Simple Tone Detector

I'm looking for a SIMPLE circuit to detect the presence of a tone -
any tone in the audible range.

My quest: Consider the poor radio dispatcher, sitting there wearing a
pair of headphones when the next town / county over tones out an
ambulance or fire department.

I'm looking to be able to detect any audible tone and use the results
to decrease the operator's headphone sound level during the tone.

I have already tried a DSP tone elimination and found that:
1) It's too effective: I need do decrease tone volume - not eliminate
it.
2) Way too complex and expensive for the seemingly simple task.

The circuit needs to be compact and inexpensive - connected between
the dispatch console headphone jack and the operator's headphones. It
must NOT interfere with normal speech!

I have spent several hours online seatching and came up empty. Any
thoughts or suggestions?

Bill Powell

Hi Bill,

One idea: Have an audio voltage controlled oscillator, locked to the
audio input with a phase locked loop. I'd try the 4046 chip. Add the
locked VCO output shifted by 180 degrees to the audio. Set the
amplitude of the counter-tone, so that it doesn't completely cancel the
output, but just attenuates it. The PLL will keep the shift constant.
You'll also have to detect when the PLL is not locked and remove the
counter-tone on this condition. No idea how well this would work... but
I'd cost only a few $, for the 4046, an opamp or two and some Rs&Cs.

Anyway, just an idea.
Cheers,
VK3FLP

"Bill Powell" wrote in message
news
Simple Tone Detector

I'm looking for a SIMPLE circuit to detect the presence of a tone -
any tone in the audible range.

My quest: Consider the poor radio dispatcher, sitting there wearing a
pair of headphones when the next town / county over tones out an
ambulance or fire department.

I'm looking to be able to detect any audible tone and use the results
to decrease the operator's headphone sound level during the tone.

I have already tried a DSP tone elimination and found that:
1) It's too effective: I need do decrease tone volume - not eliminate
it.
2) Way too complex and expensive for the seemingly simple task.

The circuit needs to be compact and inexpensive - connected between
the dispatch console headphone jack and the operator's headphones. It
must NOT interfere with normal speech!

I have spent several hours online seatching and came up empty. Any
thoughts or suggestions?

Bill Powell


You could build a circuit using a 567 tone decoder IC, but the problem is
that you will first need to know what freq the tone is. Still you could use
multiple tone decoders, feeding to a muteing circuit. If you can specify
the tones you want to mute, we may come up with a circuit to help. The 567
would output to a relay that would simply mute the audio only when the tone
is present. JTT KF4HUF .

3flp wrote:

The PLL will keep the shift constant.
You'll also have to detect when the PLL is not locked and remove the
counter-tone on this condition.

Use a CMOS switch (4016 or 4066) to mute the tone.

No idea how well this would work... but
I'd cost only a few $, for the 4046, an opamp or two and some Rs&Cs.

It works really well - I used to use this method to remove the 96 kHz
(approx) interference in "scrambled" cable TV signals.

Bob

--
Everything gets easier with practice, except getting up in the morning!

How about a simple AGC circuit. You could make the detector more sensitive
around the tone frequency (with a BP filter) so it will reduce gain more due
to tone.
OR
If you know the tone freq, just notch it 6-8-10-15 dB - whatever you need.

I'm not sure why you say the DSP has to eliminate the tone. It could easily
be made to only reduce the tone to a a specified level.

I think the tone detector-PLL is not too good a method. Getting the phase
shift to do a cancell will take an adjustable all-pass phase shifter. Not
impossible, but complex as well.

73, Steve, K9DCI

"Bill Powell" wrote in message
news
Simple Tone Detector

I'm looking for a SIMPLE circuit to detect the presence of a tone -
any tone in the audible range.

My quest: Consider the poor radio dispatcher, sitting there wearing a
pair of headphones when the next town / county over tones out an
ambulance or fire department.

I'm looking to be able to detect any audible tone and use the results
to decrease the operator's headphone sound level during the tone.

I have already tried a DSP tone elimination and found that:
1) It's too effective: I need do decrease tone volume - not eliminate
it.
2) Way too complex and expensive for the seemingly simple task.

The circuit needs to be compact and inexpensive - connected between
the dispatch console headphone jack and the operator's headphones. It
must NOT interfere with normal speech!

I have spent several hours online seatching and came up empty. Any
thoughts or suggestions?

Bill Powell

Steve N. wrote:

I think the tone detector-PLL is not too good a method. Getting the phase
shift to do a cancell will take an adjustable all-pass phase shifter. Not
impossible, but complex as well.

The PLL can correct phase as well as frequency of the cancelling tone - it's
trivially easy.

Bob

--
Everything gets easier with practice, except getting up in the morning!

Curious, I ask...going somewhat on a tangent...


"biascomms" wrote in message
. uk...
Steve N. wrote:

I think the tone detector-PLL is not too good a method. Getting the
phase
shift to do a cancell will take an adjustable all-pass phase shifter.
Not
impossible, but complex as well.

The PLL can correct phase as well as frequency of the cancelling tone -
it's
trivially easy.
Bob --
Everything gets easier with practice, except getting up in the morning!

AMEN!

Bob,
The OP said a DSP was too complex, so I was keeping with that restriction
(and assuming that he may have been an analog type of guy)
How do you propose to get the 180? An Exclusive OR type PD has *approx* 90
degrees for a type one loop and 90 should be possible with a type 2...so you
need another 90. Not impossible by any means, of course. If the VCO is a
state variable implementation or a phase shift type, you may be able to get
another 90 degree section for "free".
The PLL lock time will allow a burst of tone at the start *and end*...(see
next).
Then, when the loop is idling, with no tone to cancel you need a gating
circuit to disable its tone from the output since ther'll be no tone to
cancel and time for it to turn on/off. Sounds like a juggling exercise.
Comment?

The PLL lock time will allow a burst of tone where a notch/equalizer
won't and it sure seems a notch is very simple - a hand full of parts in a
twin "T" or possibly narrowing it up with a augmented twin-T (twin-T and
amp)...or there may be another band reject, I think based on the Wein bridge
or some other active filter topology.

Now that I think about it, I suspect there may me a way to just change where
the tone in inserted into the path to eliminate it from the dispatcher's
speaker/phones.. But that assumes access to the system and the OP may have
just wanted to stick something in the output jack for the dispatcher...
There may even be a "dispatcher tone injection level adjustment." (:-)
Perhaps it is set too high now...

This is the type of problem that I *know* has a really simple solution.
It's just knowing all the constraints and getting the clue for that answer.

73, Steve, K9DCI.

On Mon, 01 May 2006 01:34:52 GMT, Bill Powell wrote:

Simple Tone Detector

I'm looking for a SIMPLE circuit to detect the presence of a tone -
any tone in the audible range.

My quest: Consider the poor radio dispatcher, sitting there wearing a
pair of headphones when the next town / county over tones out an
ambulance or fire department.

I'm looking to be able to detect any audible tone and use the results
to decrease the operator's headphone sound level during the tone.

I have already tried a DSP tone elimination and found that:
1) It's too effective: I need do decrease tone volume - not eliminate
it.
2) Way too complex and expensive for the seemingly simple task.

The circuit needs to be compact and inexpensive - connected between
the dispatch console headphone jack and the operator's headphones. It
must NOT interfere with normal speech!

I have spent several hours online seatching and came up empty. Any
thoughts or suggestions?

Bill Powell


Seems to me a good AGC amplifier will do the trick. It will maintain a
fairly constant volume level no matter what is comming in.

73
Gary K4FMX

Gary Schafer wrote:


Seems to me a good AGC amplifier will do the trick. It will maintain a
fairly constant volume level no matter what is comming in.

Ultimately, a clipper (with a low pass filter to get rid of much of the
nastiness) might solve the problem!

Bob

--
Everything gets easier with practice, except getting up in the morning!

You might want to look into one of those public address feedback
cancellors that uses automatic audio notches. they might have
adjustment on the notch depth which might be what you need. Call an
audio warehouse and ask questions..

Bill Powell wrote:

Simple Tone Detector

I'm looking for a SIMPLE circuit to detect the presence of a tone -
any tone in the audible range.

My quest: Consider the poor radio dispatcher, sitting there wearing a
pair of headphones when the next town / county over tones out an
ambulance or fire department.

I'm looking to be able to detect any audible tone and use the results
to decrease the operator's headphone sound level during the tone.

I have already tried a DSP tone elimination and found that:
1) It's too effective: I need do decrease tone volume - not eliminate
it.
2) Way too complex and expensive for the seemingly simple task.

The circuit needs to be compact and inexpensive - connected between
the dispatch console headphone jack and the operator's headphones. It
must NOT interfere with normal speech!

I have spent several hours online seatching and came up empty. Any
thoughts or suggestions?

Bill Powell





--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

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