1 files changed, 56 insertions, 0 deletions
diff --git a/stacks/crdop/lib/afsk_demodulator.py b/stacks/crdop/lib/afsk_demodulator.py
new file mode 100644
index 0000000..5ba3d8b
--- /dev/null
+++ b/stacks/crdop/lib/afsk_demodulator.py
@@ -0,0 +1,56 @@
+"""
+AFSK demodulator — Bell 202 mark/space discrimination per symbol period.
+
+Uses per-symbol Goertzel energy at 1200 Hz and 2200 Hz (Dire Wolf class approach).
+"""
+from __future__ import annotations
+
+import math
+import struct
+from array import array
+
+from bell202_line_code import TONE_MARK, TONE_SPACE, decode_tones_to_bits
+
+_TWO_PI = 2.0 * math.pi
+
+
+def _goertzel_power(samples: array, freq: float, sample_rate: int) -> float:
+    n = len(samples)
+    if n < 8:
+        return 0.0
+    k = int(0.5 + (n * freq) / sample_rate)
+    w = _TWO_PI * k / n
+    coeff = 2.0 * math.cos(w)
+    s0 = s1 = s2 = 0.0
+    for x in samples:
+        s0 = x + coeff * s1 - s2
+        s2 = s1
+        s1 = s0
+    return s1 * s1 + s2 * s2 - coeff * s1 * s2
+
+
+class AfskDemodulator:
+    def __init__(self, sample_rate: int = 48000, baud: int = 1200) -> None:
+        self.sample_rate = sample_rate
+        self.baud = baud
+        self.samples_per_symbol = sample_rate // baud
+
+    def demodulate_pcm(self, pcm: bytes) -> list[int]:
+        """Return mark/space tone sequence from mono S16_LE PCM."""
+        count = len(pcm) // 2
+        if count < self.samples_per_symbol:
+            return []
+        samples = array("h")
+        samples.frombytes(pcm[: count * 2])
+        tones: list[int] = []
+        pos = 0
+        while pos + self.samples_per_symbol <= len(samples):
+            window = samples[pos : pos + self.samples_per_symbol]
+            pos += self.samples_per_symbol
+            p_mark = _goertzel_power(window, 1200.0, self.sample_rate)
+            p_space = _goertzel_power(window, 2200.0, self.sample_rate)
+            tones.append(TONE_MARK if p_mark >= p_space else TONE_SPACE)
+        return tones
+
+    def demodulate_to_bits(self, pcm: bytes, start_tone: int = TONE_SPACE) -> bytes:
+        return decode_tones_to_bits(self.demodulate_pcm(pcm), start_tone=start_tone)