envelope follower

[Sean Costello]

>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.

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. 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.

Anyone on the list who actually knows what I'm talking about (I really don't
- I think my idea would work, but I'm not explaining it too well), please
feel free to comment, clarify, castigate, etc.

Sean Costello

P.S. Speaking of Electronotes: after many long months of being out of the
DIY scene, I decided to order the rest of the Electronotes publications.
Bernie is still doing business - I received two large boxes of paper last
week.  He is moving to a new house, but the old mailing address works for now.