Continuous Tone-Coded Squelch System

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.

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.

67.0	94.8	131.8	171.3	203.5
69.3	97.4	136.5	173.8	206.5
71.9	100.0	141.3	177.3	210.7
74.4	103.5	146.2	179.9	218.1
77.0	107.2	151.4	183.5	225.7
79.7	110.9	156.7	186.2	229.1
82.5	114.8	159.8	189.9	233.6
85.4	118.8	162.2	192.8	241.8
88.5	123.0	165.5	196.6	250.3
91.5	127.3	167.9	199.5	254.1

--Lou Maag 14:21, 3 Jan 2005 (EST)