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I am newly licensed (Technician) and bought a UV-5RA as a cheap way to check out what's going on in my area and see if I'm finding it interesting enough to get a better UHF/VHF HT.

The other day I was listening to a conversation happening on a local repeater and when one of the guys gave their call I looked it up for the heck of it and it turned out he was near me. That made me curious to see if I could receive him directly, so I switched the radio from "channel" mode to "free-tune" mode and tuned to the repeater's receive frequency.

Turned out I could hear him. However, I could not hear a CTCSS tone on his transmission. The repeater in question uses one (I checked in the local listings and on RepeaterBook) so the person talking through the repeater had to be transmitting one. And according to various references, the tones run (approximately) from 67Hz to 250Hz.

Those frequencies are well within the human range of hearing (for example, the fundamental frequency for adult male speech is generally in the 85-180Hz range), so why didn't I hear them when listening to the repeater receiver frequency?

Similarly, when doing some UV-5R receive testing by transmitting to it from an old Motorola GMRS Talkabout (I know not to transmit back to it) I did not hear any tones, even though the Talkabout has PL turned on.

Is it as simple as the CTCSS tone is simply transmitted at a such low intensity level that it is hard to hear? Or is something else going on?

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  • $\begingroup$ I find the tones near 250 Hz quite audible on my radios. Also, they are very seldom used. A quick look at repeaterbook shows all are below 200 and most are below 150 Hz, which are much less audible on a communications receiver. $\endgroup$ – tomnexus Apr 15 at 22:58
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Many radio manufacturers are aware that people don't want to hear CTCSS tones - so they put high-pass filters on both the output (speaker) and input (microphone).

That eliminates the tone before it gets to your ears, and will also prevent an errant tone near the microphone from triggering a repeater somewhere.

Some of us might not agree that the tones are within human hearing range - at least, not all of them for all of us!

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  • $\begingroup$ But in the case I was describing I was doing free-form tuning and my radio had no idea what tone to "expect". So how could it know what tone frequencies to filter out? The tones are 65-250Hz (approx) so I don't see how the radio could just filter out everything over 65Hz (heck, the fundamental frequency of even adult male speech is 85-180Hz). $\endgroup$ – QuantumMechanic Apr 14 at 14:05
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    $\begingroup$ A "high-pass" filter blocks everything below a certain frequency -- in this case, probably around 250 Hz. Like aspirin doesn't need to know where you hurt, the filter will block all CTCSS tone frequencies. $\endgroup$ – Zeiss Ikon Apr 14 at 14:14
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    $\begingroup$ D'oh! Yes, of course a high-pass filter is blocking the lower stuff. But that leads to the similar problem. If the filter is blocking everything under 250Hz it is also blocking at least the fundamental speaking frequency and maybe also the first harmonic or two of that, which would seem like it would cause problems for intelligibility of speech. $\endgroup$ – QuantumMechanic Apr 14 at 19:42
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    $\begingroup$ Speech is intelligible and even normal sounding without any fundamental frequencies. That's why big men could talk (pitch mostly below 250 Hz) on old fashioned POTS phone lines (many with a response of 300-3k Hz). And no one would hear anything funny. Same with music. Plenty of people used to use small pocket AM radios with 2" speakers that produced almost no sounds below 300 Hz to listen to their favorite pop tunes. $\endgroup$ – hotpaw2 Apr 15 at 0:49
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    $\begingroup$ Also, the tiny, cheap speaker in the UV-5RA is unlikely to reproduce anything below 300 Hz with any reasonable fidelity. $\endgroup$ – Phil Frost - W8II Apr 15 at 13:37
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The volume of the tones is typically low enough with respect to the voice signal that you can't hear it. Occasionally the tone generator in a radio will be poorly calibrated, and you do hear it.

Usually the radio has a tone scan function, and you can use that to find out what tone is being used. You can also turn on tone squelch if you have the tone set correctly, and if it is there, the radio will not block the signal.

Repeaters frequently filter out tones on the incoming signal and then add it back in on transmit. This allows using a different tone on input and output, although this is rare. Many repeaters and nearly all regular radios just pass the tone through without attempting to filter it.

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  • $\begingroup$ Pretty sure the BaoFeng HTs don't have tone scan -- at least, I haven't seen this in the manual for mine (the 8W version). $\endgroup$ – Zeiss Ikon Apr 14 at 14:15
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    $\begingroup$ I've never designed a VHF or UHF radio so I don't know for sure, but I doubt that CTCSS tones are heard in a receiver because the volume of the tones are low, because that would make them more difficult to decode. The Wikipedia article on CTCSS says that the levels are set for 15% deviation, but I don't know how loud that is ;) I think it's far more likely that a typical receiver runs the audio through a high-pass filter after decoding the tones. +1 for mentioning tone scan! $\endgroup$ – rclocher3 Apr 14 at 15:51
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    $\begingroup$ Same question as above, though. With the fundamental frequency of (adult male) speech being 85-180Hz wouldn't blocking everything below 250Hz cause intelligibility problems? Blocking below 250Hz would take out the fundamental plus a harmonic or two. Wouldn't that be a lot to have missing? $\endgroup$ – QuantumMechanic Apr 14 at 19:45
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    $\begingroup$ Nothing sounds missing (except HiFi musically) when spectrum below 250 Hz is completely eliminated. Most small phones (without earbuds) do that, and people still use them to talk. The human brain does magic "heard sound" (re)construction. $\endgroup$ – hotpaw2 Apr 15 at 0:52
  • $\begingroup$ en.wikipedia.org/wiki/Missing_fundamental $\endgroup$ – Phil Frost - W8II Apr 15 at 13:38
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Same with SDR software. My SDR code runs on a iPhone that's connected to "HiFi" audio transducers. So I had to add a 270 Hz high-pass biquad IIR audio filtering subroutine to get rid of annoying hum when listening to NFM from local 2M repeaters. (I obviously switch that subroutine out when listening to broadcast wideband FM music.)

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