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Tuesday, March 22, 2011

Squelch Tones, Simplified

Did you know that besides just the voice you hear, many radio frequencies carry additional information that you can't even hear? The most common example of that is a squelch tone, more officially known as CTCSS, CDCSS, DPL or DCS. I'm not going to get into the technical details of how these work, but I'll give a real world example of how this affects you when you're scanning.

One of the more active Fire Buff radio networks in Eastern Massachusetts is Fire Radio Systems, which operates on 461.4000. If you have a scanner that doesn't support CTCSS codes and you program in that frequency, you may also hear one of the other area organizations that share that same frequency, such as Bass Pro Shops in Foxboro, the New England Science Center in Worcester, or the First Church of Christ, Scientist, in Boston.

None of these groups want to hear the others' radio traffic, so each group's radios are programmed with a different squelch code. In the case of FRS, the radios are programmed with a code of 136.5. The same code is sent along with each transmission, and any transmissions that don't have that code won't be heard.

Most newer scanners also support these codes. In addition to screening out unwanted traffic when programmed in correctly, if left unprogrammed, they will also display the code that is being used, which can help you identify unknown frequencies or users. If your scanner doesn't support this, it's probably time for an upgrade. I use the Uniden BC346XT and the Uniden BCD396XT. The GRE PSR-310 is a popular choice, as well.

Post any questions in the comments below and I'll try and answer them as best as I can.

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