Erica Synths EDU DIY EG Eurorack Module Kit - Manual (Page 15)

THE PSEUDO-ADSR ENVELOPE
Here’s how that would work. As you can see, I’ve basically just copied our comparator
and pasted it down at the bottom. Both the new and the original one get the input signal,
and they share the same reference voltage, but I’ve placed a high-pass filter before the
top one. That high-pass would normally turn a square wave cycle into two short voltage
spikes.
First a positive one, when the input transitions from low
to high – and then a negative one when it drops from
high to low. Now, since we’re not interested in the
negative spike, and it could cause our comparator to
glitch out under certain circumstances, I’ve decided to
eliminate it using another diode. Whenever the voltage
at the comparator’s input (i.e. after the 1 µF capacitor)
tries to go negative, the diode will open and thereby
neutralize it.
12
Okay, but what do we need the positive spike for?
Simple.
By feeding this positive spike into our
comparator, we get a quick 12 V burst right when
the envelope is triggered
. So with a fast attack, the
envelope’s curve will always start at the peak level.
You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right 12
here: https://tinyurl.com/yc2c8u5h – you can change all values by double clicking on
components.
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