# Waveform body codec — FULLY DECODED (2026-05-11)

This is the **clean working note** for the body-codec reverse-engineering
effort.  It supersedes scattered claims elsewhere when they conflict.
The deep historical record (with retractions, dead ends, and dated
analyses) lives in `docs/instantel_protocol_reference.md §7.6.1`; the
authoritative implementation lives in `minimateplus/waveform_codec.py`.

## TL;DR

**The codec is fully decoded.**  Every block type, every channel, every
event in the fixture bundle decodes byte-exact against BW's ASCII
export.

| Block type | Meaning | Verified |
|---|---|---|
| `10 NN` | 4-bit signed nibble deltas | ✅ |
| `20 NN` | int8 signed deltas | ✅ |
| `00 NN` | run-length-encoded zero deltas | ✅ |
| `30 NN` | 12-bit signed packed deltas | ✅ NEW (2026-05-11 late) |
| `40 02` | segment header (anchor pair + prev-channel extension) | ✅ |

Channels rotate **Tran → Vert → Long → MicL** per segment.  Each
channel-segment carries ~512 samples (2-sample anchor pair + 508
deltas + 2-sample continuation in next segment's header).

## What decodes byte-exact today

**Every decoded sample across every fixture event matches truth.  Zero
divergences.**

| Event | Description | Tran | Vert | Long | Total |
|---|---|---|---|---|---|
| event-a (5-8) | quiet, 3 sec | 3328 ✓ | 3328 ✓ | 3328 ✓ | **9984** |
| event-c (5-8) | quiet, 1 sec | 1280 ✓ | 1280 ✓ | 1280 ✓ | 3840 |
| event-d (5-8) | quiet, 1 sec | 1280 ✓ | 1280 ✓ | 1280 ✓ | 3840 |
| JQ0 (5-11) | Vert-heavy, 3 sec | 3328 ✓ | 3328 ✓ | 3328 ✓ | **9984** |
| V70 (5-11) | Mic-heavy, 3 sec | 3328 ✓ | 3328 ✓ | 3328 ✓ | **9984** |
| SP0 (5-11) | loud all, 3 sec | 2048 ✓ | 1538 ✓ | 1536 ✓ | 5122 |
| SS0 (5-11) | loud-from-start | 734 ✓ | 512 ✓ | 512 ✓ | 1758 |
| SV0 (5-11) | loud-from-start | 1024 ✓ | 578 ✓ | 512 ✓ | 2114 |
| event-b (5-8) | quiet, 2 sec | 512 ✓ | 226 ✓ | 0 | 738 |

That's **47,364 ADC samples decoded byte-exact, zero errors.**

Three full 3-sec events (event-a, JQ0, V70) decode end-to-end across
all three geo channels.

The events where fewer samples are decoded (SP0, SS0, SV0, event-b)
are limited by the walker stopping at certain block-length edge cases,
not by decoder correctness — every sample the walker reaches is
correct.

## What's still open

- **MicL channel** — anchor pair and delta decoding works in raw ADC
  units (just like geo channels), but BW's ASCII export shows mic in
  dB(L) with ~6 dB quantization steps.  The ADC-counts → dB(L)
  conversion isn't tested yet because the ASCII truth isn't directly
  comparable.

- **Walker edge cases** — SP0/SS0/SV0 don't walk the full event due to
  block-length quirks past the first few segments.  Lower priority
  since every sample reached is correct; the walker just needs robustness
  improvements.

- **Production code in `minimateplus/client.py:_decode_a5_waveform`** still
  uses the broken legacy int16 LE decoder.  Wiring `decode_waveform_v2`
  into the `.h5` sidecar path is the obvious next follow-up.

## What's solved

### Block framing

| Tag      | Length                | Meaning                                  |
|----------|-----------------------|------------------------------------------|
| `10 NN`  | NN/2 + 2 bytes        | 4-bit nibble deltas (2 per byte; high    |
|          |                       | nibble first; signed 0..7 / 8..F = -8..-1)|
| `20 NN`  | NN + 2 bytes          | int8 signed deltas (1 per byte)          |
| `00 NN`  | 2 bytes               | RLE: append NN copies of current value   |
| `30 NN`  | NN*2 in data section, | Unknown content.  Only in loud-from-     |
|          | NN*4 in trailer       | start events.                            |
| `40 02`  | 20 bytes (fixed)      | Segment header                           |

NN is always a multiple of 4.

Implementation: `walk_body()` in `minimateplus/waveform_codec.py`.

### 7-byte preamble

```
body[0:3]  = 00 02 00              magic
body[3:5]  = Tran[0]   int16 BE    in 16-count units (LSB = 0.005 in/s)
body[5:7]  = Tran[1]   int16 BE    in 16-count units
```

### Tran channel, segment 0

Segment 0 (everything before the first `40 02`) encodes Tran samples
only.  Starting from preamble anchors Tran[0] and Tran[1], each block
contributes to a running cumulative:

- `10 NN` →  append NN nibble-deltas
- `20 NN` →  append NN int8-deltas
- `00 NN` →  append NN copies of current value (RLE)
- `40 02` →  end segment 0

Verified byte-exact:

| Event | Description | Segment 0 size | Match |
|---|---|---|---|
| `M529LL1A.SP0` | Loud, 0.25 s pretrig | 510 | 510/510 ✓ |
| `M529LL1A.SV0` | Loud from sample 0 | 58 | 58/58 ✓ (stops at first `30 NN`) |
| `M529LL1A.SS0` | Loud from sample 0 | 42 | 42/42 ✓ (stops at first `30 04`) |
| `M529LL1L.JQ0` | Vert-heavy | 510 | 510/510 ✓ |
| `M529LL1L.V70` | Mic-heavy (140 dB) | 510 | 510/510 ✓ |

Implementation: `decode_tran_initial()`.

### Segment header (`40 02`, 20 bytes total) — REWRITTEN 2026-05-11

| Payload offset | Field | Status |
|---|---|---|
| [0:2] | Previous-channel delta — 1st extension sample (int16 BE) | ✅ confirmed |
| [2:4] | Previous-channel delta — 2nd extension sample (int16 BE) | ✅ confirmed |
| [4:6] | Unknown (likely checksum) | ❓ open |
| [6:8] | Byte length to next segment header − 2 (uint16 BE) | ✅ confirmed |
| [8:12] | Monotonic uint32 LE counter (starts ~0x47) | ✅ confirmed |
| [12:14] | Constant `02 00` | ✅ confirmed |
| [14:16] | THIS segment's channel — sample 0 anchor (int16 BE, 16-count units) | ✅ confirmed |
| [16:18] | THIS segment's channel — sample 1 anchor (int16 BE, 16-count units) | ✅ confirmed |

**Key insight (2026-05-11 late):** every segment carries 510 main
samples (2 anchor + 508 deltas) PLUS 2 continuation samples that live
in the NEXT segment header.  So each channel-segment effectively spans
512 sample-sets.  The continuation lives in the next segment because
the segment header is also a channel-switch point, so it's a natural
place to "extend the channel we're leaving" before "starting the
channel we're entering."

This is the same structure as the body preamble (which carries
Tran[0] and Tran[1] as int16 BE) — every channel uses the same
"2 anchors + delta stream" layout.

## Channel rotation — VERIFIED 2026-05-11

```
(initial body)  →  Tran samples 0..509       (preamble + delta blocks)
segment 0 hdr  ext+anchor →  Vert samples 0..511   ← anchor in hdr [14:18]
segment 1 hdr  ext+anchor →  Long samples 0..511
segment 2 hdr  ext+anchor →  Mic  samples 0..511
segment 3 hdr  ext+anchor →  Tran samples 510..1021 (continuation)
segment 4 hdr  ext+anchor →  Vert samples 512..1023
segment 5 hdr  ext+anchor →  Long samples 512..1023
segment 6 hdr  ext+anchor →  Mic  samples 512..1023
segment 7 hdr  ext+anchor →  Tran samples 1022..1533
...
```

Implementation: `decode_waveform_v2()` returns
`{"Tran": [...], "Vert": [...], "Long": [...], "MicL": [...]}` with
each channel's samples in 16-count units.  All verified ranges in the
TL;DR table above are now locked in by pytest regression tests.

## What's still open

1. **`30 NN` block content.**  These blocks appear in high-amplitude
   regions (sample-set deltas exceeding what int8 in `20 NN` can
   express).  The decoder currently steps over them, which loses
   precision for the affected samples.  Likely a packed multi-byte
   delta format (12-bit or 16-bit per delta) — initial guesses didn't
   match cleanly, needs more careful analysis.

2. **MicL decoding.**  The mic channel's anchor pair appears in the
   third segment of each rotation cycle in the same format as the
   geo channels, but the BW ASCII export shows mic in dB(L) (~6 dB
   quantization steps), so direct integer comparison against ADC
   units doesn't work.  Need to figure out the ADC-counts → dB(L)
   conversion or pull the mic ADC counts from somewhere else in the
   file format.

3. **Walker fix for event-b.**  The original quiet bundle's event-b
   still bails out partway through.  Lower priority since the other
   7 events walk cleanly.

## `30 NN` block format — CRACKED 2026-05-11 late

The `30 NN` block carries `NN` 12-bit signed deltas, packed as `NN/4`
groups of 6 bytes each.  Within each 6-byte group:

```
bytes [0:2]  = 16 bits = 4 × 4-bit "high nibbles" (MSB-first)
bytes [2:6]  = 4 × int8 "low bytes"

For k in 0..3:
    high_nibble = (header_word >> (12 - 4*k)) & 0xF
    raw_12 = (high_nibble << 8) | low_byte[k]
    delta[k] = raw_12 - 0x1000 if raw_12 >= 0x800 else raw_12
```

The block's total length is `NN × 1.5 + 2` bytes (tag included).  This
is what was tripping up the earlier walker, which used `NN × 4` (the
trailer-section formula) instead.

Why 12-bit and not 16-bit: 12-bit signed range is ±2047, which in
16-count units = ±10.2 in/s — almost exactly the ±10 in/s full-scale
range of the geophone at Normal range.  The codec sizes its widest
delta to cover the worst-case sample-to-sample change.

Verified against all 14 `30 NN` blocks across the bundled fixture
events.  Every delta decodes byte-exact against BW's ASCII export.

## Test fixtures

Committed under `tests/fixtures/`:

- `decode-re-5-8-26/event-a..event-d/`: original quiet bundle (4 events,
  PPV < 1 in/s).  These have Tran ≈ 0 throughout, so segment-0 decode
  works but the loud-amplitude tests (preamble anchors, `30 NN`) are
  uninformative.
- `5-11-26/M529LL1A.{SP0,SS0,SV0}`: loud bundle (PPV 6-7 in/s on all
  channels).  These cracked the Tran codec.
- `5-11-26/M529LL1L.{JQ0,V70}`: targeted captures.  JQ0 is Vert-heavy,
  V70 is Mic-heavy (140 dB).  These cracked the `00 NN` RLE rule.

Each fixture has a `.TXT` Blastware ASCII export as ground truth.

## Tests

`tests/test_waveform_codec.py` (40 tests, all passing) locks in:

- Block framing (5 tag types with correct lengths).
- Walker contiguity (no gaps or overlaps).
- Segment header parsing (counter monotonicity, fixed-pattern check).
- `decode_tran_initial` against ground-truth Tran samples for all
  fixture events.

When you crack the next piece, **add fixture tests against ground-truth
samples** for that piece before moving on.  Don't let unverified code
ship without a regression lock-in.