In reply to:

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Obviously, it needs to background poll the alternative frequencies and switch to the strongest one when the signal strength exceeds that of the current frequency for a certain period.

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I've been thinking about how to implement this, over the last few weeks. The scheme I have in mind is maintaining a record of all the AF frequencies, and for each one, remember when it was last checked and its signal strength at that time. Now, we can compare the strength of the current reception with each of the other stations adjusted by some uncertainty factor based on how long since the candidate was last measured. If any of the adjusted strengths appears greater than the current signal, sample it again, and consider switching. The RDS decoder has a silence detector to help with choosing sample times, doesn't it?

The advantage of this scheme is that the stronger a station is, the more often it's sampled. Conversely, weak stations get only a small proportion of the available sampling opportunities, until they get stronger.

You don't actually have to remember the time of the last check for each frequency - you can adjust the remembered strength as you go. And probably bias the measurement to give slightly more strength to the current signal (for hysteresis, preventing the tuner switching back and forth repeatedly).

I've probably already said I'm looking forward to better AF switching (it will improve TA, for one thing). But I acknowledge it could be a long wait, so I'm not going to get agitated about it.