Envelope Follower

gstopp at fibermux.com gstopp at fibermux.com
Wed Feb 28 18:40:10 CET 1996


     Oh yeah I fergot to mention the multiplier type of envelope follower - 
     basically this is made by running the audio signal into both inputs of 
     a ring modulator, and smoothing it with an integrator like Don says. 
     This acts as a full-wave rectifier with a logarithmic transfer 
     function. Probably you could get the same results with an amplitude 
     recovery rectifier followed by a log generator (right?).
     
     I remember seeing an untested design proposal way back in an old 
     Electronotes - it consisted of two multipliers, with the audio signal 
     running into a wideband 90-degree phase difference network, with the 
     SIN output going to both inputs of one multiplier, and the COS output 
     going into both inputs of the other multiplier, and the outputs of 
     each multiplier averaged together. The proposal was that the valleys 
     on the SIN multiplier's output would be filled in by the peaks of the 
     COS multiplier's output, and vice versa. This way there would be no 
     smoothing filter required, or so it seems. Would this work?
     
     Re: the heating resistor method - talk about response time! :-)
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re: Envelope Follower
Author:  don at till.com at ccrelayout
Date:    2/19/96 10:18 AM
     
The two most common methods of envelope detection are:
     
  Cheezy: Rectify the input voltage and average.
     
  Proper: Square the input voltage (ie., multiply the signal by itself), 
  average, then (optionally) take the square root of that.
     
The difference shows up in how low and high input signal voltages are 
relatively weighted in contibuting to the average.
     
(There's even a mechanical variation of the "proper" approach: Use the 
input signal to drive a resistor and measure how much it heats up.)
     
I can't find my catalog now to double check, but it appears that Serge 
is saying they don't use the "cheezy" apprach.
     
  -- Don




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