Ryan Malloy
b6a031acc2
--tune flag scans the band then tunes to a station using rtl_fm piped to aplay. Supports interactive station picker (--tune) or direct frequency (--tune 102.0).
362 lines
11 KiB
Python
Executable File
362 lines
11 KiB
Python
Executable File
#!/usr/bin/env python3
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"""FM Band Scanner — scan 87.5–108.0 MHz using rtl_power, rank stations by signal strength."""
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import argparse
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import csv
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import io
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import json
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import signal
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import shutil
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import subprocess
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import sys
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from collections import defaultdict
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from pathlib import Path
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def run_rtl_power(gain: int = 10) -> str:
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"""Execute rtl_power for a single FM band sweep and return raw CSV output.
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The scan covers 87.5–108.0 MHz in 200 kHz bins (US FM channel spacing)
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with 1-second integration time per sweep segment.
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"""
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cmd = [
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"rtl_power",
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"-f", "87.5M:108M:200k",
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"-g", str(gain),
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"-i", "1",
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"-1", # single-shot
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"-", # stdout
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]
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try:
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result = subprocess.run(cmd, capture_output=True, text=True, timeout=30)
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except FileNotFoundError:
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print("Error: rtl_power not found. Install rtl-sdr tools.", file=sys.stderr)
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sys.exit(1)
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except subprocess.TimeoutExpired:
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print("Error: rtl_power timed out after 30 seconds.", file=sys.stderr)
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sys.exit(1)
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if result.returncode != 0:
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stderr = result.stderr.strip()
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print(f"Error: rtl_power exited with code {result.returncode}", file=sys.stderr)
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if stderr:
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print(stderr, file=sys.stderr)
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sys.exit(1)
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return result.stdout
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def parse_scan(csv_data: str) -> list[tuple[float, float]]:
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"""Parse rtl_power CSV output into (frequency_mhz, power_dbm) pairs.
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rtl_power CSV format per row:
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date, time, freq_low_hz, freq_high_hz, bin_step_hz, num_samples, dBm, dBm, ...
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Each row covers a frequency range with multiple FFT bins. We compute the
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center frequency of each bin and pair it with its power reading.
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"""
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readings: list[tuple[float, float]] = []
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reader = csv.reader(io.StringIO(csv_data))
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for row in reader:
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if len(row) < 7:
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continue
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try:
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freq_low = float(row[2].strip())
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freq_high = float(row[3].strip())
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bin_step = float(row[4].strip())
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# num_samples = int(row[5].strip()) # not needed
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power_values = [float(v.strip()) for v in row[6:] if v.strip()]
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except (ValueError, IndexError):
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continue
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# Map each FFT bin to its center frequency
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for i, power in enumerate(power_values):
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freq_hz = freq_low + (i * bin_step) + (bin_step / 2)
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freq_mhz = freq_hz / 1e6
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readings.append((freq_mhz, power))
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return readings
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def aggregate_channels(readings: list[tuple[float, float]]) -> list[dict]:
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"""Aggregate raw FFT bins into 200 kHz FM channels.
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FM stations in the US are spaced at odd multiples of 100 kHz
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(87.9, 88.1, 88.3, ..., 107.9). Each occupies ~200 kHz bandwidth.
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We snap each reading to the nearest standard channel and take the
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max power across all bins in that channel.
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"""
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channel_bins: dict[float, list[float]] = defaultdict(list)
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for freq_mhz, power in readings:
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# Snap to nearest 0.2 MHz FM channel (87.5, 87.7, 87.9, ...)
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channel = round(round(freq_mhz / 0.2) * 0.2, 1)
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if 87.5 <= channel <= 108.0:
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channel_bins[channel].append(power)
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channels = []
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for freq in sorted(channel_bins):
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powers = channel_bins[freq]
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# Use max power — peak represents the carrier
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max_power = max(powers)
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channels.append({"freq_mhz": freq, "power_dbm": max_power})
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return channels
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def detect_stations(
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channels: list[dict], threshold_db: float = 10.0
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) -> tuple[list[dict], float]:
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"""Find stations that rise above the noise floor.
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The noise floor is estimated as the median power across all channels.
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A channel is flagged as a station if its power exceeds
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noise_floor + threshold_db.
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Returns (stations_sorted_by_power, noise_floor_dbm).
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"""
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if not channels:
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return [], -99.0
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powers = sorted(ch["power_dbm"] for ch in channels)
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noise_floor = powers[len(powers) // 2] # median
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stations = []
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for ch in channels:
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snr = ch["power_dbm"] - noise_floor
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if snr >= threshold_db:
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stations.append({**ch, "snr_db": round(snr, 1)})
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stations.sort(key=lambda s: s["power_dbm"], reverse=True)
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return stations, noise_floor
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def display_results(
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stations: list[dict],
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noise_floor: float,
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all_channels: list[dict] | None = None,
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show_all: bool = False,
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):
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"""Print a formatted table of scan results to the terminal."""
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term_width = shutil.get_terminal_size((80, 24)).columns
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bar_max = max(32, term_width - 42)
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items = all_channels if (show_all and all_channels) else stations
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if not items:
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print("No stations detected.")
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return
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# Bar scaling: use noise floor as baseline so every station gets a visible bar
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powers = [ch["power_dbm"] for ch in items]
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p_min = noise_floor
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p_max = max(powers)
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p_range = p_max - p_min if p_max != p_min else 1.0
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header = "FM Band Scan \u2014 87.5 to 108.0 MHz"
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print()
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print(f" {header}")
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print(f" {'═' * (len(header) + 2)}")
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print(f" {'#':>3} {'Frequency':<12} {'Power':<10} Signal")
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print(f" {'─' * 3} {'─' * 12} {'─' * 9} {'─' * bar_max}")
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block_chars = " \u2581\u2582\u2583\u2584\u2585\u2586\u2587\u2588"
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for i, ch in enumerate(items, 1):
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freq = ch["freq_mhz"]
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power = ch["power_dbm"]
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# Normalize to [0, 1]
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norm = max(0.0, min(1.0, (power - p_min) / p_range))
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bar_len = norm * bar_max
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full_blocks = int(bar_len)
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frac = bar_len - full_blocks
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frac_char = block_chars[int(frac * 8)] if frac > 0.05 else ""
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bar = "\u2588" * full_blocks + frac_char
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# Color: green for strong, yellow for mid, dim for weak
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if "snr_db" in ch and ch["snr_db"] >= 10:
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bar = f"\033[32m{bar}\033[0m" # green
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elif "snr_db" in ch:
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bar = f"\033[33m{bar}\033[0m" # yellow
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elif show_all:
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bar = f"\033[2m{bar}\033[0m" # dim
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label = f"{freq:>5.1f} MHz"
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print(f" {i:>3} {label:<12} {power:>7.1f} dBm {bar}")
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print(f" {'═' * (len(header) + 2)}")
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print(
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f" Noise floor: {noise_floor:.1f} dBm | "
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f"Stations found: {len(stations)}"
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)
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print()
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def save_json(stations: list[dict], noise_floor: float, path: str):
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"""Write scan results to a JSON file."""
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data = {
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"band": "FM",
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"range_mhz": [87.5, 108.0],
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"noise_floor_dbm": round(noise_floor, 1),
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"station_count": len(stations),
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"stations": [
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{
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"freq_mhz": s["freq_mhz"],
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"power_dbm": round(s["power_dbm"], 1),
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"snr_db": s["snr_db"],
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}
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for s in stations
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],
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}
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Path(path).write_text(json.dumps(data, indent=2) + "\n")
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print(f"Results saved to {path}")
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def pick_station(stations: list[dict]) -> float | None:
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"""Interactive station picker. Returns frequency in MHz or None to quit."""
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if not stations:
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print("No stations to choose from.")
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return None
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try:
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choice = input(" Tune to station # (or q to quit): ").strip()
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except (EOFError, KeyboardInterrupt):
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print()
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return None
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if choice.lower() in ("q", "quit", ""):
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return None
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try:
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idx = int(choice) - 1
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if 0 <= idx < len(stations):
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return stations[idx]["freq_mhz"]
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print(f" Pick 1–{len(stations)}.")
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except ValueError:
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# Maybe they typed a frequency directly
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try:
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freq = float(choice)
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if 87.5 <= freq <= 108.0:
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return freq
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print(" Frequency must be 87.5–108.0 MHz.")
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except ValueError:
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print(" Enter a station number or frequency.")
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return pick_station(stations)
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def tune_station(freq_mhz: float, gain: int = 10):
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"""Tune to an FM station using rtl_fm piped to aplay.
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rtl_fm demodulates wideband FM, resamples to 48 kHz mono,
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and pipes raw PCM to ALSA's aplay for real-time audio output.
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"""
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freq_hz = int(freq_mhz * 1e6)
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print(f"\n Tuning to {freq_mhz:.1f} MHz — Ctrl+C to stop\n")
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rtl_cmd = [
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"rtl_fm",
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"-f", str(freq_hz),
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"-M", "wbfm", # wideband FM demodulation
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"-s", "200k", # sample rate (200 kHz captures full FM channel)
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"-r", "48k", # resample output to 48 kHz
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"-g", str(gain),
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"-", # output to stdout
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]
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play_cmd = [
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"aplay",
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"-r", "48000", # 48 kHz sample rate
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"-f", "S16_LE", # signed 16-bit little-endian PCM
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"-t", "raw", # raw format (no WAV header)
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"-c", "1", # mono
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"-q", # quiet (no progress output)
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]
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rtl_proc = None
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play_proc = None
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try:
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rtl_proc = subprocess.Popen(rtl_cmd, stdout=subprocess.PIPE, stderr=subprocess.DEVNULL)
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play_proc = subprocess.Popen(play_cmd, stdin=rtl_proc.stdout, stderr=subprocess.DEVNULL)
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# Allow rtl_proc to receive SIGPIPE if play_proc exits
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rtl_proc.stdout.close()
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play_proc.wait()
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except KeyboardInterrupt:
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print("\n Stopped.")
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except FileNotFoundError as e:
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print(f" Error: {e.filename} not found.", file=sys.stderr)
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finally:
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for proc in (play_proc, rtl_proc):
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if proc and proc.poll() is None:
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proc.send_signal(signal.SIGTERM)
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proc.wait(timeout=3)
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def main():
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parser = argparse.ArgumentParser(
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description="Scan the US FM band and rank stations by signal strength."
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)
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parser.add_argument(
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"--threshold",
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type=float,
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default=10.0,
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help="Minimum dB above noise floor to flag as station (default: 10)",
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)
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parser.add_argument(
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"--gain",
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type=int,
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default=10,
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help="RF tuner gain in dB (default: 10)",
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)
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parser.add_argument(
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"--json",
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metavar="FILE",
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help="Save results to JSON file",
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)
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parser.add_argument(
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"--all",
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action="store_true",
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dest="show_all",
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help="Show all channels, not just detected stations",
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)
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parser.add_argument(
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"--tune",
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nargs="?",
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const="pick",
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metavar="FREQ",
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help="Tune to a station after scanning (optionally specify frequency in MHz)",
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)
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args = parser.parse_args()
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print("Scanning FM band (87.5–108.0 MHz)...", flush=True)
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raw = run_rtl_power(gain=args.gain)
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readings = parse_scan(raw)
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if not readings:
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print("No data received from rtl_power.", file=sys.stderr)
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sys.exit(1)
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channels = aggregate_channels(readings)
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stations, noise_floor = detect_stations(channels, threshold_db=args.threshold)
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display_results(
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stations,
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noise_floor,
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all_channels=channels,
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show_all=args.show_all,
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)
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if args.json:
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save_json(stations, noise_floor, args.json)
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if args.tune is not None:
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if args.tune == "pick":
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freq = pick_station(stations)
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else:
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freq = float(args.tune)
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if freq:
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tune_station(freq, gain=args.gain)
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if __name__ == "__main__":
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main()
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