Brad Dick EDITORIAL DIRECTOR /
07.01.2007 12:00 PM
Could interference kill DTV?

I got to seriously thinking about interference recently when both my home and work computers began to emit strange sounds. At random times, on both systems, I'd hear this buzz…dit…dit…dit through the audio system.

I was starting to think the interference was unique to Kansas City, but when I heard the identical noise while in an office in San Jose, I knew that the cause wasn't limited to Kansas.

A few minutes on the Internet led me to the likely culprit, and a couple of quick tests confirmed the offending source. GSM-based cell phones are interfering with my computers. Every time my — or a nearby — GSM phone pings the nearest tower, saying, “Here I am,” that transmission is picked up by the computer's audio system. It's not just a computer issue because a wide variety of devices are susceptible to external interference — even your digital television.

In March, the FCC's Office of Engineering and Technology (OET) released a 200-plus page report (FCC/OET 07-TR-1003) concerning interference to DTV receivers. You can read the report yourself, but it basically states: “No receiver appeared to fully achieve the ATSC recommended guidelines for interference rejection performance — guidelines that are less stringent than the receiver performance assumptions on which current DTV interference protection criteria are based.”

Reading between the lines, it says that the odds of your OTA viewers getting the perfectly clear digital image you are transmitting is as likely as finding an honest politician.

A group of familiar consumer manufacturers — Dell, EarthLink, Google, HP, Intel, Microsoft and Philips — is proposing the use of TV white space for a variety of new services and devices. Collectively known as the White Spaces Coalition, this group is lobbying the FCC heavily for particular rules regarding personal/portable devices that would transmit on those frequencies.

Not surprisingly, the MSTV and NAB have expressed a strong opposing viewpoint. In fact, these two organizations claim a high likelihood of these new devices causing massive interference to OTA DTV signals.

In their May 15, 2007, reply comments, MSTV and NAB claim that the FCC's OET report “… shows that a 100mW transmitter operating on the first adjacent channel could cause interference to DTV viewers in 80 percent to 87 percent of a TV station's service area.”

The MSTV and NAB also state that a study by the Canadian Research Centre and University of Kansas suggests that interference from white space unlicensed devices could interfere with viewers' reception in more than 95 percent of a TV station's area.

The situation probably isn't that dire, but remember who the members of the coalition are. It's composed of the same guys who want you to always back up your computer (because you know it'll fail), click on start to turn it off, and replace it every two years. Believe them? I don't think so.

Oh, and the cell phone vendor's solution to my GSM phone interference? Separate the phone from the interfered with device by placing it in another room.

Hmmm … That'll work well in my office and car.

Send comments to: editor@broadcastengineering.com



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