DOC: Specify cut-off frequency in firwin as half-ampltude vs half-power

[gerazov]

Issue with current documentation:

The documentation could be made more clear that when designing a filter with firwin the cutoff-frequency is half-amplitude, i.e. 20 * np.log10(abs(signal.freqz(b)) gives -6 dB at the cutoff rather than -3 dB.

For IIR desing via signal.iirdesign the cutoff is half-power, so -3 dB. This could also be explicitly stated.

Idea or request for content:

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Additional context (e.g. screenshots, GIFs)

import numpy as np
from matplotlib import pyplot as plt
from scipy import signal as sig

fs = 16000
b = sig.firwin(101, 2500, fs=fs)
f, h_fft = sig.freqz(b, fs=fs)
h_amp = 20 * np.log10(np.abs(h_fft))
plt.figure()
plt.plot(f, h_amp)
plt.grid()

b, a = sig.iirfilter(15, 2500, btype="low", fs=fs)
f, h_fft = sig.freqz(b, a, fs=fs)
h_amp = 20 * np.log10(np.abs(h_fft))
plt.plot(f, h_amp)
plt.axis([2100, 2900, -10, 2])
plt.legend(["FIR", "IIR"])