User intuition (16-bit) + 12-bit packing hypothesis + the int16 ADC
range constraint led to the final piece.
30 NN block format (CONFIRMED across all 14 blocks in the fixture
bundle):
NN 12-bit signed deltas packed as NN/4 groups of 6 bytes each.
Within each group:
bytes [0:2] = 16 bits = 4 × 4-bit high nibbles (MSB-first)
bytes [2:6] = 4 × int8 low bytes
delta[k] = sign_extend_12((high_nibble[k] << 8) | low_byte[k])
Block length = NN × 1.5 + 2 bytes (tag included). Earlier walker
used NN × 4 which is only correct in the TRAILER section.
Why 12-bit: ±2047 in 16-count units ≈ ±10 in/s = the geophone's
full-scale range at Normal sensitivity. The codec sizes its widest
delta to cover the worst-case sample-to-sample change.
Results: every decoded sample across all fixture events matches truth
byte-exact. ZERO divergences.
event-a: 9984 samples (full event, all 3 geos)
event-c: 3840 (full event)
event-d: 3840 (full event)
JQ0: 9984 (full event)
V70: 9984 (full event)
SP0: 5122 (walker stops early on edge cases)
SS0: 1758
SV0: 2114
event-b: 738
TOTAL: 47,364 ADC samples verified, zero errors.
Three full 3-sec events decode end-to-end across all three geo
channels. The events where fewer samples decode (SP0/SS0/SV0/event-b)
are limited by walker robustness issues past the first few segments,
NOT by decoder correctness.
64 tests pass (up from 55). Files: minimateplus/waveform_codec.py
(new 30 NN decode + corrected walker length), tests/test_waveform_codec.py
(new full-event regression tests), docs/* (updated status everywhere),
analysis/test_30nn_hybrid.py (new — the analysis script that confirmed
the format).
Claude