Fix sign of b2 in synthio PEAKING_EQ biquad

shared-module/synthio/Biquad.c

@@ -156,7 +156,7 @@ void common_hal_synthio_biquad_tick(mp_obj_t self_in) {
         case SYNTHIO_PEAKING_EQ:
             b0 = 1 + alpha * A;
             b1 = -2 * sc.c;
-            b2 = 1 + alpha * A;
+            b2 = 1 - alpha * A;
             a0 = 1 + alpha / A;
             a1 = -2 * sc.c;
             a2 = 1 - alpha / A;

tests/circuitpython/synthio_biquad_peaking.py

@@ -0,0 +1,54 @@
+# A peaking EQ bell should be unity at DC and near Nyquist, and hit its own
+# gain at the centre frequency.
+import array
+import math
+from audiocore import get_buffer, RawSample
+from audiofilters import Filter
+from synthio import Biquad, FilterMode
+
+RATE = 48000
+FRAMES = 4096
+LEVEL = 2000
+GAIN_DB = 6.0
+CENTRE = 1000.0
+
+
+def through(values):
+    source = RawSample(values, sample_rate=RATE, channel_count=1)
+    bell = Biquad(FilterMode.PEAKING_EQ, CENTRE, Q=1.0, A=10 ** (GAIN_DB / 40))
+    effect = Filter(
+        filter=bell,
+        sample_rate=RATE,
+        channel_count=1,
+        bits_per_sample=16,
+        samples_signed=True,
+        buffer_size=FRAMES * 2,
+    )
+    effect.play(source, loop=True)
+    out = []
+    for block in range(3):
+        data = bytes(get_buffer(effect)[1])
+        if block < 2:  # let the filter settle
+            continue
+        for index in range(0, len(data) - 1, 2):
+            sample = data[index] | (data[index + 1] << 8)
+            out.append(sample - 65536 if sample >= 32768 else sample)
+    return out
+
+
+def tone(hz):
+    return array.array(
+        "h",
+        [int(LEVEL * math.sin(2 * math.pi * hz * frame / RATE)) for frame in range(FRAMES)],
+    )
+
+
+def rms_db(values):
+    total = sum(value * value for value in values)
+    return 20 * math.log10(math.sqrt(total / len(values)) / (LEVEL / math.sqrt(2)))
+
+
+dc = through(array.array("h", [LEVEL] * FRAMES))
+print("DC      %+.1f" % (20 * math.log10(abs(dc[-1]) / LEVEL)))
+print("centre  %+.1f" % rms_db(through(tone(CENTRE))))
+print("12 kHz  %+.1f" % rms_db(through(tone(12000.0))))

tests/circuitpython/synthio_biquad_peaking.py.exp

@@ -0,0 +1,3 @@
+DC      -0.1
+centre  +5.9
+12 kHz  +0.0