Merge pull request matplotlib#7845 from vollbier/specgram_overlap_err

Fixed bug with default parameters NFFT and noverlap in specgram()

lib/matplotlib/axes/_axes.py

@@ -7200,8 +7200,12 @@ def specgram(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
         if not self._hold:
             self.cla()
 
+        if NFFT is None:
+            NFFT = 256  # same default as in mlab.specgram()
         if Fc is None:
-            Fc = 0
+            Fc = 0  # same default as in mlab._spectral_helper()
+        if noverlap is None:
+            noverlap = 128  # same default as in mlab.specgram()
 
         if mode == 'complex':
             raise ValueError('Cannot plot a complex specgram')

lib/matplotlib/mlab.py

@@ -1273,7 +1273,12 @@ def specgram(x, NFFT=None, Fs=None, detrend=None, window=None,
 
     """
     if noverlap is None:
-        noverlap = 128
+        noverlap = 128  # default in _spectral_helper() is noverlap = 0
+    if NFFT is None:
+        NFFT = 256  # same default as in _spectral_helper()
+    if len(x) <= NFFT:
+        warnings.warn("Only one segment is calculated since parameter NFFT " +
+                      "(=%d) >= signal length (=%d)." % (NFFT, len(x)))
 
     spec, freqs, t = _spectral_helper(x=x, y=None, NFFT=NFFT, Fs=Fs,
                                       detrend_func=detrend, window=window,