fir_taps

#!/usr/bin/python3

import sys
import argparse
import numpy as np

def lowpass(w_c, N, start=None):
	a = np.arange(N) - (float(N - 1) / 2)
	taps = np.sin(a * w_c) / a / np.pi

	if N % 2 == 1:
		taps[N/2] = w_c / np.pi

	return taps

WINDOW_FUNC = {
	'rectangular': lambda a: np.zeros(a) + 1,
	'hamming': np.hamming,
	'hanning': lambda a: np.hanning(a + 2)[1:-1]
}

def window_by_name(name):
	try:
		return WINDOW_FUNC[name]
	except KeyError:
		msg = "%r is not a known windowing function" % name
		raise argparse.ArgumentTypeError(msg)

def main():
	parser = argparse.ArgumentParser(description='Produce taps for a low-pass FIR filter.')

	parser.add_argument('-n', metavar='NUMBER', type=int, required=True,
						help='number of taps')
	parser.add_argument('-r', metavar='RATE', type=float, required=True,
						help='sample rate')
	parser.add_argument('-c', metavar='FREQ', type=float, required=True,
						help='cutoff frequency')
	parser.add_argument('-w', metavar='WINDOW', type=window_by_name, required=True,
						help='windowing function, one of: ' + ', '.join(WINDOW_FUNC.keys()))

	args = parser.parse_args()

	taps = lowpass((args.c / args.r) * 2 * np.pi, args.n) * args.w(args.n)

	np.savetxt(sys.stdout, taps)

if __name__ == "__main__":
	main()