[BUG] - Technical Audit: Fix numerical instabilities for IIR filters

neurodsp/filt/checks.py

@@ -89,16 +89,18 @@ def check_filter_definition(pass_type, f_range):
     return f_lo, f_hi
 
 
-def check_filter_properties(b_vals, a_vals, fs, pass_type, f_range,
+def check_filter_properties(filter_coefs, a_vals, fs, pass_type, f_range,
                             transitions=(-20, -3), verbose=True):
     """Check a filters properties, including pass band and transition band.
 
     Parameters
     ----------
-    b_vals : 1d array
-        B value filter coefficients for a filter.
-    a_vals : 1d array
-        A value filter coefficients for a filter.
+    filter_coefs : 1d or 2d array
+        If 1d, interpreted as the B-value filter coefficients.
+        If 2d, interpreted as the second-order (sos) filter coefficients.
+    a_vals : 1d array or None
+        The A-value filter coefficients for a filter.
+        If second-order filter coefficients are provided in `filter_coefs`, must be None.
     fs : float
         Sampling rate, in Hz.
     pass_type : {'bandpass', 'bandstop', 'lowpass', 'highpass'}
@@ -133,15 +135,6 @@ def check_filter_properties(b_vals, a_vals, fs, pass_type, f_range,
     >>> passes = check_filter_properties(filter_coefs, 1, fs, pass_type, f_range)
     Transition bandwidth is 0.5 Hz.
     Pass/stop bandwidth is 24.0 Hz.
-
-    Check the properties of an IIR filter:
-
-    >>> from neurodsp.filt.iir import design_iir_filter
-    >>> fs, pass_type, f_range, order = 500, 'bandstop', (55, 65), 7
-    >>> b_vals, a_vals = design_iir_filter(fs, pass_type, f_range, order)
-    >>> passes = check_filter_properties(b_vals, a_vals, fs, pass_type, f_range)
-    Transition bandwidth is 1.5 Hz.
-    Pass/stop bandwidth is 10.0 Hz.
     """
 
     # Import utility functions inside function to avoid circular imports
@@ -152,7 +145,7 @@ def check_filter_properties(b_vals, a_vals, fs, pass_type, f_range,
     passes = True
 
     # Compute the frequency response
-    f_db, db = compute_frequency_response(b_vals, a_vals, fs)
+    f_db, db = compute_frequency_response(filter_coefs, a_vals, fs)
 
     # Check that frequency response goes below transition level (has significant attenuation)
     if np.min(db) >= transitions[0]:

neurodsp/filt/fir.py

@@ -6,7 +6,7 @@
 from neurodsp.utils import remove_nans, restore_nans
 from neurodsp.utils.decorators import multidim
 from neurodsp.plts.filt import plot_filter_properties
-from neurodsp.filt.utils import compute_nyquist, compute_frequency_response, remove_filter_edges
+from neurodsp.filt.utils import compute_frequency_response, remove_filter_edges
 from neurodsp.filt.checks import (check_filter_definition, check_filter_properties,
                                   check_filter_length)
 
@@ -174,15 +174,14 @@ def design_fir_filter(fs, pass_type, f_range, n_cycles=3, n_seconds=None):
     f_lo, f_hi = check_filter_definition(pass_type, f_range)
     filt_len = compute_filter_length(fs, pass_type, f_lo, f_hi, n_cycles, n_seconds)
 
-    f_nyq = compute_nyquist(fs)
     if pass_type == 'bandpass':
-        filter_coefs = firwin(filt_len, (f_lo, f_hi), pass_zero=False, nyq=f_nyq)
+        filter_coefs = firwin(filt_len, (f_lo, f_hi), pass_zero=False, fs=fs)
     elif pass_type == 'bandstop':
-        filter_coefs = firwin(filt_len, (f_lo, f_hi), nyq=f_nyq)
+        filter_coefs = firwin(filt_len, (f_lo, f_hi), fs=fs)
     elif pass_type == 'highpass':
-        filter_coefs = firwin(filt_len, f_lo, pass_zero=False, nyq=f_nyq)
+        filter_coefs = firwin(filt_len, f_lo, pass_zero=False, fs=fs)
     elif pass_type == 'lowpass':
-        filter_coefs = firwin(filt_len, f_hi, nyq=f_nyq)
+        filter_coefs = firwin(filt_len, f_hi, fs=fs)
 
     return filter_coefs
 

neurodsp/filt/iir.py

@@ -1,8 +1,6 @@
 """Filter signals with IIR filters."""
 
-from warnings import warn
-
-from scipy.signal import butter, filtfilt
+from scipy.signal import butter, sosfiltfilt
 
 from neurodsp.utils import remove_nans, restore_nans
 from neurodsp.filt.utils import compute_nyquist, compute_frequency_response
@@ -42,14 +40,14 @@ def filter_signal_iir(sig, fs, pass_type, f_range, butterworth_order,
     plot_properties : bool, optional, default: False
         If True, plot the properties of the filter, including frequency response and/or kernel.
     return_filter : bool, optional, default: False
-        If True, return the filter coefficients of the IIR filter.
+        If True, return the second order series coefficients of the IIR filter.
 
     Returns
     -------
     sig_filt : 1d array
         Filtered time series.
-    filter_coefs : tuple of (1d array, 1d array)
-        Filter coefficients of the IIR filter, as (b_vals, a_vals).
+    sos : 2d array
+        Second order series coefficients of the IIR filter. Has shape of (n_sections, 6).
         Only returned if `return_filter` is True.
 
     Examples
@@ -64,42 +62,40 @@ def filter_signal_iir(sig, fs, pass_type, f_range, butterworth_order,
     """
 
     # Design filter
-    b_vals, a_vals = design_iir_filter(fs, pass_type, f_range, butterworth_order)
+    sos = design_iir_filter(fs, pass_type, f_range, butterworth_order)
 
     # Check filter properties: compute transition bandwidth & run checks
-    check_filter_properties(b_vals, a_vals, fs, pass_type, f_range, verbose=print_transitions)
+    check_filter_properties(sos, None, fs, pass_type, f_range, verbose=print_transitions)
 
     # Remove any NaN on the edges of 'sig'
     sig, sig_nans = remove_nans(sig)
 
     # Apply filter
-    sig_filt = apply_iir_filter(sig, b_vals, a_vals)
+    sig_filt = apply_iir_filter(sig, sos)
 
     # Add NaN back on the edges of 'sig', if there were any at the beginning
     sig_filt = restore_nans(sig_filt, sig_nans)
 
     # Plot frequency response, if desired
     if plot_properties:
-        f_db, db = compute_frequency_response(b_vals, a_vals, fs)
+        f_db, db = compute_frequency_response(sos, None, fs)
         plot_frequency_response(f_db, db)
 
     if return_filter:
-        return sig_filt, (b_vals, a_vals)
+        return sig_filt, sos
     else:
         return sig_filt
 
 
-def apply_iir_filter(sig, b_vals, a_vals):
+def apply_iir_filter(sig, sos):
     """Apply an IIR filter to a signal.
 
     Parameters
     ----------
     sig : array
         Time series to be filtered.
-    b_vals : 1d array
-        B value filter coefficients for an IIR filter.
-    a_vals : 1d array
-        A value filter coefficients for an IIR filter.
+    sos : 2d array
+        Second order series coefficients for an IIR filter. Has shape of (n_sections, 6).
 
     Returns
     -------
@@ -113,12 +109,12 @@ def apply_iir_filter(sig, b_vals, a_vals):
     >>> from neurodsp.sim import sim_combined
     >>> sig = sim_combined(n_seconds=10, fs=500,
     ...                    components={'sim_powerlaw': {}, 'sim_oscillation' : {'freq': 10}})
-    >>> b_vals, a_vals = design_iir_filter(fs=500, pass_type='bandstop',
+    >>> sos = design_iir_filter(fs=500, pass_type='bandstop',
     ...                                    f_range=(55, 65), butterworth_order=7)
-    >>> filt_signal = apply_iir_filter(sig, b_vals, a_vals)
+    >>> filt_signal = apply_iir_filter(sig, sos)
     """
 
-    return filtfilt(b_vals, a_vals, sig)
+    return sosfiltfilt(sos, sig)
 
 
 def design_iir_filter(fs, pass_type, f_range, butterworth_order):
@@ -146,23 +142,17 @@ def design_iir_filter(fs, pass_type, f_range, butterworth_order):
 
     Returns
     -------
-    b_vals : 1d array
-        B value filter coefficients for an IIR filter.
-    a_vals : 1d array
-        A value filter coefficients for an IIR filter.
+    sos : 2d array
+        Second order series coefficients for an IIR filter. Has shape of (n_sections, 6).
 
     Examples
     --------
     Compute coefficients for a bandstop IIR filter:
 
-    >>> b_vals, a_vals = design_iir_filter(fs=500, pass_type='bandstop',
-    ...                                    f_range=(55, 65), butterworth_order=7)
+    >>> sos = design_iir_filter(fs=500, pass_type='bandstop',
+    ...                         f_range=(55, 65), butterworth_order=7)
     """
 
-    # Warn about only recommending IIR for bandstop
-    if pass_type != 'bandstop':
-        warn('IIR filters are not recommended other than for notch filters.')
-
     # Check filter definition
     f_lo, f_hi = check_filter_definition(pass_type, f_range)
 
@@ -175,6 +165,6 @@ def design_iir_filter(fs, pass_type, f_range, butterworth_order):
         win = f_hi / f_nyq
 
     # Design filter
-    b_vals, a_vals = butter(butterworth_order, win, pass_type)
+    sos = butter(butterworth_order, win, pass_type, output='sos')
 
-    return b_vals, a_vals
+    return sos

neurodsp/filt/utils.py

@@ -1,7 +1,7 @@
 """Utility functions for filtering."""
 
 import numpy as np
-from scipy.signal import freqz
+from scipy.signal import freqz, sosfreqz
 
 from neurodsp.utils.decorators import multidim
 from neurodsp.filt.checks import check_filter_definition
@@ -40,15 +40,17 @@ def infer_passtype(f_range):
     return pass_type
 
 
-def compute_frequency_response(b_vals, a_vals, fs):
+def compute_frequency_response(filter_coefs, a_vals, fs):
     """Compute the frequency response of a filter.
 
     Parameters
     ----------
-    b_vals : 1d array
-        B value filter coefficients for a filter.
-    a_vals : 1d array
-        A value filter coefficients for a filter.
+    filter_coefs : 1d or 2d array
+        If 1d, interpreted as the B-value filter coefficients.
+        If 2d, interpreted as the second-order (sos) filter coefficients.
+    a_vals : 1d array or None
+        The A-value filter coefficients for a filter.
+        If second-order filter coefficients are provided in `filter_coefs`, must be None.
     fs : float
         Sampling rate, in Hz.
 
@@ -67,15 +69,23 @@ def compute_frequency_response(b_vals, a_vals, fs):
     >>> filter_coefs = design_fir_filter(fs=500, pass_type='bandpass', f_range=(8, 12))
     >>> f_db, db = compute_frequency_response(filter_coefs, 1, fs=500)
 
-    Compute the frequency response for an IIR filter:
+    Compute the frequency response for an IIR filter, which uses SOS coefficients:
 
     >>> from neurodsp.filt.iir import design_iir_filter
-    >>> b_vals, a_vals = design_iir_filter(fs=500, pass_type='bandstop',
-    ...                                    f_range=(55, 65), butterworth_order=7)
-    >>> f_db, db = compute_frequency_response(b_vals, a_vals, fs=500)
+    >>> sos_coefs = design_iir_filter(fs=500, pass_type='bandpass',
+    ...                               f_range=(8, 12), butterworth_order=3)
+    >>> f_db, db = compute_frequency_response(sos_coefs, None, fs=500)
     """
 
-    w_vals, h_vals = freqz(b_vals, a_vals, worN=int(fs * 2))
+    if filter_coefs.ndim == 1 and a_vals is not None:
+        # Compute response for B & A value filter coefficient inputs
+        w_vals, h_vals = freqz(filter_coefs, a_vals, worN=int(fs * 2))
+    elif filter_coefs.ndim == 2 and a_vals is None:
+        # Compute response for sos filter coefficient inputs
+        w_vals, h_vals = sosfreqz(filter_coefs, worN=int(fs * 2))
+    else:
+        raise ValueError("The organization of the filter coefficient inputs is not understood.")
+
     f_db = w_vals * fs / (2. * np.pi)
     db = 20 * np.log10(abs(h_vals))
 
@@ -109,7 +119,7 @@ def compute_pass_band(fs, pass_type, f_range):
 
     Examples
     --------
-    Compute the bandwith of a bandpass filter:
+    Compute the bandwidth of a bandpass filter:
 
     >>> compute_pass_band(fs=500, pass_type='bandpass', f_range=(5, 25))
     20.0
@@ -158,9 +168,9 @@ def compute_transition_band(f_db, db, low=-20, high=-3):
     Compute the transition band of an IIR filter, using the computed frequency response:
 
     >>> from neurodsp.filt.iir import design_iir_filter
-    >>> b_vals, a_vals = design_iir_filter(fs=500, pass_type='bandstop',
-    ...                                    f_range=(10, 20), butterworth_order=7)
-    >>> f_db, db = compute_frequency_response(b_vals, a_vals, fs=500)
+    >>> sos = design_iir_filter(fs=500, pass_type='bandstop',
+    ...                         f_range=(10, 20), butterworth_order=7)
+    >>> f_db, db = compute_frequency_response(sos, None, fs=500)
     >>> compute_transition_band(f_db, db, low=-20, high=-3)
     2.0
     """

neurodsp/tests/test_filt_checks.py

@@ -46,8 +46,18 @@ def test_check_filter_definition():
 def test_check_filter_properties():
 
     filter_coefs = design_fir_filter(FS, 'bandpass', (8, 12))
-    check_filter_properties(filter_coefs, 1, FS, 'bandpass', (8, 12))
-    assert True
+
+    passes = check_filter_properties(filter_coefs, 1, FS, 'bandpass', (8, 12))
+    assert passes is True
+
+    filter_coefs = design_fir_filter(FS, 'bandstop', (8, 12))
+    passes = check_filter_properties(filter_coefs, 1, FS, 'bandpass', (8, 12))
+    assert passes is False
+
+    filter_coefs = design_fir_filter(FS, 'bandpass', (20, 21))
+    passes = check_filter_properties(filter_coefs, 1, FS, 'bandpass', (8, 12))
+    assert passes is False
+
 
 def test_check_filter_length():
 

neurodsp/tests/test_filt_fir.py

@@ -1,5 +1,6 @@
 """Tests for FIR filters."""
 
+from pytest import raises
 import numpy as np
 
 from neurodsp.tests.settings import FS
@@ -56,3 +57,7 @@ def test_compute_filter_length():
     expected_filt_len = int(np.ceil(fs * n_cycles / f_lo)) + 1
     filt_len = compute_filter_length(fs, 'bandpass', f_lo, f_hi, n_cycles=n_cycles, n_seconds=None)
     assert filt_len == expected_filt_len
+
+    with raises(ValueError):
+        filt_len = compute_filter_length(fs, 'bandpass', f_lo, f_hi)
+

neurodsp/tests/test_filt_iir.py

@@ -20,9 +20,12 @@ def test_filter_signal_iir_2d(tsig2d):
    assert out.shape == tsig2d.shape
    assert sum(~np.isnan(out[0, :])) > 0
 
+   out, sos = filter_signal_iir(tsig2d, FS, 'bandpass', (8, 12), 3, return_filter=True)
+   assert np.shape(sos)[1] == 6
+
 def test_apply_iir_filter(tsig):
 
-    out = apply_iir_filter(tsig, np.array([1, 1, 1, 1, 1]), np.array([1, 1, 1, 1, 1]))
+    out = apply_iir_filter(tsig, np.array([1, 1, 1, 1, 1, 1]))
     assert out.shape == tsig.shape
 
 def test_design_iir_filter():

neurodsp/tests/test_filt_utils.py

@@ -1,5 +1,6 @@
 """Tests for filter utilities."""
 
+from pytest import raises
 from neurodsp.tests.settings import FS
 
 from neurodsp.filt.utils import *
@@ -19,6 +20,9 @@ def test_compute_frequency_response():
     filter_coefs = design_fir_filter(FS, 'bandpass', (8, 12))
     f_db, db = compute_frequency_response(filter_coefs, 1, FS)
 
+    with raises(ValueError):
+        f_db, db = compute_frequency_response(filter_coefs, None, FS)
+
 def test_compute_pass_band():
 
     assert compute_pass_band(FS, 'bandpass', (4, 8)) == 4.