add shifted delta coef feature

speechbox/preprocess/features.py

@@ -38,8 +38,23 @@ def mfcc_deltas_012(utterance, **kwargs):
     features[2::3] = librosa.feature.delta(mfccs, order=2, width=5)
     return features.T
 
+def sdc(frames, d, P, k, **kwargs):
+    """
+    Shifted-delta coefficients from
+    Torres-Carrasquillo, P. A. el al. (2002).
+    https://www.isca-speech.org/archive/icslp_2002/i02_0089.html
+
+    N is assumed to be frames.shape[1].
+    """
+    assert d > 0 and P > 0 and k > 0, "invalid sdc parameters d = {}, P = {}, k = {}".format(d, P, k)
+    assert frames.ndim > 1, "invalid dimensions for feature frames: {}".format(frames.ndim)
+    num_frames = frames.shape[0]
+    frames = np.pad(frames, pad_width=((0, d + k*P), (0, 0)), mode='constant', constant_values=0)
+    #TODO without loop expression
+    return np.concatenate([frames[i+2*d::P][:k] - frames[i::P][:k] for i in range(num_frames)])
 
 all_extractors = collections.OrderedDict([
     ("mfcc", mfcc),
     ("mfcc-deltas-012", mfcc_deltas_012),
+    ("sdc", sdc),
 ])