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

Mattias Rickardsson mr at analogue.org
Tue Jun 27 11:03:29 CEST 2023


On Mon, 26 Jun 2023 at 23:29, Neil Johnson <neil.johnson71 at gmail.com> wrote:

> On Mon, 26 Jun 2023 at 22:20, Mattias Rickardsson <mr at analogue.org> wrote:
>> Neil Johnson <neil.johnson71 at gmail.com> skrev:
>>> ...
>>> Purists will pop up and say that when the modulating signal is a sine
>>> wave there is no difference between the two, and that's fine (lets ignore
>>> the pi/2 difference between sin and cos, eh?).
>> Surely the amplitudes differ, right? And the modulation depth that
>> follows. Louder treble and weaker bass in a differentiated sine wave. :-)
> Only because your differentiator is rubbish!

I'm referring to the f factor here  -  the derivative of a certain sinewave
is lower than the derivative of an equal-amplitude sinewave with higher
frequency. Comparing PM and FM with the same wideband modulator signal
gives more high-frequent modulation in the PM case. My gut feeling is that
this ought to be the less desirable of the two, but still it's used in
typical FM synthesis. :-)

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