Catch FM stations using Python

FM.py

from rtlsdr import RtlSdr
import numpy as np
import scipy.signal as signal
from scipy.io.wavfile import write

sdr = RtlSdr()

F_station = int(105.9e6)   # FM Station
duration = 10
F_offset = 250000         # Offset to capture at
# We capture at an offset to avoid DC spike
Fc = F_station - F_offset # Capture center frequency
Fs = int(1140000)         # Sample rate
N = int(8192000)          # Samples to capture

# configure device
sdr.sample_rate = Fs      # Hz
sdr.center_freq = Fc      # Hz
sdr.gain = 'auto'

# Read samples
samples = sdr.read_samples(N)

# Clean up the SDR device
sdr.close()
del(sdr)

# Convert samples to a numpy array
x1 = np.array(samples).astype("complex64")

# Get a baseband signal, a signal with no modulation
fc1 = np.exp(-1.0j*2.0*np.pi* F_offset/Fs*np.arange(len(x1)))
x2 = x1 * fc1

# Decimate data
f_bw = 200000
dec_rate = int(Fs / f_bw)
x4 = signal.decimate(x2, dec_rate)
# Calculate the new sampling rate
Fs_y = Fs/dec_rate

# Turn changes in frequency into changes in amplitude
y5 = x4[1:] * np.conj(x4[:-1])
x5 = np.angle(y5)

#Filter and remove noise
d = Fs_y * 75e-6   # Calculate the # of samples to hit the -3dB point
x = np.exp(-1/d)   # Calculate the decay between each sample
b = [1-x]          # Create the filter coefficients
a = [1,-x]
x6 = signal.lfilter(b,a,x5)

audio_freq = 44100.0
dec_audio = int(Fs_y/audio_freq)
Fs_audio = int(Fs_y / dec_audio)

x7 = signal.decimate(x6, dec_audio)

#Convert to real numbers
x7 *= 10000 / np.max(np.abs(x7))
# Save to file as 16-bit signed single-channel audio samples
x7 = x7.astype('int16')

#Save audio
write('audio.wav', Fs_audio, x7)