The FCC has set aside unoccupied TV channels next to channel 37 (608-614 MHz, reserved for radio astronomy) for wireless microphone operation, making equipment that tunes across TV channels 34 to 40 (590-632 MHz) valuable going forward.
All TVBDs are prohibited from the first available channel on each side of channel 37, and two more channels are reserved in 13 major metropolitan areas.
Stereo
Wireless IEMs transmit in multiplexed (MPX) stereo FM, and understanding its limitations helps optimize their operation. The preference for stereo is that it’s easier to listen to, so it reduces listening levels and hearing fatigue, while contributing to hearing conservation.
Without going into a full description of FM multiplex circuitry – where two channels of audio are carried on a single broadcast frequency – it’s important to note that stereo FM transmissions are more susceptible to noise and distortion than mono, so its S/N ratio is inherently worse.
Musicians with personal mixers that don’t move around much, such as a backing band or a praise band, benefit from their transmitters being located nearby.
Just like audio, radio waves obey the inverse square law. If you half the distance to the transmitter, your signal power quadruples.
Further, for musicians that only want their mix in mono – and there are a few – their mix can sound cleaner in mono, especially on the far side of the stage.
Due to MPX stereo encoding, wireless stereo channel separation is only about half of what it would be with a hardwired mix, so signals panned “hard” to the 5 o’clock or 7 o’clock positions only sound like 9 and 3 o’clock.
Channel separation is further reduced below 200 Hz and above 8,000 Hz, so that guitars and voices pan better than bass, kick drum or overheads, which may be one reason why drummers, accustomed to the superior fidelity of a hardwired mix, have a strong preference for them over wireless.
Nevertheless, most other performers prefer the freedom of wireless over the fidelity of hardwired IEMs.
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Mark Frink is Associate Editor of Live Sound International.