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"""
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)
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