Issue warning when a Mel filter is all zero (#914)

* Warn if create_fb_matrix produces a column whose weights are all zeros

See also https://github.com/librosa/librosa/issues/478

test/torchaudio_unittest/functional/functional_cpu_test.py

@@ -21,6 +21,23 @@ class TestLFilterFloat64(Lfilter, common_utils.PytorchTestCase):
     device = torch.device('cpu')
 
 
+class TestCreateFBMatrix(common_utils.TorchaudioTestCase):
+    def test_no_warning_high_n_freq(self):
+        with pytest.warns(None) as w:
+            F.create_fb_matrix(288, 0, 8000, 128, 16000)
+        assert len(w) == 0
+
+    def test_no_warning_low_n_mels(self):
+        with pytest.warns(None) as w:
+            F.create_fb_matrix(201, 0, 8000, 89, 16000)
+        assert len(w) == 0
+
+    def test_warning(self):
+        with pytest.warns(None) as w:
+            F.create_fb_matrix(201, 0, 8000, 128, 16000)
+        assert len(w) == 1
+
+
 class TestComputeDeltas(common_utils.TorchaudioTestCase):
     """Test suite for correctness of compute_deltas"""
     def test_one_channel(self):

torchaudio/functional.py

@@ -313,6 +313,13 @@ def create_fb_matrix(
         enorm = 2.0 / (f_pts[2:n_mels + 2] - f_pts[:n_mels])
         fb *= enorm.unsqueeze(0)
 
+    if (fb.max(dim=0).values == 0.).any():
+        warnings.warn(
+            "At least one mel filterbank has all zero values. "
+            f"The value for `n_mels` ({n_mels}) may be set too high. "
+            f"Or, the value for `n_freqs` ({n_freqs}) may be set too low."
+        )
+
     return fb