Wireless microphones in the frequency jungle
Nov 1, 2006 12:00 PM, BY SVEN BOETCHER
At first glance, it seems like David is fighting Goliath. Small wireless microphone transmitters with just a few milliwatts of output power are struggling for their place in the frequency spectrum occupied by TV transmitter giants with thousands of watts. As we all know, David did not beat Goliath with muscles and power, but with skill. Following this analogy, wireless microphone systems must use smart technologies and clever systems to survive in an environment becoming more and more hostile for microphones.
Whether in music or theater, performers no longer expect to trail a microphone cable. The wireless microphone gives talent complete freedom of movement. Unfortunately, demands on the wireless spectrum are increasing. In-ear monitoring for artists has become in vogue. It removes the need for foldback wedges at the front of the stage and allows individual control over level, but the systems are wireless. The UHF band is becoming crowded with DTV and with pending auctions of existing analog TV slots.
The DTT squeeze
Digital TV signals transmit in the same frequency band as their analog predecessors. Wireless audio systems are allowed to operate as secondary users in vacant analog TV channels. In Europe and Africa, analog PAL television occupies only 7MHz of the 8MHz-wide UHF channel. (See Figure 1.) Until now, the remaining 1MHz gap has been used for production communication, reporters' transmitters and, to some extent, wireless microphones.
Following the introduction of DTT transmissions alongside existing analog channels, the spectrum available for wireless microphones is severely limited in many countries. (See Figure 4 on page 76.) In practice, the frequency spectrum in major cities is crowded. Figure 5 on page 76 shows a spectrum sweep taken during planning for the 2004 Olympics in Athens.
In rural areas, there may be significantly fewer transmitters on-air, so the field strength average is less. However, TV productions, live concerts and musicals are usually staged in metropolitan areas.
As many governments plan to auction off the analog TV channels after digital switchover, the spectrum available for wireless microphones could be even less as demands for wireless communication increase. A musical production in a theater may require 60 wireless microphone channels. Broadcast productions and concerts often require a similar number of channels.
Frequency efficiency
In such a crowded spectrum, no kilohertz can be wasted. And it becomes more necessary to pack transmitting frequencies as close as possible in the spectrum while not reducing the transmission safety of a system. This is not an easy task, taking into account that in a wireless microphone system there are many more unwanted signals than wanted signals.
Figure 6 shows 16 wireless microphone channels between approximately 1600 unwanted intermodulation products. Different RF signals mix on the nonlinear curve of any amplifier to make new unwanted signals — intermodulation. The different carrier frequencies produce not only harmonics (integral multiples) but also a large number of (odd number) sums and differences of integral multiples of the input frequency components. These limit the number of usable frequencies within a certain band.
The better and cleaner the transmitted signal from the wireless microphone is, the higher the density of transmitting frequencies in a wireless system can be chosen. Figure 7 shows the spectrum of a Sennheiser SK 5212 bodypack transmitter compared to a transmitter with lower spectrum efficiency.
Operating a multichannel wireless microphone system in a crowded spectrum requires the right equipment, a high level of RF experience and detailed planning. A recent example was the 2006 Eurovision Song Contest, which used 54 wireless microphone channels and 16 wireless monitor systems.
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