CTCSS Frequencies

CTCSS (Continuous Tone Coded Squelch System) enables two-way radio equipment to selectively communicate with other radios in a group. Units with the same CTCSS code will be able to communicate. Units with a different CTCSS code will not hear each other.

CTCSS, also known as 'PL' or 'Private Line' in Motorola gear, 'Channel Guard' in General Electric gear, and 'Call Guard' in E.F. Johnson gear, was developed to allow more groups of users on the same frequency. For example, 'A-1 Cab Co.', and 'Ace Cab Co.', could use the same frequency without hearing the other companies users. Each group would seem to have exclusive use of the frequency (until someone gets long-winded).

CTCSS is a low frequency tone that is combined with the voice signal from the radio transmitter. The receiving radio decodes the tone and if it matches, opens the squelch circuit allowing the voice signal to be heard. The tone is never heard because a high-pass audio filter mutes the voice frequencies below about 300 Hz.

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