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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
Download Patch File Download Diff File Expand all files Collapse all files
Software/components/drivers/si4684/src/Si4684Driver.cpp
+48 -3
View File
@@ -40,6 +40,8 @@ constexpr std::size_t kSpiReplyLeadIn = 1U;
/** FM_RSQ_STATUS field indices with kSpiReplyLeadIn (AN649 RESP5–10). */
constexpr std::size_t kFmRsqOffValid = 6U;
constexpr std::size_t kFmRsqOffReadFreq = 7U;
/** AN649 FM_RSQ_STATUS RESP8 FREQOFF: signed offset in 2 PPM units. */
constexpr std::size_t kFmRsqOffFreqOff = 9U;
constexpr std::size_t kFmRsqOffRssi = 10U;
constexpr std::size_t kFmRsqOffSnr = 11U;
@@ -64,6 +66,12 @@ constexpr std::uint16_t kSi4684I2sOutEnable = 0x8002U;
/** Si4684 volume: 0=mute, 63=max (AN649 AUDIO_ANALOG_VOLUME). */
constexpr std::uint8_t kSi4684VolumeMax = 63U;
constexpr std::uint16_t kPropFmRdsConfig = 0x3C02U;
/** AN649 FM_AUDIO_DE_EMPHASIS (0x3900): 0=75us/US (chip default), 1=50us/
* Europe, 2=disabled. FM seek band/spacing above is already the European
* 87.5-107.9 MHz/100 kHz plan, so the chip must not stay on its 75us/US
* default -- that under-de-emphasizes treble on every station. */
constexpr std::uint16_t kPropFmAudioDeEmphasis = 0x3900U;
constexpr std::uint16_t kFmAudioDeEmphasisEurope = 0x0001U;
/** AN649 FM valid tune properties (defaults RSSI 17 dBµV, SNR 10 dB). */
constexpr std::uint16_t kPropFmValidRssiThreshold = 0x3202U;
constexpr std::uint16_t kPropFmValidSnrThreshold = 0x3204U;
@@ -486,6 +494,16 @@ std::expected<void, Si4684Error> Si4684Driver::configureAfterBoot(
{0xB302U, 0x0000U},
{0xB303U, 0x0000U},
{0xB401U, 0x0002U},
// AN649 Property 0xB500 DAB_ACF_ENABLE: bit0 SOFTMUTE_ENABLE,
// bit1 COMF_NOISE_ENABLE, datasheet default 0x0003 (both on).
// Tried 0x0003 on 2026-08-23 hoping it would mask the periodic
// hiss/unintelligible-voice symptom on real DAB reception --
// live A/B test made it worse (COMF_NOISE injects synthetic
// noise on every brief signal-quality dip, and frequent
// softmute engagement chopped up speech). PE5PVB's independent
// SI4684-DAB-Receiver project also explicitly disables this
// (Set_Property(0xB500, 0x0000)), matching what real testing
// shows here -- disabled deliberately, not an oversight.
{0xB500U, 0x0000U},
};
for (const auto& prop : kDabProps) {
@@ -542,6 +560,11 @@ std::expected<void, Si4684Error> Si4684Driver::configureAfterBoot(
if (auto rds = setProperty(kPropFmRdsConfig, 0x0001U); !rds) {
return rds;
}
if (auto deEmph = setProperty(kPropFmAudioDeEmphasis,
kFmAudioDeEmphasisEurope);
!deEmph) {
return deEmph;
}
// AN649 §0x3202/0x3204: lower seek/tune validity for weak lab antennas.
if (auto rssi = setProperty(kPropFmValidRssiThreshold,
kFmValidRssiThresholdDbuV);
@@ -597,7 +620,8 @@ std::expected<void, Si4684Error> Si4684Driver::configureAfterBoot(
}
std::expected<void, Si4684Error> Si4684Driver::boot(
Si4684Band band, std::uint8_t xtalIbias, std::uint8_t xtalCtun)
Si4684Band band, std::uint8_t xtalIbias, std::uint8_t xtalCtun,
std::uint32_t xtalFreqHz)
{
if (booted_ && loadedBand_ == band) {
return {};
@@ -672,10 +696,19 @@ std::expected<void, Si4684Error> Si4684Driver::boot(
}
// ARG2=0x17(CLK_MODE=crystal,TR_SIZE), ARG3=IBIAS, ARG4-7=XTAL_FREQ
// 19.2 MHz (0x0124F800), ARG8=CTUN, ARG9=0x10 (fixed bit4=1 per AN649
// (little-endian, nominal 19.2 MHz = 0x0124F800; may be intentionally
// offset from nominal as a software crystal calibration -- see the
// xtalFreqHz doc comment), ARG8=CTUN, ARG9=0x10 (fixed bit4=1 per AN649
// §Command 0x01), ARG10-15=0 (AN649 POWER_UP argument table).
std::uint8_t powerUp[] = {
0x17, xtalIbias, 0x00, 0xf8, 0x24, 0x01, xtalCtun, 0x10,
0x17,
xtalIbias,
static_cast<std::uint8_t>(xtalFreqHz & 0xFFU),
static_cast<std::uint8_t>((xtalFreqHz >> 8) & 0xFFU),
static_cast<std::uint8_t>((xtalFreqHz >> 16) & 0xFFU),
static_cast<std::uint8_t>((xtalFreqHz >> 24) & 0xFFU),
xtalCtun,
0x10,
0x00, 0x00, 0x00, 0x18, 0x00, 0x00,
};
if (auto pu = writeCommand(Command::PowerUp, powerUp, sizeof(powerUp));
@@ -745,6 +778,17 @@ std::expected<void, Si4684Error> Si4684Driver::boot(
return {};
}
std::expected<void, Si4684Error> Si4684Driver::recalibrateXtal(
std::uint8_t xtalIbias, std::uint8_t xtalCtun, std::uint32_t xtalFreqHz)
{
if (auto ready = ensureBooted(); !ready) {
return ready;
}
const Si4684Band band = loadedBand_;
booted_ = false;
return boot(band, xtalIbias, xtalCtun, xtalFreqHz);
}
bool Si4684Driver::isBooted() const noexcept
{
return booted_;
@@ -920,6 +964,7 @@ std::expected<Si4684FmRsq, Si4684Error> Si4684Driver::readFmRsq()
static_cast<std::int8_t>(raw[kFmRsqOffSnr]),
freqInBand && chipValid,
false,
static_cast<std::int8_t>(raw[kFmRsqOffFreqOff]),
};
return rsq;
}