The slowest signalling system uses continuous sub-audio frequency tones. Known as continuous tone controlled signalling (previously squelch) system (CTCSS) its performance, in the UK, is specified in Radiocommunications Agency specification MPT 1306. The most common use for the system is to control receiver mute opening. Permitting a mute to open only on receipt of an authorized signal, its use enables privacy between users to be maintained on shared systems, common base station systems for example, and reduces the annoyance factor from interference in the absence of a signal. Thirty-two tones are permitted and assigned by the Radiocommunications Agency (Table 16.1).
Table 16.1 CTCSS tones and modulation levels67.0 110.9 146.2
71.9 114.8 151.4
77.0 118.8 156.7
82.5 123.0 162.2
88.5 127.3 167.9
94.8 131.8 173.8
103.5 136.5 179.9
107.2 141.3 186.2
192.8 203.5 210.7 218.1 225.7 233.6 241.8 250.3
Table 16.2 Modulation levels
low modulation levels
System channel spacing
Amplitude modulation Modulation depth (%) Angle modulation
25 kHz 12.5
10 to 20 10 to 20 400 to 800 200 to 400