Sylpheed/docs/re/structures/menu-bgm-loop-fields-conflict.md

🟡 The loop point IS a runtime field — and reading it contradicts the audio measurement

Classification: decoded (the fields and their values) plus an unresolved conflict (what they mean in seconds). Xenia Canary, 2026-08-30, one boot, 45 s on the menu. The port should change nothing on the strength of this page.

✅ The loop point is not absent from the format

bgm-two-stems.md says "no loop-point field has been identified in the XMA header, so a menu loop is authored". That is true of the file header and it left the wrong impression. The loop lives in the XMA decoder context, set at runtime by XMASetLoopData, and Xenia's UpdateLoopStatus already logs it — no patch was needed, only the Apu log category (--log_mask=13 --log_level=3).

ctx	wave	loop_start	loop_end	loop_count
0	3 876 864 B	3 605 682	25 640 423	255 (infinite)
1	3 930 112 B	3 539 158	26 216 351	255

Bit offsets. 8 734 records, every one after BGM_103's contexts appear — the movie's three ADV streams log none at all, i.e. they do not loop.

✅ And the semantics are visible in the trajectory. input_buffer_read_offset runs from 32 (the first packet header) upward, and 20 % of samples sit below loop_start — so the stream plays from the beginning, and loop_start is where it returns after loop_end. The first pass is longer than the cycles after it.

⚠️ No wrap was observed. Max read offset was 16.9 M / 17.2 M against a loop_end of 25.6 M / 26.2 M — the 45 s hold was too short. The jump itself is inferred from the field semantics, not watched.

🔴 Two things I predicted are refuted

1. "loop_start ≈ 0" — no. It is ~3.5–3.6 M bits, 11.4–11.6 % into the stream. I registered that prediction before the run and it is wrong.

2. A linear bits→seconds conversion — invalid, and the data proves it. Applied to each stem with its own byte_size:

	linear loop duration
ctx0	62.34 s
ctx1	63.29 s

Two stems that play sample-synchronously cannot have loop durations 0.95 s apart — they would drift a second per cycle. So the assumption fails on its own output. XMA frames are variable-length in bits, which is exactly why.

🔴 The conflict, stated rather than resolved

loop_start at 11.6 % of the stream implies a cycle of roughly [10 s, 72 s] of the 87.744 s wave. But menu-bgm-loop-measured.md put the observed wave offsets at 0.25 … 57.18 s. Both cannot be true.

⚠️ And the weakness is probably mine. That page's locator control used slices cut from the wave itself, which are exact copies — a far easier matching problem than a real capture, which differs by decoder, gain and mix. A control that is easier than the measurement does not bound the measurement's error, and music with repeated phrases is exactly where a locator aliases. The clean +5.00 s stepping shows the locator is self-consistent; it does not show it locked to the right phrase.

So the honest position is:

claim	status
the loop is a runtime field, with these values	✅ decoded
the movie streams do not loop	✅ decoded
the cycle is 61.93 s	🟡 measured from audio, and its control was too easy
where the cycle starts in the wave	🔴 contested — 0.25 s from audio, ~10 s from the field
bits → seconds	❔ needs an XMA frame walk; not done

✅ RESOLVED (2026-08-30, later) — the wrap was watched, and the length is confirmed

The conflict is settled by timing the loop instead of converting it. Tailing the Apu debug log and stamping input_buffer_read_offset as it arrives (../data/menu-bgm-wrap-timing.txt):

wraps observed	three
each	exactly its own loop_end → its own loop_start
the two contexts	wrap at the same instant, all three times
cycle	61.56 s and 62.06 s → 61.81 s

✅ The field semantics are now watched, not inferred. And the two stems wrapping together is the property the linear conversion could not deliver — 62.34 vs 63.29 s would have drifted them a second per cycle.

✅ 61.81 s against the audio measurement's 61.93 s — 0.2 % apart, from instruments sharing nothing: one is a wall clock between decoder events, the other an autocorrelation that never touched the wave.

🔴 Linearity refuted a second time, internally. The fitted rate over the clean stretch 10–60 s is 341 394 bits/s; the cycle covers 22 034 741 bits in 61.81 s = 356 491 bits/s. 4.4 % apart inside one stream — no single rate converts these offsets.

🔴 And my audio locator's placement is refuted

loop_start at 3.6 M bits is 11.6 % of the stream by any reading, and ~10.1 s at the cycle's own mean rate. menu-bgm-loop-measured.md put the loop's start at 0.25 s. That is wrong, and the reason is the one already suspected: its control matched slices cut from the wave itself, which never tested the aliasing the real problem has.

So the length was right and the placement was wrong — which is why the port's shipped loop sounds correct: it has the right duration, over the wrong span.

✅ MEASURED (2026-08-30, latest): loop_start is at 9.44 s

The fix was scheduling. Tailing the log from before the music starts cut the unsampled backlog from 616 samples spanning offsets 32…2 559 033 down to 125 spanning 32…515 239, and the first pass is then sampled like any other cycle (../data/menu-bgm-loop-start.txt).

Wraps at 96.46 / 158.33 / 220.21 s, gaps 61.87 / 61.87, both contexts together.

Two derivations, neither converting bits to seconds:

	ctx0	ctx1
(a) time to read_offset crossing loop_start, + the head at the local measured rate	9.44 s	9.44 s
(b) first pass (offset 32 → loop_end) − cycle	9.44 s	9.44 s

Four numbers, one value. The head correction is 1.33 s and uses a rate measured on 748 timestamped samples of that same stretch — not the cycle mean, and not an assumption of linearity across the stream.

So the loop region is [9.44 s, 71.31 s] of an 87.744 s wave, cycling every 61.87 s. The first 9.44 s is an intro played once; the last 16.4 s — the fade-out bgm-two-stems.md documents — is never played at all.

⚠️ The decoder reads ahead of playback, but both endpoints are read_offset events, so the lead cancels in the difference. One boot, one bank.

The superseded position

Offsets below loop_start play exactly once, before the first wrap, and this trace stamped that whole stretch at t=0.002 — 616 samples spanning offsets 32…2 559 033 in a single batch, because the trace started after the music and swallowed the log's backlog in one read. A back-extrapolation suggests ~9–13 s, but it is a linear back-extrapolation and linearity is what the same run refutes.

The fix is one line of scheduling: start the trace before tapping into the menu, so the first pass is sampled at 0.5 s like every later cycle. Not done.

What the port should do: keep 61.93, and know the span is wrong

✅ The 61.93 s length is now confirmed twice over — keep it. ⚠️ But the span is wrong: the game loops a 61.8 s window that begins ~10 s into the wave, not the first 61.93 s. A trim to [0, 61.93] therefore replays the intro every cycle and omits the tail the game does play. 🟡 The exact start is not measured, so do not re-cut on a guess — what is needed is the trace started before the music, which is one line of scheduling and is not done.