XZ 67.0 WZ 69.3 XA 71.9 WA 74.4 XB 77.0 WB 79.7 YZ 82.5 YA 85.4 YB 88.5 ZZ 91.5 ZA 94.8 ZB 97.4 1Z 100.0 1A 103.5 1B 107.2 2Z 110.9 2A 114.8 2B 118.8 3Z 123.0 3A 127.3 3B 131.8 4Z 136.5 4A 141.3 4B 146.2 5Z 151.4 5A 156.7 5B 162.2 6Z 167.9 6A 173.8 6B 179.9 7Z 186.2 7A 192.8 M1 203.5 8Z 206.5 M2 210.7 M3 218.1 M4 225.7 9Z 229.1 M5 233.6 M6 241.8 M7 250.3 0Z 254.1W5VZB home page
OK, as we were saying: For instance, you have N radio channels available, and you have X times N groups of people who want to use radios, and who don't want to listen to each other's chit-chat. Very simple, you put X number of groups on each channel, and you assign each of them a "PL tone". All of company Y's radios will transmit their assigned tone along with their voices. And while their radios will receive anything on the channel, their speakers will not turn on unless the proper tone is present. To prevent collisions, the speaker comes on whenever the mike is picked up, so the person who wants to talk will hear any traffic already on the channel, and can wait until it's clear. Of course a certain degree of cooperation is necessary, but that's easy to come by with this bit of gizmology doing most of the sorting out automatically.
Another use for PL tones is for channels that are not shared but may be subject to stray signals, especially repeaters on mountaintops or other high points. The channel may be in use far away so that snippets of signals sneak in, and using different tones in different areas will prevent false keying on distant signals. Many ham repeaters use a standard tone which is city-specific.
Yet another situation appears on crowded mountaintops, like Sandia Crest by Albuquerque. There's a ridiculous number of transmitters in a small area, and interference is not unlikely. In this situation each repeater is likely to use a different tone to minimize triggering by spurious cross-products, meaning two or more radio signals mixing in whatever corroded metallic joints they can find -- fences are good at that, but by no means the only way it happens. Basically, any funky connection has nonlinear characteristics and acts as a "crystal set", like the old-time radios. The new frequencies produced by the mixing are then re-radiated, and fences are good at that too.
Finally, all of the above situations are subject to interfering noise, both man-made and of atmospheric or other natural causes. PL tones will prevent false keying from noise.
One last thing: why would you want to use audible tones as though they were subaudible? Very simple. This scheme does work with subaudible tones too, but it takes a few cycles of the tone to trigger whatever squelch mechanism you are using, and the higher frequencies get a few cycles out in a much shorter time. Using a real subaudible tone (e.g. 25 Hz) would result in the first little bit of each transmission being noticeably "clipped" coming out of the speakers.