switch to 16-bit dumps & add a script for dumping cleaned waveforms

I can see very clear signal when I:
- tape only the contacts of the sensor to the table, and
- bend the free end into the air, and release (it snaps down)

scripts/itm-to-pcm.py

@@ -0,0 +1,70 @@
+#!/usr/bin/env python3
+'''
+Convert itm.txt dumps to pcm (wave) files
+
+usage: itmdump -f /tmp/itm.txt > dump.txt
+itm-to-pcm.py dump.txt dump.wav
+'''
+
+import wave
+import sys
+
+def read_itm(file_in):
+    f = open(file_in, 'rb')
+    while True:
+        samples = f.read(2)
+        if not samples: break
+        #if len(set(samples)) != 1: continue # corrupted
+        #yield samples[0]
+        yield samples[0] + 0x100*samples[1]
+    #for line in f:
+    #    try:
+    #        if line.startswith(b'ADC:'):
+    #            sample = int(line[len('ADC: '):].strip(), 16)
+    #            if sample < 0x1000: yield sample
+    #    except:
+    #        pass
+
+def frames_from_samples(itm):
+    b = bytearray()
+    for sample in itm:
+        sample = (sample + 0x10000) % 0x10000 # counteract Python's signed mod badness
+        lsb0 = sample % 0x100
+        lsb1 = (sample // 0x100) % 0x100
+        b.append(lsb0)
+        b.append(lsb1)
+
+    return b
+
+def clean_frames(frames):
+    # Run a median filter over the frames to clean outliers:
+    post_med = [0]*len(frames)
+    for i, _ in enumerate(frames):
+        left = max(0, i-2)
+        right = min(len(frames), i+3)
+        post_med[i] = median(frames[left:right])
+    # Remove DC component
+    dc = sum(post_med) / len(post_med)
+    post_dc = [f - dc for f in post_med]
+    # Amplify to [-0x7fff, 0x7fff]
+    prev_max = max(abs(f) for f in post_dc)
+    amp_factor = 0x7fff / prev_max
+    post_amp = [f*amp_factor for f in post_dc]
+    # Round to int
+    return [int(round(f)) for f in post_amp]
+
+def median(samples):
+    x = list(sorted(samples))
+    return x[len(x) // 2]
+
+def main():
+    _self, file_in, file_out = sys.argv
+    itm = clean_frames(list(read_itm(file_in)))
+    wave_out = wave.open(file_out, 'wb')
+    wave_out.setnchannels(1)
+    wave_out.setsampwidth(2)
+    wave_out.setframerate(1000)
+    wave_out.writeframes(frames_from_samples(itm))
+
+if __name__ == "__main__":
+    main()