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

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!