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FFT in scipy etc

From: andrew cooke <andrew@...>

Date: Thu, 4 Aug 2011 09:59:27 -0400

A quick example of generating an FTT using ipython / numpy / pylab etc.
Warning: I am not convinced this is correct.  My main reference is

Start ipython so that it will produce plots:

  > ipython -pylab
  Python 2.7 (r27:82500, Aug 07 2010, 16:54:59) [GCC]
  Type "copyright", "credits" or "license" for more information.

  IPython 0.10 -- An enhanced Interactive Python.
  ?         -> Introduction and overview of IPython's features.
  %quickref -> Quick reference.
  help      -> Python's own help system.
  object?   -> Details about 'object'. ?object also works, ?? prints more.

    Welcome to pylab, a matplotlib-based Python environment.
    For more information, type 'help(pylab)'.

  In [1]: # import what we need

  In [2]: from numpy import *

  In [3]: from pylab import *

  In [4]: # the data are a column of numbers that were sampled at 40Hz

  In [5]: y = genfromtxt('1312405054-23-result-KIV1-SHZ.txt')

  In [6]: len(y)
  Out[6]: 4000

  In [7]: # so generate an x axis

  In [8]: x=r_[0:100:1/40.0]

  In [9]: len(x)
  Out[9]: 4000

  In [10]: plot(x,y)
  Out[10]: [<matplotlib.lines.Line2D object at 0x1e46890>]

  In [11]: # now do the fft

  In [12]: s = fft(y)

  In [13]: plot(s)
  /usr/lib64/python2.7/site-packages/numpy/core/numeric.py:284: ComplexWarning: Casting complex values to real discards the imaginary part
    return array(a, dtype, copy=False, order=order)
  Out[13]: [<matplotlib.lines.Line2D object at 0x1dfc950>]

  In [14]: # that's the usual fft mess, so generate freq axis for half

  In [15]: f = 40*r_[0:2000]/4000.

  In [16]: plot(f,abs(s[0:2000])/4000)
  Out[16]: [<matplotlib.lines.Line2D object at 0x1e91090>]


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