Sound Card Spectrum Window Toggle

Controls: Spectrum Dialog >> Window
Macro: SpectWindOn

This button applies a window function to the raw waveform data before computing the spectrum. You can toggle this from the keyboard via ALT+W even when the Spectrum dialog is not active.

An optional BW Corr (Bandwidth Correction) button just below the Window button allows you to select true energy scaling instead of best peak amplitudes. The window function itself is selected using the six radio buttons at the bottom of the Window groupbox.

The FFT (Fast Fourier Transform) that is used to generate the Spectrum display can give misleading results when signal frequencies don't fall exactly on the analysis frequencies of the FFT. The 1024-point FFT used here provides 512 analysis frequencies, spread evenly from 0 hertz to the Nyquist frequency (half the sample rate).

The FFT only provides information about the amount of signal at each of these analysis frequencies. When a signal component falls between two analysis frequencies, each will show a partial response. But in addition there will be leakage responses at many adjacent frequencies, which on the display will appear as "skirts" draping away from the true frequency peak. A window function, applied to the waveform before computing the FFT, can reduce these leakage skirts, but there is a trade-off among the various possible window functions: Functions that most reduce the leakage will also broaden the response near the true peak.

You don't need a window function when the signal is generated using the Lines option for setting the Generator sine frequency, since it is exactly synchronous and produces no leakage skirts.

You also shouldn't use Window when the signal is broadband noise (no tone peaks), or when analyzing transients that fit entirely within the 1024 samples used by the FFT.

See Spectrum Window Theory for a complete discussion.

Macro Notes:

SpectWindOn=1 activates Spectrum Window, SpectWindOn=0 turns it off, and SpectWindOn=x toggles between on and off.

See also Spectrum Control Dialog