fftshift

rearranges the fft output, moving the zero frequency to the center of the spectrum

Calling Sequence

y=fftshift(x [,job])

Arguments

:x real or complex vector or matrix. : :y real or complex vector or matrix. : :job integer, dimension selection, or string ‘all’ :

Description

if x results of an fft computation y= fftshift(x) or y= fftshift(x,”all”) moves the zero frequency component to the center of the spectrum, which is sometimes a more convenient form.

If x is a vector of size n, y is the vector x([n/2+1:n,1:n/2])

If x is an m by n matrix y is the matrix x([m/2+1:n,1:m/2],[n/2+1:n,1:n/2]).

[x11 x12]                  [x22 x21]
x=[       ]        gives   y=[       ]
[x21 x22]                  [x12 x11]

y= fftshift(x,n) make the swap only along the `n`th dimension

Examples

//make a signal
t=0:0.1:1000;
x=3*`sin`_(t)+8*`sin`_(3*t)+0.5*`sin`_(5*t)+3*`rand`_(t);
//compute the fft
y=`fft`_(x,-1);

//display
`clf`_();
`subplot`_(2,1,1);`plot2d`_(`abs`_(y))
`subplot`_(2,1,2);`plot2d`_(fftshift(`abs`_(y)))

//make a 2D image
t=0:0.1:30;
x=3*`sin`_(t')*`cos`_(2*t)+8*`sin`_(3*t')*`sin`_(5*t)+..
  0.5*`sin`_(5*t')*`sin`_(5*t)+3*`rand`_(t')*`rand`_(t);
//compute the fft
y=`fft`_(x,-1);

//display
`clf`_();
`xset`_('colormap',`hotcolormap`_(256))
`subplot`_(2,1,1);`Matplot`_(`abs`_(y))
`subplot`_(2,1,2);`Matplot`_(fftshift(`abs`_(y)))

See Also

• fft fast Fourier transform.