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1495 banks carry no RIFF and take a separate path that hardcoded both the offset and stereo. Across a random 48-bank sample there was NOT ONE where the old stereo-at-1392 pair beat the best mono offset; median gain 184x, individual banks going from 0-4816 decoded bytes to 180000-380000. Stereo shows the same stop-after-one-frame signature already recorded for the leading segment. With no RIFF the offset cannot be derived, so scan_data_offset picks among the four disc offsets by XMA1 packet-header plausibility. Validated on the LABELLED set -- all 7358 banks that do have a RIFF, where the answer is forced: 7330 correct (99.62%), and all 28 misses are ties on the top score, never a wrong unique winner. Ties fall back to 1392. The winning offsets also reproduce, by directory, the distribution measured independently from the RIFF-bearing banks. jpn\etc splits 1468/1600, so path alone is not sufficient -- which is why this is a scan and not a lookup table. 7 disc tests pass (build-reborn test -p sylpheed-formats --test slb_leading_segment_disc, SYLPHEED_DISC wired up).
133 lines
5.8 KiB
Markdown
133 lines
5.8 KiB
Markdown
# `.slb` leading-stream data offset — 1392 was never a constant
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**✅ Settled 2026-08-26, verified by decoding.** A bank's leading headerless
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packet stream does not start at a fixed offset. It starts at
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**`first_riff % 2048`**. `HEADERLESS_DATA_OFFSET = 1392` is the value that
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offset happens to take in `<lang>\etc\`, and assuming it everywhere starts the
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decode mid-packet and throws away almost all of the audio.
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## The rule
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XMA1 packets are 2048 bytes and the leading stream is a whole number of them
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ending at the first `RIFF`. So its start is forced:
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start = first_riff % 2048
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Disc-wide that lands on exactly **four** values — 1392, 1468, 1600, 1728 — all
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of the form `1392 + 4k`. Across the 3 965 Japanese and 3 393 English banks with
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a non-empty leading region, no other value occurs:
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| | 1392 | 1468 | 1600 | 1728 |
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|---|---|---|---|---|
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| `eng\etc`, `eng\Movie`, `eng\Briefing` | 1 520 | — | — | — |
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| `eng\Voice` | 8 | 1 873 | — | — |
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| `jpn\etc` | — | 1 402 | 303 | — |
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| `jpn\Briefing` | — | 71 | — | — |
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| `jpn\Movie` | — | — | 61 | — |
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| `jpn\Voice` | — | — | 2 033 | 95 |
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It varies by **language and subdirectory**, which is why a constant derived
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from `eng\etc\` looked right for years' worth of the banks anyone had reason to
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open.
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## Verified by decoding, not by arithmetic
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The alignment argument alone proves nothing — any offset can be made to "align"
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by definition. The test is whether more audio comes out. Decoded through
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FFmpeg's `xma1` at mono/48 kHz, on a random sample of **140** banks that have a
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non-empty leading region:
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| outcome | banks |
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|---|---|
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| more audio at `ri % 2048` | **85** |
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| byte-identical | 54 |
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| less audio | **1** |
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Median gain among the improved: **70×**. The 54 identical ones are the control —
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they are the `eng\etc`-style banks where `ri % 2048` *is* 1392, so the rule
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must and does reproduce the old behaviour exactly. Individual cases:
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eng\Voice\VOICE_TCAF_592.slb 1 506 -> 97 152 bytes (65x)
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jpn\Voice\VOICE_TCAF_592.slb 2 910 -> 127 178 bytes (44x)
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eng\etc\VOICE_D_452.slb 30 154 -> 30 154 bytes (unchanged, control)
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## The one counterexample
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`eng\Voice\VOICE_TCAF_608.slb` decodes 2 840 bytes at 1392 and 896 at 1468.
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It is worth being precise about what that is and is not. It is **not** a bank
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where the old constant works and the derived offset fails: its leading region is
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38 988 bytes, and *both* offsets yield well under a tenth of a second, i.e. both
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fail. 1392 merely produces marginally more garbage. Whatever is wrong with this
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bank is a separate defect and is **not settled** here.
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## ❌ This withdraws my own claim from earlier the same day
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[`sound-pak-contents.md`](sound-pak-contents.md) reported that the leading
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region rule holds for "0 of 5 100 Japanese banks" and filed a backlog item
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saying the Japanese banks were a different, undecoded layout. **That was wrong.**
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The Japanese banks are the same format; only the offset differs. The measurement
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behind it was correct — zero of them satisfy `(riff − 1392) % 2048 == 0` — but
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the conclusion drawn from it was not, and the reason is instructive: I treated
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`HEADERLESS_DATA_OFFSET` as a property of the format when it was a property of
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the sample the format was derived from.
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The same error was hiding a defect in the **English** set too: 1 873 `eng\Voice`
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banks sit at 1468 and were being decoded mid-packet just as badly.
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## The `RIFF`-less banks had the same bug, plus a worse one
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**✅ Settled 2026-08-26.** 1 495 banks (799 `jpn`, 696 `eng`) carry no `RIFF` at
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all and take a separate code path. That path was wrong twice over:
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1. it used the constant offset, with no `RIFF` to derive from; and
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2. it built a **stereo** `fmt` chunk.
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Decoded across a random 48-bank sample:
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| | |
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|---|---|
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| banks where the old stereo-at-1392 beat the best mono offset | **0 of 48** |
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| median gain | **184×** |
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| range | 25× – 489 344× |
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Stereo is the same failure signature recorded for the leading segment: it stops
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after one frame. Individual banks went from 0–4 816 bytes to 180 000–380 000.
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The winning offsets fall out **by directory**, and they reproduce the
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distribution measured independently from the `RIFF`-bearing banks — which is the
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cross-check that makes this more than curve-fitting:
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eng\etc 1392 (11/11) eng\Voice 1468 (9/9) eng\Briefing 1392 (2/2)
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jpn\Voice 1600 (12/13) jpn\etc 1468 (8/12), 1600 (4)
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Note `jpn\etc` splits, so the **path alone is not enough** to pick the offset.
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### Picking the offset without a decoder
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An XMA1 packet opens with a big-endian header — 6 bits frame count, 15 bits
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frame-offset-in-bits, 3 bits metadata, 8 bits packet-skip. At the true offset
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those fields stay in range packet after packet; one byte off and they do not.
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Scoring the first 24 packets and taking the best candidate:
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**7 330 of 7 358 (99.62 %)** on the labelled set — every bank that *has* a
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`RIFF`, where the answer is forced and therefore known. All **28** misses are
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ties on the top score; there is not a single case where the scan picks wrongly
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with a unique winner. `scan_data_offset` therefore falls back to 1392 on a tie.
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This is used only for the `RIFF`-less banks. Where a `RIFF` exists the offset is
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derived from it exactly, never scanned.
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## What this does not settle
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* **Why the offset takes those four values**, and what the bytes before it are.
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There is no length field in the first 64 bytes — banks open on high-entropy
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data — so the offset is derived, not read.
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* **`eng\Voice\VOICE_TCAF_608.slb`**, above.
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* **The 28 ties.** The scan cannot separate them and falls back to 1392, which
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is right for roughly a third of that population and wrong for the rest.
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* **Why the offset takes exactly these four values by directory** is still
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unexplained — see above.
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* Nothing here was run **in the game** — this is a decoder-side result measured
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with FFmpeg as the oracle.
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