[sdiy] what is the amplitude envelope of a signal

[Scott Gravenhorst]

Hey Martin and Harry,

Pardon my lame observation, but it looks to me like the problem
is that process needs to know the true length of a cycle.  That
is, in order to avoid recognizing a cycle's secondary peaks as
primary peaks.  Almost as if a PLL is needed in the algorithm to
mark out a cycle.  While this might not work in a practical
circuit, it might be useful in a non real time algorithm.

Then again, it is early here and I'm just sipping my first cup of
coffee.  So I hope I've at least amused, if not helped and
hopefully not annoyed...  Anyway, interesting stuff.  I'm all eyes.

"Czech Martin" <Martin.Czech at micronas.com> wrote:
>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
>> > 
>> > 
>> 
>> 
>

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