Sylpheed/docs/re/structures/voice-region-starts-late.md

# 🔴 `resolve_movie_voice_region` starts **inside** the first stream — 8 of 10 multichannel regions

**Classification: decoded**, against the running decoder as ground truth for one
movie and a structural check disc-wide. Found because the port agent refused to
apply a result of mine and did the arithmetic instead.

## How it surfaced

I sent the port a stream→channel assignment indexed by `byte_size`. It did not
apply it, and said why:

| | bytes |
|---|---|
| the running decoder's three `ADV` contexts | **3 584 000** |
| the resolved `ADV` voice region | **3 114 352** |
| | **15 % too small to hold them** |

Two spans, one of which was not what the other thought it was. The disc side is
this crate's, and it is the one that was wrong.

## The gap is a whole number of packets

| | |
|---|---|
| `ctx0` declares | **632** packets = 1 294 336 B |
| the resolver's leading chunk has | **394** packets = 806 912 B |
| difference | **238 packets = 487 424 B** |

A whole number of packets is what a **start offset** looks like. Corruption does not
land on 2048-byte multiples.

## Verified against the decoder, which cannot be fitted to

Stepping the region start backwards and re-running `to_xma_riffs`
([`../data/voice-region-start-clip.txt`](../data/voice-region-start-clip.txt)):

```
-   0 packets: [806912, 1118208, 1171456]
- 237 packets: [1292288, 1118208, 1171456]
- 238 packets: [1294336, 1118208, 1171456]   <== the decoder's own three sizes
- 239 packets: [1296384, 1118208, 1171456]
- 300 packets: [57344, 45056, 1294336, 1118208, 1171456]
```

✅ **−238 is a real boundary, not the end of a sweep.** At −300 the *previous*
asset's chunks appear (57 344, 45 056) while the three `ADV` sizes stay exactly
stable. The stream begins there and something else ends just before it.

## Disc-wide

[`../data/voice-region-start-audit.txt`](../data/voice-region-start-audit.txt):

### ✅ The real population (2026-08-30, corrected)

The number this page first published — *"8 of 10 three-chunk regions start
mid-stream"* — was **not a count**. The audit that produced it was cut short, the
committed table ends mid-list with no summary line, and I read a partial file as a
complete one. The port agent's count of **25** was right.

Redone as a census that prints its population, coverage and skips together
([`../data/voice-region-chunk-census.txt`](../data/voice-region-chunk-census.txt)):

```
POPULATION: 104 movies in the manifest
COVERAGE:   95 resolved and read, 9 unresolved, 0 unreadable  (104 accounted for)
  1 chunk(s):  70 region(s)
  3 chunk(s):  25 region(s)
```

Cross-referenced against the fix's own sweep, which also ran to completion
(78 unchanged + 17 fixed + 9 skipped = 104):

| | |
|---|---|
| regions the fix changed | **17** |
| of those, three-chunk | **17 — all of them** |
| of those, one-chunk | **0** |
| three-chunk regions **not** affected | **8** — `S02A S05A S07B S11A S12A S12B S13B S15B` |

✅ **So "the defect is specific to the multichannel regions" survives, and now has
complete populations on both sides**: every affected region has three chunks, and
not one of the 70 single-chunk regions was touched. ⚠️ **But it is not true that
every three-chunk region was affected** — 8 of the 25 were already starting at a
boundary, which is what the 1.5 MB cap predicts, since a region only trips the
filter if its span exceeds it.

📌 The original "8 of 10" was wrong in its denominator and coincidentally shares a
digit with the 8 that are *unaffected*. Recorded because a number that survives
into a later document by resembling the right answer is the worst kind.

## What this means for anyone consuming a voice region

🔴 **In those 8 movies the leading chunk is a truncated first stream, not a spurious
artefact.** Any consumer that drops it as "the leading chunk that matches nothing"
is discarding most of a real stream — and any measurement made *on* it (levels,
correlations against the other chunks) was made on a fragment.

⚠️ **This includes measurements in this corpus.** My own
[`intro-audio-decomposed.md`](intro-audio-decomposed.md) assignment used `ADV`'s
clipped chunk 0; the quantitative argument there survives because it quotes a
**ratio** test explicitly chosen to be immune to the clipping, but the absolute
level for chunk 0 was measured over 62 % of the stream.

## ✅ WHY — and it is fixed (2026-08-30, later)

The predecessor trailer does **not** land 238 packets into the next asset. It is
never consulted: a second condition on the start filter threw it away.

```rust
.filter(|&s| s < end && end - s < 1_500_000)   // "only within one bank"
```

`ADV`'s predecessor sits **3 618 816 B** before `end`, so the filter rejects it and
`start` falls back to `anchor` — **a TOC offset, which is not a stream boundary at
all**. That is the whole mechanism, and it explains the shape of the defect
exactly: it strikes regions *larger than 1.5 MB*, which is why the multichannel
three-stream regions are hit and the single-stream ones never are.

| | |
|---|---|
| `ADV` predecessor trailer | 433 425 776 |
| + descriptor and padding | 17 040 B |
| = stream start | **433 442 816** — the −238-packet start, to the byte |

**17 of the 95 resolving movies** took the fallback.

### ✅ The fix, and its disc-wide check

Dropping the cap (keeping `s < end`):

```
anchor (today)          start 433930240  ->  [806912, 1118208, 1171456]
predecessor (proposed)  start 433425776  ->  [1294336, 1118208, 1171456]   MATCHES
```

| | movies |
|---|---|
| unchanged | **78** |
| fixed cleanly — first chunk grows, every later chunk byte-identical | **17** |
| **changed in any other way** | **0** |

Zero. In all 17 the *only* difference is a larger first chunk, which is what a
corrected start looks like and what pulling in a neighbouring asset does not.
[`../data/voice-region-cap-sweep.txt`](../data/voice-region-cap-sweep.txt).

**Landed** in `media.rs`, with a regression test pinned to the **running decoder's**
byte_sizes rather than to this crate's own output —
`adv_voice_region_holds_all_three_decoded_streams`. That distinction is the point:
every internal check passed happily while a third of a stream was missing, so only
an external number could have caught it.

⚠️ **Exact clips for the other 16 are still not independently verified.** The sweep
shows their first chunk grows and their tails are untouched, which is strong; but
`ADV` is the only one with a decoder measurement behind it.