Thanks for all the comments!

I am familiar with Verance and have worked with Civolution on audio and
video watermarking. They are indeed sophisticated (and expensive) systems.

I've also seen a bit about the Nielson Portable People Meter (
https://en.wikipedia.org/wiki/Portable_People_Meter ) which adds
identifying audio to broadcast stations for ratings. The spectral plots
I've seen of PPM show fairly high level tones added to the audio but are
psychoacoustically masked by adjacent frequency audio.

I've also considered CTSS (continuous tone coded squelch) methods where a
continuous subaudible tone identifies the signal. A modification of that
could be subaudible FSK where I'd be able to identify multiple signals
using the subaudible FSK and a UART. That technique is not suitable for
the normal radio use of CTSS where you need to identify the signal (to
unsquelch the radio) very quickly. Here, I could identify the signal in a
minute or so.

On "talk off," early ham radio DTMF (dual tone multi frequency or Touch
Tone) detectors suffered from this since they only looked for the presence
of the tones. Better ones compared the in-band level to the out of band
level for the tones. Voice would have near equal in-band to out-of-band
levels while DTMF tones had high in-band signals compared to the
out-of-band. In the application I'm looking at, though, there will be a
lot of out-of-band audio.

Methods of detecting signals below the noise or interfering signal level
are interesting. Spread spectrum is one method. There's also the JT
methods described at https://physics.princeton.edu/pulsar/k1jt/wsjt.html .
These use frequency shift keying at a slow rate (0.372 seconds per baud)
with multiple frequencies to get more bits per baud. This allows data
recovery with a signal to noise ratio of =9628 to =9624 dB in 2500 Hz
bandwidth.

Thanks for the ideas!

Harold


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