Continuous Tone Coded Squelch System for repeater control. Nine tones have been allocated, each identified by a letter. The sub-audile tone frequencies are as follows
A = 67.0 Hz B = 71.9 Hz C = 77.0 Hz D = 82.5 Hz E = 88.5 Hz F = 94.8 Hz G = 103.5 Hz H = 110.9 Hz J = 118.8 Hz
The CTCSS system, when available, will be an alternative to the standard 1750 Hz tone-burst access, NOT a substitute for it. The current standatd will remain unchanged.
A station equipped with CTCSS facilities, will be able to access the repeater with EITHER a 1750 Hz tone-urst OR a continuous CTCSS tone.
When the repeater detects the correct CTCSS tone, the receivers squelch will open at the lowest level, thus providing full receiver sensitivity for the local user. This does not mean that those not equipped with CTCSS tone facilities will not be able to use the repeater, but it will mean that they will not have the benefit of full sensitivity of the repeaters receiver. This function, however, will not be detrimental to users inside the repeaters normal coerage area. The advantage of this system is that during "lift" conditions, the repeater will not respond to weak, interfering signals, that do not contain the correct CTCSS tone, from users of other repeaters on the same frequency.
In conjunction with the access system the repeater will constantly radiate the CTCSS tone while the repeater is in "relay mode." That means, that if you have a CTCSS decoder/detector fitted to your receiver, your receiver can be set to respond only to the signal bearing the correct tone, so you won't have to listen to the repeaters beacons.
When a repeater is fitted with the CTCSS system, it will identify itself with an additional letter added to it's callsign in Morse Code. The letter used will signify the tone frequency it transmits. In the case of GB3BC this is the letter F after it's callsign.
NOTE : GB3BC uses the "valid CTCSS tone detected" purely as an alternative to the 1750 Hz tone-burst for access, it does not affect the receiver sensitivity at the present time. It does give you an unlimited time-out. You will however, be cut of for approximately six seconds by the time-out "T's" signifying that you have reached the 4 minutes of "waffle time" given for normal 1750 Hz access users. If this facility is heard being regularly abused, the committee reserves the right to re-install full time-out operation.
]]>In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. It is sometimes called tone squelch. Where more than one user group is on the same channel (called co-channel users), CTCSS filters out other users if they are using a different CTCSS tone or no CTCSS.
Receivers equipped with a CTCSS circuit usually have a switch that selects normal mode or CTCSS mode. When enabled, the CTCSS radio circuit, instead of opening the receive audio for any signal, causes the two-way radio receiver's audio to open only in the presence of the normal RF signal AND the correct sub-audible audio tone (sub-audible meaning that the receiver circuitry can detect it, but is not apparent to the users in the audio output). This is akin to the use of a lock on a door. A carrier squelch or noise squelch receiver not configured with CTCSS has no lock on its door and will let any signal in. A receiver with CTCSS circuitry (and with it enabled) locks out all signals except ones encoded with the correct tone. CTCSS can be regarded as a form of in-band signaling.
Link to RSGB for others
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