CTCSS tones: Why don't we hear them?

[Blaise]

OK, this is driving me nuts.  Normal human hearing spans from 20 Hz to 20kHz (Outliers from 12 Hz to 28kHz, but those are rare).  CTCSS tones are listed as ranging from approximately 6Hz to 260 Hz, all *well* within the range of the average human ear.

So riddle me this, Batman(Batmen? Batfolk?): Why do we not all hear low-pitched buzzing during radio calls made with CTCSS tones in use?

 

[SteveShannon]

2 minutes ago, Blaise said:

OK, this is driving me nuts.  Normal human hearing spans from 20 Hz to 20kHz (Outliers from 12 Hz to 28kHz, but those are rare).  CTCSS tones are listed as ranging from approximately 6Hz to 260 Hz, all *well* within the range of the average human ear.

So riddle me this, Batman(Batmen? Batfolk?): Why do we not all hear low-pitched buzzing during radio calls made with CTCSS tones in use?

 

The radio that receives them filters them out.

[Blaise]

Just now, Sshannon said:

The radio that acts upon them filters them out.

Doesn't that mean that any sounds in that range that we *intend* be transmitted (like "Hey Mike, listen to the sound this machine is making.  Does that mean it's going to explode?) will also be filtered out of transmission, if they happen to be within whatever range the radio electronics is unable to discriminate between of the CTCSS tone?

[SteveShannon]

7 minutes ago, Blaise said:

Doesn't that mean that any sounds in that range that we *intend* be transmitted (like "Hey Mike, listen to the sound this machine is making.  Does that mean it's going to explode?) will also be filtered out of transmission, if they happen to be within whatever range the radio electronics is unable to discriminate between of the CTCSS tone?

Yes, tones below 260 Hz are filtered out intentionally by radios designed to receive and decode them.  If you listen with a radio receiver that has no such filtering the tones could be pretty annoying.  Think of hearing the 60 Hz hum of a loose transformer as part of every message, or the sound of a mosquito or buzz of a bee. 

Here’s an aftermarket CTCSS filter:

https://www.masterscommunications.com/products/filter/plf15.html

[Blaise]

15 minutes ago, Sshannon said:

Yes, tones below 260 Hz are filtered out intentionally by radios designed to receive and decode them.  If you listen with a radio receiver that has no such filtering the tones could be pretty annoying. 

Oh, wow, so they filter out *ALL* sounds below that line, not just the expected tone?  I mean I guess that makes sense to some degree, or you'd hear everyone else's tones on an open channel, but that's a fair chunk of our hearing range missing!  I wonder why they don't use higher frequencies.  Anything over 28kHz would be literally unhearable, and a frequency that high can transmit a lot of data, if need be...

[SteveShannon]

4 minutes ago, Blaise said:

Oh, wow, so they filter out *ALL* sounds below that line, not just the expected tone?  I mean I guess that makes sense to some degree, or you'd hear everyone else's tones on an open channel, but that's a fair chunk of our hearing range missing!  I wonder why they don't use higher frequencies.  Anything over 28kHz would be literally unhearable, and a frequency that high can transmit a lot of data, if need be...

Filtering out just one tone is almost impossible.  There’s always a certain width of frequencies that are also filtered out and it’s easiest to simply build a high pass filter rather than try to isolate a single frequency.  Also, keep in mind that the bandwidth allowed for narrowband is only 6.25 KHz, so everything transmitted has to fit within that. For wideband it goes to 12.5 kHz, so no audio frequency higher than about that will be transmitted.

[Blaise]

11 minutes ago, Sshannon said:

keep in mind that the bandwidth allowed for narrowband is only 6.25 KHz, so everything transmitted has to fit within that. For wideband it goes to 12.5 kHz, so no audio frequency higher than about that will be transmitted.

Welp, now my brain has exploded.  I need to go read more theory...

[KAF6045]

4 hours ago, Blaise said:

Welp, now my brain has exploded.  I need to go read more theory...

Pretty much any service that is not broadcast AM or FM limits the voice band to approximately 6000Hz (Amateur AM and any remaining shortwave broadcasters run in a 6kHz bandwidth when including both sidebands, single sideband often runs less than 3kHz -- 2.4kHz is one of the settings on my rigs, or 2700Hz minus 300Hz). The services are designed for communication, and most vocal formants fall into the octave of 300-3000Hz -- they are NOT designed for musical fidelity (and definitely do not pass all the higher harmonics in the 3000-30000Hz octave [note: I'm using octave in terms of order of magnitude: 30-300, 300-3000, 3000-30000, even though the 20000-30000 is pretty much inaudible to humans... NOT musical octave where an octave is a doubling -> 440A to 880A, etc. 300-3000Hz covers about 2.5 music octaves]

Broadcast FM has a 200kHz channel spacing; not certain how much is actually used for signal deviation. Broadcast AM uses 10kHz spacing (5kHz per sideband -- so just one sideband is almost the entire width of shortwave double-sideband AM).

 

[nokones]

8 hours ago, Sshannon said:

Yes, tones below 260 Hz are filtered out intentionally by radios designed to receive and decode them.  If you listen with a radio receiver that has no such filtering the tones could be pretty annoying.  Think of hearing the 60 Hz hum of a loose transformer as part of every message, or the sound of a mosquito or buzz of a bee. 

Here’s an aftermarket CTCSS filter:

https://www.masterscommunications.com/products/filter/plf15.html

I always liked the hum of 192.8 Hz PL tone.

[axorlov]

KAF6045 gave very clear correct answer. In voice applications (including phone network) it is usually only 300-3000Hz (often 300-2300Hz) is processed, everything else is filtered out. This is why tones are called "sub-audible". And since nothing on this planet is perfect, including filtering, many radios will let you enjoy the higher tones, barely hearable.

[gortex2]

So back in the xstal base days we added alot of "pl" boards. You could definitely here the hum/tone when it was over modulated. If its set for the right level you wont hear it as said above. On wideband it was normally 700 hz we were shooting for on lowband. If you got over 750 you could definately hear the hum in the background. 

[WRKC935]

On 1/30/2023 at 10:39 AM, Sshannon said:

Filtering out just one tone is almost impossible.  There’s always a certain width of frequencies that are also filtered out and it’s easiest to simply build a high pass filter rather than try to isolate a single frequency.  Also, keep in mind that the bandwidth allowed for narrowband is only 6.25 KHz, so everything transmitted has to fit within that. For wideband it goes to 12.5 kHz, so no audio frequency higher than about that will be transmitted.

I don't know.  They filter 2175Hz out pretty well for the guys using tone remote control.

[SteveShannon]

27 minutes ago, WRKC935 said:

I don't know.  They filter 2175Hz out pretty well for the guys using tone remote

So, Midian uses a notch filter:

”Notch Filter: As a notch filter, the NFBF-10 will remove the undesirable tone from the incoming audio while passing the rest of the audio. The NFBF-10A is commonly used to remove 2175 Hz keying tone from tone remote radio dispatch systems going into a voice recorder. The NFBF-10C is used to remove CTCSS tones from audio.”

A notch filter has a response that affects nearby tones as well, but with a lesser effect. 
Here are three different graphs of frequency responses for notch filters.  As I mentioned earlier, none of them affect only a single tone; all have some effect on adjacent frequencies, although the very sharp one would not have much, but still it would be a segment of voice frequencies that would not be reproduced. Probably not terribly noticeable.

 

[tweiss3]

Many of the commercial radios have a jumper you can change to filter the bottom end out or not which may be needed for data. An example, Vertex Standard VX2500, out of the DB9 connector on the back:

Normal is to filter everything 300 Hz and below out, but for packet, you need to open it up.