Erica Synths EDU DIY EG Eurorack Module Kit - Manual (Page 7)

THE SIMPLEST ENVELOPE
Since a square wave is really just an oscillation between
a high- and a low level voltage, this would indeed cause
our filter to open up and close down. Even better – it
would do so rhythmically as the oscillator cycles
through its phases. And while this does work, it has two
severe limitations. First: the opening and closing
movements are always instant, with no way of making
them any more gradual. This might be what you want in
some contexts, but often, you’d need something less
abrupt. And second: We can’t change the rhythmic
pattern – all we get is a constant staccato.
So what can we do about that? Well, the most obvious thing is to try and make the rising
and falling edges less steep.
In envelope terms, we’d say that we want to slow down
the attack and extend the release . And while this may sound complicated, it’s actually
anything but.
All we need are two components: a resistor and a capacitor, set up like this. Now, if 3
you’ve worked with analog filters before, this setup should look strikingly familiar. That’s
because this is really just a bog-standard passive low pass filter . And what it does to
our square wave is exactly what we said we’re after: it takes the rising and falling edges
and makes them less steep.
Here’s how it works. Once the voltage at the input switches from low to high, a current
will be forced through the resistor and into the capacitor, slowly filling it up. As the
capacitor is being charged, the voltage at the output slowly rises until the cap is
completely filled up, and the input- and output voltages align. Then, when the input
voltage swings low, the whole process reverses. Now the capacitor will push its contents
through the resistor and into the input, so to speak. This happens because the voltage on
the right is much higher than the voltage on the left. As the capacitor empties out slowly,
Read more about resistors and capacitors in the components & concepts appendix (page 3
26/27).
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