733fe76ed5
Increase fft_size from 512 (default) to 2048 for better NFM channel isolation, and add squelch_snr_threshold = 15.0 dB per channel to reject noise-triggered recordings. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2.9 KiB
Context
RTLSDR-Airband monitors 3 CB channels (26.985, 27.035, 27.075 MHz) via a single RTL-SDR dongle on a Pi 400. Recordings split per transmission produce many false positives — short noise-triggered files, often on multiple channels simultaneously.
Current config: fft_size commented out (defaults to 512), no squelch_snr_threshold set (defaults to 9.54 dB). Both contribute to over-sensitivity.
Goals / Non-Goals
Goals:
- Reduce false-positive recordings caused by noise, static, and weak signals
- Achieve this through RTLSDR-Airband config tuning only — no post-processing
Non-Goals:
- Eliminating 100% of false positives (some noise will always slip through)
- Automated cleanup scripts or post-processing pipelines
- Changes to gain, bandwidth, or sample rate (these are working adequately)
Decisions
1. Set fft_size = 2048
Decision: Uncomment and increase fft_size from the current default of 512 to 2048.
Rationale: With 3 NFM channels spaced 10 kHz apart using 8 kHz bandwidth, a 512-point FFT produces bins too wide for clean separation. This causes energy from one channel (or wideband noise) to bleed into adjacent channels, triggering simultaneous false recordings. The simultaneous cross-channel triggers observed in the recordings confirm this.
2048 provides 4x the spectral resolution. Going to 4096 would be even better but may stress the Pi 400's CPU; 2048 is a safe starting point.
Alternatives considered:
- 4096: Best selectivity but higher CPU cost. Can try later if 2048 isn't sufficient.
- 1024 (the previously commented-out value): Moderate improvement but may not fully resolve cross-channel bleed.
2. Add squelch_snr_threshold = 15.0 per channel
Decision: Set an explicit SNR threshold of 15 dB on each channel.
Rationale: The default 9.54 dB is calibrated for aviation AM monitoring where signals are cleaner. CB at 27 MHz has a higher noise floor and more atmospheric interference. 15 dB is a reasonable starting point that should reject most noise while still capturing normal-strength CB transmissions.
Alternatives considered:
- 20-25 dB: More aggressive, but risks missing weaker legitimate transmissions. Can increase later if 15 isn't enough.
squelch_threshold(absolute dBFS): Harder to tune and doesn't adapt to changing noise floor conditions. The SNR-relative approach is more robust.
Risks / Trade-offs
- [Weak signals missed] → Higher squelch threshold will drop marginal transmissions. Acceptable trade-off — false positives are currently more disruptive than missed weak signals.
- [CPU increase from larger FFT] → 2048 uses more CPU than 512. Pi 400 quad-core should handle this comfortably, but worth monitoring after deployment.
- [Iterative tuning needed] → These values are starting points. May need adjustment after observing results over a day or two.