Provided by: tcllib_1.21+dfsg-1_all 
NAME
math::fourier - Discrete and fast fourier transforms
SYNOPSIS
package require Tcl 8.4
package require math::fourier 1.0.2
::math::fourier::dft in_data
::math::fourier::inverse_dft in_data
::math::fourier::lowpass cutoff in_data
::math::fourier::highpass cutoff in_data
________________________________________________________________________________________________________________
DESCRIPTION
The math::fourier package uses the fast Fourier transform, if applicable, or the ordinary transform to
implement the discrete Fourier transform. It also provides a few simple filter procedures as an
illustration of how such filters can be implemented.
The purpose of this document is to describe the implemented procedures and provide some examples of their
usage. As there is ample literature on the algorithms involved, we refer to relevant text books for more
explanations. We also refer to the original Wiki page on the subject which describes some of the
considerations behind the current implementation.
GENERAL INFORMATION
The two top-level procedures defined are
• dft data-list
• inverse_dft data-list
Both take a list of complex numbers and apply a Discrete Fourier Transform (DFT) or its inverse
respectively to these lists of numbers. A "complex number" in this case is either (i) a pair (two
element list) of numbers, interpreted as the real and imaginary parts of the complex number, or (ii) a
single number, interpreted as the real part of a complex number whose imaginary part is zero. The return
value is always in the first format. (The DFT generally produces complex results even if the input is
purely real.) Applying first one and then the other of these procedures to a list of complex numbers will
(modulo rounding errors due to floating point arithmetic) return the original list of numbers.
If the input length N is a power of two then these procedures will utilize the O(N log N) Fast Fourier
Transform algorithm. If input length is not a power of two then the DFT will instead be computed using
the naive quadratic algorithm.
Some examples:
% dft {1 2 3 4}
{10 0.0} {-2.0 2.0} {-2 0.0} {-2.0 -2.0}
% inverse_dft {{10 0.0} {-2.0 2.0} {-2 0.0} {-2.0 -2.0}}
{1.0 0.0} {2.0 0.0} {3.0 0.0} {4.0 0.0}
% dft {1 2 3 4 5}
{15.0 0.0} {-2.5 3.44095480118} {-2.5 0.812299240582} {-2.5 -0.812299240582} {-2.5 -3.44095480118}
% inverse_dft {{15.0 0.0} {-2.5 3.44095480118} {-2.5 0.812299240582} {-2.5 -0.812299240582} {-2.5 -3.44095480118}}
{1.0 0.0} {2.0 8.881784197e-17} {3.0 4.4408920985e-17} {4.0 4.4408920985e-17} {5.0 -8.881784197e-17}
In the last case, the imaginary parts <1e-16 would have been zero in exact arithmetic, but aren't here
due to rounding errors.
Internally, the procedures use a flat list format where every even index element of a list is a real part
and every odd index element is an imaginary part. This is reflected in the variable names by Re_ and Im_
prefixes.
The package includes two simple filters. They have an analogue equivalent in a simple electronic circuit,
a resistor and a capacitance in series. Using these filters requires the math::complexnumbers package.
PROCEDURES
The public Fourier transform procedures are:
::math::fourier::dft in_data
Determine the Fourier transform of the given list of complex numbers. The result is a list of
complex numbers representing the (complex) amplitudes of the Fourier components.
list in_data
List of data
::math::fourier::inverse_dft in_data
Determine the inverse Fourier transform of the given list of complex numbers (interpreted as
amplitudes). The result is a list of complex numbers representing the original (complex) data
list in_data
List of data (amplitudes)
::math::fourier::lowpass cutoff in_data
Filter the (complex) amplitudes so that high-frequency components are suppressed. The implemented
filter is a first-order low-pass filter, the discrete equivalent of a simple electronic circuit
with a resistor and a capacitance.
float cutoff
Cut-off frequency
list in_data
List of data (amplitudes)
::math::fourier::highpass cutoff in_data
Filter the (complex) amplitudes so that low-frequency components are suppressed. The implemented
filter is a first-order low-pass filter, the discrete equivalent of a simple electronic circuit
with a resistor and a capacitance.
float cutoff
Cut-off frequency
list in_data
List of data (amplitudes)
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please
report such in the category math :: fourier of the Tcllib Trackers
[http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for enhancements you may have for
either package and/or documentation.
When proposing code changes, please provide unified diffs, i.e the output of diff -u.
Note further that attachments are strongly preferred over inlined patches. Attachments can be made by
going to the Edit form of the ticket immediately after its creation, and then using the left-most button
in the secondary navigation bar.
KEYWORDS
FFT, Fourier transform, complex numbers, mathematics
CATEGORY
Mathematics
tcllib 1.0.2 math::fourier(3tcl)