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
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