The Continuous Tone Coded Squelch System, commonly referred to as CTCSS, has been in used in the land mobile radio arena from the late 1960’s. It is known by a number of different trade names such as Private Line® (PL) by Motorola, Channel Guard® (CG) by General Electric and generically as tone squelch.
It is a use of sub-audible tones that are transmitted along with the speech portion of the transmission which allows more than one agency (or fleet) to use the same radio frequency without causing undue interference to another agency on the that frequency. Receivers for agency XYZ are set to only open their audio squelch when the proper sub-audible frequency tone is part of the transmission.
Today the sharing of frequencies by agencies is less common than it once was, CTCSS is more commonly used by repeater systems to prevent noise or interference from causing the repeater squawk obnoxiously, and by receivers as an extra measure of squelch (for instance, to prevent engine noise from breaking squelch).
The land mobile industry started with some 38 sub-audible frequencies this has increased over the years to the more generally accepted 50. There is no generic standard tone number assignment or code letter to go with a particular tone; however, below is a chart of the commonly accepted 50 tones used at this time.
In addition to the standard tones, some manufacturers have made available additional tone frequencies specific to their own products, but not available to products from other manufacturers.
Standard CTCSS Tones
- Note: 150.0 is the standard tone used by military radios in the 30-88 MHz band.
Non-Standard CTCSS Tones (Kenwood)
Sound Card Decoding Programs
These programs will display the a detected CTCSS tone when connected to the audio output of a radio receiver.