Zero-frequency shift
Perform a shift in order to move the frequency 0 to the center of the input.
fftshift(x, MARGIN = 2)
x |
input data, specified as a vector or matrix. |
MARGIN |
dimension to operate along, 1 = row, 2 = columns (default).
Specifying |
If x is a vector of N elements corresponding to N time
samples spaced by dt, then fftshift(x) corresponds to
frequencies f = c(-seq(ceiling((N-1)/2), 1, -1), 0, (1:floor((N-1)/2)))
* df, where df = 1 / (N * dt). In other words, the left and right
halves of x are swapped.
If x is a matrix, then fftshift operates on the rows or columns
of x, according to the MARGIN argument, i.e. it swaps the the
upper and lower halves of the matrix (MARGIN = 1), or the left and
right halves of the matrix (MARGIN = 2). Specifying MARGIN =
c(1, 2) swaps along both dimensions, i.e., swaps the first quadrant with the
fourth, and the second with the third.
vector or matrix with centered frequency.
Vincent Cautaerts, vincent@comf5.comm.eng.osaka-u.ac.jp,
adapted by John W. Eaton.
Conversion to R by Geert van Boxtel, G.J.M.vanBoxtel@gmail.com.
ifftshift
Xeven <- 1:6 ev <- fftshift(Xeven) # returns 4 5 6 1 2 3 Xodd <- 1:7 odd <- fftshift(Xodd) # returns 5 6 7 1 2 3 4 fs <- 100 # sampling frequency t <- seq(0, 10 - 1/fs, 1/fs) # time vector S <- cos(2 * pi * 15 * t) n <- length(S) X <- fft(S) f <- (0:(n - 1)) * (fs / n); # frequency range power <- abs(X)^2 / n # power plot(f, power, type="l") Y <- fftshift(X) fsh <- ((-n/2):(n/2-1)) * (fs / n) # zero-centered frequency range powersh <- abs(Y)^2 / n # zero-centered power plot(fsh, powersh, type = "l")
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