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.

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 799 jpn / 696 eng banks with no RIFF at all are untouched by this; they go down the headerless path and were not re-examined.
• Nothing here was run in the game — this is a decoder-side result measured with FFmpeg as the oracle.