lfilter: add an optional arg clamp (#600)

* lfilter: add an optional arg `clamp`

to give users control on the clamping behavior within the range [-1, 1],
which was hardcoded before.

Fixes #596

* doc string formatting

* lint

* doc string again

Co-authored-by: Vincent QB <vincentqb@users.noreply.github.com>

test/test_functional.py

@@ -28,6 +28,15 @@ def test_simple(self):
 
         torch.testing.assert_allclose(output_waveform[:, 3:], waveform[:, 0:-3], atol=1e-5, rtol=1e-5)
 
+    def test_clamp(self):
+        input_signal = torch.ones(1, 44100 * 1, dtype=self.dtype, device=self.device)
+        b_coeffs = torch.tensor([1, 0], dtype=self.dtype, device=self.device)
+        a_coeffs = torch.tensor([1, -0.95], dtype=self.dtype, device=self.device)
+        output_signal = F.lfilter(input_signal, a_coeffs, b_coeffs, clamp=True)
+        self.assertTrue(output_signal.max() <= 1)
+        output_signal = F.lfilter(input_signal, a_coeffs, b_coeffs, clamp=False)
+        self.assertTrue(output_signal.max() > 1)
+
 
 class TestLfilterFloat32CPU(_LfilterMixin, unittest.TestCase):
     device = torch.device('cpu')

torchaudio/functional.py

@@ -673,21 +673,23 @@ def phase_vocoder(
 def lfilter(
         waveform: Tensor,
         a_coeffs: Tensor,
-        b_coeffs: Tensor
+        b_coeffs: Tensor,
+        clamp: bool = True,
 ) -> Tensor:
     r"""Perform an IIR filter by evaluating difference equation.
 
     Args:
-        waveform (Tensor): audio waveform of dimension of `(..., time)`.  Must be normalized to -1 to 1.
-        a_coeffs (Tensor): denominator coefficients of difference equation of dimension of `(n_order + 1)`.
-                                Lower delays coefficients are first, e.g. `[a0, a1, a2, ...]`.
+        waveform (Tensor): audio waveform of dimension of ``(..., time)``.  Must be normalized to -1 to 1.
+        a_coeffs (Tensor): denominator coefficients of difference equation of dimension of ``(n_order + 1)``.
+                                Lower delays coefficients are first, e.g. ``[a0, a1, a2, ...]``.
                                 Must be same size as b_coeffs (pad with 0's as necessary).
-        b_coeffs (Tensor): numerator coefficients of difference equation of dimension of `(n_order + 1)`.
-                                 Lower delays coefficients are first, e.g. `[b0, b1, b2, ...]`.
+        b_coeffs (Tensor): numerator coefficients of difference equation of dimension of ``(n_order + 1)``.
+                                 Lower delays coefficients are first, e.g. ``[b0, b1, b2, ...]``.
                                  Must be same size as a_coeffs (pad with 0's as necessary).
+        clamp (bool, optional): If ``True``, clamp the output signal to be in the range [-1, 1] (Default: ``True``)
 
     Returns:
-        Tensor: Waveform with dimension of `(..., time)`.  Output will be clipped to -1 to 1.
+        Tensor: Waveform with dimension of ``(..., time)``.
     """
     # pack batch
     shape = waveform.size()
@@ -731,7 +733,10 @@ def lfilter(
         o0.addmv_(windowed_output_signal, a_coeffs_flipped, alpha=-1)
         padded_output_waveform[:, i_sample + n_order - 1] = o0
 
-    output = torch.clamp(padded_output_waveform[:, (n_order - 1):], min=-1., max=1.)
+    output = padded_output_waveform[:, (n_order - 1):]
+
+    if clamp:
+        output = torch.clamp(output, min=-1., max=1.)
 
     # unpack batch
     output = output.reshape(shape[:-1] + output.shape[-1:])