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

mskala at ansuz.sooke.bc.ca mskala at ansuz.sooke.bc.ca
Tue Feb 4 15:39:20 CET 2020

On Tue, 4 Feb 2020, Mattias Rickardsson wrote:
> Den mån 3 feb. 2020 01:30 <mskala at ansuz.sooke.bc.ca> skrev:
>       People think that linear FM doesn't
>       shift the perceived pitch, and in order to help with that, it's common to
>       make the input AC coupled.
> Intriguing formulation there - do you mean it actually shifts the perceived pitch? :-)

Pitch perception is very complicated, hard to predict, and may be
different for different listeners or different contexts.  Also, pitch is
itself logarithmic in frequency, so the statement that linear FM
(preserving the arithmetic average of the instantaneous frequency) doesn't
shift pitch and exponential FM (preserving the average of the log of the
instantaneous frequency, both of these assuming no DC offset in the
modulating waveform) does shift pitch, seems to depend on the ear
perceiving frequency changes differently depending on their speed.  That
is possible, but then we have to ask at what speed of frequency change the
ear switches between the two measurement bases.

The complicated inharmonic waveforms that come from some FM patches may
not exactly have a perceived pitch at all.  And Shepard tones demonstrate
that pitch may be perceptible while not being a well-behaved number.

So I think the blanket statement that linear FM doesn't shift perceived
pitch and exponential does, is inaccurate.  It'd be better to say that
linear FM may be less likely to shift perceived pitch.

Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before tribes.

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