Multiplies a series with a Flattop window.
FLATTOP(series, ampflag, type, "sym")
series 
 
A series or array.  
ampflag 
 
Optional. An integer, the amplitude correction flag:
 
type 
 
Optional. An integer, the window generation method:
 
"sym" 
 
Optional. A string, the symmetry flag:

FLATTOP(N, ampflag, type, "sym")
N 
 
An integer, the length of the window.  
ampflag 
 
Optional. An integer, the amplitude correction flag:
 
type 
 
Optional. An integer, the window generation method:
 
"sym" 
 
Optional. A string, the symmetry flag:

A series or array.
W1: gsin(1000, .001, 45)
W2: spectrum(flattop(W1))
The MAX of W2 == 1.0 at 45 Hertz.
W3: flattop(1000, "symmetric")
Generates a 1000 point symmetric flattop window.
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 = flattop(s) * rms(s) / rms(flattop(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 periodic flattop window is constructed by the formula of the form:
where N is the length of the window and n is the n^{th }point (1 <= n <= N).
The symmetric flattop window is constructed by the formula of the form:
where N is the length of the window and n is the n^{th} point (1 <= n <= N).
The type parameter specifies the values of the flattop window coefficients. The following types are supported:
For type 0, ISO 184311 (default):
For type 1, two point flattop window:
For type 2, alternate four point flattop window:
For type 3, three point HP P301 flattop window:
For type 4, HP four point window:
For type 5, modified HP P401 five point window:
For type 6, Rohde & Schwartz four point window:
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 N1 point periodic window is effectively created and the N^{th} point is set to the first point.
"Periodic" or "iso" creates a periodic window function useful in spectrum analysis applications where the starting zero is preserved and the trailing zero is removed. "Periodic" or "iso" conforms to the ISO 184311 standard for windowing functions.
The Hamming, Hanning, Flattop and Blackman windows are part of the family of cosine window functions. The ISO 184311 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 N1.
Because the ISO 184311 standard specifies a periodic flattop window, FLATTOP uses "periodic" as the default for the "sym" flag.
See GFLATTOP to generate a Flattop window.
See FLATTOPWIN for an implementation of the alternative 4 term flattop window (type == 2).