The selective system utilises Frequency Shift Keying (FSK) as the
modulation technique. The design of the encoder/decoder allows
individual channels to be set for operation on either the Australian
Standard Selcall format(Codan compatible, based on CCIR 493-4) or the
true CCIR493-4 format and its associated tones. Tones used are as
follows:

Australian Standard Selcall format (Codan compatible):
High tone:- 1870 Hz Low tone:- 1700 Hz Centre tone:- 1785 Hz