July 2002
Intermediate to advanced
344 pages
8h 37m
English
Content preview from Newnes Radio and RF Engineering Pocket Book, 3rd Edition
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197
16.3 Digital signalling
FFSK
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.
DCS
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, 2
9
= 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 ...
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