#!/usr/bin/env python3
"""
Real-time FM audio demodulator for HackRF One.

* Centre frequency : 100.1 MHz (edit FREQ_HZ below if needed)
* RF sample rate   : 2 MS/s
* Audio sample rate: 48 kS/s (pushed straight to your speakers)

Dependencies
------------
pip install numpy scipy pyhackrf sounddevice

Tested with:
* HackRF One firmware 2024.02.1
* pyhackrf 0.6.7
* NumPy 2.0,  SciPy 1.13
* sounddevice 0.4.6 (PortAudio backend)

Press Ctrl-C to quit.
"""

import numpy as np
import scipy.signal
import sounddevice as sd
from math import gcd
from hackrf import HackRf

# ------------------ editable constants ------------------ #
FREQ_HZ    = 100_100_000          # 100.1 MHz – change to your station
RF_RATE    = 2_000_000            # 2 MS/s I-Q
AUDIO_RATE = 48_000               # 48 kS/s mono
RF_GAIN_DB = 20                   # front-end gain in dB
BUF_SYMS   = 262_144              # ≈131 ms at 2 MS/s
DEEMPH_TC  = 75e-6                # 75 µs FM de-emphasis
# -------------------------------------------------------- #


def fm_demodulate(iq: np.ndarray) -> np.ndarray:
    """Wide-band FM discriminator + de-emphasis + resample."""
    iq -= np.mean(iq)                                   # DC-remove
    audio = np.diff(np.unwrap(np.angle(iq)))            # phase diff
    audio = audio[::40]                                 # decimate 2 MHz → 50 kHz

    # 75 µs de-emphasis (simple IIR)
    alpha = np.exp(-1.0 / (AUDIO_RATE * DEEMPH_TC))
    b0, a1 = 1.0 - alpha, alpha
    for i in range(1, audio.size):
        audio[i] = b0 * audio[i] + a1 * audio[i-1]

    audio = np.tanh(5 * audio)                          # soft-clip

    fs_in = RF_RATE // 40                               # 50 kHz
    if fs_in != AUDIO_RATE:                             # resample to 48 kHz
        g = gcd(fs_in, AUDIO_RATE)
        audio = scipy.signal.resample_poly(audio,
                                           AUDIO_RATE // g,
                                           fs_in // g)
    return audio.astype(np.float32)


def main():
    hackrf = HackRf()
    hackrf.open()
    hackrf.set_sample_rate(RF_RATE)
    hackrf.set_freq(FREQ_HZ)
    hackrf.set_lna_gain(RF_GAIN_DB)
    hackrf.set_vga_gain(0)
    hackrf.set_amp_enable(False)
    hackrf.start_rx_mode()

    print(f"► Receiving {FREQ_HZ/1e6:.3f} MHz – playing live audio…")

    sd.default.samplerate = AUDIO_RATE
    sd.default.channels   = 1
    stream = sd.OutputStream(dtype='float32')
    stream.start()

    try:
        while True:
            raw = hackrf.receive(BUF_SYMS)              # bytes: I0 Q0 I1 Q1…
            u8  = np.frombuffer(raw, dtype=np.uint8).astype(np.float32) - 128.0
            iq  = (u8[::2] + 1j * u8[1::2]) / 128.0     # complex f32 −1…+1
            audio = fm_demodulate(iq)
            stream.write(audio)
    except KeyboardInterrupt:
        pass
    finally:
        stream.stop()
        hackrf.stop_rx_mode()
        hackrf.close()


if __name__ == "__main__":
    main()