stupid envelope follower idea

[Harry Bissell]

Ian Fritz wrote:

> Hi Terry --
>
> > RMS detection is a means of calculating the "area under the curve" of the
> > waveform.
>
> Huh?  :-)
>
> The area under the curve of a waveform is zero. (Assuming ac coupling, or,
> equivalently, ignoring the zero frequency Fourier component.)

No fair... if we are talking envelope detection, we assume absolute value.  Area
under
the curve is ... er...area under the curve.... ;^)

>
>
> > It is commonly done by squaring the instantaneous points of the
> > waveform with an analog multiplier.  The peak voltage squared yields the
> > RMS power.
>
> Well -- it's (1) square the voltage, (2) average over a cycle, then (3) take
> the square root.

RMS detection is usually used in things like compressors to the the human
perception
of equal amplitude... IMHO it does not work well and I never personally use that
detection method.  It is a nice way to have an "average" level detection, but in
real life
it is slow. If I wanted to ride gain on a complete audio signal (like a CD...)
it would be good. But on individual waveforms I think its too slow. I can't wait
full cycles. (or even half cycles...

>
>
> But since the instantaneous power is independent of the phases of the
> various Fourier components, the phase shifts you are concerned about don't
> matter. They change the instantaneous waveform amplitude, but not the RMS
> value.
>
> > I hope that's right, I'm going by memory here.   The problem
> > with that approach might be the time altered waveform might not have the
> > same "area under the curve" as the original waveform.  It should be
> > possible to do a mathmatical analysis or simulation to answer your
> > question, maybe someone on the list has the means to do so, unfortunately
> I
> > don't.
>
> I think the math is simple. The RMS voltage is the square root of the sum of
> the squares of the Fourier amplitudes. Phases don't matter.

OTOH: I bet after you break the signal into bands, RMS each, and sum them
together...
that the envelope follower looks a lot like the original signal... unfollowed
(so to speak)
But its worth a try. Now I'll have to try and do a simulation on this... that
should keep
me busy for a while...

H^) harry

>
>
>   Ian