Two agencies sharing the same frequency but different tones, work how?

[RedPenguin]

Alright, I understand how agencies can sometimes share frequencies, even if they are not related to each other in any way, shape, or form.

Though, I was told once, that even if another agency has a different tone, you have to still monitor without the tone, so that you don't get interference with each other.

So what's the purpose of the tone, if technically you have to listen to the other agencies traffic anyway, not just yours?

 

[DiGiTaLD]

The FCC requires that you monitor before transmitting. Most commercial radios have a provision (if installed correctly) to disable CTCSS/DCS decoding before transmitting, such as picking up the hand microphone from the clip, or pressing the monitor button on the base microphone before pressing the transmit switch.

You don't have to listen to the other users' traffic all the time, just right before you transmit. Of course, even though this is the rule, in many setups I have seen lately it is not observed.

 

[zz0468]

The theory is that you don't have to listen to ALL of the other agencies traffic, only when you monitor before transmitting. It's a nuisance reducer, not a nuisance eliminator.

edit: I hate when someone else posts the same thing while I'm writing my response!

 

[gmclam]

There are several scenarios out there. The typical thing I experience is 2 agencies, each 40 miles from me, but in opposite directions (80 miles from each other). They are on the same frequency, but use different CT/DC tones. In this situation each should not get any interference from the other, and if they have their tones enabled for receive, will not even hear each other.

Another scenario involves repeaters. Perhaps the frequency to my repeater is shared by different agencies buy my repeater only looks for a tone of 100.0. In this case only my traffic gets repeated. However, if someone on that same input frequency is closer to my repeater (or has a stronger signal) than someone in my agency, then my repeater will never hear my remote.

Take that last situation and go back to simplex operation without a repeater. If there are 4 of us on the frequency, 2 using tone "A" and 2 using tone "B", each receiver would still have to have the strongest signal from the desired transmitter for us to all converse at once. Again, the strongest signal at the receiver wins. If you have CT/PL mode on, you'll just hear nothing if someone with an "incorrect" tone is being received. If you have CT/PL off, then you'll hear something, but not the person you want.