CTCSS (Continuous Tone Coded Squelch System)
Is a generic trade term used by the mobile radio industry, and is used to accept desired
signals and reject undesired signals on a radio channel. It is also known as PL (Private Line)
to Motorola users and Channel Guard to GE/Ericsson users.
The system involves the use of a sub-audible tone (lower than 300 Hz in frequency) injected
into the voice modulation line of the transmitter and a matching tone decoder after the FM
discriminator (detector) in the receiver. When activated, the presence of a CTCSS tone on a
carrier, opens the audio squelch (Mute) circuit of the receiver, allowing the signal modulation
to pass through to the audio amplifier. If the correct sub-audio tone is not present then the
audio squelch does not open and nothing is heard even though the radio has received the
signal.
The sub-audible tone modulates the transmitter at about +/-1.5kHz deviation whereas the
voice level is about +/-3.5kHz deviation. This results in transmission of normal voice signals
that carry with them, a very low level of continuous audio tone at a specific frequency. The
tone is selected from a frequency range of 67.0 Hz to 254.1 Hz and is designed to fall below
the normal audio pass band of 300Hz-3kHz in a radio, so that it is not heard. When received
by another radio, this sub-audio tone is detected and used to open the audio squelch gate
(mute). This only occurs if both radios have been set to use the same sub-audio tone.
If the radio you are receiving a transmission from generates a sub-audible tone when it
transmits, then you can use CTCSS and block all signals except those coming from this
station.
Useful in keeping interference from paging transmitters etc, out of your receiver or distant
stations during times of good propagation.
CTCSS will not block unwanted signals arriving at your receiver at the same time as a desired
signal. It will only keep the mute closed unless the correct sub-audible tone is heard.
Sometimes, what sounds like audio hum, may be heard from a radio, where the transmitter
deviation of the CTCSS is set too high.
Once equipped with CTCSS, your radio will still receive all transmissions whether they
carry the correct tone or not. However, the only transmissions that you will actually
hear are those carrying the correct tone. At sufficient signal strength, any transmission
not carrying the correct tone won’t open the mute on your radio but will block the
receiver from hearing weaker signals. These weaker signals may in fact be the ones
you are listening out for and you will be prevented from hearing them by a stronger
signal that doesn’t open your mute because it has either no tone or the wrong tone
present. Unless you are looking at the radio display at the time, you won’t be aware
that this is happening. CTCSS needs using with care.
Tone Squelch (CTCSS on transmit only) is useful to access a repeater equipped with Tone
squelch access. The repeater hears the sub-audible tone and opens up the receiver squelch
to pass your signal and ignores all other signals. Under these circumstances, there is no need
to have CTCSS enabled on your receivers and by so doing, you will hear all transmissions
made on that channel but only those that transmit the correct tone will go through the
repeater.
All MRT’s have their own nominal CTCSS tone allocated and it would possibly be a good idea
to also have a general CTCSS tone that all teams can use so allowing multiple teams to
access the same repeater for example.
CTCSS is not permitted on the UKSAR team working channels due to other users of these
channels not necessarily having the ability to employ this system. The Coastguard and other
marine users have never used CTCSS and if it were to be employed by MRT’s then
communications with some organisations would be restricted. However, when using
repeaters, OfCom will normally insist on the use of CTCSS to prevent accidental use of the
repeater by other users of the channel. This method of signal encoding is permitted on the
repeater and re-broadcast channels used by MRT’s in the UK.
A more recent variation on CTCSS called DCS (Digital Coding System) uses the same subaudio frequencies and is permitted on our channels. Note that the two systems are not
mutually compatible even though the tone frequencies used are the same for each. Where
use of CTCSS is permitted, DCS can be used instead.
Team
Patterdale MRT
Team working
channel
24b
161.800 MHz
CTCSS
Tone
67 Hz
Keswick MRT
62b
160.725 MHz
77 Hz
Langdale Ambleside MRT
63a
156.175 MHz
88.5 Hz
Kendal MSRT
63b
160.775 MHz
103.5 Hz
Coniston MRT
64a
156.225 MHz
110.9 Hz
Penrith MRT
64b
160.825 MHz
118.80Hz
Wasdale MRT
85b
161.875 MHz
127.3 Hz
Cockermouth MRT
63b
160.775 MHz
136.5 Hz
Duddon & Furness MRT
64b
160.825 MHz
146.2 Hz
Kirby Stephen MRT
85b
161.875 MHz
156.4 Hz
.
The above CTCSS tones have been allocated specifically to LDSAMRA MRT’s by the MRC.
No provision appears to have yet been made for teams not in the LDSAMRA region who may
also be using re-broadcast. The only advice the Comms Sub Committee can offer on this is
that should you decide to use a CTCSS tone in order to reduce interference or restrict access
to your re-broadcast device, then you should use the tone suggested above.