[sdiy] what is the amplitude envelope of a signal

[Czech Martin]

Harry,

your reply shows that you listened carefully to your 
results. These are exactly the problems I came across.

For such applications the usual circuits/ideas do not perform
too well, or they are even useless.

Your "eye" approach is into the same direction of what
I tryed, unfortunately this is a very intelligent
approach. For the human beeing it is not too hard
to draw the envelope, but automatons have problems...

m.c.

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Harry 
> Bissell Jr
> Sent: Mittwoch, 1. Oktober 2003 01:29
> To: Richard Wentk; Synth-DIY list
> Subject: RE: [sdiy] what is the amplitude envelope of a signal
> 
> 
> OK then... lets make a formal definition  ;^P
> 
> The "envelope" of a signal is what your 'eye'
> would process looking at the rectified or absolute
> value of a signal.
> 
> The envelope would be a 'best fit' curve to that.
> 
> Single constant generators do not work (for me)
> I'm processing a guitar, which has the following
> nasty habits
> 
> 1) The positive and negative half cycles do not
> have equal amplitude... and this change is variable
> over time. Usually there is one large peak of one
> polarity, followed by two smaller peaks of the
> opposite
> polarity.
> 
> 2) The guitar has a normal exponential decay, but the
> decay time can be changed easily (on the fly) by
> adding
> damping to the strings in small or even large amounts.
> The biggest problem is that if you mute the strings...
> it will take forever for a single constant to get to
> the
> correct value.  The next note will start at the wrong
> value... maybe a too high value if the next note is
> more quiet.
> 
> The ripple vs speed tradeoff is a killer.  Any ripple
> (which the 'best fit' or 'eyeball' curve conveniently
> filters out) will cause noticible intermodulation
> distortion especially if you are using the CV to drive
> a filter.  Any lag will destroy the accuracy of the
> original waveform.  Now maybe your guitar, or drum
> has a real noticible attack time.
> 
> This is a real nightmare problem.  If I could stand
> it, 
> I'd think about running some delay in the signal to
> allow the circuit to 'look into the future' and
> adapt... but I fear even a millisecond is too much
> 
> H^) harry
> 
> 
> --- Richard Wentk <richard at skydancer.com> wrote:
> > At 13:08 30/09/2003 +0200, Czech Martin wrote:
> > > > If you're getting signal breakthrough it means
> > either there's
> > > > some signal
> > > > breakthrough in the circuit due to a design or
> > construction
> > > > fault, or the
> > > > filtering isn't working properly, or both. The
> > output should
> > > > be a slow *DC*
> > > > variation. There should be no effective signal
> > content above
> > > > 100Hz at the
> > > > very maximum, and 10Hz would be more typical for
> > many applications.
> > >
> > >That is a good question. A lowpass with 100Hz or
> > even 10Hz
> > >gives problems with signals that have a short
> > attack.
> > 
> > But are those problems severe enough to make a
> > musical difference? I can't 
> > think of many applications where a dual
> > time-constant standard follower 
> > circuit wouldn't produce useful results. And people
> > have been using single 
> > constant followers since Bob Moog's days without
> > worrying too much about it.
> > 
> > > > The only way to create a perfect envelope
> > follower is to do
> > > > it digitally in
> > > > non-real time, where you can go forward and
> > backtrack with an
> > > > adaptive
> > > > time-constant and some clever logic.
> > >
> > >This is what I tryed, now I found that I can not
> > write down
> > >a formal description of "envelope", because I do
> > not know
> > >what it really is.
> > >
> > >Like I said, I thought I knew exactly what it is
> > about, but
> > >when it comes to a formal description (algorithm) I
> > know
> > >nothing about.
> > 
> > That's because it's an ad hoc thing that doesn't
> > have a formal definition. 
> > However you do it you'll always have a trade-off
> > between responsiveness and 
> > accuracy, especially on transients.
> > 
> > If you want a digital interpretation, pick your time
> > constant, sample 
> > sections equivalent to its duration, calculate the
> > RMS value in each (not 
> > too hard digitally) and then interpolate the values
> > to create a curve. Use 
> > cubic instead of linear interpolation for a smoother
> > result.
> > 
> > For adaptive tracking lower the time constant by a
> > few factors whenever you 
> > see a transient (i.e. a very rapid change in RMS
> > level), shift the start of 
> > each RMS window to the beginning of the transient,
> > and then increase the 
> > time constant again during the release or decay
> > period. This won't track 
> > reverse transients but it will handle most other
> > things.
> > 
> > You can include a reverse transient check if you
> > want to get really picky.
> > 
> > This solution won't work in real time because
> > there's some lookahead and 
> > lookback is involved, but it will give you the best
> > possible results 
> > otherwise.
> > 
> > Richard
> > 
> > 
> 
>