How does SoundCheck's Heterodyne analysis algorithm work?

The SoundCheck heterodyne analysis algorithm measures the amplitude of a sinusoidal signal using the same technique that a radio receiver uses to tune broadcasts. This is done by taking the input signal and multiplying it by a sine wave of a known frequency in the time domain. In basic Fourier analysis it is known that by 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 selected window that allows them to customize their preferences between absolute amplitude accuracy and rejection of background noise. SoundCheck's heterodyne algorithm takes less computational time than generic ones because it reads the frequencies required to scan the test signal from the stimulus step earlier on in the sequence.  

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