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White Space Update for Wireless Microphones

by Tim Heneveld on May 21, 2009

Several months ago (10-13-08) I wrote about the current state of the wireless microphone frequency spectrum and how the actions of congress and the FCC are shaping the future of wireless technology. Well, several notable updates and changes have been made since that time. You may recall that February 17, 2009, was the date set for the transition to DTV (Digital TV). Congress (in all its wisdom) decided that it would extend the deadline to June 12, 2009.

Back at the end of 2008, no one was really sure what restrictions were going to be placed on wireless microphone users. As it turns out, the latest documents from the FCC indicate that the entire RF spectrum from 698-806 MHz (commonly called the 700 MHz spectrum) will most likely be “off limits” for wireless microphone users starting June 12, 2009 (not official yet, but that’s the current best guess). And like it or not, wireless microphone users are considered “secondary users” by the FCC, while television and cell phones users hold rank as “primary users.” For now, at least, wireless microphone users will be allowed to operate in the UHF television band between TV channels 14-51 (470-698 MHz), and in the VHF television band between TV channels 7-13 (174-216 MHz). There are no current proposals before the FCC that would limit the use of this space by wireless microphone users, but this “wireless real estate” is getting pretty crowded. Finding usable channels in this spectrum as everybody moves into this area will likely only get more difficult and result in more interference.

White Space Devices (WSD) were allowed to go on sale after February 17, 2009, but realistically we don’t expect to see any commercially available devices before the end of 2009. All white space devices will be required to avoid transmitting on any active TV channel or channels used by wireless microphones. Before transmitting anything, the WSD will have to determine its precise location and consult a database of channels that are safe to use. This database is not yet set up and there are no current guidelines for how a wireless mic user would register into the database. The FCC is currently accepting proposals for administration of this database, so if you’ve got some extra time I’m sure they would be happy to hear how you would run it! In theory, if the device is within one kilometer of a location where wireless microphones are known to be in use, the device must move to a different channel or cease transmitting. WSDs will have to check the database at least once per day, whenever they are powered on, and whenever their location changes. WSDs will also have to utilize the yet unproven ability to sense other devices that are transmitting in the spectrum so they don’t step all over somebody else. Given the general past history of things like government managed databases and wireless devices (like cell phones) working reliably, I’m not exactly betting that all of this is going to work real smoothly.

A better bet is that the promised database will either 1) never come online or 2) it will be useless when it does. Here are several reasons why such a database could be problematic:

  • Who is going to enforce the system when abuses occur (say, for example, if someone registers more frequencies than they’re actually using)?
  • What will happen when a cell phone user drives by a church or a school using wireless microphones? If the frequencies are in conflict, the phone would need to instantly switch to a new frequency, or there could be serious interference.
  • The FCC currently has a system for registering wireless microphones, but many (if not most) end users do not go out of their way to register their devices. If an end user buys a wireless mic, and it works as expected, the user is happy. If the microphone doesn’t work as expected, the user is likely to return the product.
  • What if the wireless microphone is registered but being used in a different location (say, an offsite event)? How far can you go from the original point of registration before you have to re-register it?
  • Even if the FCC could administer a high tech database, such as would be needed for this application, there are so many variables and things that could go wrong, that it would likely be useless.

Clearly the FCC has its work cut out for itself, and there are no quick fixes to solving this complex white space predicament (short of banning wireless microphones from said frequencies). While things may look like they will work in the short term, there are some potentially sticky long-term issues here.  As an independent audio-video consulting firm, it’s our job to do what’s best for the end user, and for now, “what’s best” is still up for debate. We are interested in your thoughts on this topic. Leave a comment below, and let us know what you think.

Tim Heneveld

Tim Heneveld

Tim Heneveld, CTS-D, has worked in almost every aspect of the Audio-Video and lighting industry. From System Design to Project Management to Studio and Live Engineering. Tim has overseen audio, video, and lighting systems as well as operations, and provided recording studio and live sound engineering around the world for recording artists of all genres. Tim is a Certified Crestron and Extron Programmer, and is the Senior AVL Design Consultant for Acoustics By Design, in Grand Rapids, Michigan.

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