  In this paper it is shown that the mathematical form which most precisely
describes the Blazhko RR Lyrae light curves is connected to almost periodic
functions and not to the mathematics of modulation. That is, the Blazhko effect
is more than a simple external modulation of the pulsation signal. The
mathematical framework of almost periodic functions predicts a new observable
effect: a shift of the Fourier harmonics of the main pulsation frequency from
the exact harmonic position. This phenomenon is called as harmonic detuning
effect (HDE).The published deviations of the harmonics of V445 Lyr are
explained with this effect. HDE is also found for V2178 Cyg, another Blazhko
star observed by the Kepler space telescope. HDE is detectable only if the
phase variation part of the Blazhko effect is of large amplitude and
non-periodic enough, additionally, the time span of the observed light curve is
sufficiently long for obtaining precise frequencies. These three conditions
restrict the number of stars showing detectable HDE and explain why this effect
has not been discovered up to now.
