The SoundCheck heterodyne analysis algorithm measures the amplitude of a sinusoidal signal using the same heterodyne technique that a radio receiver uses to tune broadcasts. That is the input signal is multiplied, in the time domain, by a sine wave of a known frequency. A basic principle of Fourier analysis is that multiplying a signal by a sinusoid of a known frequency results in shifting the frequency by known quantity. The output of the shifted signal can then be passed through a narrow-band filter tuned to the known frequency before then being measured by an RMS level meter. In the case of SoundCheck the signal is modulated to be effectively DC and then measured using a very short FFT with a user selectable window that allows the user to optimize between absolute amplitude accuracy and rejection of background noise. The heterodyne algorithm in SoundCheck has a further advantage over a generic heterodyne analyzer in that it does not need to scan, the frequencies to search the signal for are already known from the stimulus step.
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