Broadcasters are innately familiar with the need to share spectrum. For many years, TV stations in the same market have been coordinating the wireless microphone frequencies used by their ENG crews to prevent interference when covering a breaking news event. But soon, a new class of commercial and consumer devices will begin sharing the same “white spaces” used by those wireless microphones. To continue delivering the kind of audio quality that viewers expect, broadcast engineers who use wireless microphones will need to take advantage of some new tools and techniques.
Meet the new neighbors
The new TV-band devices (TVBDs) come in two flavors. Fixed devices are typically mounted on a pole or other structure, and provide broadband access to other devices nearby. They can transmit with up to 1W of power and use an antenna with up to 6dB of gain, for an effective output power of 4W. Permitted power levels decrease as the antenna height increases.
Personal/portable devices could be used inside a home or carried like a smartphone. Their output power is limited to 100mW, and it is reduced to 40mW on channels that are adjacent to one occupied by a TV station.
These new TVBDs have only been deployed on a limited basis so far, with the first installations limited to one county near Wilmington, NC, and one county near Roanoke, VA. It is expected that wider deployment will begin later this year. Only two fixed devices have been certified by the FCC as of this writing, but approvals for additional products are in the pipeline.
Unlicensed TVBDs are only allowed to operate on TV channels that aren’t occupied by broadcast stations or other authorized users (like public safety agencies) in a particular location. The device is told which channels it can use by an FCC-authorized TV Bands Device Database. The device contacts a database (through a cellular connection, for example), tells the database its precise location, and the database sends back a list of available channels. Alternatively, an installer can set the initial operating channel manually.
Once operating, if the device is unable to contact a database for more than 24 hours, or if no channels are available, the device cannot transmit. If a portable device moves more than 330ft, it has to contact the database again to see if the original list of channels is still valid.
Clearly, these new TVBDs could potentially interfere with wireless microphones used by broadcasters in the field. Depending on your location, TVBDs could go on-air in your town next month or next year. Casual users of wireless mics may be content to take their chances with interference, but broadcasters and production companies can’t tolerate unpredictable audio dropouts.