1 files changed, 50 insertions, 0 deletions
diff --git a/stacks/crdop/lib/bell202_line_code.py b/stacks/crdop/lib/bell202_line_code.py
new file mode 100644
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+++ b/stacks/crdop/lib/bell202_line_code.py
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+"""
+Bell 202 frequency-toggle line code (1200 baud AFSK layer).
+
+On-air rule: bit 0 → toggle mark/space tone; bit 1 → hold current tone.
+We avoid the legacy term in API names — this is Bell 202 / amateur packet radio.
+"""
+from __future__ import annotations
+
+MARK_HZ = 1200.0
+SPACE_HZ = 2200.0
+BAUD_1200 = 1200
+
+TONE_MARK = 1
+TONE_SPACE = 0
+
+
+def encode_bits_to_tones(bits: bytes, start_tone: int = TONE_SPACE) -> list[int]:
+    """Map HDLC bit stream to mark/space tone sequence (one tone per bit)."""
+    tone = start_tone
+    out: list[int] = []
+    for byte in bits:
+        for bit_i in range(7, -1, -1):
+            bit = (byte >> bit_i) & 1
+            if bit == 0:
+                tone ^= 1
+            out.append(tone)
+    return out
+
+
+def decode_tones_to_bits(tones: list[int], start_tone: int = TONE_SPACE) -> bytes:
+    """Recover bit stream from mark/space tone sequence."""
+    if not tones:
+        return b""
+    prev = start_tone
+    bits: list[int] = []
+    for tone in tones:
+        if tone not in (TONE_MARK, TONE_SPACE):
+            continue
+        bits.append(0 if tone != prev else 1)
+        prev = tone
+    buf = bytearray()
+    for i in range(0, len(bits), 8):
+        chunk = bits[i : i + 8]
+        if len(chunk) < 8:
+            break
+        val = 0
+        for b in chunk:
+            val = (val << 1) | b
+        buf.append(val)
+    return bytes(buf)