- RRC_ALPHA 0.75→0.50: narrower noise BW, ~0.6 dB SNR gain
- COSTAS_KI 3.5e-7: maintain ζ≈0.68 (1e-6 caused loop instability)
- Soft confidence: use biphase_i.abs() instead of full vector magnitude
so OSD confidence is aligned with bit-decision sign; suppresses
false groups under noise with residual Costas phase error
- OSD(2) in locked mode: corrects ≤2-bit errors after block sync
- Search mode: hard decode only for Block A; OSD(1) in search yielded
~13% false Block A rate per bit, letting wrong clock candidates
accumulate false groups as fast as the correct candidate
- Incumbent candidate tracking (best_candidate_idx): the winning
candidate updates best_state at equal score; challengers need strictly
higher score; best_score tracks incumbent even on no-state-change
groups so challengers can't leapfrog on a single false group
- blocks_to_chips: add NRZI (NRZ-Mark) pre-encoding so the differential
biphase decoder recovers actual data bits rather than XOR-of-pairs
- Add blocks_to_chips_round_trips_all_groups test: verifies all 16 blocks
across 4 PS segments round-trip correctly without BPSK modulation
[fix](trx-backend-soapysdr): lower pilot lock threshold for weak-signal RDS
- PILOT_LOCK_THRESHOLD 0.25→0.20, add PILOT_LOCK_ONSET=0.30 constant
- Pilot reference engages at coherence ≥0.36 (was ≥0.45)
WIP: end_to_end_clean_signal_decodes_ps still failing (13/15 pass).
Decoder skips segment 2 due to ISI from rectangular test chips through
RRC receive filter. chips_to_rds_signal needs RRC pulse shaping.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
Tech 1: replace one-pole baseband LPF with FIR RRC matched filter
(alpha=0.75, 4-chip span) — largest single measured improvement per
empirical comparison (gr-rds RRC vs plain FIR: 32/38 vs 18/38 stations).
Tech 2: 19 kHz pilot x3 -> 57 kHz coherent carrier reference via the
triple-angle formula; fed from the WFM pilot Costas PLL when
pilot_lock_level > 0.5, clearing to NCO fallback otherwise.
Tech 3/7/8: OSD(2) soft-decision block decoder replaces hard CRC check.
Per-bit soft magnitudes accumulated in Candidate::block_soft[26].
decode_block_soft() searches Hamming distance 0/1/2 (352 trials total)
and returns the minimum Euclidean-cost valid codeword; ~2-3 dB gain.
Tech 4: 8th-order 57 kHz BPF (4 cascaded biquads at Q=5) in wfm.rs
replaces the previous single Q=10 biquad; ~6x steeper ACI stopband.
Tech 5: Costas loop with tanh soft phase detector drives the RDS carrier
NCO when no pilot reference is available (P+I, B_L ~20 Hz).
Tech 6: Block A PI field LLR accumulation — signed per-bit LLR summed
over 3 independent Block A observations before committing the PI value,
correcting weak-signal false locks without delaying strong-signal lock.
Tech 9: 8-tap complex CMA blind equalizer applied to IQ samples before
FM discrimination; constant-modulus error (|y|^2 - R^2) drives tap
adaptation without a training sequence, suppressing adjacent-channel
interference at the source.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
sjg