[med-svn] [python-mne] 242/376: adding low and high pass filters

[Yaroslav Halchenko]

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yoh pushed a commit to annotated tag v0.1
in repository python-mne.

commit 722ad1f2ed0280948a4e21551e55c3408b26f1db
Author: Alexandre Gramfort <alexandre.gramfort at inria.fr>
Date:   Tue May 10 15:44:33 2011 -0400

    adding low and high pass filters
---
 mne/filter.py            | 130 ++++++++++++++++++++++++++++++++++++++++++++++-
 mne/tests/test_filter.py |  12 +++++
 2 files changed, 141 insertions(+), 1 deletion(-)

diff --git a/mne/filter.py b/mne/filter.py
index a6a8be4..c2f4d73 100644
--- a/mne/filter.py
+++ b/mne/filter.py
@@ -43,6 +43,8 @@ def band_pass_filter(x, Fs, Fp1, Fp2):
     Fs1 = Fp1 - 0.5 in Hz
     Fs2 = Fp2 + 0.5 in Hz
     """
+    Fp1 = float(Fp1)
+    Fp2 = float(Fp2)
 
     # Default values in Hz
     Fs1 = Fp1 - 0.5
@@ -53,7 +55,7 @@ def band_pass_filter(x, Fs, Fp1, Fp2):
     # Make x EVEN
     Norig = len(x)
     if Norig % 2 == 1:
-        x = np.c_[x, 1]
+        x = np.r_[x, 1]
 
     # Normalize frequencies
     Ns1 = Fs1 / (Fs / 2)
@@ -74,3 +76,129 @@ def band_pass_filter(x, Fs, Fp1, Fp2):
     x = x[:Norig]
 
     return xf
+
+
+def low_pass_filter(x, Fs, Fp):
+    """Lowpass filter for the signal x.
+
+    An acausal fft algorithm is applied (i.e. no phase shift). The filter
+    functions is constructed from a Hamming window (window used in "firwin2"
+    function) to avoid ripples in the frequency reponse (windowing is a
+    smoothing in frequency domain)
+
+    Parameters
+    ----------
+    x : 1d array
+        Signal to filter
+    Fs : float
+        sampling rate
+    Fp : float
+        cut-off frequency
+
+    Returns
+    -------
+    xf : array
+        x filtered
+
+    Notes
+    -----
+    The passbands (Fp1 Fp2) frequencies are defined in Hz as
+      -------------------------
+                              | \
+                              |  \
+                              |   \
+                              |    \
+                              |     -----------------
+                              |
+                              Fp  Fp+0.5
+
+    """
+    Fp = float(Fp)
+
+    assert x.ndim == 1
+
+    # Make x EVEN
+    Norig = len(x)
+    if Norig % 2 == 1:
+        x = np.r_[x, 1]
+
+    # Normalize frequencies
+    Ns = (Fp + 0.5) / (Fs / 2)
+    Np = Fp / (Fs / 2)
+
+    # Construct the filter function H(f)
+    N = len(x)
+
+    B = signal.firwin2(N, [0, Np, Ns, 1], [1, 1, 0, 0])
+
+    # Make zero-phase filter function
+    H = np.abs(fft(B))
+
+    xf = np.real(ifft(fft(x) * H))
+    xf = xf[:Norig]
+    x = x[:Norig]
+
+    return xf
+
+
+def high_pass_filter(x, Fs, Fp):
+    """Highpass filter for the signal x.
+
+    An acausal fft algorithm is applied (i.e. no phase shift). The filter
+    functions is constructed from a Hamming window (window used in "firwin2"
+    function) to avoid ripples in the frequency reponse (windowing is a
+    smoothing in frequency domain)
+
+    Parameters
+    ----------
+    x : 1d array
+        Signal to filter
+    Fs : float
+        sampling rate
+    Fp : float
+        cut-off frequency
+
+    Returns
+    -------
+    xf : array
+        x filtered
+
+    Notes
+    -----
+    The passbands (Fp1 Fp2) frequencies are defined in Hz as
+                   -----------------------
+                 /|
+                / |
+               /  |
+              /   |
+    ----------    |
+                  |
+          Fp-0.5  Fp
+
+    """
+    Fp = float(Fp)
+
+    assert x.ndim == 1
+
+    # Make x ODD
+    Norig = len(x)
+    if Norig % 2 == 0:
+        x = np.r_[x, 1]
+
+    # Normalize frequencies
+    Ns = (Fp - 0.5) / (Fs / 2)
+    Np = Fp / (Fs / 2)
+
+    # Construct the filter function H(f)
+    N = len(x)
+
+    B = signal.firwin2(N, [0, Ns, Np, 1], [0, 0, 1, 1])
+
+    # Make zero-phase filter function
+    H = np.abs(fft(B))
+
+    xf = np.real(ifft(fft(x) * H))
+    xf = xf[:Norig]
+    x = x[:Norig]
+
+    return xf
diff --git a/mne/tests/test_filter.py b/mne/tests/test_filter.py
new file mode 100644
index 0000000..98f5ff1
--- /dev/null
+++ b/mne/tests/test_filter.py
@@ -0,0 +1,12 @@
+import numpy as np
+from numpy.testing import assert_array_almost_equal
+
+from ..filter import band_pass_filter, high_pass_filter, low_pass_filter
+
+def test_filters():
+    a = np.random.randn(1000)
+    Fs = 1000
+    bp = band_pass_filter(a, Fs, 4, 8)
+    lp = low_pass_filter(a, Fs, 8)
+    hp = high_pass_filter(lp, Fs, 4)
+    assert_array_almost_equal(hp, bp, 2)

-- 
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