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confirmed three ways Chasing where the event code comes from, sub_821C5580 turned out to do two things worth having. For Q6 it arms the outer gate: li r11,3 ; stw r11,16(r28) is exactly the value sub_821C7850 tests before dispatching, which answers the "what does this+16 == 3 gate on" question I left open last iteration. It then passes r5 = [r27+4], an event read out of a structure, so the vocabulary is still not enumerable. Of the four callers of sub_821C7850, three pass constants -- 0, 0 and 5 -- and this one does not. The accident is a few instructions earlier: li r5, 1103 into a sound-play call. 1103 is a BGM cue id, BGM_103. That closes a residual I had written off as undecodable: which bank the menu plays. The cue table genuinely cannot say, since its BGM entries are numeric -- but the code can, and it checks out three independent ways. The census says BGM_103.slb is two waves of 3876864 and 3930112 bytes; the XMA probe at the main menu saw two stereo streams of 3876864 and 3930112 bytes. Byte for byte on both. That also corrects me. bgm-two-stems.md said those observed sizes matched no bank's declared waves and inferred the decoder gets a window rather than a whole wave. Wrong on both counts -- I had checked only the BGM_0xx rows of the census because that was the block on screen. METHOD gets it: check a measurement against the whole reference set, not the part you happened to be reading.
154 lines
7.6 KiB
Markdown
154 lines
7.6 KiB
Markdown
# A music bank is two stems of one performance, played together
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**Status:** ✅ `CONFIRMED` for the structure — **decoded**, with a disc-wide check
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over all 32 BGM banks. 🟡 for the *role* of the second wave — **measured** by
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signal analysis, and the two readings that survive are named below. ❔ which bank
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is the menu's music is **undecodable from the disc**, with the reach stated.
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Answers [MISSION Q10](../../port/MISSION.md). It also **withdraws the premise**: the
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handoff said `BGM_001.slb` is *three* sub-waves of 10 KB / 4.47 MB / 4.67 MB. The
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10 KB is the bank **header**, not a wave.
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## The structure — decoded
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```
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BGM_001.slb (9 178 040 B)
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0 BANK id=1001 data_size=9175992 hdr = 5 blocks (10240 B) 16bit/2ch
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10240 RIFF data@14336 size=4466688 (2181 packets) 25697 B/s → 173.82 s
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4481024 seek 2181 packets
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4491264 RIFF data@4495360 size=4673536 (2282 packets) 26887 B/s → 173.82 s
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9168896 seek 2282 packets
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```
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A bank is a 10 240-byte header and then **exactly two waves**, and the two always
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have the **same duration** — different byte sizes and different bitrates, same
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number of seconds. Duration is `data_size / PsuedoBytesPerSec` (the u32 at
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`RIFF+0x20`; `RIFF+0x24` is the sample rate, 48 000 Hz except `BGM_020`–`023`
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at 44 100).
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### Disc-wide
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`tools/re-capture/bgm_wave_census.py`, full output in
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[`../data/bgm-wave-census.txt`](../data/bgm-wave-census.txt):
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> **28 of the 32 BGM entries** read straight off their own pak entry as two waves
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> of equal duration — equal to 0.01 s, over lengths from 37 s to 277 s.
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The other four — `BGM_106`…`BGM_109` — are **not counterexamples**, they are the
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known leading-region straddle ([`slb-data-offset.md`](slb-data-offset.md)): those
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entry windows start mid-bank, so an entry contains the tail of the previous bank,
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one whole wave of its own, and the `BANK` header of the *next* one.
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`slb_segment_phase.py bank` shows it directly — the entry named `BGM_107.slb`
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contains `BANK id=1108`. Realigning across the boundaries with the `seek` packet
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counts, which pin each join exactly (bank 1107: a `seek` for 1164 packets and a
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2 383 872-byte wave = 1164 packets), restores the same shape:
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| bank | wave 0 | wave 1 |
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|---|---|---|
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| 1107 | 2 383 872 B → 65.18 s | 2 418 688 B → 65.18 s |
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| 1108 | 3 696 640 B → 128.91 s | 3 696 640 B → 128.91 s |
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| 1109 | 2 887 680 B → 81.50 s | 2 932 736 B → 81.50 s |
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Bank 1106's wave 0 begins in unclaimed bytes before its entry and is known only
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by its `seek` count (1 718 packets), so it is 🟡 rather than measured.
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**So: intro + loop is dead, and so is two halves of one piece.** Both would give
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unequal durations; 32 banks give equal ones.
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## The role of the second wave — measured
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Decoded `BGM_001`'s two waves to PCM and compared them.
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| | wave 0 | wave 1 |
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|---|---|---|
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| duration | 173.809 s | 173.809 s |
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| first sound / last sound | 0.001 s / **167.663 s** | 0.003 s / **167.663 s** |
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| RMS | 6 824 | 3 961 |
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| L↔R correlation | 0.61 | **0.14** |
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| energy below 200 Hz | 16.5 % | **6.7 %** |
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| energy 1–4 kHz | 31.1 % | 42.5 % |
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**They are sample-synchronous.** Transient-envelope cross-correlation searched
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over ±5 s peaks at **lag +0.00 s** (0.5624), and both waves stop at the *same
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millisecond*, 167.663 s. Two takes, two halves or two alternates would not do
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that. They are two stems of one performance and are meant to sound **at the same
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time**.
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**Therefore concatenating them is wrong**, and not subtly: it plays the piece
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twice, the second time as a bass-less secondary stem.
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### 🟡 Which kind of second stem — two readings survive
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* **The rear pair of a 4-channel mix.** Fits every number: quieter, far more
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decorrelated between L and R, and with the bass managed away to the front pair.
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* **A second intensity layer** to be mixed in or crossfaded.
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**Not settled, and the obvious field does not settle it.** `ChannelMask` is
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`0x0002` on *both* waves, and [`sound-slb.md`](sound-slb.md) already records that
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this game writes meaningless channel metadata (movie voices declare 2 channels
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over mono content). What would settle it is a runtime observation — whether the
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game submits both waves to the mixer at once — which needs an emulator with audio
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this container does not have.
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## ❔ Which bank is the menu's music — undecodable, and here is the reach
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The cue table binds cue names to sound ids and banks
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([`sound-cue-table.md`](sound-cue-table.md)), and its **32 BGM cues are named
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`BGM_001` … `BGM_109`** — pure numbers, no `TITLE`, no `MENU`, no semantic name
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anywhere. Looked in: the `SOUNDS` record (5 798 cues), `FILES` (5 135 bank
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paths), and the bank headers themselves (a `BANK id` that simply repeats the cue
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number, `id=1001` for `BGM_001`). Nothing names a screen.
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So the menu↔bank binding has to come from the running game — the same route Q8's
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event bindings need — and until then the port is **choosing** a track, not
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transcribing one.
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## What the port needs to know
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* a music bank is **two waves that play together**, not one track and not a
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sequence — do not concatenate;
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* both are full length; align them at sample 0;
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* **the track is not a seamless loop.** `BGM_001` fades out and is followed by
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**6.15 s of silence** (last sound 167.663 s of 173.809 s; the final second
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before silence is at RMS 168 against 4 788 at the head). Looping the wave as
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stored gives a fade-out and six seconds of nothing every cycle. No loop-point
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field has been identified in the XMA header, so a menu loop is **authored**.
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## ✅ Confirmed at runtime — two stereo streams decode at once
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The page above said the "played together" reading needed "a runtime observation
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of whether the game submits both waves to the mixer at once", and that this
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container could not do it. It can: `--xma_param_probe=true` logs every XMA stream
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the decoder is handed.
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Sitting on the **main menu**, two stereo 48 kHz streams were decoding
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simultaneously — **1 893 packets / 3 876 864 B** and **1 919 packets /
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3 930 112 B**. Two stereo streams alive at the same moment on a screen with one
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piece of music playing is the direct observation this page was missing: **a music
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bank's two waves are simultaneous, not sequential.** Concatenating them is wrong,
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now measured as well as inferred.
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🟡 It still does **not** separate the two readings — surround-rear pair versus a
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second intensity layer — because both predict simultaneity.
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## ✅ The menu's music is `BGM_103` — three independent routes agree
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An earlier version of this section said the two observed byte sizes "do not match
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any bank's declared wave sizes". **That was wrong** — I had checked only the
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`BGM_0xx` rows. They match `BGM_103` exactly, and the code names it:
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| route | evidence |
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|---|---|
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| **static, code** | `GamePart_Title`'s phase handler `sub_821C5580` does `li r5, 1103` into a sound call (`sub_8217ACF8`, with `r4 = 4`) — **cue 1103 = `BGM_103`** |
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| **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 |
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| **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 |
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Byte-for-byte on both waves. So:
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* **which bank the menu plays is decoded** — `BGM_103`, from the executable, since
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the cue *table*'s BGM entries are numeric and name no screen;
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* and the two-stems-play-together finding is now confirmed **on a named bank**,
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with the runtime stream sizes equal to the bank's declared wave sizes — so the
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game hands the decoder the whole wave, not a window, which is the opposite of
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what the earlier note guessed.
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