《CTCSS PL TONES》.pdfVIP

CTCSS Introduction Long ago and not so far away, Motorola came up with a way to get more than one Land Mobile customer on the same frequency at almost the same time. They figured that different customers could coexist on the same frequency if they did not have to listen to each other routinely. They invented Continuous Tone Coded Squelch System or CTCSS for short and patented it as PL short for Private line. Other manufacturers, finding that the system was absolutely necessary to stay competitive came up with Channel Guard, Quiet Channel, Call Guard, and many other names for the same thing to avoid lawsuits for marketing a patented system. The manufacturers of amateur equipment seem to have settled on tone for encode only and tone squelch for encode/decode. Most of the amateur VHF and UHF equipment manufactured in the last ten years has at least encode capability (standard or optional) and many have decode capability (standard or optional). When it is available, it is simply a plug in circuit board. Aftermarket encoders and encoder/decoders can be added to virtually any transceiver since they have now been developed smaller than a postage stamp. The system is based on a subaudible tone injected after the audio stages into the transmitter during encode and the tone is detected before the audio circuits in the receiver. The decoder switch is then used to perform some function, usually to unmute the receiver when the tone is decoded. In the commercial equipment, the audio bandwidth tends to be narrower than our amateur equipment and there are circuits installed to filter out the tones so they are truly subaudible. Most of our amateur equipment transmits and receives a much broader audio bandwidth and has no special tone filters, so most hear the tones. The lower the tone frequency the less audible it tends to be. Details on CTCSS The system is designed around a set of relatively low frequency tones (32