Many analogue modulated systems use digital signalling in the form of 1200 bit/s fast frequency shift keying which is faster than 5-tone but less robust.
Digitally coded squelch (DCS) is a digital, but slower, alternative to CTCSS. It is not in current use in the UK. DCS continuously repeats the constant binary bit pattern of a 23 bit word. Golay error coding reduces the number of data bits to 12 of which 3 initiate the Golay sequence leaving 9 information bits, i.e. a 9 bit word. With a 9 bit word, 29 = 512 codes are possible but, because of problems with the frequencies produced, the codes with 7 or more 1s or 0s in succession are discarded leaving 104 ‘clean codes’. The transmitted rate is 134 bit/s and the highest fundamental frequency is therefore 67 Hz. The lowest fundamental frequency is produced when 6 zeros follow 6 ones = 1/6th of the fundamental frequency = 11.7 Hz. The important harmonics are:
third = 201 Hz fifth = 335 Hz seventh = 469 Hz
The seventh harmonic is well within the audio pass band of most receivers but at 3 octaves from the fundamental is relatively weak.