Sylpheed/docs/re/structures/slb-data-offset.md

.slb leading-stream data offset — 1392 was never a constant

✅ Settled 2026-08-26, verified by decoding. A bank's leading headerless packet stream does not start at a fixed offset. It starts at first_riff % 2048. HEADERLESS_DATA_OFFSET = 1392 is the value that offset happens to take in <lang>\etc\, and assuming it everywhere starts the decode mid-packet and throws away almost all of the audio.

The rule

XMA1 packets are 2048 bytes and the leading stream is a whole number of them ending at the first RIFF. So its start is forced:

start = first_riff % 2048

Disc-wide that lands on exactly four values — 1392, 1468, 1600, 1728 — all of the form 1392 + 4k. Across the 3 965 Japanese and 3 393 English banks with a non-empty leading region, no other value occurs:

	1392	1468	1600	1728
eng\etc, eng\Movie, eng\Briefing	1 520	—	—	—
eng\Voice	8	1 873	—	—
jpn\etc	—	1 402	303	—
jpn\Briefing	—	71	—	—
jpn\Movie	—	—	61	—
jpn\Voice	—	—	2 033	95

It varies by language and subdirectory, which is why a constant derived from eng\etc\ looked right for years' worth of the banks anyone had reason to open.

Verified by decoding, not by arithmetic

The alignment argument alone proves nothing — any offset can be made to "align" by definition. The test is whether more audio comes out. Decoded through FFmpeg's xma1 at mono/48 kHz, on a random sample of 140 banks that have a non-empty leading region:

outcome	banks
more audio at ri % 2048	85
byte-identical	54
less audio	1

Median gain among the improved: 70×. The 54 identical ones are the control — they are the eng\etc-style banks where ri % 2048 is 1392, so the rule must and does reproduce the old behaviour exactly. Individual cases:

eng\Voice\VOICE_TCAF_592.slb    1 506 ->  97 152 bytes   (65x)
jpn\Voice\VOICE_TCAF_592.slb    2 910 -> 127 178 bytes   (44x)
eng\etc\VOICE_D_452.slb        30 154 ->  30 154 bytes   (unchanged, control)

The one counterexample

eng\Voice\VOICE_TCAF_608.slb decodes 2 840 bytes at 1392 and 896 at 1468.

It is worth being precise about what that is and is not. It is not a bank where the old constant works and the derived offset fails: its leading region is 38 988 bytes, and both offsets yield well under a tenth of a second, i.e. both fail. 1392 merely produces marginally more garbage. Whatever is wrong with this bank is a separate defect and is not settled here.

❌ This withdraws my own claim from earlier the same day

sound-pak-contents.md reported that the leading region rule holds for "0 of 5 100 Japanese banks" and filed a backlog item saying the Japanese banks were a different, undecoded layout. That was wrong. The Japanese banks are the same format; only the offset differs. The measurement behind it was correct — zero of them satisfy (riff − 1392) % 2048 == 0 — but the conclusion drawn from it was not, and the reason is instructive: I treated HEADERLESS_DATA_OFFSET as a property of the format when it was a property of the sample the format was derived from.

The same error was hiding a defect in the English set too: 1 873 eng\Voice banks sit at 1468 and were being decoded mid-packet just as badly.

The RIFF-less banks had the same bug, plus a worse one

✅ Settled 2026-08-26. 1 495 banks (799 jpn, 696 eng) carry no RIFF at all and take a separate code path. That path was wrong twice over:

1. it used the constant offset, with no RIFF to derive from; and
2. it built a stereo fmt chunk.

Decoded across a random 48-bank sample:

banks where the old stereo-at-1392 beat the best mono offset	0 of 48
median gain	184×
range	25× – 489 344×

Stereo is the same failure signature recorded for the leading segment: it stops after one frame. Individual banks went from 0–4 816 bytes to 180 000–380 000.

The winning offsets fall out by directory, and they reproduce the distribution measured independently from the RIFF-bearing banks — which is the cross-check that makes this more than curve-fitting:

eng\etc 1392 (11/11)   eng\Voice 1468 (9/9)   eng\Briefing 1392 (2/2)
jpn\Voice 1600 (12/13) jpn\etc 1468 (8/12), 1600 (4)

Note jpn\etc splits, so the path alone is not enough to pick the offset.

Picking the offset without a decoder

An XMA1 packet opens with a big-endian header — 6 bits frame count, 15 bits frame-offset-in-bits, 3 bits metadata, 8 bits packet-skip. At the true offset those fields stay in range packet after packet; one byte off and they do not. Scoring the first 24 packets and taking the best candidate:

7 330 of 7 358 (99.62 %) on the labelled set — every bank that has a RIFF, where the answer is forced and therefore known. All 28 misses are ties on the top score; there is not a single case where the scan picks wrongly with a unique winner. scan_data_offset therefore falls back to 1392 on a tie.

This is used only for the RIFF-less banks. Where a RIFF exists the offset is derived from it exactly, never scanned.

What this does not settle

• Why the offset takes those four values, and what the bytes before it are. There is no length field in the first 64 bytes — banks open on high-entropy data — so the offset is derived, not read.
• eng\Voice\VOICE_TCAF_608.slb, above.
• The 28 ties. The scan cannot separate them and falls back to 1392, which is right for roughly a third of that population and wrong for the rest.
• Why the offset takes exactly these four values by directory is still unexplained — see above.
• Nothing here was run in the game — this is a decoder-side result measured with FFmpeg as the oracle.