envelope follower

[Don Tillman]

   Date: Thu, 30 May 1996 14:52:27 +0200
   From: Rene Schmitz <uzs159 at ibm.rhrz.uni-bonn.de>

   Nononono! The thing is you measure an amplitude of a signal whose
   frequency you *know*, and you design the circuit for exactly and
   only that frequency, and it has to be a sinewave. (So it is more an
   AM demodulator; fixed carrier; variing amplitude) This was what was
   assumed in the earlyer post. (Divided audio bands...)  Ok, I should
   have said this more clearly, but it can be seen from the maths!!
   Erm, thats what I meant by *discrete* frequency.

I too was confused by the initial description... Splitting an audio
signal into seperate bands immediately made me think that the
post-rectifier filters were going to be optimized for each band, which
by itself would work pretty well, but that's not what was being
proposed.  (Although, the proposed circuit's performance improvement
should probably be compared to that solution.)

The squared-90-degree approach is very cool, and you can implement
that approach without splitting the signal into frequency bands by
using a 90-degree phase difference network (like on frequency
shifters).  It doesn't have to be nearly as accurate as on a frequency
shifter so the parts count wouldn't be high.

Of course if you're building a Vocoder, you'll be splitting the signal
into bands anyway.

Here's a parts saving variation on the proposed idea: Depending on the
band-pass filters you'll be using, you can get a 90-degree phase
difference by tapping off at different points in the filter.  

  -- Don