software EGs / PICs

Dave Halliday dave.halliday at
Thu Jun 19 07:45:58 CEST 1997

>>What got me thinking was a discussion about envelope generation.  This 
>>is an application where a PIC would be perfect.  Choose one with the 
>>8-bit A/D / D/A onboard, use a chunk of memory ( external EEROM ) for
>>shape tables and you can have as complex and as fast a risetime as you 
>>wish.  The Analog / DIgital section can output several channels of 
>>envelope - laging, modulated, different waveshapes, etc...  You can 
>>also use the Analog inputs for reading the panel pots - there is also 
>>a serial input so communication ( updates ) is possible.
>     Very cool idea.  I'm not that sure I understand how the PIC would 
>     shape the analog waveform, but I get the basic concept of it... and I 
>     like the idea of having EEROM memory to store waveforms...

I was thinking that the PIC would respond to an input ( CV and Gate or 
MIDI ) take a quick look at the position of a couple panel pots
and proceed to spit out a stored waveform - your envelope.

The waveform could be either a "generic" exponential or linear one 
( based on a small number of stored data points and interpolated ) or 
a complex arbitrary one based on a couple K-words worth of sample in 
EEROM.  The neat thing is that your circuit could be accepting input 
from another analog source and adding the two, it could be generating 
several identical waveforms except one would be inverted, delayed by 
nnn milliseconds etc...

I am imagining using this lag feature especially.  Use it to control a 
VCF and VCA - the quality of the sound will change with it's amplitude 
but there will be a fraction of a second for it to evolve before the 
volume goes down.  With two VCA's, you could have the fundamental OSC 
decay first and the harmonic one decay a little after.  Or two VCF's 
with different sound sources...

Quick Watson, the game's afoot!   <grin>

>     I've thought of adding a motorized control section to my PAIA FatMan 
>     so I can automate my pots -- and the heart of the system would be a 
>     Parallax BasicStamp -- just something that I can hack around with, 
>     make the pots move, etc. by way of serial commands sent by my PC (2400 
>     baud) to the BasicStamp.

>     I've started looking for automated pots -- if anyone has any 
>     suggestions, let's here 'em!

The only ones I have seen have been in the $30 range and the operation 
is really slow with ZERO feedback as to position.  YOu would have to 
use one of the two tracks to derive any positional informaition.

There are a number of cheap ways to get around this - how about using 
the old Calrex Opto cells which were in Craig Anderton's book - a
light dependent resistor and an LED in a single light-tight package.  
Put two of them together and they do not track very well but...
Another option would be using some form of gain stage ( CA3080 )
The last option ( for low frequencies only ) would be to use a CMOS 
switch, running at several tens of KHz with a variable duty cycle.
--- Via Silver Xpress V4.4 [Reg]

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