ppg: Pull the PPG signal processing into a seperate library

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

1 files changed, 170 insertions, 0 deletions

diff --git a/wasp/ppg.py b/wasp/ppg.py
new file mode 100644
index 0000000..324392d
--- /dev/null
+++ b/wasp/ppg.py
@@ -0,0 +1,170 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+# Copyright (C) 2020 Daniel Thompson
+
+"""Photoplethysmogram (PPG) Signal Processing
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Algorithms and signal processing primatives that can be used to convert
+raw PPG signals into something useful.
+"""
+
+import array
+import micropython
+
+@micropython.viper
+def _compare(d1, d2, count: int, shift: int) -> int:
+    """Compare two sequences of (signed) bytes and quantify how dissimilar
+    they are.
+    """
+    p1 = ptr8(d1)
+    p2 = ptr8(d2)
+
+    e = 0
+    for i in range(count):
+        s1 = int(p1[i])
+        if s1 > 127:
+            s1 -= 256
+
+        s2 = int(p2[i])
+        if s2 > 127:
+            s2 -= 256
+
+        d = s1 - s2
+        e += d*d
+    return e
+
+class Biquad():
+    """Direct Form II Biquad Filter"""
+
+    def __init__(self, b0, b1, b2, a1, a2):
+        self._coeff = (b0, b1, b2, a1, a2)
+        self._v1 = 0
+        self._v2 = 0
+
+    def step(self, x):
+        c = self._coeff
+        v1 = self._v1
+        v2 = self._v2
+
+        v = x - (c[3] * v1) - (c[4] * v2)
+        y = (c[0] * v) + (c[1] * v1) + (c[2] * v2)
+
+        self._v2 = v1
+        self._v1 = v
+        return y
+
+class PTAGC():
+    """Peak Tracking Automatic Gain Control
+
+    In order for the correlation checks to work correctly we must
+    aggressively reject spikes caused by fast DC steps. Setting a
+    threshold based on the median is very effective at killing
+    spikes but needs an extra 1k for sample storage which isn't
+    really plausible for a microcontroller.
+    """
+    def __init__(self, start, decay, threshold):
+        self._peak = start
+        self._decay = decay
+        self._boost = 1 / decay
+        self._threshold = threshold
+
+    def step(self, spl):
+        # peak tracking
+        peak = self._peak
+        if abs(spl) > peak:
+            peak *= self._boost
+        else:
+            peak *= self._decay
+        self._peak = peak
+
+        # rejection filter (clipper)
+        threshold = self._threshold
+        if spl > (peak * threshold) or spl < (peak * -threshold):
+            return 0
+
+        # booster
+        spl = 100 * spl / (2 * peak)
+
+        return spl
+
+class PPG():
+    """
+    """
+
+    def __init__(self, spl):
+        self._offset = spl
+        self.data = array.array('b')
+
+        self._hpf = Biquad(0.87033078, -1.74066156, 0.87033078,
+                                       -1.72377617, 0.75754694)
+        self._agc = PTAGC(20, 0.971, 2)
+        self._lpf = Biquad(0.11595249, 0.23190498, 0.11595249,
+                                      -0.72168143, 0.18549138)
+
+    def preprocess(self, spl):
+        """Preprocess a PPG sample.
+
+        Must be called at 24Hz for accurate heart rate calculations.
+        """
+        spl -= self._offset
+        spl = self._hpf.step(spl)
+        spl = self._agc.step(spl)
+        spl = self._lpf.step(spl)
+        spl = int(spl)
+
+        self.data.append(spl)
+        return spl
+
+    def _get_heart_rate(self):
+        def compare(d, shift):
+            return _compare(d[shift:], d[:-shift], len(d)-shift, shift)
+
+        def trough(d, mn, mx):
+            z2 = compare(d, mn-2)
+            z1 = compare(d, mn-1)
+            for i in range(mn, mx+1):
+                z = compare(d, i)
+                if z2 > z1 and z1 < z:
+                    return i
+                z2 = z1
+                z1 = z
+
+            return -1
+
+        data = memoryview(self.data)
+
+        # Search initially from ~210 to 30 bpm
+        t0 = trough(data, 7, 48)
+        if t0 < 0:
+            return None
+
+        # Check the second cycle ...
+        t1 = t0 * 2
+        t1 = trough(data, t1 - 5, t1 + 5)
+        if t1 < 0:
+            return None
+
+        # ... and the third
+        t2 = (t1 * 3) // 2
+        t2 = trough(data, t2 - 5, t2 + 4)
+        if t2 < 0:
+            return None
+
+        # If we can find a fourth cycle then use that for the extra
+        # precision otherwise report whatever we've found
+        t3 = (t2 * 4) // 3
+        t3 = trough(data, t3 - 4, t3 + 4)
+        if t3 < 0:
+            return (60 * 24 * 3) // t2
+        return (60 * 24 * 4) // t3
+
+    def get_heart_rate(self):
+        if len(self.data) < 200:
+            return None
+
+        hr = self._get_heart_rate()
+
+        # Clear out the accumulated data
+        self.data = array.array('b')
+
+        return hr