In Reply to: CTCSS Services posted by Cliff on November 04, 2000 at 08:38:04:
: I'm interested in experimenting listening to services using CTCSS tones around North King, SOuth Snohomish county. Just to figure out how they work and sound. ANyone have some services that are busy and use those tones?
You might want to start off with a basic understanding of CTCSS (or "Channel Guard" / "PL" tones as GE & Moneyrola call them)) tones and what they are used for .
The primary purpose is to require a receiver to see something more significant than a carrier signal before opening squelch of the receiver, whether that receiver be in a repeater, or a users car / hand (as in mobile and handheld radios)
A typical configuration will require a receiver to see both a carrier, with the correct tone modulated on the carrier before the receiver opens up.
In a repeater, this means that the repeater will not repeat until a user keys a radio that also transmits the tone. This is programmed into radios these days, so the user doesn't know they are really doing this. One logical next step is to allow multiple repeaters to use the same frequency pair, usually at differing locations providing differing coverage areas. All of the system users can hear whatever repeater was used, and a user can "select" which site to use for best coverage to them. Oregon State Police uses this setup extensively throughout the state where troopers sharing a patrol area can't always get into the same site. Portland area has at least 3 repeaters, prob more, on the 154.935 freq that covers much of the tri-county area. Again, along I-84 headed east from Pendleton over the pass (I forget the name) theres a mess of repeaters covering the corridor, all transmitting on the same freqency, so officers will hear each others traffic.
Another logical use of tones is kind of the inverse, where one repeater uses many tones, to segregate (kind of) different users. This was the foundation of the "community shared" repeaters on UHF. The vast majority of UHF repeaters in the 460.600 (or so) to 464.975 region. This way, different companies could shre a repeater, and if all of the users required their tone to be received to open squelch, then company "a" doesn't hear company "b"'s traffic, and so on. This caused several problems. One is, if you don't hear the other company, and you want to use the radio, what do you normally do? That's right, pick up the microphone, key up, and WALK ALL OVER your neighbor. Good neighbors would monitor for a minute first. Most commercial radios turn off the tone squelch function when the mic is lifted off the hang up box (which is why its a hang up box, instead of a hang up clip, although some grounded the clip ...)
The funny part about this use is, casual, maybe naive users, usually think their conversation is "private" because of the use of the tone. Not so, simply a tool to allow others to "filter" traffic they hear. Turn off the filter, and you hear everything.
As mentioned in a previous post, FRS radios come equipped with CTCSS tones, which, by the way, stands for "Continuous Tone Coded Squelch System". Once again, this allows you to "filter out" both other users of the channel, as well as the intermod that these cheap radios are so susceptible to. Heck, in this RF environment, even expensive radios can have intermod problems, yet another use for tones.
Ham radio repeaters also use tone, mostly to insure the repeater is free of intermod. In the western washington area, the most used tone on UHF HAM repeaters is 103.5 (Moneyrola 1A) on the input. Listen to 444.650, and I believe you will hear a 131.8 tone. You will get that one anywhere in the Puget Sound Area. Listen commute hours for traffic.
Hope this helps.