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02.23.2006 12:00AM
Another British Set-Top Box Calls for Help
Last week's news out of the UK., of a Freeview set-top box transmitting a distress signal initially appeared to be a delayed report on the incident described here previously in the Jan.17 RF Report.

As it turned out, this was a new case of a terrestrial DTV set-top box sending a distress signal. In this episode, Mary Donaldson's Freeview DTV set-top box in her Plymouth, Devon home originated the distress calls. The device was transmitting distress signals that were picked up by satellite and the RAF Aeronautical Rescue Co-ordination Centre at Kinloss, Scotland. A BBC News article, TV box launches sea rescue search, said coast guard units at Falmouth were alerted and sent out two lifeboat rescue teams.

The BBC article quotes an Ofcom (the United Kingdom telecommunications regulator) spokesman: "Any box that is capable of receiving is capable of transmitting, and they somehow flipped from receiving to sending. These are two instances in the millions of boxes being used. It's a very small number, and we don't believe it is a widespread problem." A Freeview spokesman said the fault was with the manufacturer of the boxes and they would wait for the results of an Ofcom study before planning a recall of Freeview boxes.

None of the articles reviewed described the nature of the signal from the Freeview box, other than to state that it was transmitting on the international distress frequency of 121.5 MHz. Emergency locator transmitters (ELTs) amplitude modulate the 121.5 MHz signal with a swept tone between 300 Hz to 1000 Hz at a 2 to 3 Hz repetition rate. While some spurious signal emission from digital equipment isn't unusual, if the Freeview boxes modulated the signal in this manner, this would be a very unusual coincidence.


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