# [CM] fm and transposition

Juan Reyes juanig@ccrma.Stanford.EDU
17 Aug 2004 21:08:28 -0700

```I am not sure but this might bring back the old issue of Phase
Modulation and why Phase Modulation is a good implementation of FM.

In FM, if remember well, the carrier frequency is not always the
fundamental and also if you have a modulation index which is not an
integer and greater than 1 you might have "fold-over" of lateral
frequencies which might be the case of the fm-bell and low frequencies.
Therefore the spectra is not the same at different frequencies. A good
guess might be to try changing the modulation index or the C:M ratio so
that the spectra (amplitudes of partials) remains the same at different
frequencies.

--* Juan

On Tue, 2004-08-17 at 04:15, Oded Ben-Tal wrote:
> I'm using clms fm-bell instrument to produce sounds to be used in an
> experiment. Each short sound is presented in 3 transpositions. I'm
> extracting two numbers out of the sounds RMS amplitude and a mean
> frequency.(*) So here is my question: RMS
> amplitude and mean frequency values of the transposed sounds are not
> simple transpositions. in other words if 1a and 1b are two sounds using
> fundamental freq. f0, 2a and 2b are copies of the clm code with f0 shifted
> up. the RMS amplitude of 1a is larger than 1b but the reverse is true of
> 2a and 2b. (and similarly for mean frequency).
> I'm trying to understand why is that so. Or if it shouldn't be to track my
> error.
>
> thanks
> Oded
>
> (*)For the mean frequency I'm averaging windowed fft of the
> sound
> weighted by local amplitude (the trail end of the sound is soft such that
> the average frequency is that of the noise; the sounds are not single
> pitch so I'm not using fundamental frequency).
>
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