fftb(data [, axes, /allaxes])
fftb, data [, axes, /allaxes]
Returns a discrete backward Fourier transform of the real
data along the indicated
axes. By default,
the backward transform (from frequency space) is applied in the first
dimension. The returned values are of type
subroutine form replaces
data with the transformed
axes is specified, then it is taken to contain the
axes along which transformation is to be done. No axis may appear
more than once. The axes need not be in any specific order. If
/allaxes is specified, then all axes are treated. If neither
/allaxes are specified, then the first
axis is assumed. The data may have arbitrary positive dimensions.
The returned array has the same dimensions as
data are assumed to be stored as ’half-complex’
sequences, in order of increasing frequency. For each frequency,
first the real and then the imaginary part is given, except that no
imaginary part is given for values that are always real, i.e., for
frequency zero and (for even-length sequences) also for the highest
represented frequency (equal to half of the sequence length).
Note that only one half of the relevant frequencies are represented in
data, because the missing half can be constructed from
the provided half by complex conjugation.
x = fftb(y) is the opposite of
y = fft(x), and
= fftb(y, /amplitudes) is the opposite of
y = fft(x,
The calculation speed of the algorithm depends strongly on the number of data points in the dimension in which the algorithm is applied. For quickest results, that number of data points should only have factors equal to small prime numbers, such as 2, 3, and 5.
See also: power, sc, scb