As the amplitude of a modulating voltage to an exponential VCO is increased, the pitch of the modulated signal arises, making dynamic depth FM unrealistic. The pitch rise is proportional to I/o, the first of the modified Bessel functions. Higher order terms involving additional modified Bessel functions can be used to compute the entire spectrum. Various methods of correcting for the pitch shift are possible, but the most useful solution to the dynamic depth FM problem with exponential VCO's is to add some form of auxiliary linear control.
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