Most FRS and GMRS radio’s have a built in CTCSS (Continuous Tone-Coded Squelch System) which when employed, forces a radio in receive mode to squelch the audio output when the selected tone is not present on a received signal. This allows an FRS/GMRS radio user to set up any number of radio’s on the same channel with the same privacy tone such that the radio’s will only unsquelch if and only if the received signal has the tone present.
Privacy tones actually have nothing to do with privacy. Using a privacy tone does not prevent others from eavesdropping on your conversation. Rather than calling them privacy tones as the makers of FRS/GMRS radio equipment do, they can more appropriately be called “silence” tones. This is because all that these tones do is silence your radio when the person transmitting on your channel is not using the same (or any) privacy tone.
The reason that you cannot hear a privacy tone when it is being used on a channel you are receiving is because your FRS/GMRS radio uses a high-pass audio filter to remove all of the audio below a certain frequency. This effectively blocks the tones from being output on the radio’s speaker.
So how does it work? In the following example table, all of the people are using FRS radio’s on channel 2.
|Name||privacy code||Who this person can hear|
There are a lot of FRS/GMRS radio manufacturers including Motorola, Uniden, Cobra, Midland, and many others. Unfortunately, these manufacturers have not gotten together and defined a standard mapping of their privacy numbers to the tone frequencies. Motorola has consistently provided a list of 38 PL tones and some manufacturers implement a subset of this list.
|Motorola (and others)
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