Next: Functions and Variables for FFTPACK5, Previous: Introduction to fast Fourier transform [Contents][Index]
Translates complex values of the form r %e^(%i t) to the form
a + b %i, where r is the magnitude and t is the phase.
r and t are 1-dimensional arrays of the same size.
The array size need not be a power of 2.
The original values of the input arrays are
replaced by the real and imaginary parts, a and b, on return.
The outputs are calculated as
a = r cos(t) b = r sin(t)
polartorect is the inverse function of recttopolar.
load("fft") loads this function. See also fft.
Translates complex values of the form a + b %i to the form
r %e^(%i t), where a is the real part and b is the imaginary
part. a and b are 1-dimensional arrays of the same size.
The array size need not be a power of 2.
The original values of the input arrays are
replaced by the magnitude and angle, r and t, on return.
The outputs are calculated as
r = sqrt(a^2 + b^2) t = atan2(b, a)
The computed angle is in the range -%pi to %pi.
recttopolar is the inverse function of polartorect.
load("fft") loads this function. See also fft.
Computes the inverse complex fast Fourier transform.
y is a list or array (named or unnamed) which contains the data to
transform. The number of elements must be a power of 2.
The elements must be literal numbers (integers, rationals, floats, or bigfloats)
or symbolic constants,
or expressions a + b*%i where a and b are literal numbers
or symbolic constants.
inverse_fft returns a new object of the same type as y,
which is not modified.
Results are always computed as floats
or expressions a + b*%i where a and b are floats.
If bigfloat precision is needed the function bf_inverse_fft can
be used instead as a drop-in replacement of inverse_fft that is
slower, but supports bfloats.
The inverse discrete Fourier transform is defined as follows.
Let x be the output of the inverse transform.
Then for j from 0 through n - 1,
x[j] = sum(y[k] exp(-2 %i %pi j k / n), k, 0, n - 1)
As there are various sign and normalization conventions possible, this definition of the transform may differ from that used by other mathematical software.
load("fft") loads this function.
See also fft (forward transform), recttopolar, and
polartorect.
Examples:
Real data.
(%i1) load ("fft") $
(%i2) fpprintprec : 4 $
(%i3) L : [1, 2, 3, 4, -1, -2, -3, -4] $
(%i4) L1 : inverse_fft (L);
(%o4) [0.0, 14.49 %i - .8284, 0.0, 2.485 %i + 4.828, 0.0,
4.828 - 2.485 %i, 0.0, - 14.49 %i - .8284]
(%i5) L2 : fft (L1);
(%o5) [1.0, 2.0 - 2.168L-19 %i, 3.0 - 7.525L-20 %i,
4.0 - 4.256L-19 %i, - 1.0, 2.168L-19 %i - 2.0,
7.525L-20 %i - 3.0, 4.256L-19 %i - 4.0]
(%i6) lmax (abs (L2 - L));
(%o6) 3.545L-16
Complex data.
(%i1) load ("fft") $
(%i2) fpprintprec : 4 $
(%i3) L : [1, 1 + %i, 1 - %i, -1, -1, 1 - %i, 1 + %i, 1] $
(%i4) L1 : inverse_fft (L);
(%o4) [4.0, 2.711L-19 %i + 4.0, 2.0 %i - 2.0,
- 2.828 %i - 2.828, 0.0, 5.421L-20 %i + 4.0, - 2.0 %i - 2.0,
2.828 %i + 2.828]
(%i5) L2 : fft (L1);
(%o5) [4.066E-20 %i + 1.0, 1.0 %i + 1.0, 1.0 - 1.0 %i,
1.55L-19 %i - 1.0, - 4.066E-20 %i - 1.0, 1.0 - 1.0 %i,
1.0 %i + 1.0, 1.0 - 7.368L-20 %i]
(%i6) lmax (abs (L2 - L));
(%o6) 6.841L-17
Computes the complex fast Fourier transform.
x is a list or array (named or unnamed) which contains the data to
transform. The number of elements must be a power of 2.
The elements must be literal numbers (integers, rationals, floats, or bigfloats)
or symbolic constants,
or expressions a + b*%i where a and b are literal numbers
or symbolic constants.
fft returns a new object of the same type as x,
which is not modified.
Results are always computed as floats
or expressions a + b*%i where a and b are floats.
If bigfloat precision is needed the function bf_fft can be used
instead as a drop-in replacement of fft that is slower, but
supports bfloats. In addition if it is known that the input consists
of only real values (no imaginary parts), real_fft can be used
which is potentially faster.
The discrete Fourier transform is defined as follows.
Let y be the output of the transform.
Then for k from 0 through n - 1,
y[k] = (1/n) sum(x[j] exp(+2 %i %pi j k / n), j, 0, n - 1)
As there are various sign and normalization conventions possible, this definition of the transform may differ from that used by other mathematical software.
When the data x are real,
real coefficients a and b can be computed such that
x[j] = sum(a[k]*cos(2*%pi*j*k/n)+b[k]*sin(2*%pi*j*k/n), k, 0, n/2)
with
a[0] = realpart (y[0]) b[0] = 0
and, for k from 1 through n/2 - 1,
a[k] = realpart (y[k] + y[n - k]) b[k] = imagpart (y[n - k] - y[k])
and
a[n/2] = realpart (y[n/2]) b[n/2] = 0
load("fft") loads this function.
See also inverse_fft (inverse transform),
recttopolar, and polartorect.. See real_fft
for FFTs of a real-valued input, and bf_fft and
bf_real_fft for operations on bigfloat values. Finally, for
transforms of any size (but limited to float values), see
fftpack5_fft and fftpack5_real_fft.
Examples:
Real data.
(%i1) load ("fft") $
(%i2) fpprintprec : 4 $
(%i3) L : [1, 2, 3, 4, -1, -2, -3, -4] $
(%i4) L1 : fft (L);
(%o4) [0.0, 1.811 %i - .1036, 0.0, 0.3107 %i + .6036, 0.0,
0.6036 - 0.3107 %i, 0.0, (- 1.811 %i) - 0.1036]
(%i5) L2 : inverse_fft (L1);
(%o5) [1.0, 2.168L-19 %i + 2.0, 7.525L-20 %i + 3.0,
4.256L-19 %i + 4.0, - 1.0, - 2.168L-19 %i - 2.0,
- 7.525L-20 %i - 3.0, - 4.256L-19 %i - 4.0]
(%i6) lmax (abs (L2 - L));
(%o6) 3.545L-16
Complex data.
(%i1) load ("fft") $
(%i2) fpprintprec : 4 $
(%i3) L : [1, 1 + %i, 1 - %i, -1, -1, 1 - %i, 1 + %i, 1] $
(%i4) L1 : fft (L);
(%o4) [0.5, 0.5, 0.25 %i - 0.25, (- 0.3536 %i) - 0.3536, 0.0, 0.5,
(- 0.25 %i) - 0.25, 0.3536 %i + 0.3536]
(%i5) L2 : inverse_fft (L1);
(%o5) [1.0, 1.0 %i + 1.0, 1.0 - 1.0 %i, - 1.0, - 1.0, 1.0 - 1.0 %i,
1.0 %i + 1.0, 1.0]
(%i6) lmax (abs (L2 - L));
(%o6) 0.0
Computation of sine and cosine coefficients.
(%i1) load ("fft") $
(%i2) fpprintprec : 4 $
(%i3) L : [1, 2, 3, 4, 5, 6, 7, 8] $
(%i4) n : length (L) $
(%i5) x : make_array (any, n) $
(%i6) fillarray (x, L) $
(%i7) y : fft (x) $
(%i8) a : make_array (any, n/2 + 1) $
(%i9) b : make_array (any, n/2 + 1) $
(%i10) a[0] : realpart (y[0]) $
(%i11) b[0] : 0 $
(%i12) for k : 1 thru n/2 - 1 do
(a[k] : realpart (y[k] + y[n - k]),
b[k] : imagpart (y[n - k] - y[k]));
(%o12) done
(%i13) a[n/2] : y[n/2] $
(%i14) b[n/2] : 0 $
(%i15) listarray (a);
(%o15) [4.5, - 1.0, - 1.0, - 1.0, - 0.5]
(%i16) listarray (b);
(%o16) [0, - 2.414, - 1.0, - .4142, 0]
(%i17) f(j) := sum (a[k]*cos(2*%pi*j*k/n) + b[k]*sin(2*%pi*j*k/n),
k, 0, n/2) $
(%i18) makelist (float (f (j)), j, 0, n - 1);
(%o18) [1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0]
Computes the fast Fourier transform of a real-valued sequence
x. This is equivalent to performing fft(x), except that
only the first N/2+1 results are returned, where N is
the length of x. N must be power of two.
No check is made that x contains only real values.
The symmetry properties of the Fourier transform of real sequences to
reduce he complexity. In particular the first and last output values
of real_fft are purely real. For larger sequences, real_fft
may be computed more quickly than fft.
Since the output length is short, the normal inverse_fft cannot
be directly used. Use inverse_real_fft to compute the inverse.
Computes the inverse Fourier transform of y, which must have a
length of N/2+1 where N is a power of two. That is, the
input x is expected to be the output of real_fft.
No check is made to ensure that the input has the correct format. (The first and last elements must be purely real.)
Computes the inverse complex fast Fourier transform. This is the
bigfloat version of inverse_fft that converts the input to
bigfloats and returns a bigfloat result.
Computes the forward complex fast Fourier transform. This is the
bigfloat version of fft that converts the input to
bigfloats and returns a bigfloat result.
Computes the forward fast Fourier transform of a real-valued input
returning a bigfloat result. This is the bigfloat version of
real_fft.
Computes the inverse fast Fourier transform with a real-valued
bigfloat output. This is the bigfloat version of inverse_real_fft.
Next: Functions and Variables for FFTPACK5, Previous: Introduction to fast Fourier transform [Contents][Index]