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Fix Si4684 pitch distortion: uncalibrated crystal (CTUN/XTAL_FREQ)

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Root cause of FM/DAB "stonata" audio: xtalCtun=31/nominal XTAL_FREQ
defaults were never measured against this board's actual crystal
(Abracon ABM8-19.200MHZ-10-1-U-T, CL=10pF, two external 15pF load
caps). Fixed via CTUN trim by ear (31->0) plus a new live calibration
loop that reads the Si4684's own FM_RSQ FREQOFF field (broadcast
carriers are GPS-locked, so it's a free precision frequency reference)
to trim XTAL_FREQ with no lab equipment. Converged to CTUN=0,
XTAL_FREQ=19199750 Hz, residual -3/-4 ppm cross-checked on two
stations.

New tools/infrastructure (kept, not one-off):
- Si4684Driver::recalibrateXtal() + POST /api/tuner/xtal-calibrate:
  live crystal re-trim without an ESP32 reflash.
- FM_RSQ FREQOFF exposed as "freqoff_ppm" in GET /api/tuner/status.
- tools/si4684_xtal_calibration.py: automates the trim loop.
- tools/si4684_antenna_calibration.py: AN851 Appendix A ANTCAP/VARM/VARB
  sweep tool (same session, separate calibration).
- CONFIG_ESP32_I2S_TEST_TONE (off by default): isolates Si4684-specific
  audio issues from shared-downstream ones by writing a tone directly
  over the I2S bus the Si4684 also uses.
- SIGMA_WRITE_REGISTER_BLOCK/sigma_safeload_block now retry on NACK and
  verify via read-back instead of firing I2C writes blind.
- FM de-emphasis set to European 50us (was left at the US 75us default).
- DAB_ACF_ENABLE restored to its previous 0x0000 with a citation
  explaining why (tested the datasheet default of 0x0003, made things
  audibly worse).

See docs/si4684-rf-investigation-report.md's 2026-08-23 entry for the
full elimination chain and docs/TODO.md for follow-up work (persisting
calibration results to EEPROM instead of requiring a firmware edit).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
michele
2026-08-23 20:30:42 +02:00
co-authored by Claude Sonnet 5
parent a91c63fc31
commit fae2305164
24 changed files with 1309 additions and 29 deletions
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Software/main/hardware_bootstrap.cpp
+15 -1
View File
@@ -160,7 +160,21 @@ std::expected<void, HardwareBootError> HardwareBootstrap::boot()
return {};
}
if (auto tunerResult = gSi4684.boot(si4684::Si4684Band::Dab); !tunerResult) {
// xtalCtun=0, xtalFreqHz=19199750 (2026-08-23): the compiled-in
// defaults (ctun=31, xtal=19200000 nominal) were never measured
// against this board's actual crystal (Abracon ABM8-19.200MHZ-10-1-U-T,
// CL=10pF per part number, plus two external 15pF load caps per the
// schematic). CTUN=0 was found audibly best via A/B listening (0/5/31),
// then xtalFreqHz was trimmed properly using the chip's own FM_RSQ
// FREQOFF measurement (tools/si4684_xtal_calibration.py) against two
// real, GPS-locked broadcast carriers 87.6/105.1 MHz -- converged to
// -3 to -4 ppm residual on both (cross-check confirms it's the
// crystal, not something frequency-dependent), down from +70 ppm
// uncorrected. See POST /api/tuner/xtal-calibrate to re-trim live if
// this ever needs revisiting (e.g. after a board/crystal change).
if (auto tunerResult =
gSi4684.boot(si4684::Si4684Band::Dab, 72U, 0U, 19199750U);
!tunerResult) {
ESP_LOGE(kTag, "Si4684 boot failed: error %d", static_cast<int>(tunerResult.error()));
return std::unexpected(HardwareBootError::Si4684BootFailed);
}