docs: def zero_crossing_last(frames):
      """Finds the last zero crossing in frames"""
      frames = N.array(frames)
  
      crossings = N.where(N.diff(N.sign(frames)))
      # crossings = N.where(frames[:n] * frames[1:n + 1] < 0)
  
      if len(crossings[0]) == 0:
          print "No zero crossing"
          return len(frames) - 1
      return crossings[0][-1]