[sdiy] Coupling for FM?

[Antti Huovilainen]

On Fri, 6 Oct 2006 ceme at cox.net wrote:

> What are the (electric and (most importantly) sonic) differences between 
> audio FM from a VCO to a VCF with and without a capacitor between?

Drift and no drift. Almost no difference in practise if the capacitor is 
large enough and your FM signal has no DC offset (well trimmed saw or 
triangle, pulse is out of question).

> control. Why is this and how can I calculate the capacitance necessary 
> for a particular circuit? Along these same lines, what about VCO to VCO 
> FM and the AC/DC coupling involved?  Why and how?

The capacitor acts as simple 1st order highpass filter. Just place the 
-3dB sufficiently below lowest frequency you want to pass (at say 10 Hz 
for example).
For VCO FM you pretty much need it there to avoid unacceptable tuning 
instability (also the reason why almost all digital synths actually do 
phase modulation instead of FM).

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
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