[sdiy] PLL examples

[JH.]

> Wouldn't it be necessary to limit this to "real" peaks somehow?
> 
> Thinking of e.g. a low pass filtered square wave - formally, there are
> lots of peaks due to ringing at each edge.

After a positive peak, a flipflop is set, and only with a negative peak
of enough size, the flipflop is reset. No chance for ringing to trigger
anything, unless the ringing is larger than the fundamental. 
"Enough size" is determined with lossy peak detectors,
which "remember" the size of the previous peak.

This is just from memory - analyse that Boss circuit to see if it fits.

JH.