add fft calc example

docs/fft.py

@@ -0,0 +1,35 @@
+import numpy as np
+import matplotlib.pyplot as plt
+import scipy.fftpack
+
+# Fe = sample rate
+# N = samples count
+def plot(Fe, N, x, y):
+  plt.subplot(2, 1, 1)
+  plt.plot(x, y)
+
+  yf = scipy.fftpack.fft(y)
+  xf = np.linspace(0.0, Fe/2, N/2)
+
+  plt.subplot(2, 1, 2)
+  plt.plot(xf, 2.0/N * np.abs(yf[:N/2]))
+  plt.show()
+
+def sample():
+  Fe = 1000
+  N = Fe
+  x = np.linspace(0.0, 1.0, N)
+  y = np.sin(50.0 * 2.0*np.pi*x) + 0.5*np.sin(80.0 * 2.0*np.pi*x)
+
+  plot(Fe, N, x, y)
+
+def real_sound_weave(freqHz):
+  Fe = 16000
+  durationMs = 100
+  N = Fe * durationMs / 1000
+  x = np.linspace(0.0, N, N)
+  y = np.sin(2.0 * np.pi * x / (Fe / float(freqHz))) * 0x7FFF
+
+  plot(Fe, N, x, y)
+
+real_sound_weave(4500)