From 58aa313d06058734c8e79ffce7867242fbff3af8 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 4 Mar 2026 21:51:05 +0000
Subject: [PATCH 1/2] Initial plan


From 5eb290da9c2e4d44409282daddbb23d87c8cced8 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 4 Mar 2026 21:54:54 +0000
Subject: [PATCH 2/2] Implement sos2ss, sos2tf, and sos2zp functions in
 transfer.py

Co-authored-by: cokelaer <778821+cokelaer@users.noreply.github.com>
---
 src/spectrum/transfer.py | 88 ++++++++++++++++++++++++++++++++++++++--
 test/test_transfer.py    | 40 ++++++++++++++++++
 2 files changed, 125 insertions(+), 3 deletions(-)

diff --git a/src/spectrum/transfer.py b/src/spectrum/transfer.py
index 11f6632..bd36bce 100644
--- a/src/spectrum/transfer.py
+++ b/src/spectrum/transfer.py
@@ -2,9 +2,9 @@
 
 import numpy as np
 
-__all__ = ["tf2zp",  'eqtflength', 'latc2tf', 'latcfilt',
-    'ss2zpk', 'tf2sos', 'tf2ss', 'tf2zpk', 'zpk2ss', 'zpk2tf']
-
+__all__ = ['eqtflength', 'latc2tf', 'latcfilt',
+    'sos2ss', 'sos2tf', 'sos2zp',
+    'ss2zpk', 'tf2zp', 'tf2sos', 'tf2ss', 'tf2zpk', 'zpk2ss', 'zpk2tf']
 
 """to be done
 
@@ -249,3 +249,85 @@ def zpk2ss(z, p, k):
     return scipy.signal.zpk2ss(z,p,k)
 
 
+def sos2tf(sos):
+    """Return a single transfer function from a series of second-order sections.
+
+    :param array_like sos: Array of second-order filter coefficients, must have
+        shape ``(n_sections, 6)``. Each row corresponds to a second-order
+        section, with the first three columns providing the numerator
+        coefficients and the last three providing the denominator coefficients.
+
+    :return:
+        * b : ndarray Numerator polynomial coefficients.
+        * a : ndarray Denominator polynomial coefficients.
+
+    .. doctest::
+
+        >>> import scipy.signal
+        >>> from spectrum.transfer import sos2tf
+        >>> sos = scipy.signal.butter(4, 0.2, output='sos')
+        >>> b, a = sos2tf(sos)
+
+    .. note:: wrapper of scipy function sos2tf
+    .. seealso:: :func:`sos2ss`, :func:`sos2zp`
+    """
+    import scipy.signal
+    return scipy.signal.sos2tf(sos)
+
+
+def sos2zp(sos):
+    """Return zeros, poles, and gain of a series of second-order sections.
+
+    :param array_like sos: Array of second-order filter coefficients, must have
+        shape ``(n_sections, 6)``. Each row corresponds to a second-order
+        section, with the first three columns providing the numerator
+        coefficients and the last three providing the denominator coefficients.
+
+    :return:
+        * z : ndarray Zeros of the transfer function.
+        * p : ndarray Poles of the transfer function.
+        * k : float System gain.
+
+    .. doctest::
+
+        >>> import scipy.signal
+        >>> from spectrum.transfer import sos2zp
+        >>> sos = scipy.signal.butter(4, 0.2, output='sos')
+        >>> z, p, k = sos2zp(sos)
+
+    .. note:: wrapper of scipy function sos2zpk
+    .. seealso:: :func:`sos2ss`, :func:`sos2tf`
+    """
+    import scipy.signal
+    return scipy.signal.sos2zpk(sos)
+
+
+def sos2ss(sos):
+    """Convert digital filter second-order section parameters to state-space form.
+
+    :param array_like sos: Array of second-order filter coefficients, must have
+        shape ``(n_sections, 6)``. Each row corresponds to a second-order
+        section, with the first three columns providing the numerator
+        coefficients and the last three providing the denominator coefficients.
+
+    :return:
+        * A : ndarray State-space representation of the IIR filter.
+        * B : ndarray State-space representation of the IIR filter.
+        * C : ndarray State-space representation of the IIR filter.
+        * D : ndarray State-space representation of the IIR filter.
+
+    .. doctest::
+
+        >>> import scipy.signal
+        >>> from spectrum.transfer import sos2ss
+        >>> sos = scipy.signal.butter(4, 0.2, output='sos')
+        >>> A, B, C, D = sos2ss(sos)
+
+    .. note:: Implemented by chaining :func:`sos2tf` and ``scipy.signal.tf2ss``.
+    .. seealso:: :func:`sos2tf`, :func:`sos2zp`
+    """
+    import scipy.signal
+    b, a = sos2tf(sos)
+    return scipy.signal.tf2ss(b, a)
+
+
diff --git a/test/test_transfer.py b/test/test_transfer.py
index 56b9971..3db10b1 100644
--- a/test/test_transfer.py
+++ b/test/test_transfer.py
@@ -89,6 +89,46 @@ def test_ss2zpk():
     assert k == kp
 
 
+def test_sos2tf():
+    from scipy import signal
+    sos = signal.butter(4, 0.2, output='sos')
+    b, a = transfer.sos2tf(sos)
+    # Verify round-trip: sos2tf then tf2zpk should match sos2zpk
+    z_tf, p_tf, k_tf = signal.tf2zpk(b, a)
+    z_sos, p_sos, k_sos = signal.sos2zpk(sos)
+    assert_almost_equal(abs(k_tf), abs(k_sos), decimal=5)
+    assert len(z_tf) == len(z_sos)
+    assert len(p_tf) == len(p_sos)
+
+
+def test_sos2zp():
+    from scipy import signal
+    sos = signal.butter(4, 0.2, output='sos')
+    z, p, k = transfer.sos2zp(sos)
+    z_ref, p_ref, k_ref = signal.sos2zpk(sos)
+    assert_almost_equal(np.sort(np.abs(z)), np.sort(np.abs(z_ref)), decimal=5)
+    assert_almost_equal(np.sort(np.abs(p)), np.sort(np.abs(p_ref)), decimal=5)
+    assert_almost_equal(k, k_ref, decimal=5)
+
+
+def test_sos2ss():
+    from scipy import signal
+    sos = signal.butter(4, 0.2, output='sos')
+    A, B, C, D = transfer.sos2ss(sos)
+    # Verify state-space matrices are returned with correct shapes
+    n_sections = sos.shape[0]
+    order = 2 * n_sections
+    assert A.shape == (order, order)
+    assert B.shape == (order, 1)
+    assert C.shape == (1, order)
+    assert D.shape == (1, 1)
+    # Verify round-trip: ss2zpk should recover poles/zeros matching sos2zpk
+    z_ss, p_ss, k_ss = signal.ss2zpk(A, B, C, D)
+    z_sos, p_sos, k_sos = signal.sos2zpk(sos)
+    assert_almost_equal(np.sort(np.abs(p_ss)), np.sort(np.abs(p_sos)), decimal=5)
+    assert_almost_equal(abs(k_ss), abs(k_sos), decimal=5)
+
+