MMI Technical Resource
   Updated 5 March 2003 CTCSS Explained   

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  • General
    What are the differences between CTCSS, Private Line (PL), Channel Guard (CG), and tone squelch? They're all the same! CTCSS (Continuous Tone Coded Squelch System) and tone squelch are the generic terms for Motorola's "Private Line" (PL) or GE's "Channel Guard" (CG). Just as the name above implies, CTCSS is a continuous tone that is so low in frequency and amplitude (volume) that it is almost impossible to hear with the un-aided ear. This subaudible tone is sent over the air with a tone encoder to the receiving station which, in turn, has a tone decoder. The decoder doesn't let any sound through to the speaker until it hears the specific tone it was programmed to listen for. This allows users to be on one frequency without causing interference to more distant stations using different subaudible tones.

    Squelch Tail Elimination
    Some other common terms you might hear in conjunction with CTCSS is "reverse burst" or squelch tail eliminator. Reverse burst was started back in the old days where the tone was conceived and detected by PL reeds in the radios (i.e. Motorola Micor). To generate the PL, a resonant reed vibrates at a particular frequency causing an electronic sine wave. When the receiving radio hears this PL, its decoding reed only vibrates at a particular frequency, indicating resonance and giving a valid PL signal. When the transmit PL goes away, the reed will still vibrate for a few hundred milliseconds causing a fairly long squelch tail (white noise burst). In order to eliminate this squelch tail at the end of a transmission, the user unkeys the radio and it automatically sends a reverse burst by changing the phase of the PL signal 180 degrees for about 200 milliseconds before the transmitter drops out. This causes the receive PL reed to come to a quick stop and mute the receiver before the signal goes away. Now days, a lot of radios still use reverse burst for eliminating squelch tails. Other radios either do not have any kind of squelch tail eliminator or they just turn off their PL about 200 milliseconds before transmitter shut-down.

    The following graphic shows PTT, Transmitter, and Tone coordination.



    Here is a picture of a complete CTCSS Encoder/Decoder that is programmable for 64 different subaudible tones. The old Motorola PL reeds, alone, are about the size of this board.

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