The Continuous Tone Coded Squelch System. This is the formal name for a method of radio operation also known a number of marketing names, like Motorola's Private Line (PL) system or simply as "tone squelch". For all the fancy talk around it (especially since it comes standard on just about every FRS radio you can purchase today), CTCSS is a relatively simple system.

With CTCSS, a specific low-frequency sub-audible audio tone is sent along with the voice communication on the radio channel. If the receiving radio or repeater system recognizes the tone being sent, it will open the squelch and allow the transmitting station to be heard on the other side. Conversely, no other stations but ones transmitting the correct tone will be heard on the receiver side.

The Electronic Industries Association (EIA) and Bell Labs designed the tones used so that none were a harmonic of any other, to keep the encode and decode circuits simpler. Motorola later added their own two letter designators to the tones for their own system, but the frequencies are the same.

This system has since been superceded in state of the art by Digital Coded Squelch or DCS, but is still largely the state of the practice for many commercial and amateur radio systems.

As a side-note, sub-audible is something of a misnomer. Two-way radio speakers are designed to reduce the frequencies in this range in much the same way that a good bass-enhancing stereo system is designed to accentuate them. If you were to plug your scanner output into a home stereo system and dead-key a radio transmitting a CTCSS tone, it would be quite audible.

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