Speedup _make_wav

When numpy is installed, ~30x speedup; otherwise, ~5x speedup

IPython/lib/display.py

@@ -130,7 +130,6 b' class Audio(DisplayObject):'
     @staticmethod
     def _make_wav(data, rate, normalize):
         """ Transform a numpy array to a PCM bytestring """
-        import struct
         from io import BytesIO
         import wave
 
@@ -145,7 +144,7 b' class Audio(DisplayObject):'
         waveobj.setframerate(rate)
         waveobj.setsampwidth(2)
         waveobj.setcomptype('NONE','NONE')
-        waveobj.writeframes(b''.join([struct.pack('<h',x) for x in scaled]))
+        waveobj.writeframes(scaled)
         val = fp.getvalue()
         waveobj.close()
 
@@ -170,12 +169,17 b' class Audio(DisplayObject):'
         
         max_abs_value = np.max(np.abs(data))
         normalization_factor = Audio._get_normalization_factor(max_abs_value, normalize)
-        scaled = np.int16(data / normalization_factor * 32767).tolist()
-        return scaled, nchan
+        scaled = data / normalization_factor * 32767
+        return scaled.astype('<h').tostring(), nchan
 
 
     @staticmethod
     def _validate_and_normalize_without_numpy(data, normalize):
+        import array
+        import sys
+
+        data = array.array('f', data)
+
         try:
             max_abs_value = float(max([abs(x) for x in data]))
         except TypeError:
@@ -183,9 +187,11 b' class Audio(DisplayObject):'
                 'supported if numpy is not installed')
 
         normalization_factor = Audio._get_normalization_factor(max_abs_value, normalize)
-        scaled = [int(x / normalization_factor * 32767) for x in data]
+        scaled = array.array('h', [int(x / normalization_factor * 32767) for x in data])
+        if sys.byteorder == 'big':
+            scaled.byteswap()
         nchan = 1
-        return scaled, nchan
+        return scaled.tobytes(), nchan
 
     @staticmethod
     def _get_normalization_factor(max_abs_value, normalize):