Yet some more phasemodulation thoughts

[mbartkow at ET.PUT.Poznan.PL]

Why not use phase shifters for this job ?

Lets consider a series of OTA-based phase shifters. These have such
advantage over others that can be linearly tuned within a wide fre-
quency range. Applying the same control voltage as one going to VCO
input one achieves a phase shift that is constant over pitch changes
of the VCO. Now modulating the PS control voltage by the audio signal
the phase of the signal is modulated as desired for the Yammy (or should
I spell it "yummy" ?) sound :-)

The number of phase shift stages certainly determines the maximum
modulation depth. I don't know yet, if it is better to tune the particular 
sections to different "center" frequencies (i.e. the frequency where the 
phase shift of the stage is pi/2), which would allow to achieve more linear
phase/frequency characteristics and more linear phase shift dependence on
the modulation input. On the other hand, if phase shift depends linearly
on the frequency it is just a continuous delay line. So maybe a step-like
phase characteristics is better, whereby all the spectral components are 
shifted in the same way ? This is easier to build, but the shape of the
signal is destroyed as in all nonlinear-phase systems. It might lead to
more dramatic effect of the modulation. What do you think? 


regards,

MB


PS. I have got a reprint of the original J. Chowning's article on FM synthesis.
Although he refers to it as "frequency modulation", it is obviously a phase
modulation in his own formulas. In fact, these two are to some extent equiv-
alent, since after appropriate scalling of the modulation depth, or index they 
give identical output.

--

Maciej Bartkowiak, PhD
========================================================================
Institute of Electronics and Telecommunication     fax: (+48 61) 8782572
Poznan University of Technology          phone: (+48 61) 8791016 int.171
Piotrowo 3A                             email: mbartkow at et.put.poznan.pl
60-965 Poznan POLAND               http://www.et.put.poznan.pl/~mbartkow
========================================================================