Import microphone calibration files (.cal / factory .txt)

[Skeyelab]

Summary

Support loading microphone calibration files globally so frequency-dependent corrections apply across analysis widgets.

Motivation

Measurement mics often ship with factory cal files (e.g. serial-specific .txt with *1000Hz -38.6 sensitivity header + Hz/dB table). REW-style .cal files are also common. Today Friture only supports a scalar dB offset; cal file frequency data is unused.

Example factory file (24161.txt):

*1000Hz  -38.6
20.00  0.1
...
20000.00  2.5

Requirements

• Load cal file from Settings → Input calibration (global, not per-dock)
• Parse factory *1000Hz header format and REW-style .cal (freq/gain lines; optional sensitivity header)
• Apply frequency correction globally to spectrum, octave, and spectrogram displays
• Persist cal file path (and parsed data) in settings; reload on startup
• Scalar SPL offset remains via existing global offset / Calibrate… workflow (interface gain still captured empirically unless we add preamp gain field later)

Out of scope (follow-ups)

• Per-dock cal file override
• Auto-computing dBFS offset from sensitivity without known interface gain
• Phase correction from cal files

Test plan

• Unit tests for cal file parser (factory + REW samples)
• Test interpolated correction at known frequencies
• Integration: spectrum/octave output shifts by file correction at a bin

[Skeyelab]

Implemented on branch `feature/global-calibration`:

• Parser (`friture/mic_cal_file.py`): factory `*1000Hz -38.6` headers + REW `.cal` / `Sensitivity … dBFS`
• Global settings: Browse/Clear mic cal file under Input calibration
• Applied globally to FFT spectrum, octave spectrum, and spectrogram (scalar offset + subtract freq correction)
• Level meters still use scalar global offset; use Calibrate… for absolute SPL

Verified parsing `24161.txt` (sensitivity −38.6 dB @ 1 kHz, 257 frequency points).