# BLACKMAN

## Purpose:

Multiplies a series with a 3 term Blackman window.

## Syntax:

BLACKMAN(series, ampflag, "sym")

series

-

A series or array.

ampflag

-

Optional. An integer, the amplitude correction flag:

 0 : do not correct amplitude (default) 1 : correct amplitude 2 : correct RMS amplitude 3 : correct mean squared amplitude

"sym"

-

Optional. A string, the symmetry flag:

 "symmetric" : Starting and ending points are equal (default). "periodic" : Periodically extended window where the symmetric end point is removed. Conforms to the ISO standard. "iso" : Same as "periodic".

## Alternate Syntax:

BLACKMAN(N, ampflag, "sym")

N

-

An integer, the length of the window.

ampflag

-

Optional. An integer, the amplitude correction flag:

 0 : do not correct amplitude (default) 1 : correct amplitude 2 : correct RMS amplitude 3 : correct mean squared amplitude

"sym"

-

Optional. A string, the symmetry flag:

 "symmetric" : Starting and ending points are equal (default). "periodic" : Periodically extended window where the symmetric end point is removed. Conforms to the ISO standard. "iso" : Same as "periodic".

## Returns:

A series or array.

## Example:

W1: gsin(1000, .001, 45)

W2: spectrum(blackman(W1))

W3: spectrum(blackman(W1, 1))

The MAX of W2 == 0.419 and the MAX of W3 == 1.0. The amplitude of the spectrum in W3 has been corrected to take into account amplitude effects of the symmetric 3 term Blackman window.

where n is the nth point (1 <= n <= N) and N is the number of points to generate.

## Example:

W4: blackman(1000, "periodic")

Creates a 1000 point periodic Blackman window that conforms to the ISO 18431-1 standard.

where n is the nth point (1 <= n <= N) and N is the number of points to generate.

## Remarks:

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 = blackman(s) * rms(s) / rms(blackman(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 same value as the first point.

"Periodic" or "iso" creates a periodic window function useful in spectrum analysis applications. "Periodic" or "iso" conforms to the ISO 18431-1 standard for windowing functions.

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 Blackman window:

See GBLACKMAN to generate a Blackman window.

See BLACKMANHARRIS to multiply a series with a 4 term Blackman-Harris window.