DTV INTERFERENCE
by Edwin J. Somers, CAS

The startup of digital television in the United States is generating lots of confusion and misinformation. I will attempt to simplify the issues as related to wireless microphone interference problems.

The FCC has issued licenses and frequency assignments to all the television stations in the United States that have agreed to start transmitting by the year 2002. In addition, the FCC has required the broadcasters to have 38 stations active in the ten major markets. What this means to those of us in Los Angeles is that there are ten digital television channels, most of which are actively simulcasting at this time. A contact at ABC reports that they are simulcasting with a frame doubler with all their programming except for one HDTV program on Sunday evenings.

There are a total of 22 DTV channel assignments in the LA basin. If you want to look at the list of assignments for the entire United States, you may view or download the document from the Internet. The URL is: http://www.fcc.gov/Bureaus/Mass_Media/Orders/1998/fcc98315.pdf

This photograph is a view of NBC's DTV signal on our spectrum monitor. It is channel 36, which covers from 602-608 MHz. The vertical lines are one MHz wide and the horizontal lines are 10 dBm in amplitude. The center frequency is 605 MHz. What you can also see here is a signal that pretty much has a constant amplitude across the entire 6 MHz UHF TV channel.

We have found that it is impossible to use a wireless microphone on any active digital television channel. As you can imagine, we are getting very good at changing frequencies on the various models of wireless microphones.

So what does this all mean to you? What are the options if you go to use your trusty wireless one day, and it has a usable range of about 10 feet?

First thing to do is verify that your receiver is being de-sensed by a DTV signal. You can check your list or you can contact the Sales or Service Department at LSC. If you are on a DTV frequency, you have a few options: 1) have the unit re-crystalled in our Service Department, 2) put the unit up for consignment sale to be sold to out-of-LA users, or 3) keep it for your own use in areas other than Los Angeles.

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