Sylpheed/docs/re/structures/bgm-two-stems.md

A music bank is two stems of one performance, played together

Status: ✅ CONFIRMED for the structure — decoded, with a disc-wide check over all 32 BGM banks. 🟡 for the role of the second wave — measured by signal analysis, and the two readings that survive are named below. ❔ which bank is the menu's music is undecodable from the disc, with the reach stated.

Answers MISSION Q10. It also withdraws the premise: the handoff said BGM_001.slb is three sub-waves of 10 KB / 4.47 MB / 4.67 MB. The 10 KB is the bank header, not a wave.

The structure — decoded

BGM_001.slb   (9 178 040 B)
      0   BANK  id=1001  data_size=9175992  hdr = 5 blocks (10240 B)  16bit/2ch
  10240   RIFF  data@14336    size=4466688  (2181 packets)   25697 B/s → 173.82 s
4481024   seek  2181 packets
4491264   RIFF  data@4495360  size=4673536  (2282 packets)   26887 B/s → 173.82 s
9168896   seek  2282 packets

A bank is a 10 240-byte header and then exactly two waves, and the two always have the same duration — different byte sizes and different bitrates, same number of seconds. Duration is data_size / PsuedoBytesPerSec (the u32 at RIFF+0x20; RIFF+0x24 is the sample rate, 48 000 Hz except BGM_020–023 at 44 100).

Disc-wide

tools/re-capture/bgm_wave_census.py, full output in ../data/bgm-wave-census.txt:

28 of the 32 BGM entries read straight off their own pak entry as two waves of equal duration — equal to 0.01 s, over lengths from 37 s to 277 s.

The other four — BGM_106…BGM_109 — are not counterexamples, they are the known leading-region straddle (slb-data-offset.md): those entry windows start mid-bank, so an entry contains the tail of the previous bank, one whole wave of its own, and the BANK header of the next one. slb_segment_phase.py bank shows it directly — the entry named BGM_107.slb contains BANK id=1108. Realigning across the boundaries with the seek packet counts, which pin each join exactly (bank 1107: a seek for 1164 packets and a 2 383 872-byte wave = 1164 packets), restores the same shape:

bank	wave 0	wave 1
1107	2 383 872 B → 65.18 s	2 418 688 B → 65.18 s
1108	3 696 640 B → 128.91 s	3 696 640 B → 128.91 s
1109	2 887 680 B → 81.50 s	2 932 736 B → 81.50 s

Bank 1106's wave 0 begins in unclaimed bytes before its entry and is known only by its seek count (1 718 packets), so it is 🟡 rather than measured.

So: intro + loop is dead, and so is two halves of one piece. Both would give unequal durations; 32 banks give equal ones.

The role of the second wave — measured

Decoded BGM_001's two waves to PCM and compared them.

	wave 0	wave 1
duration	173.809 s	173.809 s
first sound / last sound	0.001 s / 167.663 s	0.003 s / 167.663 s
RMS	6 824	3 961
L↔R correlation	0.61	0.14
energy below 200 Hz	16.5 %	6.7 %
energy 1–4 kHz	31.1 %	42.5 %

They are sample-synchronous. Transient-envelope cross-correlation searched over ±5 s peaks at lag +0.00 s (0.5624), and both waves stop at the same millisecond, 167.663 s. Two takes, two halves or two alternates would not do that. They are two stems of one performance and are meant to sound at the same time.

Therefore concatenating them is wrong, and not subtly: it plays the piece twice, the second time as a bass-less secondary stem.

🟡 Which kind of second stem — two readings survive

• The rear pair of a 4-channel mix. Fits every number: quieter, far more decorrelated between L and R, and with the bass managed away to the front pair.
• A second intensity layer to be mixed in or crossfaded.

Not settled, and the obvious field does not settle it. ChannelMask is 0x0002 on both waves, and sound-slb.md already records that this game writes meaningless channel metadata (movie voices declare 2 channels over mono content). What would settle it is a runtime observation — whether the game submits both waves to the mixer at once — which needs an emulator with audio this container does not have.

❔ Which bank is the menu's music — undecodable, and here is the reach

The cue table binds cue names to sound ids and banks (sound-cue-table.md), and its 32 BGM cues are named BGM_001 … BGM_109 — pure numbers, no TITLE, no MENU, no semantic name anywhere. Looked in: the SOUNDS record (5 798 cues), FILES (5 135 bank paths), and the bank headers themselves (a BANK id that simply repeats the cue number, id=1001 for BGM_001). Nothing names a screen.

So the menu↔bank binding has to come from the running game — the same route Q8's event bindings need — and until then the port is choosing a track, not transcribing one.

What the port needs to know

• a music bank is two waves that play together, not one track and not a sequence — do not concatenate;
• both are full length; align them at sample 0;
• the track is not a seamless loop. BGM_001 fades out and is followed by 6.15 s of silence (last sound 167.663 s of 173.809 s; the final second before silence is at RMS 168 against 4 788 at the head). Looping the wave as stored gives a fade-out and six seconds of nothing every cycle. No loop-point field has been identified in the XMA header, so a menu loop is authored.