[tzs]

I'm not sure they cannot also do FM. The argument I usually see against FM is that demodulating FM is much more complicated, but that is not really true. Demodulating FM accurately is much more complicated, but if you'll accept a fairly distorted result then an AM demodulator can be used to demodulate FM.

You just tune the AM demodulator off center from the FM signal. This is called slope detection. If you have an SDR you can play around with this. Here's an article on slope detection [1].

I think this only works if the bandwidth of the AM detector is fairly small compared to the FM signal. I have no idea what the bandwidth of tooth filling receivers is.

[1] http://www.radio-electronics.com/info/rf-technology-design/f...