Add: Doc & tools about coherence

README.md

@@ -97,7 +97,7 @@ If you want to change the alsa settings, You can use `sudo alsactl --file=/etc/v
 
 ## 6-Mics Circular Array Kit
 
-[![](https://user-images.githubusercontent.com/3901856/37193936-d727868a-23a6-11e8-8096-2c531ff479fc.png)]()
+[![](https://user-images.githubusercontent.com/3901856/37268348-6adef768-2600-11e8-8861-588b1c3ea142.png)]()
 
 The 6 Mics Circular Array Kit uses ac108 x 2 / ac101 x 1 / micphones x 6, includes 8 ADCs and 2 DACs.
 
@@ -195,7 +195,7 @@ plughw:CARD=seeed8micvoicec,DEV=0
 In contrast to 6-Mics Circular Array Kit for Raspberry Pi,
 the difference is only first 4 input channels are valid capturing data.
 
-#### Test:
+### Usage:
 ```bash
 #for ReSpeaker 2-mic
 #It will capture sound an playback on hw:1
@@ -221,6 +221,22 @@ aplay -D ac101 a.wav
 **Note: for developer using 6-Mics Circular Array Kit(or 4-Mics Linear Array Kit) doing capturing & playback the same time,
 capturing must be start first, or else the capturing channels will miss order.**
 
+### Coherence
+
+Estimate the magnitude squared coherence using Welch’s method.
+![4-mics-linear-array-kit coherence](https://user-images.githubusercontent.com/3901856/37268469-2869dd34-2601-11e8-9a4e-a831b3af82fe.png)  
+Note: 'CO 1-2' means the coherence between channel 1 and channel 2.
+
+```bash
+# How to get the coherence of the captured audio(a.wav for example).
+sudo apt install python-numpy python-scipy python-matplotlib
+python tools/coherence.py a.wav
+
+Requirement of the input audio file:
+- formt: WAV(Microsoft) signed 16-bit PCM
+- channels: >=2
+```
+
 ### uninstall seeed-voicecard
 If you want to upgrade the driver , you need uninstall the driver first.
 

tools/coherence.py

@@ -0,0 +1,44 @@
+"""
+Estimate the magnitude squared coherence estimate,
+
+- requirements
+    sudo apt install python-numpy python-scipy python-matplotlib
+"""
+
+
+import sys
+import wave
+import numpy as np
+from scipy import signal
+import matplotlib.pyplot as plt
+
+if len(sys.argv) < 2:
+    print('Usage: python {} audio.wav'.format(sys.argv[0]))
+    sys.exit(1)
+
+wav = wave.open(sys.argv[1], 'rb')
+channels = wav.getnchannels()
+frames = wav.readframes(wav.getnframes())
+fs = wav.getframerate()
+wav.close()
+
+print("channels: %d" % channels)
+print("rate    : %d" % fs)
+print("frames  : %d" % wav.getnframes())
+
+array = np.fromstring(frames, dtype='int16')
+
+ch0 = array[0::channels]
+
+fig, ax = plt.subplots()
+
+for ch in range(1, channels):
+    f, c = signal.coherence(ch0, array[ch::channels], fs, nperseg=1024)
+    ax.semilogy(f, c, label="CO 1-%d" % (ch + 1))
+
+legend = ax.legend(loc='lower right', shadow=True, fontsize='small')
+
+plt.xlabel('frequency [Hz]')
+plt.ylabel('Coherence')
+plt.show()
+