bandpass.py

import wave
import numpy as np


# band-pass filter https://plot.ly/python/fft-filters/
def bandpass(lowfreq, highfreq, framerate, frames):
    # fL = 3900 / framerate
    # fH = 4200 / framerate
    fL = lowfreq / framerate
    fH = highfreq / framerate
    b = 0.08
    N = int(np.ceil((4/b)))
    if not N % 2:  # no odd numbers
        N += 1
        n = np.arange(N)

    # low-pass filter
    hlpf = np.sinc(2 * fH * (n - (N - 1) / 2.))
    hlpf *= np.blackman(N)
    hlpf = hlpf / np.sum(hlpf)

    # high-pass filter
    hhpf = hlpf = np.sinc(2 * fL * (n - (N - 1) / 2.))
    hlpf *= np.blackman(N)
    hlpf = hlpf / np.sum(hhpf)
    hhpf = -hhpf
    hhpf[int((N - 1) / 2)] += 1

    h = np.convolve(hlpf, hhpf)
    new_signal = np.convolve(frames, h)
    return new_signal


fd = wave.open('/home/cblades/beeping_clip.aiff', 'rb')
n_samples = fd.getnframes()
framerate = fd.getframerate()
samplewidth = fd.getsampwidth()
print(samplewidth)
channels = fd.getnchannels()
assert(channels == 1)

frames = fd.readframes(n_samples)
frames = np.fromstring(frames, dtype='<i2')
fd.close()

# filter out anything not in ~4000Hz range
new_signal = bandpass(3900, 4200, framerate, frames)

out = wave.open('filtered_beeping.wav', 'wb')
out.setnchannels(channels)
out.setsampwidth(samplewidth)
out.setframerate(framerate)
out.writeframes(new_signal.tobytes())
out.close()
fd.close()