old_contents: # Copyright (c) 2006-2014 Sippy Software, Inc. All rights reserved.
  #
  # All rights reserved.
  #
  # Redistribution and use in source and binary forms, with or without modification,
  # are permitted provided that the following conditions are met:
  #
  # 1. Redistributions of source code must retain the above copyright notice, this
  # list of conditions and the following disclaimer.
  #
  # 2. Redistributions in binary form must reproduce the above copyright notice,
  # this list of conditions and the following disclaimer in the documentation and/or
  # other materials provided with the distribution.
  #
  # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
  # ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  # WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  # DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
  # ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  # (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
  # ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  from threading import local
  
  class recfilter(object):
      lastval = 0.0
      a = None
      b = None
  
      def __init__(self, fcoef, initval):
          #print 'recfilter::init()'
          self.lastval = float(initval)
          self.a = 1.0 - float(fcoef)
          self.b = float(fcoef)
  
      def apply(self, x):
          self.lastval = self.a * float(x) + self.b * self.lastval
          return self.lastval
  
  class recfilter_ts(local, recfilter):
      def __init__(self, *args):
          #print 'recfilter_ts::init()'
          recfilter.__init__(self, *args)
  
new_contents: # Copyright (c) 2006-2014 Sippy Software, Inc. All rights reserved.
  #
  # All rights reserved.
  #
  # Redistribution and use in source and binary forms, with or without modification,
  # are permitted provided that the following conditions are met:
  #
  # 1. Redistributions of source code must retain the above copyright notice, this
  # list of conditions and the following disclaimer.
  #
  # 2. Redistributions in binary form must reproduce the above copyright notice,
  # this list of conditions and the following disclaimer in the documentation and/or
  # other materials provided with the distribution.
  #
  # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
  # ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  # WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  # DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
  # ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  # (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
  # ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  from threading import local
  from math import exp, pi
  
  def calc_f_coef(Fs, Fc):
      if Fs < Fc * 2.0:
          raise ValueError('The cutoff frequency (%f) should be less ' \
            'than half of the sampling rate (%f)' %  (Fc, Fs))
      return exp(-2.0 * pi * Fc / Fs)
  
  class recfilter(object):
      lastval = 0.0
      a = None
      b = None
  
      def __init__(self, fcoef, initval):
          #print 'recfilter::init()'
          self.lastval = float(initval)
          self.a = 1.0 - float(fcoef)
          self.b = float(fcoef)
  
      def apply(self, x):
          self.lastval = self.a * float(x) + self.b * self.lastval
          return self.lastval
  
  class recfilter_ts(local, recfilter):
      def __init__(self, *args):
          #print 'recfilter_ts::init()'
          recfilter.__init__(self, *args)
  
subject: Add utility function calc_f_coef(), which calculates recursive filter coefficient based on the sampling frequency and desired filter cutoff frequency.