Filter and fps checks: -fl 0 and short clips crash, -fl above Nyquist silently skips the filter, a wrong container fps is not caught

[joeljose]

Severity: Medium. Crashes happen after all the processing is done, and wrong output is produced silently.

Findings (all reproduced)

input	result
-fl 0 / -fl -5 / -fh 0	ValueError: filter critical frequencies must be greater than 0, a raw traceback raised after every frame has been processed
13–27 frames with -fl 100 -fh 800	ValueError: The length of the input vector x must be greater than padlen… (sosfiltfilt padding). The frame_count > 12 guard is too low; 28 frames works
≤ 12 frames with a filter	the filter is skipped silently
-fl ≥ Nyquist	prints Warning: … skipping filter and writes unfiltered audio
-fh above Nyquist	quietly clamped to 0.99·Nyquist (acceptable, but should be reported up front)

The fps trap

The README notes that MIT's sample AVIs report ~30 fps although they were captured at 2200 fps. If a user forgets --fps 2200:

• Nyquist becomes 15 Hz, so a typical -fl 100 becomes the "skipping filter" case above, and the output is unfiltered;
• the WAV is written at a 30 Hz sample rate, which most players can't play.

Nothing warns about this. Also, save_wav(..., int(fps)) truncates fractional rates (29.97 → 29).

Suggested fix

• Validate everything in main() before reading frames:
  • -fl > 0, -fh > 0, -fl < Nyquist;
  • error, rather than warn, when -fl ≥ Nyquist;
  • print the effective band.
• Replace frame_count > 12 with the real requirement: compute padlen from the SOS and fall back to sosfilt (or padtype=None) or a clear error for shorter clips.
• Warn loudly when the effective fps is below ~500 Hz, e.g. "Visual Microphone needs high-speed video; the container reports 30 fps — did you mean --fps 2200?".
• Write the WAV with round(fps), or resample to a standard rate (e.g. 8 or 16 kHz) with scipy.signal.resample_poly while keeping the native rate as an option. Many players reject 2200 Hz WAVs.

Acceptance criteria

• Each case above exits early with a clear message, or processes correctly.
• Unit tests cover each case.