Recent changes in FCC policy are affecting operation of wireless microphone and monitoring systems in the United States. It's important for production professionals to understand these changes and apply best practices to ensure reliable operation of their equipment.
Wireless mics primarily operate on frequencies in the UHF TV spectrum. They are considered licensed broadcast auxiliary devices a broadcaster or broadcast content provider may operate on locally vacant TV channels. For example, channel 25 (536MHz-542MHz) is not used for TV broadcast in Boston. Therefore production may operate approximately eight wireless mics tuned to different frequencies between 536MHz and 542MHz.
It's important to remember that two separate and distinct issues have been progressing in parallel: the digital dividend and the TVBD ruling, aka the white space debate.
This relates to the reallocation of TV channels 52-69 (698MHz to 806MHz), generically called the 700MHz band. Once analog TV terminates, all full-power TV broadcast will be consolidated below channel 52. The 700MHz range will partially be used for emergency communications in channels 63, 64, 68 and 69. The rights to use the majority of the remaining channels were auctioned to telecom companies such as AT&T, Verizon and Qualcomm to provide what is being termed as advanced wireless services (AWS). This raised billions of dollars for the federal government and was therefore called the digital dividend. Although the FCC has not made a final ruling (as of this publication date), it looks imminent that wireless mics will have to vacate this range. The pro audio industry has been lobbying for a grace period to allow operation of existing equipment beyond the DTV transition date.
The 700MHz band can be subdivided as:
Emergency communication channels: 63-64 (764MHz-776MHz) and 68-69 (794MHz-806MHz).
TVBD ruling (the white space debate)
Auctioned spectrum: 698MHz-758MHz and 776MHz-788MHz. The telecom companies have not completed building the infrastructure for their AWS. AWS will probably become active at the end of this year in major cities and will then eventually spread. Functional, 700MHz wireless mics will continue to work reliably for many months after the DTV transition date, maybe even years in some parts of the country.
Block D: 758MHz-763MHz and 788MHz-793MHz scheduled for future auction. Block D was envisioned to be used as a private/public partnership. It did not solicit a minimum bid in the FCC auction, probably because the winner would have had to share it with municipal agencies. Therefore, it is not allocated to any one entity yet. Functionally, these frequencies look clear for wireless mics for the foreseeable future, until the FCC successfully auctions Block D.
The bottom line is wireless mic users will lose access to the 700MHz band. The bright side is that as more analog stations terminate their broadcasts, this will open some TV channels below 698MHz. (See Figure 1 on page 14. )
In November 2008, the FCC released its rules allowing a new class of unlicensed consumer electronic products to operate in locally unused TV channels, just as wireless mics have done for years. These forthcoming products have previously been referred to as white space devices (WSD) but are now called TV band devices (TVBDs). They will mainly be used as broadband access devices.
TVBDs are categorized as:
These are allowed to operate with effective radiating power up to 4W on channels 2-51, with the exceptions of channels 3, 4 and 37.
Due to their mobile nature, these devices are the most concerning for production professionals. However, portable TVBDs are restricted to channels 21-51 and are also not allowed in channel 37. (Channel 37 is a reserved channel for radio astronomy and medical telemetry.) They are limited to 100mW operating power or 40mW if operating in a channel adjacent to an active station. This moderate power will reduce their range and therefore the possibility to cause interference.
Licensed operation of wireless mics takes precedence over TVBDs. TVBDs must coordinate around active licensed wireless mic systems. The rules include several safeguards to avoid interference to wireless microphones.
TVBDs must include the ability to scan the airwaves to sense wireless microphones (in addition to TV stations). Until they can demonstrate through proof of performance that they can reliably sense wireless mics and avoid causing interference, they must also use a geolocation/database system. TVBDs must use location sensing in conjunction with a database of registered broadcast license assignments. The database will also include a list of protected areas for wireless microphones such as entertainment venues and sporting events. TVBDs must first access the database to obtain a list of permitted channels in the area before operating. A TVBD that lacks this capability can operate only under the direct control of a capable TVBD.
Personal/portable devices will be barred from channels from 14-20 (470MHz-512MHz). In addition, in 13 major markets where certain channels between 14 and 20 are used for land mobile (municipal and public safety) operations, two channels between 21 and 51 will be reserved and available for wireless microphones. These will be the first open (non-TV) channels above and below channel 37.
This means, at minimum, 16 wireless mic or monitoring systems (eight in each TV channel) can be used simultaneously in any venue. When using equipment with high linearity (extreme suppression of harmonic distortion known as intermodulation) the number increases to at least 20 systems (10 in each TV channel). Protected areas will be able to operate many more channels.
Multistage and studio properties can also effectively increase the number of systems in use through:
- Physical distance and transmitter output power management
This can be augmented by a balance of other techniques such as shifted coordinated frequency sets (same frequency spacing but offset by 100kHz or more), zone isolation (natural or enhanced shielding between rooms), directional antennas and filtered distribution systems.
- Time multiplexing
This involves using systems in different rooms at different times.
There are a couple of techniques that can be used to ensure maximum protection from portable TVBDs. If a city has three consecutive vacant channels, operate wireless mics in the middle channel. (See Figure 2 on page 14.) This will force the TVBD to operate on a channel adjacent to an active TV broadcast, which means it will have to operate at its lower 40mW output power. (See Figure 3.)
A portable TVBD approaching a production area should sense wireless audio systems as soon as possible. The effective radiating power of mobile wireless mic transmitters is often diminished by shadowing and body absorption, especially with a body pack transmitter. Conversely, a monitoring system with a stationary transmitter using an antenna fixed in a high position provides a more stable signal. If it is operating at the maximum allowable power of 250mW, a portable TVBD should sense it from much farther away compared with lower power, mobile wireless mics. (See Figure 4.) This approach is a bit different from what was often recommended in the past. However, this technique allows a monitoring system to act as a beacon, adding a level of protection for wireless mics within the same channel.
The reduction in available spectrum plus forthcoming TVBDs creates new challenges for production professionals. However, through careful planning and adherence to best practices, even large, multichannel wireless audio systems will be able to operate reliably.
Joe Ciaudelli is a consultant for the professional products industry team at Sennheiser.