[sdiy] what is the amplitude envelope of a signal

[Richard Wentk]

At 16:28 30/09/2003 -0700, Harry Bissell Jr wrote:
>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.

But this depends on horizontal zoom level!  You're going to see variations 
depending on how you display the signal.

>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.

Okay, but an RMS approximation to this would still give a useful signal, if 
maybe not a perfectly accurate one.

>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.

You could control this with transient detection/processing.

>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

That's why it's best done digitally. :-)

Still - you can get *useful* results from a plain old rectifier/smoother 
circuit. This may not be accurate in engineering terms, but you can still 
do creative and musically relevant things with it

There are also various PPM/RMS metering circuits which (literally) produce 
a good approximation of how your eyes would like to see volume changes.

If the standard simple circuit isn't working for you, one of those would be 
the next step.

Richard