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DADiSP Worksheet Functions > Function Categories > Fourier Transforms and Signal Processing > FLATTOPWIN
Multiplies a series with an alternate 4 term Flattop window.
FLATTOPWIN(series, ampflag, "sym")
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series |
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A series or array. |
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ampflag |
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Optional. An integer, the amplitude correction flag:
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"sym" |
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Optional. A string, the symmetry flag:
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FLATTOPWIN(N, ampflag, "sym")
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N |
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An integer, the window length. |
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ampflag |
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Optional. An integer, the amplitude correction flag:
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"sym" |
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Optional. A string, the symmetry flag:
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A series or array.
W1: gsin(1000, .001, 45)
W2: spectrum(flattopwin(W1))
W1 is multiplied by an alternate symmetric flattop window. The MAX of
where N is the length of the window and n is the nth point
W3: flattopwin(1000, "periodic")
Generates a 1000 point alternate periodic flattop window.
where N is the length of the window and n is the nth point (1 <= n <= N).
The Flattop window preserves the amplitude of a series at the expense of frequency resolution. It will accurately measure the amplitude of a series at any frequency, even if the frequency lies between FFT bins.
If ampflag == 1, the correction factor is the mean of the spectral window. This assures that the spectrum of a sinusoid of amplitude A has a peak of A.
If ampflag == 2, the correction is applied as follows:
w = flattopwin(s) * rms(s) / rms(flattopwin(s))
This assures that:
sqrt(area(psd(w))) == rms(s) approximately
If ampflag == 3, the correction is applied as follows:
w = winfun(s) / sqrt(mean(win * win)
where win is the windowing function.
The "sym" flag controls the window symmetry as follows:
"Symmetric" sets the last point to be the same value as the first point. For an N point symmetric window, a N-1 point periodic window is effectively created and the Nth point is set to the first point.
"Periodic" creates a periodic window function useful in spectrum analysis applications.
The Hamming, Hanning, Flattop and Blackman windows are part of the family of cosine window functions. The ISO 18431-1 standard periodic form of these windowing functions are defined by:
where K is the number of window coefficients and N is the length of the window. The symmetric form of the window can be constructed by setting N to N-1.
For the alternate 4 point flattop window:
However, these coefficients do not conform to the ISO standard for a flattop window. See FLATTOP for an ISO 18431-1 compatible flattop window.
See GFLATTOPWIN to generate an alternate 4 point Flattop window.