Wireless microphones, both high-band VHF (174-216MHz, TV channels 7-13) and UHF (470-806MHz, TV channels 14-69), have always had to operated on frequencies (channels) not used by nearby TV stations in order to avoid interference from TV transmissions. Channels not used by your local TV station are referred to as “white spaces” because they appear blank on charts of TV channels (channels 2-51, excluding 37) used in various geographical areas. The Federal Communications Commission (FCC) has now opened up the wireless microphone white space channels to other types of wireless transmitters for broadband access use, e.g., telephones and laptop computers. These additional wireless transmitters (referred to by the FCC as TVBDs—short for Television Band Devices) are prohibited from interfering with licensed (under Part 74) wireless microphone use in the “white spaces” core TV bands.
Various measures have been taken to ensure that TVBDs will not interfere with wireless microphone use in the core TV bands:
- TVBDs must incorporate “spectrum sensing” technology that checks for channels being used by wireless microphones, licensed or unlicensed. TVBDs must avoid transmitting on those channels.
- Spectrum Sensing Technology is still being developed. The technology is expected to function as follows: The TVBD will check 30 seconds prior to transmitting to make sure the channel is free from wireless microphone use. After starting to transmit, it must check at least once every minute and cease transmitting on that channel within 2 seconds of detecting a wireless microphone transmission. How (and if) this currently mandated technology is actually workable on a practical basis is not yet fully understood. If not, the viability of TVBDs may be re-evaluated and requirements may change. The timetable for this process is still unclear.
- Certain channels will be prohibited to TVBDs and are therefore safe for all wireless microphone use.
Two “safe” channels are in each of the 13 major metropolitan TV markets (Boston, Chicago, Cleveland, Dallas/Ft. Worth, Detroit, Houston, Los Angeles, Miami, New York/NE NJ, Philadelphia, Pittsburgh, San Francisco/Oakland, and Washington D.C./MD/VA) where certain other wireless services operate on channels 14 through 20 (470-512MHz). The “safe” channels are the first channels not used by TV broadcasting in the particular market on each side of channel 37 (608-614MHz), e.g., channel 36 (602-608 MHz) and channel 38 (614-620MHz). For example, if channel 36 is used for TV but channels 38 and 35 are not, the “safe” channels would be channels 35 (596-602MHz) and 38.
Wireless microphone users may register for free with an internet database (to be established), noting the geographic location, dates, times (minimum one hour) and channel of the intended use. Registration must take place at least 24 hours before use and may be for temporary or permanent use. TVBDs must check the database daily and avoid transmitting within one kilometer of the designated location and times on the registered “safe” channel.
There are two types of TVBDs: Fixed devices that are required to use an outdoor antenna, and personal portable devices. Fixed devices operate at higher power than personal portable devices. Because personal portable TVBDs are prohibited from using channels 2 through 20, and fixed TVBDs are prohibited from using channels 14-20 in geographic areas where other specified wireless services are used and also prohibited from using channels adjacent to those used by your local TV stations (including stations using channels within the range 2 through 20), wireless microphones should have several “safe” channels in the 2 through 20 range that are not used by TV or other wireless service operations.
TVBDs are prohibited from using channels 52 through 69 (698-806MHz)—channels conditionally available for unlicensed wireless microphone use until June 12, 2010. None of these channels are being used for TV. However, these channels will be used by public safety and commercial licensees.
Channels 2 through 51 (54-698MHz), excluding channel 37 (608-614MHz), also are conditionally available for unlicensed wireless microphone use up to 50 milliwatts (all Nady devices). Again, however, TVBDs are prohibited from using only some of those channels.
The FCC is currently deciding whether to expand the eligibility for license to use wireless microphones free of interference from TVBDs. Licenses are presently limited to TV and radio broadcast, cable and motion picture entities.
TVBDs must pass a lengthy FCC certification process before they may be marketed, and are not expected to begin entering the market before mid-2010. There are still considerable technical hurdles to overcome before these devices are approved for use within the designated guidelines.
Wireless microphones offering user-select frequencies over a broad band (e.g. 20-36MHz or more), compared to fixed frequency units, will have the most flexibility of use. Flexibility will be limited more in some areas than others based on the tunabilty of frequency synthesized PLL systems to open bands. However it is anticipated that fixed frequency VHF systems (170-172MHz and 174-216MHz) and UHF systems in the band 470-512MHz and 596-622MHz (with geographic restrictions noted above) will also be viable for quite some time. Fixed frequency units are also usable in the bands 902-928MHz and 944-952MHz. Although both of these bands are primarily intended for other uses, wireless mics are permitted at these frequencies since they are low-power transmitters. As a practical matter, wireless mics rarely, if ever, encounter interference from other devices sharing these bands.
For more information about recent FCC changes in rules governing wireless microphone use, please visit the FCC website.