[sdiy] what is the amplitude envelope of a signal

[Harry Bissell Jr]

Check out my EDN Design Idea
"Envelope follower combines fast response, lowest
ripple"

for a circuit that will outperform both full wave
averaging and peak detector circuits.  It is very
fast in attack, and has a hold function that will
delay the release.  With actual music signals it
works quite well.

There IS a major issue with deciding just how "low"
the lowest frequency you intend to process. With
an instrument like bass guitar or guitar... this is
well defined. For wideband "music" is is hard to
determine where the frequency domain stops and the
time
domain begins...  A kick drum every 500ms would give
you a pretty good idea of the issue.

Tailoring the clock rate is necessary.

IMHO the "triple rectifier" idea is a loser.  It would
work if positive and negative half cycles were equal
in amplitude... something that is almost NEVER true
for a guitar waveform.  The rectified peaks simple DO
NOT
fill in the blanks like the theory says.

There is an idea in Electronotes using a tapped delay
with each tap rectified and added... in some ways it
is similar to my multiple peak detectors.  My method
assures that the largest peak is always aquired and
held for a period longer than a full cycle of in input
waveform... making the 'good' data always available.
Besides... you'd probably need a BBD to make the delay
:^P

In practice, I find that the extension of the signal
past the 'actual' decay point is not usually audible
... especially considering the exponential decay of
most sources anyway.  You CAN hear a delay in attack
but its
much harder to hear a delay in the decay.

H^) harry



--- Czech Martin <Martin.Czech at micronas.com> wrote:
> Yes, that is what I also tryed.
> The peaks of a rectified signal seems easy,
> but if you define them as local maxima 
> things get messy when noise comes into play.
> Connecting such peaks will not look like the
> envelope, but
> more like the rectified signal.
> So, only the larger peaks should be used, but what
> does
> that mean in a formal way?
> 
> The filtering approach must guarantee to have not
> too much
> lag, and especially an unsymmetrical lag does not
> make sense,
> since the incomming signal could start slow and end
> very suddenly.
> 
> But I will dig out this EN paper.
> Thanx!
> 
> As I said: what seemed to be so easy turns out to be
> very difficult!
> 
> 
> 
> m.c.
> 
> > -----Original Message-----
> > From: owner-synth-diy at dropmix.xs4all.nl
> > [mailto:owner-synth-diy at dropmix.xs4all.nl]On
> Behalf Of Scott Bernardi
> > Sent: Montag, 29. September 2003 15:03
> > To: Synth-DIY list
> > Subject: Re: [sdiy] what is the amplitude envelope
> of a signal
> > 
> > 
> > How about an interpolation between the peaks of a
> rectified signal?  
> > Interpolation means that if you take the peak of
> the upper half of a 
> > bipolar waveform (that's half wave rectification),
> and 
> > connect them with 
> > lines, the resulting shape would be your envelope.
> In the analog 
> > implementation the interpolation is usually
> accomplished  by 
> > filtering, 
> > and the rectification is usually made full wave. 
> > The trick in making a decent envelope follower is
> choosing the time 
> > constant for the filter. It needs to be fast
> enough to be be able to 
> > follow transients on the envelope, but too fast
> and it will 
> > follow the 
> > peaks and valleys of the waveform cycles
> themselves 
> > (appearing as ripple 
> > in the envelope).  This is a tradeoff.
> > By the way, check out an all analog implementation
> of an envelope 
> > follower that uses three cascaded FWR's with a "dc
> restorer" 
> > circuit to 
> > get 3x improved ripple rejection while retaining
> transient 
> > response at 
> >
>
http://home.comcast.net/~sbernardi/elec/og2/og2_envfollower.ht
> ml. This 
> is an Electronotes design.
> In the digital domain, I guess you would want to
> take a moving average 
> of the samples on the full wave rectified waveform
> (which you get by 
> taking the absolute value of the sample voltages)
> where the averages are 
> taken at a rate of say one half the lowest signal
> frequency you want to 
> process.
> I'm sure this has been thought out by someone before
> that knows more of 
> what they are talking about than I do.
> 
> Czech Martin wrote:
> 
> >Following the (long, excuse me) e-mail about my FS
> experiments
> >you can see that I have some little signal
> processing tools.
> >But one: envelope follower.
> >
> >I thought that programming this was an easy thing,
> everbody
> >knows what the amplitude envelope of a signal is,
> and the
> >methods for analog circuits are well known
> (rectifier, filter,
> >bleeder).
> >
> >Unfortunately it turns out that extracting the
> amplitude envelope
> >of an audio signal is not so easy.
> >I used speech as sample signal, and all my
> experiments showed
> >that considerable feedthrough of the original
> speech into
> >the envelope signal was present.
> >
> >Which led me to think more about these envelpes.
> >If I think about them, I feel I know them.
> >If I try to explain I do not know them (stolen from
> Augustinus).
> >
> >What the heck is the amplitude envelope of an audio
> signal?
> >(please give a formal description, i.e. something
> so precise
> >that programming would be easy).
> >
> >m.c.
> >
> >
> >  
> >
> 
> -- 
> Scott Bernardi
> sbernardi at comcast.net
> 
> 
> 
>