[sdiy] VCO Linear FM input - DC or AC coupled?

Mattias Rickardsson mr at analogue.org
Mon Jun 26 22:26:40 CEST 2023

Good point! Maybe it works that way, if you manage to have the full audio
band covered by the lower "differentiator" parts of the frequency response
of the practical active differentiator shown here:


Den mån 26 juni 2023 21:21Amos <controlvoltage at gmail.com> skrev:

> My interpretation of what I read in the SSI 2130 datasheet was that an
> audio signal capacitively coupled to its FM input was effectively
> modulating it by the derivative of the audio signal, thus giving you PM. Is
> there some additional nuance involved?
> On Mon, Jun 26, 2023 at 2:53 PM Mattias Rickardsson <mr at analogue.org>
> wrote:
>> On Mon, 26 Jun 2023 at 20:39, ben gebhardt via Synth-diy <
>> synth-diy at synth-diy.org> wrote:
>>> Isn't this just the difference between phase modulation and frequency
>>> modulation?
>> No, there's a derivative involved.
>> Frequency is the derivative of phase. Hence,
>> PM with a signal is the same thing as FM with the derivative of the
>> signal.
>> Having AC coupling on linear FM inputs is because any DC component will
>> bend the pitch away. Ideally I guess you want low enough DC-cutoff to leave
>> your modulating signal waveforms untouched, but fast enough to get rid of
>> DC components introduced while patching around... or something like that.
>> :-)
>> /mr
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