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I'm interested in listening to the local EMS/Fire transmissions around here and was wondering if I have to configure the CTCSS/PL tones on the radio in order to hear those transmissions.

I understand that a PL tone is used to create sub-banding in the same frequency to basically subdivide a single frequency into about 38 private "channels".

What happens if I don't configure a PL tone on my radio? Will I be able to hear all transmissions on that frequency even if some particular group is using a specific PL tone? I'm using a Baofeng UV-5RB.

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Your understanding of tone squelch is wrong. Tone squelch is simply an extension to the squelch system.

Normally, the squelch opens whenever the received signal is stronger than the squelch threshold.

With a tone squelch (and I use that term broadly here to refer to CTCSS, DCS and any other possible, similar schemes) configured for receive squelch, the squelch opens only if the received signal is strong enough and the configured tone is present in the demodulated audio. (The tone is filtered out before the audio is fed to the speaker or headphones; hence "subaudible" in the context of the radio transmission.)

All the issues with FM capture and so on remain exactly the same with or without tone squelch, so you do not get any additional "channels". (Also consider Can two FM radios transmit two separate PL tones at the same time, and be received by two radios with matching PL tones?) All that happens is that you can filter out some of the transmissions that you are not interested in. This is particularly useful in situations where the received signal is relatively weak, as you can leave the signal strength squelch turned all the way down and rely on the tone squelch to only open for useful transmissions.

With that said, we can answer your question:

If tone squelch is not configured on the receiver, practically any FM receiver will open the squelch whenever the received signal is strong enough, regardless of the presence of any particular subaudible tone or code, because it is not filtering based on any particular subaudible tone.

Hence, if you do not set up tone squelch on your radio, or your radio does not support tone squelch (applies primarily to older equipment), you should be able to hear all transmissions as they sound on the air.

It isn't entirely uncommon that radios allow separate tone squelch for transmit and receive. This can be useful, for example, if your local repeater input is tone squelch operated (to reduce interference) but it does not re-transmit the tone on the repeater's transmit (output) side.

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  • $\begingroup$ A much better description and does help me re-affirm what I thought was going on with the concept of CTCSS. Thank you. $\endgroup$ – Andrew Sep 11 '15 at 21:55
  • $\begingroup$ @Andrew As an aside, you are of course free to accept any answer you prefer (and I appreciate that you picked mine), this is why it's usually a good idea to wait a while before accepting an answer. Acceptance indicates to the community that you feel your question has been satisfactorily answered; this tends to reduce the chance of it receiving further answers. Particularly on low-traffic sites, it can be a good idea to wait up to a few days before accepting an answer. $\endgroup$ – a CVn Sep 12 '15 at 8:08
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Basically yes, most radios will open the squelch for all activity if a RX tone isn't specified. Your assumption that you can hear EMS/Fire transmissions is flawed however. Most of these services use TETRA which encrypts all traffic, so even if you could tune to these with your radio, which you might not be able to, you won't hear anything legible.

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    $\begingroup$ I'm aware that my local EMS/Fire/PD are moving to Project 25 Phase I/Phase II gear, but they still have some dispatch using the VHF/UHF spectrum. Thanks for the assumed failure on my part though; helps keep me on my toes.. $\endgroup$ – Andrew Sep 11 '15 at 15:13
  • $\begingroup$ Most of these services in the US are moving to such a system. CTCSS and DCS are, however, used around the world, and since the question was asked without a tag (or any text) suggesting that the question was specific to any single territory, it would be best not to make such assumptions when answering. $\endgroup$ – Scott Earle Sep 13 '15 at 1:36

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