envelope follower

[Rene Schmitz]

At 11:14 01.10.1997 -0700, Sean Costello wrote:
>
>About this circuit you described...
>
>As is, it is only useable for a single frequency.  HOWEVER, it seems that
>you could build a 90 degree constant phase shift network, where the input is
>run through two allpass networks that produce 2 signals that are 90 degrees
>(+- 0.5 degree) apart for a given audio range.  This technique is used in
>the construction of frequency shifters; see the Electronotes frequency
>shifter in the Electronotes Preferred Circuit Collection for an example of a
>constant 90 degree phase differencing network.

Mathematically this is a Hilbert transformation!
(Can be easy implemented in dsp, by the way...)

>Not a simple circuit, but it
>seems like it could be used in the construction of a very accurate envelope
>follower, in addition to frequency shifting.

Unfortunately not! I had thought about this my self, but recent investigation
shows, that when you square a signal like a1*sin (w1*t)+ a2*sin(w2+t) you get 
a1^2*sin^2(w1*t)+2*a1*sin(w1*t)*a2*sin(w2*t)+a2^2 sin^2(w2*t)
since (a+b)^2 = a^2+2ab+b^2  :-< (the old binomial pitfall)
even if you have the cos by a phase shifting network, this doesn't
get you where you want.

(Hmmm...It would be nice if the equation (a+b)^2=a^2+b^2 were true, you
could easily 
 pitch up a signal by an octave simply by squaring, and maths would be
easier too!)

I proposed the thing above only because of the use of divided and small enough
audio bands.
So we better *forget* it unless we are designing a
up-to-heaven-complex-vocoder. 

I suggest here again, to use a different circuit, (seen on some elektor book)
peak detector circuit (capacitor charged up to the peak of the signal to be
detected),
use a sample and hold that samples this voltage in the zero crossing, and
discharge 
the peak-hold cap then for the next peak.
This way you have very good accurracy, and the DC envelope is only a little 
"steppy". This can be cured by a LPF at about 100 Hz.
I attached a small gif image of the block diag, it may be ugly but clarifies
things.

Anyone used anything similar?!
(If there is any interest I could put a schematic on my web "site")

Bye 
  René
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