Spectrum reallocation and the TVBD ruling
Jun 1, 2009 12:00 PM, By Joe Ciaudelli
What does it mean for wireless microphone users?
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.
New approach
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.
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