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Module « scipy.signal »

Fonction cheb1ord - module scipy.signal

Signature de la fonction cheb1ord

def cheb1ord(wp, ws, gpass, gstop, analog=False, fs=None) 

Description

help(scipy.signal.cheb1ord)

Chebyshev type I filter order selection.

Return the order of the lowest order digital or analog Chebyshev Type I
filter that loses no more than `gpass` dB in the passband and has at
least `gstop` dB attenuation in the stopband.

Parameters
----------
wp, ws : float
    Passband and stopband edge frequencies.

    For digital filters, these are in the same units as `fs`. By default,
    `fs` is 2 half-cycles/sample, so these are normalized from 0 to 1,
    where 1 is the Nyquist frequency. (`wp` and `ws` are thus in
    half-cycles / sample.)  For example:

        - Lowpass:   wp = 0.2,          ws = 0.3
        - Highpass:  wp = 0.3,          ws = 0.2
        - Bandpass:  wp = [0.2, 0.5],   ws = [0.1, 0.6]
        - Bandstop:  wp = [0.1, 0.6],   ws = [0.2, 0.5]

    For analog filters, `wp` and `ws` are angular frequencies (e.g., rad/s).
gpass : float
    The maximum loss in the passband (dB).
gstop : float
    The minimum attenuation in the stopband (dB).
analog : bool, optional
    When True, return an analog filter, otherwise a digital filter is
    returned.
fs : float, optional
    The sampling frequency of the digital system.

    .. versionadded:: 1.2.0

Returns
-------
ord : int
    The lowest order for a Chebyshev type I filter that meets specs.
wn : ndarray or float
    The Chebyshev natural frequency (the "3dB frequency") for use with
    `cheby1` to give filter results. If `fs` is specified,
    this is in the same units, and `fs` must also be passed to `cheby1`.

See Also
--------
cheby1 : Filter design using order and critical points
buttord : Find order and critical points from passband and stopband spec
cheb2ord, ellipord
iirfilter : General filter design using order and critical frequencies
iirdesign : General filter design using passband and stopband spec

Examples
--------
Design a digital lowpass filter such that the passband is within 3 dB up
to 0.2*(fs/2), while rejecting at least -40 dB above 0.3*(fs/2). Plot its
frequency response, showing the passband and stopband constraints in gray.

>>> from scipy import signal
>>> import matplotlib.pyplot as plt
>>> import numpy as np

>>> N, Wn = signal.cheb1ord(0.2, 0.3, 3, 40)
>>> b, a = signal.cheby1(N, 3, Wn, 'low')
>>> w, h = signal.freqz(b, a)
>>> plt.semilogx(w / np.pi, 20 * np.log10(abs(h)))
>>> plt.title('Chebyshev I lowpass filter fit to constraints')
>>> plt.xlabel('Normalized frequency')
>>> plt.ylabel('Amplitude [dB]')
>>> plt.grid(which='both', axis='both')
>>> plt.fill([.01, 0.2, 0.2, .01], [-3, -3, -99, -99], '0.9', lw=0) # stop
>>> plt.fill([0.3, 0.3,   2,   2], [ 9, -40, -40,  9], '0.9', lw=0) # pass
>>> plt.axis([0.08, 1, -60, 3])
>>> plt.show()

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