CTCSS (Continuous Tone Coded Squelch System) is a term that is increasingly encountered in amateur radio practice. In professional practice, the abbreviation PL (Private Line, Motorola) or Channel Guard (GE/Ericsson) is also used. It serves to distinguish between desired and undesired signals on a frequency. The principle is as follows: in the transmitter, a chosen low-level sub-tone is added to the modulating signal. Behind the detector in the receiver, there is a circuit that analyzes the presence of this sub-tone; if it finds it, the signal passes through the squelch to NF zosilňovača, kde sa zosilní na úroveň potrebnú pre reproduktor. Subtón pritom nie je počuteľný, pretože jeho úroveň na vysielacej strane je veľmi nízka a navyše pred NF zosilňovačom je ešte zaradená horná priepusť, ktorá tieto subtóny účinne potláča.
Modulačný zdvih subtónov v úzkopásmovom FM transmitters is typically set to +/-0.4 to 0.8 kHz, while the modulation deviation of voice is about 5 kHz. The frequencies of the sub-tones range from 67.0 to 254.1 Hz, and their accuracy must be +/-0.5 Hz.
The significance of CTCSS in practical operation is manifold – from simplex connections to crossband operations via satellite transponder.
In simplex operation, a group of users sets the same sub-tone, and they do not have to listen to the transmissions of other radio amateurs on the same frequency (if they do not set the same sub-tone). However, the rules of mutual interference apply as in any FM operation.
In repeater technology, CTCSS can be used in two ways. If unwanted interference increases at the repeater site, which randomly activates the repeater, the operator is forced to set a higher level in normal squelch, which prevents unwanted activations of the repeater, but also users with a weaker signal must increase their ERP (Effective Radiated Power) to continue operating through the repeater.
A combination of classic squelch with CTCSS is more advantageous. Signals without CTCSS will be evaluated by classic squelch based on signal strength, with sensitivity adjustable to, for example, 3uV, while signals with CTCSS subtones are decoded by the repeater at full sensitivity – for example, 0.4uV.
Another option is for the repeater itself to transmit a signal with CTCSS. This is beneficial in areas where users have two or more repeaters on the same frequency. This way, they can select the one they are interested in when receiving.
The CTCSS system cannot block interfering signals that are fed into the receiver along with the desired signal containing a CTCSS subtone. Thus, even seemingly unintelligible mixtures of signals can pass through the CTCSS system – but only if they contain a subtone that the circuits can evaluate.
Satellite transponders advantageously use CTCSS activation, achieving significant energy savings. The transponder's transmitter only activates when a useful signal is fed into the input. This allows for full transponder sensitivity while minimizing the chance of the transmitter being activated by interfering signals.
Almost all today's FM devices are equipped with a CTCSS encoder. A decoder can be purchased as an add-on, but there are also devices fully equipped with CTCSS. Some can also search for CTCSS subtones in the signal and display their presence.
You can find more information 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