# A music bank is two stems of one performance, played together **Status:** βœ… `CONFIRMED` for the structure β€” **decoded**, with a disc-wide check over all 32 BGM banks. 🟑 for the *role* of the second wave β€” **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](../../port/MISSION.md). 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 β€” 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 β†’ 173.82 s 4481024 seek 2181 packets 4491264 RIFF data@4495360 size=4673536 (2282 packets) 26887 B/s β†’ 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** ⚠️ **Our own reader disagreed with this page until 2026-08-29, and the page was right.** `slb::to_xma_riffs` was emitting that 10 240-byte header as a third sub-wave, so `sound_bank_riffs("BGM_103.slb")` returned **three** β€” which the port caught while exporting the menu music. The header is not a wave (it decodes to 0.009 s and is 99.1 % zero); the cause was a modulus that assumes a bank header is shorter than one 2048-byte packet, and it is fixed with a disc-wide 28/28 check β€” [`slb-bank-header-not-a-wave.md`](slb-bank-header-not-a-wave.md) β€” 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`](../data/bgm-wave-census.txt): > **28 of the 32 BGM entries** read straight off their own pak entry as two waves > of equal duration β€” equal to 0.01 s, over lengths from 37 s to 277 s. The other four β€” `BGM_106`…`BGM_109` β€” are **not counterexamples**, they are the known leading-region straddle ([`slb-data-offset.md`](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 β€” 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 β†’ 65.18 s | 2 418 688 B β†’ 65.18 s | | 1108 | 3 696 640 B β†’ 128.91 s | 3 696 640 B β†’ 128.91 s | | 1109 | 2 887 680 B β†’ 81.50 s | 2 932 736 B β†’ 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 β€” 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 Β±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 β€” 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`](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 β€” whether the game submits both waves to the mixer at once β€” which needs an emulator with audio this container does not have. ## ❔ Which bank is the menu's music β€” undecodable, and here is the reach The cue table binds cue names to sound ids and banks ([`sound-cue-table.md`](sound-cue-table.md)), and its **32 BGM cues are named `BGM_001` … `BGM_109`** β€” 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 β€” the same route Q8's event bindings need β€” 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 β€” do not concatenate; * both are full length; align them at sample 0; * **the track is not a seamless loop *as stored*.** `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 end-to-end gives a fade-out and six seconds of nothing every cycle. πŸ”΄ **But "no loop-point field has been identified … so a menu loop is authored" is REFUTED (2026-08-30), and this bullet said it for days after.** There is no loop point in the **file header**; there is one in the **XMA decoder context**, written at runtime via `XMASetLoopData`, and Xenia already logs it. For `BGM_103` it is **`[9.44 s, 71.31 s]`, cycling every 61.87 s** β€” measured by watching three wraps, so the game never reaches the fade at all. That is *why* the stored tail looks unusable: it is never played. [`menu-bgm-loop-fields-conflict.md`](menu-bgm-loop-fields-conflict.md) ⚠️ Reach: measured on `BGM_103` only. Whether every bank carries loop bounds is untested β€” but "the format has nowhere to put one" is dead. ## βœ… Confirmed at runtime β€” two stereo streams decode at once The page above said the "played together" reading needed "a runtime observation of whether the game submits both waves to the mixer at once", and that this container could not do it. It can: `--xma_param_probe=true` logs every XMA stream the decoder is handed. Sitting on the **main menu**, two stereo 48 kHz streams were decoding simultaneously β€” **1 893 packets / 3 876 864 B** and **1 919 packets / 3 930 112 B**. Two stereo streams alive at the same moment on a screen with one piece of music playing is the direct observation this page was missing: **a music bank's two waves are simultaneous, not sequential.** Concatenating them is wrong, now measured as well as inferred. 🟑 It still does **not** separate the two readings β€” surround-rear pair versus a second intensity layer β€” because both predict simultaneity. ## βœ… The menu's music is `BGM_103` β€” three independent routes agree An earlier version of this section said the two observed byte sizes "do not match any bank's declared wave sizes". **That was wrong** β€” I had checked only the `BGM_0xx` rows. They match `BGM_103` exactly, and the code names it: | route | evidence | |---|---| | **static, code** | `GamePart_Title`'s phase handler `sub_821C5580` does `li r5, 1103` into a sound call (`sub_8217ACF8`, with `r4 = 4`) β€” **cue 1103 = `BGM_103`** | | **static, disc** | [`../data/bgm-wave-census.txt`](../data/bgm-wave-census.txt): `BGM_103.slb` = two waves of **3 876 864** and **3 930 112** bytes, 87.75 s each | | **runtime** | the XMA probe at the **main menu** saw two stereo 48 kHz streams decoding simultaneously, of **3 876 864** and **3 930 112** bytes | Byte-for-byte on both waves. So: * **which bank the menu plays is decoded** β€” `BGM_103`, from the executable, since the cue *table*'s BGM entries are numeric and name no screen; * and the two-stems-play-together finding is now confirmed **on a named bank**, with the runtime stream sizes equal to the bank's declared wave sizes β€” so the game hands the decoder the whole wave, not a window, which is the opposite of what the earlier note guessed.