The OSD-lite decoder was the source of FT2 false positives. It tries
685 CRC-14 checks across 5 passes (1 + 16 + 120 per pass), giving a
~4% chance of accepting random noise as a valid decode.
The reference implementation (decode174_91) verifies OSD results
against the received signal; the trx-rs OSD-lite only checked CRC.
Add ft2_count_hard_errors_vs_llr() which counts how many of the 174
coded bits in an OSD candidate disagree with the received hard
decisions. A legitimate correction disagrees in very few positions;
a false CRC match on noise disagrees in ~40-50 parity positions.
Reject OSD results with more than 36 hard errors.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>
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