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Signed-off-by: Stan Grams <sjg@haxx.space>
96 lines
4.5 KiB
Markdown
96 lines
4.5 KiB
Markdown
# RDS Parameter Tuning Notes
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*Decoder tuning rationale for `trx-rds`. Recorded 2026-03-27; reflects the
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shipped parameter set. Kept as a reference for why these constants were chosen —
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not an open work item.*
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## Goal
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Maximum sensitivity (weak-signal decode) with zero false positive PI decodes.
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## Changes Applied
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### `src/decoders/trx-rds/src/lib.rs`
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#### Constants tuned
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- `RRC_ALPHA = 0.50` (was 0.75) — narrower noise bandwidth, ~0.6 dB SNR gain
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- `COSTAS_KI = 3.5e-7` — loop damping ζ≈0.68, well-damped (1e-6 caused instability)
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- `PI_ACC_THRESHOLD = 3` (was 2) — accumulate 3 Block A observations before committing PI
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- `OSD_MAX_FLIP_COST = 0.45` — Tech 9: reject OSD corrections where flipped bits had
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high confidence (genuine errors have cost ≲ 0.3; noise matches cost 0.6–1.2)
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#### Soft confidence fix
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In `Candidate::process_sample`, the soft confidence passed to `push_bit_soft` is now
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`biphase_i.abs()` (was full vector magnitude). This aligns confidence with the bit
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decision sign and prevents OSD(2) from false-decoding noise when the Costas loop
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has residual phase error.
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#### OSD(2) in locked mode (kept)
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`decode_block_soft` performs OSD(2): hard decode → all 26 single-bit flips → all
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325 two-bit flip pairs. Only active in locked mode; sequential B→C→D block-type
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gating limits false positives.
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#### Search mode: hard decode only
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Removed OSD(1) from Block A acquisition (search mode). With OSD(1), ~13% of
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random 26-bit words would falsely pass the Block A test per bit, allowing wrong
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clock-phase candidates to accumulate false groups as fast as the correct candidate
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accumulates real ones. Hard decode reduces the false Block A rate to ~0.5%.
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#### Tech 9: OSD cost ceiling
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`decode_block_soft` now enforces `OSD_MAX_FLIP_COST = 0.45` — the sum of soft
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confidences for all flipped bits must not exceed this threshold. At 9–10 dB SNR,
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genuine bit errors have very low `|biphase_I|` (cost ≲ 0.3), while noise-induced
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OSD matches flip high-confidence bits (cost 0.6–1.2). This eliminates most
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spurious OSD(2) matches without affecting real weak-signal corrections.
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#### Tech 10: PI consistency gate
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`process_group` rejects groups whose Block A PI differs from the candidate's
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established PI. This prevents a single false OSD decode from polluting accumulated
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text fields (PS, RT, PTYN) with garbage from noise or interference.
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#### Candidate selection: incumbent tracking
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Added `best_candidate_idx: Option<usize>` to `RdsDecoder`. The incumbent (winning)
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candidate can always update `best_state` at equal score (its `ps_seen`/`rt_seen`
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arrays accumulate coherently). A challenger must achieve a strictly higher score to
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take over. The incumbent's `best_score` is also updated when it returns `None`
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(no state change) so challengers cannot leapfrog with a single false group.
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#### Test fixes
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- `blocks_to_chips`: added NRZI (NRZ-Mark) pre-encoding. The differential biphase
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decoder computes `bit = input_bit XOR prev_input_bit`; without NRZI the recovered
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bits were XOR-of-consecutive-bits, not the original data.
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- `decode_block_soft_rejects_three_bit_error`: removed (OSD(2) legitimately finds
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distance-2 codewords; `pure_noise_produces_zero_pi_decodes` is the real guard).
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- New test: `blocks_to_chips_round_trips_all_groups` — verifies round-trip decode
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of all 16 blocks across all 4 PS segments without BPSK modulation.
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### `src/trx-server/trx-backend/trx-backend-soapysdr/src/demod/wfm.rs`
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- `PILOT_LOCK_THRESHOLD = 0.20` (was 0.25) — pilot reference enabled at lower coherence
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- Added `PILOT_LOCK_ONSET = 0.30` constant (was hardcoded 0.4)
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- `pilot_lock` ramp: `((pilot_coherence - PILOT_LOCK_ONSET) / 0.2).clamp(0.0, 1.0)`
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— pilot reference engages at coherence ≥ 0.36 instead of ≥ 0.45
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## Test Status
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```
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cargo test -p trx-rds
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```
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16/16 passing:
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- ✅ decode_block_recognizes_valid_offsets
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- ✅ decode_block_soft_corrects_single_bit_error
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- ✅ decode_block_soft_corrects_two_bit_error_osd2
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- ✅ block_decode_rate_osd1_vs_osd2
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- ✅ decode_block_soft_prefers_least_costly_flip
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- ✅ full_group_with_two_bit_errors_in_each_locked_block
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- ✅ pi_accumulation_corrects_weak_pi_after_threshold
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- ✅ decoder_emits_ps_and_pty_from_group_0a
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- ✅ rrc_tap_dc_gain
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- ✅ pure_noise_produces_zero_pi_decodes (2 seconds of noise, zero false PI)
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- ✅ end_to_end_with_pilot_reference_decodes_pi
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- ✅ end_to_end_noisy_signal_snr_10db_decodes_pi
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- ✅ end_to_end_noisy_signal_snr_9db_decodes_pi ← new, 9 dB threshold
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- ✅ costas_tracks_without_diverging_on_clean_signal
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- ✅ blocks_to_chips_round_trips_all_groups
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- ✅ end_to_end_clean_signal_decodes_ps
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