed to be used simultaneously is therefore absolutely essential for the problem-free operation of a multichannel wireless system.

Incorrect positioning of the antennas and recei- ver is just as frequently a source of undesirable interference. It is essential to ensure a mini- mum distance of 5 feet (1.5 m) from large metal objects such as lighting gantries and stage decorations (especially wire mesh). You should also avoid placing antennas in wall niches to prevent shadowing. Radio signals reflected or shadowed by walls, ceilings or metal structures also weaken the useful signal, thus resulting in improper functioning of the

radio equipment (see WMS 40 illustration on page 6/7). The interference from electrical appliances that cause electrosmog (such as computers and lighting equipment) can be par- ticularly disturbing during deep fades. During a deep fade that changes only slowly, a tone code squelch prevents unwanted noise from lasting too long.

On the other hand, a conventional muting cir- cuit is unable to differentiate between “friend” (the right frequency) and “foe” (unwanted sig- nals). If the level of interference is too high, it may interrupt the audio path during noisy deep fades. Most receivers use both types of circuit: a fast muting circuit to eliminate short bursts of noise, and a tone code squelch to reject persis- tent noise. Since both types of circuit act like a hard gate on the audio signal, there will always be some residual switching noise.

To ensure problem-free operation, always ob- serve the following basic rules when setting up a multichannel wireless system as opposed to a single channel application. Always position the receiving antennas within the far-near differ- ence range (see page 45). Nevertheless, make sure there is always an unobstructed line of sight between the transmitter and receiver during the performance. Also, the better the

audio signal fed to the transmitter and the high- er the signal/noise ratio of the transmitter and receiver, the better your wireless system will work. Basically you should always set the trans- mitter audio input gain first. The signal-to-noise ratio is the ratio between the amplitude of the wanted signal and the noise amplitude; it is a logarithmic expression for the purity of a signal. With radio transmission, the signal/noise ratio depends on the amplitude of the audio signal. The stronger the audio signal, the better the sig- nal-to-noise ratio. This is why it is always a good idea to make sure not to set the audio input gain of the transmitter too low.

In order to enhance the signal/noise ratio, the audio signal passes through a pre-emphasis cir- cuit in the transmitter and a corresponding de- emphasis circuit in the receiver. The amplitude of the signal is not evenly distributed over the frequency spectrum. Higher frequencies have a lower amplitude than lower frequencies, resul- ting in a lower signal-to-noise ratio for higher frequencies than for lower ones. FM demodula- tion generates more high-frequency noise. The pre-emphasis circuit boosts higher frequencies ahead of the radio link, whilst the de-emphasis circuit in the receiver attenuates them by a cor- responding amount.

Automatic frequency setup

During concerts and other performances, a number of applian- ces emit electromagnetic waves that may disrupt your transmis- sion channels (outside interference). Thanks to the “Environment Scan” function, AKG wireless systems are able to localize such “jammers”, switching to suitable interference-free frequencies instead.

In other words, the system automatically searches for gaps in the fre-quency spectrum – that is, frequencies where no interfe- rence is de-tected – occupying them with its own carrier fre- quencies.

Important: Run Environment Scan during the soundcheck and note the results. Do another test shortly before the performance, as there will almost certainly be new sources of interference, such as television and radio transmitters, or mobile phones in the audience. This gives you time to correct any problems that may have arisen.

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