[sdiy] Why is Linear FM usually (always?) AC Coupled?

David G Dixon dixon at mail.ubc.ca
Mon Feb 3 01:28:38 CET 2020

The reason is because you don't want the base pitch to shift as a result of
the Linear FM signal input.  Linear FM is a good way to get subtle vibrato
effects, and obviously, this should be around the base pitch.


Having said that, I never AC couple my linear FM inputs.  It has never been
a problem, probably because I take pains to ensure that none of my VCO or
LFO outputs have offsets.



From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Pete
Sent: Sunday, February 02, 2020 4:07 PM
To: synth-diy at synth-diy.org DIY
Subject: [sdiy] Why is Linear FM usually (always?) AC Coupled?


I was just discussing this with a friend, and his speculation was that it
would prevent any DC component from shifting the center of the modulation
away from the note that was being played.  And while that seems plausible,
I've never heard anyone explicitly say that was why.  To be fair I don't
think I've heard anyone explain why.

So: is there a reason linear FM inputs are typically AC coupled?  Have we
stumbled on the right answer?

And if so, why would Exponential FM be left DC coupled?  So you could



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