1 files changed, 51 insertions, 0 deletions
diff --git a/stacks/crdop/lib/afsk_modulator.py b/stacks/crdop/lib/afsk_modulator.py
new file mode 100644
index 0000000..1a6bf68
--- /dev/null
+++ b/stacks/crdop/lib/afsk_modulator.py
@@ -0,0 +1,51 @@
+"""
+Continuous-phase AFSK modulator — Bell 202 mark 1200 Hz / space 2200 Hz.
+"""
+from __future__ import annotations
+
+import math
+import struct
+from array import array
+
+from bell202_line_code import MARK_HZ, SPACE_HZ, TONE_MARK, encode_bits_to_tones
+
+_TWO_PI = 2.0 * math.pi
+
+
+class AfskModulator:
+  def __init__(self, sample_rate: int = 48000, baud: int = 1200) -> None:
+      self.sample_rate = sample_rate
+      self.baud = baud
+      self._phase = 0.0
+
+  def _tone_freq(self, tone: int) -> float:
+      return MARK_HZ if tone == TONE_MARK else SPACE_HZ
+
+  def modulate_bits(self, bits: bytes, start_tone: int = 0) -> bytes:
+      """Return mono S16_LE PCM for the given bit stream."""
+      tones = encode_bits_to_tones(bits, start_tone=start_tone)
+      samples_per_symbol = self.sample_rate // self.baud
+      out: array[int] = array("h")
+      for tone in tones:
+          freq = self._tone_freq(tone)
+          step = _TWO_PI * freq / self.sample_rate
+          for _ in range(samples_per_symbol):
+              sample = int(0.7 * 32767.0 * math.sin(self._phase))
+              out.append(sample)
+              self._phase += step
+              if self._phase >= _TWO_PI:
+                  self._phase -= _TWO_PI
+      return struct.pack(f"<{len(out)}h", *out)
+
+  def steady_tone(self, tone: int, duration_s: float) -> bytes:
+      """Calibration tone (Dire Wolf -x style mark/space hold)."""
+      n = int(self.sample_rate * duration_s)
+      freq = self._tone_freq(tone)
+      step = _TWO_PI * freq / self.sample_rate
+      out: array[int] = array("h")
+      for _ in range(n):
+          out.append(int(0.7 * 32767.0 * math.sin(self._phase)))
+          self._phase += step
+          if self._phase >= _TWO_PI:
+              self._phase -= _TWO_PI
+      return struct.pack(f"<{len(out)}h", *out)