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

Amos controlvoltage at gmail.com
Mon Jun 26 21:21:25 CEST 2023


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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