CTCSS – Continuous Tone Coded Squelch System
CTCSS (Continuous Tone Coded Squelch System) is a concept, which is becoming more common in amateur radio practice. In professional practice, the abbreviation PL is also used (Private Line, Motorola) or Chanel Guard (GE/Ericsson). It serves to distinguish between desirable and undesirable signals on a frequency. The principle is as follows: in the transmitter, a selected low-level subtone is added to the modulation signal. There is a circuit behind the detector in the receiver, which analyzes the very presence of this subtone, if he finds it, the signal goes through the squelch to the NF amplifier, where it is amplified to the level required by the speaker. The subtone is not audible, because its level on the transmitting side is very low and, in addition, there is a high-pass filter in front of the NF amplifier, which effectively suppresses these undertones.
The modulation pitch of the subtones in a narrowband FM transmitter is set to approx +/-0,4 to 0,8 kHz, while the modulating pitch of the voice is approx 5 kHz. Subtone frequencies are from 67,0 do 254,1 Hz and their accuracy must be +/-0,5 Hz.
The importance of CTCSS in practical operation is manifold – from simplex connections to crossband operation via satellite transponder.
In simplex operation, a group of users sets the same subtone and does not have to listen to the broadcasts of other radio amateurs on the same frequency (if they don't set the same subtone). However, when broadcasting, rules about mutual interference apply, as with any FM operation.
In converter technology, CTCSS can be used in two ways. If unwanted interference increases at the transmitter location, which is randomly activated by the converter, the operator is forced to set a higher level during normal squelch, thus preventing unwanted activations of the converter, but also users with a weaker signal must increase the ERP (effective radiated power), so that they can continue to work through the converter.
A combination of classic squelch with CTCSS is more advantageous. Signals without CTCSS will be evaluated by the classic squelch according to the signal strength, while the sensitivity can be set to, for example, 3uV, while signals with CTCSS subtone decoder at full sensitivity of the converter – for example 0.4uV.
The second option is, so that the converter itself transmits a signal with CTCSS. It is advantageous in areas, where the user has two or more converters on the same frequency. This way, he can choose the one at the reception, in which he is interested.
The CTCSS system cannot block interfering signals fed to the receiver along with the desired signal containing the CTCSS subtone. Thus, even a seemingly unreadable mixture of signals can pass through the CTCSS system – but only if there is a subtone in it, which the circuits can evaluate.
Satellite transponders preferably use CTCSS activation, which achieves a non-negligible energy saving. The transponder transmitter is only switched on in case, that a useful signal is applied to the input. This allows the full sensitivity of the transponder to be used and at the same time minimizes the possibility, to have his transmitter activated by jamming signals.
Almost all FM devices today are equipped with a CTCSS encoder. The decoder can be purchased as an add-on, but devices fully equipped with CTCSS are also appearing. Some can also search for subtones in the CTCSS signal and display their presence.
More information can be found at:
www.directcon.net/pacres/ctcss.htm
www.bloomfield-family.org/HamClasses/CTCSS.pdf
www.eecis.udel.edu/~dra/pl.html
www.panix.com/~clay/scanning/index.cgi?pl-tones
florida-repeaters.org/ctcss.htm
