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