debug-no-signal

Debug No Signal

A receiver that runs status: ok but produces silence or garbage has a DSP fault, not a crash. Find it systematically; do not randomly tweak parameters.

Method (systematic-debugging applied to RF)

1. Reproduce and name the symptom. Re-run and state precisely what's wrong: silent audio, broadband noise, a tone at the wrong pitch, low level.
2. See, don't guess. Render a spectrogram (and/or psd_plot) of the channelized capture — the stage right after freq_xlating_lowpass — and measure it. Most faults are visible there.
3. Check hypotheses cheapest-first, changing ONE thing per iteration:
   • Wrong offset or sign. After channelizing, the target should sit at 0 Hz. If it's offset by ±X, your center_offset value or its sign is wrong (center_offset = target_freq − capture_center_freq).
   • Over/under-decimation. The post-decimation rate must be ≥ ~2× the signal bandwidth, and for FM must equal the demodulator's quad_rate. Check sample_rate / decimation.
   • Filter too narrow/wide. cutoff below the signal bandwidth clips it (muffled/silent); far too wide lets in neighbors (noisy).
   • Levels / wrong channel. measure the channelized capture: very low SNR means you channelized empty spectrum — re-check the target frequency from survey-spectrum.
   • Demod/rate mismatch. For nbfm_rx, quad_rate must be an integer multiple of audio_rate; a wav_sink sample_rate must match the audio stream rate.
4. Re-verify after each change the same way build-receiver verifies. Stop when the output is correct.
5. If no vocabulary block fixes it (e.g. the signal isn't FM at all), report the gap → escape-hatch.

Judgment

• One change at a time. If you change three params and it works, you've learned nothing and it'll break next time.
• Trust the spectrogram over your mental model of the math.