AKG Acoustics WMS 4000 manual HOW Multichannel Technology Works

AKG WMS MULTICHANNEL TECHNOLOGY

HOW MULTICHANNEL TECHNOLOGY WORKS

FREQUENCY MANAGEMENT, INTERFERENCE, AND PRACTICAL REMEDIES

A knowledge of the laws of physics governing the propagation of radio waves is essential if one is to gain maximum benefit from the ad- vantages of wireless transmission technology. Radio waves are electromagnetic waves that are used as a carrier for the transmission of signals.

In physical terms, electromagnetic waves transport energy through space. In a vacuum, they are propagated at the speed of light (in other media almost the speed of light, 300,000 km/s). The relationship between fre- quency and wavelength is defined by the follo- wing equation:

l = c/f

(wavelength = speed of light / frequency)

The speed of electromagnetic waves and their ability to pass through obstacles varies depen- ding on their wavelength. Due to the growing popularity of wireless communications such as wireless, radio/television broadcasting, mobile telephony, and wireless LAN networks, the density of the radio signals around us is rapid- ly increasing (see illustration below). Conse- quently, there is also a greater risk of distur- bances caused by RF interference and electro- smog.

Dropouts are the result of zero RF field strength at a receiving antenna. One reason may simp- ly be excessive separation between transmitter and receiver so the receiver will capture no sig-

nal. Another reason may be multipath recep- tion: as the signal is reflected several times, the resulting multiplied signals arrive at the receiving antenna from different directions and in different phases. When this occurs, they may cancel one another out, resulting in signal dropout. Another type of dropouts may be cau- sed by interference that will become audible as noise if the carrier signal is sufficiently weake- ned. In order to prevent this, most receivers are equipped with a muting circuit. Provided this circuit is fast enough, and the muting thres- hold is just above the electrosmog level, it can switch off the audio circuit whilst the signal level drops as described above. It is not possi- ble to completely suppress dropout, which is often accompanied by a crackling sound.

The best way to prevent interference is to eli- minate the source of unwanted signals such as computers. If this is not possible, the solution may be switching the microphone system to another radio frequency (see illustration above right).

Electrosmog is generated by electronic equip- ment such as lighting systems, computers, and other digital equipment. In practice it is advis- able to keep as far away as possible from lighting equipment, computers, fax machines, etc., and not to install other electronic equip- ment in a rack along with the wireless micro- phone unit. If interference occurs, the usual solution is to switch to another frequency.

Apart from intermodulation and digital noise,

the electro-smog inherent in a multichannel system is the sideband noise of the transmit- ters and receivers. Even the best oscillator is unable to generate a signal entirely without phase shift, consisting of a single, infinitely narrow spectral line. All oscillators have a noise skirt whose spectral density decreases with increasing frequency separation from the line. In the case of carrier signals on adjacent fre- quencies, the noise skirts and the carriers may overlap. For example, if a performer with a transmitter gets very close to the receiving antenna, it is possible for the sideband noise to open a muted channel. This can be avoided only with the aid of an additional tone-coded squelch circuit.

In practice it is advisable to keep as far away as possible from lighting equipment, and not to install other electronic equipment in a rack along with the wireless microphone unit. If interference occurs, the usual solution is to switch to another frequency.

Where several wireless systems are to be used simultaneously in the same place, interference may result from intermodulations due to the non-linear distortion of combined carrier fre- quencies (see illustration on the right).

Like most other wireless microphone systems, AKG WMS systems use frequency modulation. This involves changing (modulating) a carrier frequency in step with an audio signal (see also WMS 40, page 9).

The electromagnetic wave spectrum

The electromagnetic wave spectrum is becoming increasingly crowded by a wide variety of applications.

Wireless microphones generally share the same frequency range as televi- sion broadcasts. Allocated bands for UHF wireless microphones are within the 470 to 862 MHz range, in Europe 650 to 863 MHz and in the United States 470 to 806 MHz.

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