old_contents: """
  Doppler
  =======
  
  Doppler shift module.
  """
  from __future__ import division
  
  
  def velocity_from_doppler_shift(c, f1, f2):
      """
      Calculate velocity based on measured frequency shifts due to Doppler shift.
      The assumption is made that the velocity is constant between the observation times.
      
      .. math:: v = c \cdot \\left( \\frac{f_2 - f_1}{f_2 + f_1} \\right)
      
      
      :param c: Speed of sound :math:`c`.
      :param f1: Lower frequency :math:`f_1`.
      :param f2: Upper frequency :math:`f_2`.
      
      """
      return c * (f2 - f1) / (f2 + f1)
  
new_contents: """
  Doppler
  =======
  
  Doppler shift module.
  """
  from __future__ import division
  
  SOUNDSPEED = 343.0
  """Speed of sound
  """
  
  def velocity_from_doppler_shift(f1, f2, c=SOUNDSPEED):
      """Calculate velocity based on measured frequency shifts due to Doppler shift.
      
      :param c: Speed of sound :math:`c`.
      :param f1: Lower frequency :math:`f_1`.
      :param f2: Upper frequency :math:`f_2`.
      
      .. math:: v = c \cdot \\left( \\frac{f_2 - f_1}{f_2 + f_1} \\right)
      
      The assumption is made that the velocity is constant between the observation times.
      
      """
      return c * (f2 - f1) / (f2 + f1)
  
  
  def frequency_shift(frequency, velocity_source, velocity_receiver, soundspeed=SOUNDSPEED):
      """Frequency shift due to Doppler effect.
      
      :param frequency: Emitted frequency :math:`f`.
      :param velocity_source: Velocity of source :math:`v_s`. Positive if the source is moving away from the receiver (and negative in the other direction).
      :param velocity_receiver: Velocity of receiver :math:`v_r`. Positive if the receiver is moving towards the source (and negative in the other direction);
      :param soundspeed: Speed of sound :math:`c`.
      
      .. math:: f = \\frac{c + v_r}{c + v_s} f_0
      
      """
      return (soundspeed + velocity_receiver) / (soundspeed + velocity_source) * frequency
  
subject: Add simple equation to calculate Doppler shift