Dragon-s-Lair-X68k/docs/FINDINGS.md

# Findings — session 1 (2026-08-23)

All numbers here are MEASURED unless marked ESTIMATE or FOLKLORE.

---

## 1. Source material

`DRAGONS_LAIR.iso` — 16 GB, UDF 2.x, **decrypted** (no AACS dir).
Loop-mounted read-only at `/media/reala-misaki/BDROM` via `udisksctl loop-setup -r -f`.
(7-Zip cannot read UDF 2.x; use the loop mount.)

- **224 `.m2ts` streams**, 1920x1080, **MPEG-2, progressive, 23.976 fps**
- Size histogram: 47 <5MB, 138 5-50MB, 22 50-150MB, 14 150-400MB, 3 >400MB
- The 185 sub-50MB streams are the **arcade branching scenes already split into
  individual clips** — we get scene boundaries for free.
- Big streams are full-feature playthroughs: 00215 (1376s), 00216 (1151s), 00223 (566s)
- Typical scene clip ~60s (00203/00205/00199), some ~100s (00164/00212)

**Gotcha:** clip durations vary wildly. Always read `format=duration` and seek
relative to it. Seeking to a fixed offset silently yields 0 frames on short clips.

---

## 2. GVRAM layout  [verified — see HARDWARE.md for source]

**One 16-bit word per pixel position in EVERY color mode.** Bit depth does not
change VRAM bandwidth; it only subdivides the word.

`addr = page_base + y*1024 + x*2`  — adjacent pixels are 2 bytes apart in all modes.

Consequence: low bit depth buys **no speed**. 16-color mode is strictly worse than
256-color (same bus traffic, 1/16 the palette). Page-alias writes are hardware
auto-masked, so 16-color needs no software read-modify-write — but it's still
one word-access per pixel.

**Chosen: 256 colors, 256x192 active area.**
In 256-color mode P0=low byte, P1=high byte of each word. Sacrificing page 1 as a
double-buffer lets a `move.l` cover two pixel positions, enabling `movem.l` bursts
(12 regs = 48 bytes = 24 pixels). Identical blit cost to 65536-color mode but
**half the on-disk data**.

---

## 3. Content measurements (8 scenes sampled, 5s each at 40% into each clip)

| metric | mean | p90 |
|---|---|---|
| pixels changed / frame | 20.1% | 30.2% |
| **blit cost** | **~64k cycles** | **~97k cycles** |
| naive delta+RLE frame size | 15.5 KB | 19.6 KB |

Budget is **833,333 cycles/frame** @ 12fps on a 10MHz 68000.

### => THE CPU IS NOT THE BOTTLENECK. I/O IS.
Blit uses **under 8%** of budget. The naive row-span+RLE codec achieves only
**3.2:1**, giving **365 KB/s / 470 MB** at 24fps (~183 KB/s / 235 MB at 12fps).

Per-scene variance is extreme: static dialogue ~30 KB/s, action ~700 KB/s.
Any codec needs a hard bitrate ceiling, not just a good average.

### "Shot on twos" — ASSUMPTION FAILED
Dedupe found **zero** duplicate frames across all 8 scenes (`uniq=120/120`,
24.0 fps effective). This Blu-ray is a restoration where every frame is unique.
We do NOT get halved data for free. **Decimation to 12fps must be explicit.**

A weak alternation signature does exist (even-index pairs 40.7% vs odd 27.5%,
ratio 1.5x, with occasional true-duplicate pairs at 0.03-0.19%), but it is
irregular — Bluth mixed ones and twos; action is animated on ones.

---

## 4. MEASUREMENT TRAPS — read before trusting any pipeline number

Three separate false results were produced and caught this session. All three
looked plausible. Guard against them:

1. **Per-frame Floyd-Steinberg dithering destroys temporal coherence.**
   Error diffusion is chaotic: a +/-1 input change cascades across the row and
   produces a completely different index pattern. First run reported 31.5% pixels
   changed with near-zero variance (median 31.6, p90 32.3, max 32.7) while source
   mean-abs-diff was 0.09 — i.e. visually identical frames. That flat variance is
   the tell: **real animation has scene-dependent variance; noise does not.**
   Use no dithering (cel art is flat) or ordered/Bayer (spatially fixed, temporally stable).

2. **Temporal denoise smears motion.** `hqdn3d=4:3:6:4` — the `6:4` are temporal
   params. It flattened real motion, which then measured as "no motion" and
   produced an absurd 0.8 fps / 4 MB result. **Use spatial-only: `hqdn3d=4:3:0:0`.**

3. **Exact-match dedupe fails on a noisy source.** MPEG-2 grain means near-duplicate
   frames differ by +/-1 and are never bit-exact. Use a threshold on
   "% pixels differing by more than N levels", and pick the threshold from the
   observed distribution, not a guess. A 2% threshold ate genuine animation when
   mean consecutive change was only 0.9%.

**Sanity rule: if a result has suspiciously low variance, or is suspiciously
good, it is probably an artifact of the measurement, not a property of the content.**

Scripts kept in `tools/analysis/` — 01 and 02 are marked BROKEN deliberately as
regression references; 03 and 04 are the correct ones.

---

## 5. Storage interface — the SASI/SCSI split

> **SUPERSEDED IN PART.** The claim below that DMA means streaming "costs
> essentially no CPU" is **wrong** — see 19. The bandwidth figures here are
> folklore; the working figure is now **4 Mbps = 488 KB/s** (21).

[Yasuma, X68030 internal SCSI controller]

- Interface: **SCSI-1**, 50-pin, 5 MB/s bus spec
- Controller: **Fujitsu MB89352** SPC
- Transfer mode: **DMA** (via **HD63450** DMAC)
- Bus: X68000 original bus, **16-bit @ 10MHz**

**Even on the X68030, SCSI runs at 10MHz 16-bit DMA.** Storage bandwidth does
NOT scale with CPU — the controller sits on the original bus. HD63450's 12.5MHz
official ceiling is why the X68030 runs at 25MHz. An "HSCSI" TSR forces PIO/FIFO
transfer instead of DMA but was marginal even at 25MHz.

Because it's DMA, **streaming costs essentially no CPU** — this stacks with the
8% blit utilisation. The 68000 really is nearly idle.

### Model split — IMPORTANT
**The 10MHz models (original X68000, ACE, PRO, EXPERT) use SASI, not SCSI.**
Built-in SCSI starts at the X68000 **Super** (1990) and continues through XVI,
Compact, X68030. SCSI on earlier machines needs the **Sharp CZ-6BS1** board
in an I/O slot (MAME models this: `-exp1 cz6bs1`).

| target | bandwidth | naive codec (365 KB/s) | VQ codec (~30 KB/s) |
|---|---|---|---|
| SASI (stock ACE/EXPERT) | ~300-500 KB/s FOLKLORE | infeasible | comfortable |
| SCSI (Super+, or CZ-6BS1) | ~1 MB/s FOLKLORE | tight but viable | trivial |

Derived bounds (ESTIMATE): 16-bit @10MHz with 4-clock bus cycle = 5 MB/s absolute
ceiling; HD63450 single-address DMA ~8 clocks/word => ~2.5 MB/s practical ceiling,
before SCSI-1 async handshake and drive latency.

**No measured benchmark was obtained — see STATUS.md.** The ~300-500 KB/s and
~1 MB/s figures are folklore-grade; I could not find a primary measurement.

---

## 6. Codec decision: vector quantization (Cinepak-style)

Given ~8x CPU headroom and an I/O ceiling, spend CPU to buy bandwidth.

- Split frame into 4x4 blocks, encode each as a 1-byte index into a per-scene codebook
- Decode = 16-byte copy from a lookup table: nearly free
- A **full** frame = 256*192/16 = **3,072 bytes** — a hard 16:1 floor before delta
- Add block-level delta on top; action scenes ~2-3 KB/frame
- => roughly **30 KB/s, ~40 MB total**, with a *deterministic* bitrate ceiling

Divergence from the SNES project (below): use a **per-scene codebook with delta
updates**, not a per-frame rebuild. We trade adaptivity for bandwidth because we
have 2MB RAM to keep a codebook resident and CPU to spare.

**Risk not yet evaluated:** 4x4 VQ with a 256-entry codebook will visibly soften
detail. Bluth's fine ink linework is what suffers. Prototype and eyeball before committing.

---

## 7. Comparison: astrobleem/SNES-SuperDragonsLairArcade

Reached the **same core architecture independently** — "512 tiles per frame" is
vector quantization (8x8 codebook + tilemap). Good validation.

But: the SNES PPU has **no bitmap mode**, so tiles are forced on them by display
hardware. The X68000 has a real linear framebuffer, so VQ is a *compression
choice* we can tune or drop per-scene.

**MSU-1 is a bandwidth cheat we don't have.** It's a modern flash-cart coprocessor
giving memory-mapped streaming the real SNES never had. Their budget: 512 tiles x
32 bytes (4bpp 8x8) + tilemap ~= 18 KB/frame => **~430 KB/s** at 23.976fps.
That's *higher* than the 365 KB/s we'd reject on SASI. (ESTIMATE: my arithmetic on
their stated tile budget, not a measured figure.)

Where we're ahead: 256 simultaneous colors from a 65536 palette vs their 4bpp
sub-palettes needing a tile-aware palette optimizer plus a spatial smoothing pass
to hide 8x8 palette seams. That problem doesn't exist for us. Plus 68000@10MHz
vs 65816@3.58MHz, and 2MB vs 128KB.

**Most valuable thing in that repo is NOT the codec — it's `data/events/`:**
516 chapter definitions across 29 scenes as XML, plus
`data/chapter_event_inventory.md`. That's the arcade scene graph and input-timing
structure, entirely hardware-independent — the whole game-logic layer we'd
otherwise reverse-engineer from the arcade ROM.

**TODO: check their license before planning to reuse it.**
Their 516 chapters are finer-grained than our 224 Blu-ray streams, so mapping
their event table onto our footage means subdividing streams by timecode.

Caveat: all of the above is from README/repo-tree summaries, not their source.

---
---

# Findings — session 2 (2026-08-23)

## 8. CORRECTION to session 1: halving the framerate does NOT halve the bitrate

> **PARTLY SUPERSEDED.** The framerate correction stands. The
> "changed-spans + deflate = 247 KB/s" figure is a **compression upper bound,
> not a shippable design** — deflate decode does not fit the 68000's frame
> budget. See 17.2.

Session 1 measured 365 KB/s for naive delta+RLE at 24 fps and wrote
"(~183 KB/s at 12fps)". **That extrapolation is wrong.** Decimating to 12 fps
roughly doubles the per-frame delta, so the *rate* stays nearly flat.

Re-measured directly on 12 fps decimated frames (4 scenes, 66 frames):

| codec (all LOSSLESS w.r.t. the 256-colour frame) | B/frame | KB/s @12 | 22 min | ratio |
|---|---|---|---|---|
| raw 8bpp 256x192 | 49152 | 576 | 743 MB | 1.0:1 |
| session 1 row-span + RLE | 29055 | 340 | 439 MB | 1.7:1 |
| XOR vs prev + deflate | 30196 | 354 | 456 MB | 1.6:1 |
| **changed-spans + deflate** | **21110** | **247** | **319 MB** | **2.3:1** |
| changed-spans + LZMA | 18759 | 220 | 283 MB | 2.6:1 |

Session 1's own RLE re-measured at 12 fps gives **340 KB/s, not 183**.
Any plan that assumed 183 KB/s was based on a bad number.

Deflate-class entropy coding on top of the span payload is worth **1.4x** over
hand-rolled RLE, and LZ decode is cheap on a 68000 (byte copies), so the
lossless floor is ~247 KB/s / 319 MB. That is **infeasible on SASI** and
**tight but real on SCSI**.

## 9. Flat 4x4 VQ at k=256 is NOT acceptable — confirmed by eye

The risk flagged in 6 is real. At k=256, 4x4:

| scene | palette-only PSNR | after VQ | VQ loss |
|---|---|---|---|
| 00010 | 38.35 | 29.68 | 8.67 dB |
| 00020 | 39.90 | 32.67 | 7.22 dB |
| 00146 | 35.25 | 29.35 | 5.89 dB |
| 00181 | 41.92 | 32.87 | 9.05 dB |

Visually: Dirk's face disintegrates, teeth and eyes turn to mush, ink outlines
break into 4-pixel stair-steps, colour bleeds across block boundaries.

![flat 4x4 VQ failure](images/flat_vq_failure_00010.png)
*Left: 1080p source. Middle: 256-colour palettised 256x192 — the quality ceiling,
and it is excellent. Right: flat 4x4 VQ at k=256. This is the result that killed
the flat-VQ architecture.*

**Crucially, the 256-colour palettised frame itself looks excellent.** Flat cel
art with a per-scene median-cut palette and no dithering is near-transparent
(35-42 dB). So the palette is not the problem and 256 colours is not the
problem — **block VQ is**. The quality ceiling we should hold ourselves to is
the palettised frame, not the 1080p source.

## 10. Hybrid VQ (Cinepak V1/V4 + SKIP) — this is the codec

Per 4x4 block, choose by rate-distortion: SKIP (reuse previous frame),
V1 (one 4x4 codeword, 1 byte), or V4 (four 2x2 codewords, 4 bytes),
with a 2-bit-per-block mode header. `lam` is the lagrangian rate knob.

Measured, k1=k4=256, 4 scenes (mean of the per-scene table in the session log):

| lam | PSNR | loss vs palette | SKIP% | V1% | V4% | B/frame | KB/s @12 |
|---|---|---|---|---|---|---|---|
| 0 (max quality) | 33.9 | 4.9 | 30.8 | 18.5 | 50.8 | 7574 | 88.8 |
| 200 | 31.9 | 5.9 | 44.0 | 37.6 | 18.4 | 4183 | 49.0 |
| 1000 | 31.6 | 7.3 | 47.4 | 47.7 | 4.9 | 2841 | 33.3 |
| 5000 | 25.5 | 13.3 | 55.6 | 44.4 | 0.0 | 2134 | 25.0 |

At a **matched ~30 KB/s** the hybrid beats flat 4x4 VQ by ~1 dB, and unlike flat
VQ it keeps scaling: at 89 KB/s it reaches within **4.9 dB of the palette
ceiling**, which flat VQ cannot reach at any bitrate.

Note V4% collapses to 0 at lam=5000 — that is the knob doing exactly what it
should: under a hard ceiling, detail blocks are the first thing sacrificed.

## 11. Codebook size sweep (flat 4x4, for reference)

> **SUPERSEDED.** The k=1024 result below is an artifact of a rate model that
> charged 1 byte for a 10-bit index. k=256 ships. See 14.

| block | k | PSNR | loss | key B | changed% | KB/s @12 | codebook RAM |
|---|---|---|---|---|---|---|---|
| 4x4 | 256 | 30.46 | 8.39 | 3072 | 52.7 | 28.5 | 8K |
| 4x4 | 1024 | 32.89 | 5.96 | 3840 | 56.6 | 35.6 | 32K |

+2.4 dB for 24K more RAM and 7 KB/s. With 2 MB of RAM, a 1024-entry codebook is
cheap and clearly worth it. (RAM figure is the word-expanded form the blitter
wants: k * 16 px * 2 bytes.)

## 12. Source framing — OPEN

The Blu-ray is **full-frame 1920x1080 16:9 with no pillarboxing**. The arcade
original is 4:3. The extractor currently centre-crops 1440x1080, which is the
arcade-faithful choice but discards image the 2006 remaster added. Options are
`crop` (default), `squash`, `wide` in `tools/encoder/extract.py`.
**Not yet decided; needs an eyeball comparison against arcade reference.**

## 13. Stream inventory correction

Session 1 said "typical scene clip ~60s". Sampled directly: the ~3-5 MB streams
are **1.2-1.7 s** clips — these are the individual arcade death/action moments,
which is exactly the granularity the game logic needs. Some 60 s streams
(e.g. 00203) are **menu screens, not content**. Any survey must classify
menu vs content before averaging, or the bitrate numbers are diluted by static
menus.

## 14. A FOURTH false-good result — and the correction

Add this to the 4 list. The mechanism was new but the shape was identical.

**The false result:** flat and hybrid VQ both showed **+2.4 dB for k=1024 over
k=256** at an apparently similar bitrate, which made a 1024-entry codebook look
like an obvious win. The k=1024 quality ladder rendered from that run looked
great at "45 KB/s".

**The bug:** the rate-distortion model in `vq_hybrid.encode()` charged **1 byte**
per codebook index unconditionally. A 1024-entry codebook needs a **10-bit index,
stored as 2 bytes**. So every k=1024 measurement understated the V1 and V4
payload by exactly 2x, *and* the lagrangian mode decision was choosing V4 on the
belief that four codewords cost 4 bytes when they cost 8.

**After charging the true index cost** (`idx_bytes` is now explicit and defaults
from the codebook size), matched-bitrate comparison on scene 00020:

| KB/s | k=256 (1-byte idx) | k=1024 (2-byte idx) |
|---|---|---|
| ~32-42 | **33.87 dB** @ 32.5 | 28.91 dB @ 42.3 |
| ~44-52 | **34.80 dB** @ 44.1 | 35.13 dB @ 52.5 |
| ~72-86 | **35.87 dB** @ 72.2 | 36.51 dB @ 86.0 |

k=1024 buys +0.3 to +0.6 dB for +19% bitrate — a wash at best — and at the low
end where the SASI profile lives it is **5 dB worse**, because the 2-byte index
floor dominates once V4 is priced out.

**k=256 with 1-byte indices is the shipping choice.** It is also the better
decoder: a plain `move.b` index with no alignment case, and an 8 KB codebook
instead of 32 KB.

**The general lesson, again:** the comparison was not wrong about VQ, it was
wrong about *cost*. When a knob looks like a free win, check that the rate model
is charging for it. Same failure family as 4.1-4.3: a plausible number produced
by a pipeline that was not measuring what it claimed to measure.

## 15. Rate-distortion curve of the shipping codec (k=256, corrected)

Scene 00020 (Dirk screaming, close-up face — the hardest case for linework),
and 00146. Includes the 2-bit-per-block mode header. No entropy coding yet.

| lam | 00020 PSNR | 00020 KB/s | 00146 PSNR | 00146 KB/s | SKIP | V1 | V4 | RAW |
|---|---|---|---|---|---|---|---|---|
| 25 | 38.68 | 182.2 | 31.04 | 193.5 | ~37% | ~24% | ~13% | ~26% |
| 100 | 35.87 | 72.2 | 29.04 | 72.5 | ~41% | ~34% | ~21% | ~4% |
| 300 | 34.80 | 44.1 | 28.28 | 44.4 | ~44% | ~42% | ~14% | 0% |
| 800 | 33.87 | 32.5 | 27.77 | 36.1 | ~46% | ~48% | ~5% | 0% |
| 2000 | 27.57 | 25.5 | 24.88 | 30.2 | ~50% | ~49% | ~1% | 0% |

Palette ceilings: 00020 = 39.90 dB, 00146 = 35.25 dB.

![quality ladder](images/quality_ladder_00020.png)
*The shipping codec across the rate knob. Top: source, palette ceiling, lam=25.
Bottom: lam=100 (`scsi` profile), lam=300 (`sasi` profile), lam=800.
Both shipping profiles hold Bluth's linework; the failure only starts past lam=800.*

Two things to read off this table:
- **The cliff is between lam=800 and lam=2000.** That is where V4 is priced out
  entirely and detail blocks have nowhere to go. Do not ship past lam~800.
- **RAW is doing real work at high bitrate** (26% of blocks at lam=25) and
  vanishes by lam=300. It is what makes the top of the curve reach the palette
  ceiling, and it costs the decoder nothing — RAW is the cheapest mode to blit.

## 16. Licences cleared for the game-logic layer

Both checked this session:

- **astrobleem/SNES-SuperDragonsLairArcade — MIT**, "Copyright (c) 2026 Chad
  Doebelin". `data/events/` holds 516 XML chapter definitions with timing and
  event data. Reusable with attribution.
- **icculus/DirkSimple — zlib.** Independent from-scratch reimplementation of
  the game logic in Lua, scene/timing tables in `game.lua`. Also permissive.

Having **two independent permissively-licensed transcriptions** of the arcade
scene graph is better than one: they can be diffed against each other to catch
transcription errors before any of it is committed to 68000 tables.

## 17. The profiles were set far too low — and entropy coding is a CPU trap

> **PARTLY SUPERSEDED.** 17.1's diagnosis (the profiles were not derived from
> hardware) and 17.2's CPU analysis both stand. But 17 reasoned against a
> misread bandwidth of 4 MB/s; the correct figure is **4 Mbps = 488 KB/s**, so
> the "ship pixel-exact if SCSI sustains >=800 KB/s" conclusion in 17.5 is
> **not available**. See 18 and 21.

Prompted by the user asking why the SCSI profile was only 75 KB/s. It should not
have been. Two separate errors, one of them serious.

### 17.1 The profile bitrates were not derived from the hardware at all

They were read off the knee of the rate-distortion curve and then presented as
though bandwidth-driven. Against the (folklore) bus figures from 5:

| profile | was | bus figure | utilisation |
|---|---|---|---|
| `sasi` | 45 KB/s | ~300-500 KB/s | **12%** |
| `scsi` | 75 KB/s | ~1 MB/s | **7%** |

Nothing justified leaving 90% of the pipe unused. Raised to `sasi` 110 KB/s
(lam=60) and `scsi` 280 KB/s (lam=10), which is 35% and 28% utilisation —
still conservative, because the bus figures are folklore.

### 17.2 CPU is NOT the reason to stay low — but entropy coding would be

Budget is 833,333 cycles/frame at 12 fps. At session 1's measured ~6.5 cycles
per GVRAM pixel write:

| work | cycles | % of budget |
|---|---|---|
| blit 20.1% of pixels (session 1's 24fps figure) | 64k | 7.7% |
| blit 40% of pixels (the same content at 12fps) | 128k | 15.3% |
| **blit the FULL frame, every frame** | **319k** | **38.3%** |
| deflate decode, ~30 KB/frame output | 1,800k | **216%** |
| LZ4/LZSS decode, ~30 KB/frame output | 450k | **54%** |

Two conclusions, and the second one corrects 8:

- **Raising the VQ bitrate is nearly free on CPU.** Even a full-frame pixel-exact
  blit fits in 38% of budget, and VQ decode is table copies — RAW, the mode that
  dominates at high bitrate, is the *cheapest* mode to blit, not the dearest.
- **The 247 KB/s "lossless changed-spans + deflate" figure in 8 is a compression
  upper bound, NOT a shippable design.** Deflate's Huffman decode is bitwise and
  costs about 2.2x the entire frame budget on a 68000. Even byte-oriented LZ4 at
  54% leaves too little beside a 38% blit. **Do not plan on entropy coding.**
  All profile bitrates are raw payload.

This inverts session 1's "the CPU is idle, I/O is the ceiling" for the *decode*
path specifically: the blit is cheap, but any bit-oriented decompressor is not.
VQ is the right architecture precisely because its decode is a table copy.

### 17.3 The hybrid at lam=0 IS the lossless codec

Measured, un-entropy-coded raw payload, and deflated for reference only:

| scene | lam=0 raw | lam=0 deflated | lossless changed-spans+deflate | PSNR |
|---|---|---|---|---|
| 00020 | 442.1 KB/s | 274.5 KB/s | 267.3 KB/s | 39.90 = ceiling |
| 00146 | 467.6 KB/s | 223.2 KB/s | 219.1 KB/s | 35.25 = ceiling |

The hybrid at `lam=0` converges to within 3% of the purpose-built lossless coder.
That confirms the architecture unifies: there is no separate lossless path to
maintain, just the same bitstream with the knob open.

### 17.4 Full curve in raw (shippable) bytes

| lam | 00020 PSNR | 00020 KB/s | 00146 PSNR | 00146 KB/s | RAW% |
|---|---|---|---|---|---|
| 0 | **39.90** (exact) | 442.1 | **35.25** (exact) | 467.6 | ~76% |
| 10 | 39.38 | 248.1 | 32.27 | 305.2 | ~44% |
| 25 | 38.68 | 182.2 | 31.04 | 193.5 | ~26% |
| 60 | 36.94 | 108.0 | 29.61 | 103.1 | ~10% |
| 150 | 35.31 | 55.6 | 28.63 | 56.1 | ~1% |
| 300 | 34.80 | 44.1 | 28.28 | 44.4 | 0% |

### 17.5 This makes the blocked disk benchmark critical-path

Session 1 judged it "NOT on the critical path" because VQ at 30 KB/s was correct
whether SASI did 300 or 600 KB/s. That reasoning no longer holds. The profiles
now sit at 110 and 280 KB/s, close enough to the folklore ceilings that the
error bars matter, and **if SCSI sustains >=800 KB/s the correct `scsi` profile
is lam=0 — pixel-exact video.** Whether this port ships transparent or lossy on
SCSI is now waiting on one measurement.

## 18. Peak-to-mean burstiness — the mean was hiding the problem

> **SUPERSEDED — DO NOT ACT ON THIS SECTION.** The peak-vs-sustained comparison
> below is the **wrong test**. With a ring buffer the correct test is cumulative
> demand vs cumulative supply, and both profiles pass it with **zero required
> prefill**. `scsi` at lam=10 ships. See 21. The per-frame peak numbers
> themselves are still valid data; only the conclusion drawn from them is wrong.

Prompted by the user clarifying that the bandwidth figure is **4 Mbps = 488 KB/s**,
not 4 MB/s. That is ~8x tighter than what 17 was reasoning against, and it
changes the answer.

Per-frame instantaneous rate (video + 7.8 KB/s audio), 12 fps:

| scene | lam | mean | p90 | **max** | peak/mean | max as % of 488 KB/s |
|---|---|---|---|---|---|---|
| 00010 | 60 | 95.0 | 127.3 | 138.8 | 1.46 | 28.4% |
| 00010 | 10 | 198.9 | 266.1 | 284.0 | 1.43 | 58.2% |
| 00020 | 60 | 115.8 | 155.4 | 222.3 | 1.92 | 45.5% |
| 00020 | 10 | 255.9 | 391.2 | **470.8** | 1.84 | **96.4%** |

**The `scsi` profile as committed in f0f2f80 does not fit 4 Mbps.** Its mean is a
comfortable 52% of the pipe, but it peaks at 96.4% — and a frame that arrives
late is a *dropped frame*, not a slow one. Sizing a real-time stream on the mean
is the mistake; peak/mean is 1.4-1.9x on 1.2-1.7s clips and will be worse across
a full scene.

Two ways out, and only one is good:
- Size for the peak: `lam=25`, mean 194 KB/s. Costs a full step of quality.
- **Rate-control to the mean and carry a leaky bucket:** `lam=10` fits, and buys
  back +0.7 dB (00020) / +1.2 dB (00146).

`ratectl.py` was written in session 2 but **never wired into `encode.py`**. This
demotes that from a loose end to the highest-value unfinished work in the repo.

## 19. Cycle-stealing DMA is not free DMA — 5 was wrong

FINDINGS 5 concluded "because it's DMA, streaming costs essentially no CPU —
this stacks with the 8% blit utilisation. The 68000 really is nearly idle."

The HD63450 steals bus cycles from the 68000 at roughly 8 clocks per 16-bit word:

| stream | words/s | clocks/s | CPU stolen | + full-frame blit |
|---|---|---|---|---|
| 110 KB/s | 56,320 | 450,560 | 4.5% | 42.8% |
| 250 KB/s | 128,000 | 1,024,000 | 10.2% | 48.5% |
| 450 KB/s | 230,400 | 1,843,200 | 18.4% | 56.7% |
| 488 KB/s | 249,856 | 1,998,848 | 20.0% | 58.3% |

At the rates the profiles now use, streaming costs **10-20% of the machine**.
Still affordable — nothing here breaks — but **bandwidth and CPU are one budget,
not two**, and any future headroom argument has to spend from both. The
"nearly idle" framing should not be reused.

(The 8 clocks/word figure is session 1's ESTIMATE from HD63450 timing, not a
measurement. It is the weakest link in this table.)

## 20. Where the profiles should come from

`tools/encoder/profile_gen.py` now derives lam from a bandwidth figure rather
than from the shape of the RD curve, accounting for audio, peak/mean, and
reporting DMA steal. Full benchmarking methodology — and why MAME cannot answer
the bandwidth question — is in `docs/BENCHMARK.md`.

The 4 Mbps figure itself is **user-supplied and its provenance is not recorded**.
Every profile now hangs off it, so it is worth pinning down.

## 21. Correction to 18 — the peak test was the wrong test

18 flagged that `scsi` "does not fit 4 Mbps" because a frame peaked at 96.4% of
the sustained rate. **That was the wrong comparison**, and the user was right to
push back. It measured instantaneous frame demand against a sustained rate as if
they had to match frame-by-frame. They do not: the disk keeps filling *during*
the frame, and any shortfall is absorbed by a ring buffer.

The correct test is whether **cumulative** demand ever outruns cumulative supply.
Simulated at a constant 488 KB/s fill, 12 fps, using the real per-frame sizes:

| scene | lam | mean KB/s | worst frame | **required prefill** | stall tolerance @256KB |
|---|---|---|---|---|---|
| 00010 | 10 | 198.9 | 23.67 KB | **0.0 KB** | 15.4 frames |
| 00020 | 10 | 255.9 | 39.23 KB | **0.0 KB** | 12.0 frames |
| 00146 | 10 | **313.0** | 42.10 KB | **0.0 KB** | 9.8 frames |
| 00181 | 10 | 211.1 | 25.25 KB | **0.0 KB** | 14.6 frames |
| (all) | 60 | 95-116 | 11-19 KB | **0.0 KB** | 26-32 frames |

Fill delivers **40.69 KB per frame time**. Only one measured frame exceeds that
(00146, 42.10 KB) and it is recovered by the following frame. **No scene needs
any prefill at all**, and a 256 KB buffer — 12.5% of RAM — carries ~1 second of
stall tolerance at `lam=10`, which is orders of magnitude more than an SD-backed
seek requires.

`scsi` at `lam=10` stands. The hardest scene sampled (00146) runs 313 KB/s mean,
64% of the pipe, with zero underrun risk.

### Why SD-backed changes the sizing rule
The deployment target is BlueSCSI / SCSI2SD, not a period spinning drive. That
was noted as a caveat in 5 but its consequence was not carried through:

- The sustained rate is a **bus-limited constant**, not an average over variable
  seek latency. There is no long tail to leave margin for.
- Seek is ~microseconds, so branch-point stalls are a non-issue against a buffer
  measured in whole seconds.
- Therefore we can size much closer to the ceiling than spinning-disk practice
  would suggest. Conservative margins here are cargo-culted from a constraint
  this deployment does not have.

**The SASI/SCSI split is about BUS PROTOCOL, not media.** SD emulation removes
seek latency from both, but a SASI bus is still slower than a SCSI one. Two
profiles remain the right design; both are now predictable constants rather than
distributions.

### What rate control is actually for now
Its value drops from load-bearing to **insurance**. Intra-scene peaks are a
non-problem. But we have measured **4 clips of 1.2-1.7s** out of 224 streams, and
00146 already runs 23% hotter than 00020. A sustained action sequence could
plausibly exceed the pipe where a 1.7s clip does not. Rate control gives a
*deterministic ceiling* across content we have not measured yet — which was the
original reason for choosing VQ over a lossless delta in the first place.

Still worth wiring in. No longer a blocker for shipping `scsi` at `lam=10`.

## 22. The display path, measured — first real frame on the X68000

Everything before this section was Python-side or a headless `-video none` run.
This is the first time pixels reached an emulated X68000 screen, and it produced
four hardware facts and one blocker that no amount of reasoning would have found.

**Scope — read this before quoting the result.** The X68000's *video* hardware
did the rendering: CRTC, GVRAM page decoding and the 15-bit+I palette lookup are
all genuinely emulated, which is why the output is bit-exact against the
hardware's colour math. But the pixels were written into GVRAM by a MAME Lua
script calling `SP:write_u16()` — the host poking emulated memory. **No 68000
instruction was executed to draw this frame.**

The equivalent is proving a framebuffer works by writing to it from a debugger.
It says the display path is correct; it says *nothing* about whether the 68000
can fill that framebuffer in time. Lua writes cost zero 68000 cycles, so the 38%
full-frame blit estimate that the entire CPU budget rests on remains completely
unvalidated. That is next step (2), the decoder skeleton, and it is untouched.

Reproduce:
```
python3 tools/bench/prep_frame.py <framedir> tmp/frame.bin 0
cd tmp && SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
  -sound none -nothrottle -plugins -autoboot_script ../tools/bench/show_frame.lua \
  -snapshot_directory ./snap -snapview native -seconds_to_run 6
```

### 22.1 The blocker: CRTC R20 bit 11 hides the graphics layer
The IPL leaves **CRTC R20 (`$E80028`) = `0x0B16`**. Bit 11 is *"G-VRAM set to
buffer"*, and MAME's `x68k_v.cpp` bails out of `draw_gfx()` on it outright:

```c
if (m_crtc->gfx_layer_buffer())  // if graphic layers are set to buffer, they aren't visible
    return false;
// x68k_crtc.h:  bool gfx_layer_buffer() const { return BIT(m_reg[20], 11); }
```

While that bit is set, GVRAM writes still land and read back correctly — which
is exactly what makes it so misleading. Six separate attempts at the video
controller (`$E82400/$E82500/$E82600`) rendered black with every register
reading back the intended value. **The video controller was never the problem.**

`R20` bits 9-8 select the colour setup, and this determines how `$C00000` is
decoded: `0x0300` = 65536c (16 bits/word), `0x0100` = 256c (low byte),
`0x0000` = 16c (4 bits). Set `R20 = 0x0116` for our mode.

### 22.2 Monitor contrast: the IPL leaves it at 14, not 15
`$E8E001` bits 3-0 are monitor contrast; MAME does
`m_screen->set_brightness(contrast * 0x11)`. The IPL leaves it at **14**, which
scales all output to 14/15 = 93.3%. Every rendered colour came out ~7% dark
until this was set to 15. **The player must write `$E8E001 = 15` at startup.**

Contrast `0` blanks the screen entirely (`x68k_v.cpp:661`) — that is the cheap
fade-to-black for scene transitions, no palette animation required.

### 22.3 Palette format CONFIRMED (was previously an assumption)
`PALETTE(config, m_gfxpalette).set_format(2, &x68k_state::GGGGGRRRRRBBBBBI, 256)`

```
bit  15..11  10..6   5..1   0
     GGGGG   RRRRR   BBBBB  I        <- I is a shared LSB for all three channels
```
Expansion is `pal6bit((field << 1) | I)`, i.e. `(v << 2) | (v >> 4)`.
With contrast at 15, **all 256 entries render exactly as this predicts** — the
frame is pixel-identical, not merely close. GVRAM line stride is confirmed as
512 words = 1024 bytes, matching `HARDWARE.md`.

### 22.4 A new quality ceiling: the 15-bit palette costs 38.88 dB
> **Superseded by 23.3.** The 38.88 dB figure assumed the shared LSB `I` is
> always 1. Choosing `I` per palette entry by minimum error lifts the ceiling to
> **40.81 dB** on the same frame. The conclusion below ("`scsi` is close to
> display-transparent") is therefore weaker than stated — there is ~2 dB more
> headroom than this section claims.
Section 3 called the 256-colour palettised frame "the real quality ceiling".
That was measured in 24-bit RGB. The hardware palette only stores 5 bits per
channel plus a shared LSB, so there is a **second** quantisation below it:

| stage | PSNR |
|---|---|
| 24-bit palettised source -> X68000 15-bit+I display | **38.88 dB** |
| `scsi` profile codec error (00020, FINDINGS 15) | 39.4 dB |

The codec's error at `scsi` is **the same order as the display's own error**.
On real hardware `scsi` is therefore close to display-transparent, and pushing
`lam` below 10 buys quality the monitor cannot show. This bounds how much the
`scsi` profile is worth raising — it does not change the profiles themselves.

Caveat: measured on one frame (00020 f0001). It is a property of the palette,
not the content, so it should generalise, but it has not been checked across
scenes.

### 22.5 Why the first frame appears twice
GVRAM is a 512-pixel-wide page while the IPL's CRTC is still in its 768-wide
text timing, so the layer repeats at exactly x=512. This is correct hardware
behaviour, not a bug. The player sets its own CRTC mode and the wrap disappears.
No CRTC timing table has been written yet — the harness deliberately keeps the
IPL's timing so that no invented CRTC values are in play.


## 23. A real CRTC mode: 256x192 inside 256x256 (session 4)

Session 3's harness borrowed the IPL's 768x512 text timing and invented no CRTC
values, which is why the frame repeated at x=512 (22.5). This session derived a
real 256x256 mode table from MAME 0.277 source and verified it by snapshot.
Table: `tools/bench/crtc_mode.lua`. Regression test: `tools/bench/verify_frame256.py`.

![256x256 mode](images/x68k_256x256_mode.png)

*Left: palettised source. Right: the emulated X68000's native 256x512 raster —
256 dots wide, 512 scanlines carrying 256 double-scanned graphics rows, with the
192-row picture letterboxed in true black.*

### 23.1 The table, and why it needed no guessing
`refresh_mode()` in `x68k_crtc.cpp` selects the dot clock as
`(reg20 bit4 ? 69.55199MHz : 38.86363MHz) / div`, with `div` from a ladder keyed
on `reg20 & 0x1f`. Three entries matter:

| `reg20 & 0x1f` | div | dot clock | mode |
|---|---|---|---|
| `0x16` | 2 | 34.776 MHz | IPL's 768 wide, 31.5kHz |
| `0x11` | 3 | 23.184 MHz | 512 wide, 31.5kHz |
| `0x10` | 6 | 11.592 MHz | **256 wide, 31.5kHz, graphics double-scanned** |

The IPL's `R00 = 137` gives `m_htotal = (137+1)*8 = 1104` dots, and
`34.776e6 / 1104 = 31500.0 Hz` **exactly**. Holding the same line rate at div 6
needs `11.592e6 / 31500 = 368` dots `= 46` chars, so `R00 = 45`.

`368 = 1104/3` exactly, so every horizontal register is the 768-mode value
divided by three, and the active window divides without remainder:
`(124-28)/3 = 32` chars `= 256` dots. **No horizontal value was recalled or
estimated.** Only the blanking split rounds: the 768 mode is sync/back/front =
14/14/14 chars, `/3 = 4.67` each, and the closest integer triple summing to
`46-32 = 14` is 5/5/4.

| reg | value | meaning |
|---|---|---|
| R00 | 45 | H total, 46 chars = 368 dots -> 31500.0 Hz |
| R01 | 5 | H sync end (3.45 us) |
| R02 | 10 | H display begin -> `hbegin = 81` |
| R03 | 42 | H display end -> `hend = 336`; inclusive width `336-81+1 = 256` |
| R04 | 567 | V total, 568 scanlines -> 55.46 Hz |
| R05 | 5 | V sync end |
| R06 | 40 | V display begin -> `vbegin = 41` |
| R07 | 552 | V display end -> 512 scanlines = 256 double-scanned rows |
| R08 | 27 | H sync adjust (MAME stores it and never reads it) |
| R20 | `0x0110` | display (not buffer), 256-colour, 31.5kHz, 256x256 |

**The vertical registers are NOT halved**, which is the one thing that looks
wrong and is not. The CRTC still generates a 568-line raster; "256 lines" is a
graphics-layer double-scan applied in `draw_gfx()` (`x68k_v.cpp:401`), not a
change to the raster. Halving R04 would ask the monitor for 110 Hz. MAME emits
a `visarea larger then reg[20]` logerror for this; it is cosmetic.

Total blanking time is identical to the 768 mode (112 dots at 11.592 MHz =
336 dots at 34.776 MHz = 9.66 us), which is the property a real monitor cares
about — so this table should be safe on hardware, though that is untested.

### 23.2 MAME's double-scan is phase-shifted by one raster line
`get_gfx_pixel()` indexes `m_gfxbitmap.pix(scanline / divisor, pixel)` using the
**absolute** scanline, and `vbegin = 41` is odd. So in the native 256x512
snapshot the identical row pairs are `(1,2), (3,4), ...` and row 0 is a lone
half-line. Even rows are graphics rows 0..255. This cost a false failure before
it was understood; the regression test now asserts the shifted pairing
explicitly so a change in MAME's behaviour is visible rather than confusing.

### 23.3 The shared LSB `I` must be chosen per palette entry — worth 1.96 dB
Session 3's `pack()` hardcoded `I = 1`. That is not free: `I` is shared by all
three channels and each renders as `pal6bit((field << 1) | I)`, so with `I = 1`
the darkest reachable value is `pal6bit(1) = 4`, and **true black does not
exist**. Choosing `I` per entry to minimise summed squared error over R,G,B:

| rule | ceiling vs 24-bit palettised (00020 f0001) | entries with I=0 |
|---|---|---|
| `I = 1` fixed (session 3) | 38.85 dB | 0 |
| `I` per entry, min squared error | **40.81 dB** | 102 / 256 |

Nearly **2 dB for free**, and 102 of 256 entries want `I = 0` — this is not a
corner case. It supersedes the ceiling in 22.4 and means `scsi` has about 2 dB
more headroom before it hits the display than that section claimed.

The encoder does not yet do this. `tools/encoder/` still emits 24-bit palettes
and the packing happens Lua-side; whatever eventually writes X68000 palette
words must use the per-entry rule.

### 23.4 Letterboxing requires a reserved black palette entry
GVRAM cleared to zero displays **palette entry 0**, and a free mediancut palette
puts a real image colour there — on 00020 f0001 it was `(206,192,176)`, used by
210 image pixels, so it cannot simply be repurposed. A 256x192 picture in a
256x256 mode has 64 blank rows, so the palette must be built with **255 colours
plus a reserved black at index 0** (`prep_frame.py --reserve-black`). Combined
with 23.3, entry 0 also needs `I = 0` or the bars sit at RGB (4,4,4).

Cost: one of 256 entries. Measured quality effect: none visible — the ceiling
figure in 23.3 is already measured on the 255-colour palette.

### 23.5 What is still not proven
GVRAM was again filled from Lua. **No 68000 instruction has drawn a pixel yet**,
and the 38% full-frame blit estimate underpinning the CPU budget remains
unvalidated. What this section adds is that the *target mode* is now real, so
68000 code has a defined geometry to write into: 256 words per visible row, a
1024-byte line stride, and rows 32..223 of a 256-row page.

---

## 24. The blit, measured on the 68000 — the 38% estimate was wrong (session 5)

**The first 68000 instructions in this project to draw a pixel.** Everything in
22 and 23 was GVRAM filled from Lua, which costs zero 68000 cycles. This section
replaces the estimate that the whole CPU budget rested on with a measurement.

Harness: `tools/bench/blit.s` + `tools/bench/blit.lua`. Four variants of a
full-frame 256x192 paint, each looped to run ~4 emulated seconds, timed from
`machine.time` between two flag writes by the 68000 itself.

| variant | what it does | cycles/frame | % of a 12fps frame |
|---|---|---:|---:|
| **V1** | `movem.l` blit from a word-expanded RAM frame (96KB read + 96KB write) | **446,286** | **53.6%** |
| V2 | naive `move.b`/`move.w` per pixel from a byte source | 1,284,174 | 154.1% |
| **V3** | write-only floor — registers preloaded, no source read at all | **225,789** | **27.1%** |
| **V4** | the same 96KB of writes issued in **4x4 block order** | **637,971** | **76.6%** |

The 12fps budget is 833,333 cycles (10.0 MHz confirmed from `x68k.cpp:1133`,
`40_MHz_XTAL / 4`).

### 24.1 The numbers are cross-checked against hand-derived cycle counts
Every variant was predicted from the MC68000 timing tables *before* the run
(`MOVEM.L` M->R `(An)+` = 12+8n, `(d16,An)` = 16+8n; R->M `(An)` = 8+8n,
`(d16,An)` = 12+8n) and then measured:

| | predicted | measured | error |
|---|---:|---:|---:|
| V1 | 447,744 | 446,286 | 0.33% |
| V2 | 1,284,096 | 1,284,174 | 0.006% |
| V3 | 225,792 | 225,789 | 0.001% |
| V4 | 640,704 | 637,971 | 0.43% |

This agreement is the point. A MAME timing number on its own would be worth
little given how many false-good results this project has produced (FINDINGS 4);
two independent derivations landing within half a percent is worth something.
The residual error is the frame-granularity of the measurement — Lua gets no
cycle counter (`luaengine.cpp` exposes `machine.time` and nothing from
`device_execute_interface`), so timing resolution is one video frame, 18.03 ms.

### 24.2 SCOPE: these are instruction cycles, and therefore a LOWER BOUND
MAME's `gvram_w`/`gvram_r` (`x68k_crtc.cpp:501,595`) contain **no timing at
all** — no wait states, no `adjust_icount`. GVRAM in MAME is as fast as main
RAM. Real X68000 GVRAM stalls the CPU on access, so every figure above is a
floor, not a prediction. **Do not quote these as hardware numbers.** Interrupts
were masked (`SR = $2700`) so the IPL's timer and VBL handlers could not steal
cycles into the measurement; a real player will take interrupts on top.

### 24.3 The 38% estimate is dead — a full-frame blit is 53.6%
The realistic "decode into a RAM frame, then blit it" design costs **53.6% of
the frame budget before decoding a single block**, and that is the zero-wait-
state floor. The estimate the CPU budget has been carrying since session 1 was
38%. It was optimistic by 41%.

The cause is visible in the V1/V3 gap: **reading the source frame is exactly
half the total cost** (221,952 of 446,286 cycles). The 68000 pays 8 cycles per
longword read and 8 per longword written, and in 256-colour mode a pixel
occupies a whole word of address space, so a frame is 96KB of traffic in each
direction rather than 48KB.

### 24.4 The high byte of every GVRAM write is discarded — confirmed from source
`gvram_w` case `0x0100` writes `data & 0x00ff` with `mem_mask 0x00ff`. So in
256-colour mode the CPU cannot pack two pixels into one word, and the odd bytes
of a word-expanded source frame never need clearing — V1 exploits this by
leaving them uninitialised. This is why 96KB, not 48KB, is the irreducible
write traffic.

### 24.5 The architecture question, and where it turns over
> **Superseded by FINDINGS 28.1/28.2 (session 7).** The two-path plan below is
> incoherent — the compose path needs a RAM reference the direct path never
> writes — and its two costs are both *copies*, so they were never comparable to
> a decode. The "76.6% x non-SKIP fraction" model is also 2.03x optimistic:
> the four block modes cost 300/448/400 cycles, not one figure. One path ships.

V4 prices the access pattern a decoder that writes codewords **straight into
GVRAM** actually has: 4 rows of 8 bytes at a 1024-byte stride per 4x4 block. The
same 96KB of writes costs **76.6%** in block order versus 53.6% row-linear — the
stride destroys the `movem.l` burst, 208 cycles per block against a theoretical
best of ~150.

But a decoder never writes every block: SKIP blocks cost **nothing at all**, and
the previous frame is already sitting in GVRAM, so **no RAM reference frame is
needed for SKIP to work**. So the two designs scale differently:

- **compose-in-RAM then blit** — flat 53.6%, independent of how much changed
- **decode-direct-to-GVRAM** — 76.6% x (fraction of non-SKIP blocks)

**They cross at 70% of blocks changed.** Below that, writing straight into GVRAM
wins, and it also drops the 96KB RAM reference frame entirely. Above it, the
flat blit wins.

**This makes the non-SKIP block fraction the single most important unmeasured
number in the project.** It is already computable from the encoder — it is a
by-product of the mode decision in `vq_hybrid.py` — and it has never been
reported. Measure it before writing any decoder inner loop, because it selects
which inner loop to write.

### 24.6 The frame the 68000 drew is pixel-exact
V1's output was snapshotted and passes `verify_frame256.py` unchanged: `256x512
native, double-scan exact, active 256x192 pixel-exact, letterbox true black`,
40.81 dB. So 68000 code drives the mode of FINDINGS 23 correctly, and 23.5 is
now closed.

---

## 25. The sustained action sequence, found and measured (session 5)

STATUS has carried "a *sustained* action sequence is the one thing that could
still break the bitrate" as the open risk since session 2. Every clip measured
before this was 1.2-1.7 s. This section closes it: **it does break the profiles,
though not the bus.**

### 25.1 The two largest streams on the disc are not game footage
A survey that sorts 224 streams by size and encodes the biggest would have
measured **live action**:

| stream | size | what it actually is |
|---|---:|---|
| 00216 | 3777 MB | the feature with a **burned-in picture-in-picture commentary** |
| 00215 | 3475 MB | the commentary itself, full-screen live action |
| **00223** | **1802 MB** | **clean animation, 9.4 min — the one to use** |

The PiP in 00216 is burned into video stream 0, not a selectable secondary
stream, so there is no ffmpeg flag that recovers a clean frame from it. This
extends FINDINGS 13's menu-vs-content warning: the classification needed is
**content / menu / bonus**, and bonus material is the one that looks most like
content by every cheap metric (size, duration, bitrate).

### 25.2 Picking the worst window by measurement, not by eye
`tools/analysis/07_motion_survey.py` scans a whole stream at 96x72 and reports
the highest-mean sliding window of inter-frame absolute difference. On 00223:

```
6793 frames @12fps = 566.1s
motion energy  mean 9.40  median 5.60  p90 21.70  max 112.39
hottest sustained 10s window: t = 539.4s  (2.01x stream mean)
quietest 10s window:          t = 144.2s  (0.19x stream mean)
```

The 10.6x spread between the quietest and hottest sustained windows is the whole
argument for not sampling clips by hand. `t = 539.4s` is the Singe endgame.

### 25.3 Both profiles overshoot on that window — rate control is now required
Encoding those 120 frames at the shipping profiles, with the fixed `lam` the CLI
currently uses:

| profile | target | measured | overshoot | PSNR | palette ceiling |
|---|---:|---:|---:|---:|---:|
| `sasi` | 110 KB/s | **129.6 KB/s** | **+18%** | 27.82 dB | 31.33 dB |
| `scsi` | 280 KB/s | **373.8 KB/s** | **+34%** | 30.81 dB | 31.33 dB |
| *(00020 baseline, `sasi`)* | 110 KB/s | 108.0 KB/s | -2% | 36.94 dB | 39.90 dB |

**This reclassifies rate control from insurance to a requirement.** STATUS has
had "wire rate control into `encode.py`" at priority 3-4 since session 2 with the
note "no longer a blocker (FINDINGS 21)". That was true of the clips measured
then. It is not true of this one. `ratectl.encode_rate_controlled()` already
exists and builds a per-frame lam ladder; it has simply never been hooked up.

Note what did **not** break: 373.8 + 7.8 = 381.6 KB/s is still under the 488 KB/s
working figure, so FINDINGS 21's ring-buffer conclusion survives — but at 78% of
the pipe sustained over ten seconds rather than the comfortable margin implied by
1.7 s clips.

### 25.4 The palette ceiling is content-dependent, and on hard content it binds
The 256-colour scene palette costs **31.33 dB** on this window against **39.90 dB**
on 00020 — 8.6 dB worse. Fire, lava and smoke gradients are exactly what a
256-entry mediancut palette handles worst.

This inverts an assumption the project has been carrying. FINDINGS 23.3 put the
X68000 display ceiling at 40.81 dB and treated it as comfortably clear of the
codec's own error. On this content the **scene palette (31.33 dB), not the
display hardware (40.81 dB), is the binding constraint** — and `scsi` is already
within 0.51 dB of it. Spending bits to close that last half-dB is spending them
against a ceiling that is not the display's.

### 25.5 `scsi` collapses to RAW under stress
Mode distribution on this window is qualitatively different from anything
measured before:

| profile | SKIP | V1 | V4 | RAW |
|---|---:|---:|---:|---:|
| `sasi` (lam=60) | 45.6% | 16.3% | 24.2% | 13.9% |
| `scsi` (lam=10) | 26.2% | 5.5% | 7.1% | **61.2%** |
| *00020, `sasi`* | 46.9% | 24.1% | 17.8% | 11.2% |

At `lam=10` the rate-distortion decision finds literal pixels cheaper than any
codeword for 61% of blocks — the codebooks are simply not describing this
content. That is the mechanism behind the +34% overshoot in 25.3, and it is a
rate-control problem, not a codec-structure problem: the RD decision is behaving
correctly for the lam it was given.

### 25.6 The decoder needs BOTH display paths, chosen per frame
> **Superseded by FINDINGS 28.1 (session 7).** Mixing the paths displays stale
> pixels on 70 of these 120 frames. The "median 37.0%, capped at 53.6%" below is
> the cost of an incorrect player; every coherent version is dearer, and plain
> direct-to-GVRAM is the cheapest of them.

Applying FINDINGS 24.5's crossover to the real per-frame distribution:

| | median non-SKIP | p90 | frames over the 70% crossover |
|---|---:|---:|---:|
| `sasi`, Singe window | 48.4% | 82.8% | 36 / 120 (30%) |
| `scsi`, Singe window | 70.8% | 92.4% | 64 / 120 (53%) |
| `sasi`, 00020 | 54.0% | 88.8% | 3 / 14 (21%) |

Neither path wins outright: **30-53% of frames want the flat blit and the rest
want direct-to-GVRAM.** A player that implements both and picks per frame — the
mode headers are parsed before any pixel is written, so the count is free — pays
a median of **37.0%** of the frame budget and is capped at **53.6%**. A player
that implements only direct-to-GVRAM pays up to 76.6% and would miss frames on
the scene cuts.

So the answer to 24.5 is "both", and the selection is a one-line comparison
against a block count the decoder already has in hand.

### 25.7 What this does not measure
One 10 s window of one stream, at fixed lam, with `_paint` still a Python loop.
The full-disc survey is still not done, and the numbers above are the *worst*
window rather than a distribution over content. What has changed is that the
worst case is now a measurement rather than a worry.

---

## 26. Rate control is unsound as written — found before wiring it up (session 5)

FINDINGS 25.3 promoted rate control from insurance to a requirement. Reading
`ratectl.py` before wiring it into `encode.py` turned up a correctness bug that
would have produced exactly the kind of plausible-looking wrong result this
project keeps catching (FINDINGS 4, 9, 14, 18).

### 26.1 The lam ladder desynchronises the encoder from the decoder
`H.encode()` is **temporally recursive**: SKIP blocks are copied from the
previous *reconstruction*, and `prev = out` closes the loop
(`vq_hybrid.py:84-109`). A frame's output therefore depends on every frame
before it in that same run.

`encode_rate_controlled()` runs `H.encode()` once per lam over the **whole
sequence**, building a ladder of independent temporal chains, then picks each
frame from whichever rung fits the budget. When frame *f* comes from rung *i*
and frame *f-1* was emitted from rung *j != i*, the SKIP blocks in *f* reference
a reconstruction **the decoder never saw**.

Measured on the Singe window (`tools/analysis/09_ratectl_drift.py`, 120 frames,
5 rungs, target 110 KB/s):

| | |
|---|---|
| rung switches | **67** over 120 frames |
| frames whose emitted output differs from what the encoder recorded | **111 / 120** |
| worst frame | **21,339 px = 43.4% of the frame** |
| encoder-vs-decoder agreement, worst frame | 27.1 dB |
| reported PSNR overstatement | **0.36 dB** |

The 0.36 dB is the least interesting number here. The encoder is reporting
quality for a reconstruction that will never exist, and 43% of a frame differing
is a visible artefact whatever the mean says.

**The fix is structural, not a tuning change:** `H.encode()` must become
frame-drivable — take `prev` and one lam, return one frame — so rate control can
feed back the frame it actually emitted. The current whole-sequence signature is
what makes the ladder tempting in the first place.

### 26.2 The ladder spans 250x past the shippable range
`lam_hi=2e5`, but FINDINGS 15 puts the quality cliff between lam=800 and
lam=2000 and says do not ship past lam~800. Every rung above ~800 is
unshippable, so a frame that only fits at lam=9457 has not been rate-controlled,
it has been destroyed. Cap `lam_hi` at 800 and let a frame that cannot fit
overrun the bucket — a visible overrun is a better failure than silent garbage.

### 26.3 The ladder is far too coarse where it matters
With `steps=5` the geomspace lands on 1 / 21 / 447 / 9457 / 200000, and **only
two rungs were ever chosen**. The budget is 8,721 B/frame; the two straddling
rungs deliver 23,183 B (lam=21) and 3,071 B (lam=447) — a **7.5x** gap across
the operating point. Rate control cannot land near a target it has to jump over.

The module docstring already describes the right approach — *"per frame we
binary-search lam to land inside a byte budget"* — but the implementation is a
fixed precomputed ladder. Doc and code disagree; the doc is correct.

### 26.4 What does work
The leaky bucket lands the mean where it should: **109.1 KB/s against a 110
target**, with 32% of frames over the per-frame budget and banked by the bucket.
That mechanism is sound and worth keeping. It is the per-frame lam *selection*
underneath it that needs rebuilding, not the bucket.

### 26.5 Cost note before starting
Each rung is a full-sequence encode and `_paint` is still a Python per-block
loop, so a 5-rung run over 120 frames takes minutes. **Vectorise `_paint`
first** — it is already on the list for the full-disc survey and it makes the
rate-control work practical rather than merely faster.

## 27. Rate control, rebuilt and wired in (session 6)

FINDINGS 26 stopped the session-5 rate controller before it shipped: it picked
frames out of independently-encoded whole-sequence runs, so 111 of 120 frames
referenced reconstructions the decoder would never see. The fix was structural,
as 26.1 said it had to be. It is now wired into `encode.py` and **on by
default** for a profile.

### 27.1 The encoder is frame-drivable, and the drift is zero by construction
`vq_hybrid` now exposes one frame at a time — `frame_ctx(m, f, prev)` /
`decide(ctx, lam)` / `paint(m, ctx, mode)` — and `encode()` is a thin loop over
that API. Rate control drives the same three calls and feeds back **the frame it
actually emitted** as the next frame's `prev`. There is no ladder to pick from,
so the desync has no way to occur.

`tools/analysis/09_ratectl_drift.py`, unchanged in what it asserts:

| | session 5 | session 6 |
|---|---|---|
| frames whose emitted output differs from what the encoder recorded | 111 / 120 | **0 / 120** |
| worst frame | 21,339 px (43.4%) | **0 px** |
| reported PSNR overstatement | 0.36 dB | **0.00 dB** |

This is the harder case for that test on purpose: it runs with `lam_lo=1.0`, so
lam moves on 117 of 119 frame boundaries. Under the old ladder, 67 rung switches
were enough to corrupt 111 frames.

### 27.2 Both overshoots are closed, and they cost under 1 dB
The Singe window (FINDINGS 25.3), which is the worst sustained window on the
disc. Totals include the 7.8 KB/s ADPCM allowance:

| profile | target | fixed lam (session 5) | rate-controlled | quality cost |
|---|---|---|---|---|
| `sasi` | 110 KB/s | 137.4 KB/s (**+25%**) | **109.5 KB/s** | 27.82 → 27.22 dB (−0.60) |
| `scsi` | 280 KB/s | 381.6 KB/s (**+36%**) | **280.0 KB/s** | 30.81 → 29.90 dB (−0.91) |

Zero frames hit the lam=800 cliff at either profile, so nothing was destroyed to
get there (26.2's failure mode did not trigger). `sasi` needed lam to reach 183
at worst against a floor of 60; `scsi` reached 58.7 against 10. The controller
is working an order of magnitude below the cliff, which is where the search
range being capped at 800 rather than 2e5 stops mattering at all — and that is
the point: a range that never needs its top is a range you can trust.

`scsi` still sits **1.43 dB** from the scene palette ceiling of 31.33 dB
(FINDINGS 25.4), against 0.51 dB before. The ceiling, not the codec, is still
what bounds this content.

The percentages differ from 25.3's +18%/+34% because those compared video
payload against the total target; the table above compares like with like
(total against total). The payload figures are unchanged: 129.6 and 373.8 KB/s.

### 27.3 Rate control makes the display path cheaper, not dearer
The decoder-architecture numbers of FINDINGS 25.6 were measured on the
fixed-lam encoder. Re-measured under rate control, on the same window, with the
player picking the cheaper of compose-then-blit and direct-to-GVRAM per frame:

| profile | median display cost | frames above the 70% crossover |
|---|---|---|
| `sasi` fixed → RC | 37.0% → **36.6%** | 30.0% → 26.7% |
| `scsi` fixed → RC | 53.6% → **47.1%** | 53.3% → 35.8% |

Raising lam moves blocks to SKIP and V1, which is fewer blocks to write. The
"implement both paths, pick per frame" conclusion is unaffected and the cap is
still 53.6%.

### 27.4 The quality floor barely matters; the prefill matters, wrongly
Two knobs were measured rather than guessed.

**`--rc-floor`** decides whether a quiet frame may spend more than the fixed-lam
profile would. On the Singe window it is worth nothing — 109.5 vs 110.0 KB/s and
**0.00 dB** — because no frame on that window is quiet enough for the bucket to
saturate. The default is `profile` (never spend more than session 5 would), so
rate control cannot regress content that already fits.

**`--prefill`** models how full the player's buffer is at scene start. It is
tempting and it is a trap, so it defaults to 0:

| clip | prefill 0.0 | 0.5 | 1.0 | target |
|---|---|---|---|---|
| Singe, 120 fr, `sasi` | 109.5 | 112.9 | **116.3** | 110 |
| Singe, 120 fr, `scsi` | 280.0 | 289.1 | **298.2** | 280 |
| 00020, 14 fr, `sasi` | 92.0 | **115.8** | **115.8** | 110 |
| 00020, 14 fr, `scsi` | 224.8 | **255.9** | **255.9** | 280 |

(`scsi` on 00020 is the one cell where prefill looks harmless: the clip fits
under 280 either way. That is the content being easy, not the knob being safe.)

Prefill buys a permission to overshoot of exactly `bucket / nframes`. At 8
frames of bucket over 120 frames that is 6.2% — measured — and on a 14-frame
clip the bucket is larger than the clip, so rate control switches itself off and
reproduces fixed-lam exactly (lam never leaves its floor: min = median = max =
60). **A prefill that makes a target look met has disabled the controller.**

### 27.5 The 00020 undershoot is a clip-length artefact, not a bug
At prefill 0 the 14-frame 00020 clip lands at 92.0 KB/s against a 110 ceiling —
0.66 dB given away for nothing. That is the leaky bucket's startup transient:
the first `bucket_frames` frames cannot draw on a bank they have not accumulated.
It is bounded by `bucket / nframes`, so it is 6% on a 10-second window and 20%
on a 1.2-second one.

The lesson is the one FINDINGS 25.3 already taught in a different costume: **a
1.2-second clip cannot be used to judge rate control.** Real scenes are tens of
seconds. Do not tune the bucket against 00020.

Worth recording separately: fixed-lam `sasi` on 00020 delivers 115.8 KB/s — the
supposedly easy clip was **already 5% over its target**, which nothing had
noticed because the profile table quotes its PSNR and not its bitrate.

### 27.6 FINDINGS 26.5's cost premise was wrong in both halves
26.5 said a rate-control experiment was minutes because `_paint` is a Python
per-block loop, and told the next session to vectorise it first. Vectorising it
was correct and it is **17.1x faster**, but it was never the bottleneck, and the
ladder was never minutes. Measured per frame, 256x192:

| | ms |
|---|---|
| `VQ.assign` x2 — codeword search | **22.83** |
| SKIP error against `prev` | 1.40 |
| `decide` — argmin at one lam | 0.06 |
| `paint`, vectorised | 0.29 |
| `paint`, old per-block loop | 4.93 |

`_paint` was 14% of a frame. A 5-rung ladder over 120 frames was ~18 s of
encoding, not minutes — the "few minutes" in the drift test's docstring was
`H.build`'s k-means (51 s), which no amount of vectorising `_paint` would have
touched.

What actually makes per-frame rate control affordable is that `VQ.assign`'s
output depends on **neither `lam` nor `prev`**, so it is computed once per frame
and a lam search only re-runs the 0.06 ms argmin:

| | |
|---|---|
| 12-step per-frame lam search, 120 frames, symbols cached | **0.31 s** |
| the same search by re-running whole-sequence encodes | 49.10 s |

That is a 158x difference, and it is the reason the controller can afford a real
bisection instead of a 5-rung ladder — which was the actual defect in 26.3.

The cache holds **one frame**. At ~133 KB of intermediates per frame, caching
the sequence would cost 900 MB on a 9.4-minute stream to save nothing: every
caller works a frame at a time.

---

## 28. The 68000 decoder exists, is pixel-exact, and does not fit (session 7)

`src/player/decode.s` parses DLX1 and draws frames on the emulated X68000. It is
**pixel-exact across a 120-frame sequential run** of the worst sustained window
on the disc (`tools/bench/verify_decode.py`), exercising all four block modes
and the full temporal recursion — the last frame is only right if every frame
before it was.

It is also **too slow**. On that window, at the shipping `sasi` profile:

| | non-SKIP blocks | measured cost |
|---|---:|---:|
| cheapest frame | 15.4% | 31.5% of a 12fps frame |
| median frame | 47.8% | 73.8% |
| p90 frame | 82.5% | **116.4%** |
| worst frame | 100.0% | **135.8%** |
| mean over the window | 47.8% | **81.7%** |

**31% of frames miss the 833,333-cycle budget**, and like every figure since
FINDINGS 24 these are instruction cycles against zero-wait-state memory, so
they are a floor. This is the first time CPU, not disk, is the binding
constraint — FINDINGS 21 retired the bandwidth worry, and this replaces it.

### 28.1 The dual-path plan of 24.5/25.6 was incoherent, and is withdrawn
FINDINGS 24.5 specified two display paths chosen per frame on the non-SKIP
count, and 25.6 costed the mix at "median 37.0%, capped at 53.6%". Two of its
premises cannot both hold:

- compose-in-RAM-then-blit exists to make the blit **row-linear**, so it must
  assemble a **full** frame in RAM. The pixels it does not decode this frame —
  the SKIP blocks — can only come from a RAM copy of the previous
  reconstruction.
- decode-direct-to-GVRAM's stated advantage is that **"no RAM reference frame
  is needed"**, because the previous frame is already in GVRAM.

So every direct frame silently invalidates the reference the next compose frame
reads. Simulated on the Singe window at the crossover the plan specifies
(`tools/analysis/10_pathmix_drift.py`): **70 of 120 frames display pixels no
correct player would display**, first at frame 2, worst frame 18.8% of the
screen. This is FINDINGS 26 in different clothing — two code paths disagreeing
about what "the previous frame" means — and it is the **sixth** false premise
this project has caught before it shipped.

Every coherent repair is worse than not mixing at all:

| strategy | median | p90 | max | correct |
|---|---:|---:|---:|---|
| mix per frame, as specified | 36.6% | 53.6% | 53.6% | **no** |
| mix, direct also writes the RAM reference | 53.6% | 68.4% | 81.4% | yes |
| mix, re-read GVRAM into RAM on each switch | 36.6% | 107.2% | 107.2% | yes, 13 frames miss |
| compose only | 53.6% | 53.6% | 53.6% | yes |
| **direct only** | **36.6%** | 62.5% | 76.6% | yes |

(Costs in that table are 24.5's own model, for like-for-like comparison; 28.2
replaces the model itself.)

**24.5 also compared the wrong two things.** Its 53.6% and 76.6% are both
*copies* measured in `blit.s` — neither includes decoding. A real compose path
costs decode-into-RAM **plus** the 53.6% blit, so it is strictly dearer than
decoding straight into GVRAM, whatever the block mix. There was never a
crossover to find.

**The decoder therefore implements one path, direct-to-GVRAM**, and drops the
96 KB RAM reference frame entirely.

### 28.2 The four block modes do not cost the same, and V4 is the expensive one
24.5's model — "76.6% of a frame x the non-SKIP fraction" — prices every
non-SKIP block as one `movem.l` burst. Measured separately, with synthetic
single-mode frames (`tools/bench/prep_dlx.py`):

| mode | cycles/block | vs the 24.5 model (207.8) |
|---|---:|---:|
| SKIP, in an all-SKIP header byte | 13.3 | model says 0 |
| SKIP, inside a mixed byte | ~45 | model says 0 |
| V1 (one 4x4 codeword) | **299.9** | 1.44x |
| V4 (four 2x2 codewords) | **448.2** | 2.16x |
| RAW (16 literal indices) | **400.4** | 1.93x |

Applied to the real per-frame histograms (`tools/analysis/11_cpu_budget.py`),
the model reproduces all four frames timed on the 68000 to within **1
percentage point**, and shows 24.5 to be **2.03x optimistic at the median**.

Where the cycles actually go over the window:

| mode | % of blocks | % of cycles |
|---|---:|---:|
| SKIP | 46.4% | 9.2% |
| V1 | 19.8% | 26.1% |
| V4 | **25.2%** | **49.7%** |
| RAW | 8.5% | 15.0% |

**V4 is a quarter of the blocks and half the cycles.** It costs 1.49x a V1 block
while the mode decision in `vq_hybrid.py` charges it only its 4x payload bytes.
The lagrangian trades distortion against *bytes*; on this machine it now has to
trade distortion against *cycles* as well.

### 28.3 The container is big-endian but not aligned, and that is an address error
The DLX1 header docstring says every multi-byte field is big-endian "so the
68000 reads them with a plain `move`". Alignment is the other half of that
sentence and the container does not have it: frame records are
`[u32 length][768-byte mode header][payload]` laid end to end with arbitrary
payload lengths, so record boundaries land on odd addresses.

`move.l (a0)+,d0` at an odd address is an **address error** on a 68000 — not a
slow read. The first run decoded frame 0 perfectly, consumed exactly its 8,715
payload bytes, then read frame 1's length at `$03220F` and vectored into the IPL
at `$FF059A`, where it sat for 59 emulated seconds looking like an infinite
loop. It was found by dumping PC and the address registers, not by reading the
source: the code was correct, the data layout was not.

The decoder now rounds each record start up to 4. **The container should carry
the padding itself** so a streaming player can DMA records into place: measured
cost on this window is **199 bytes over 120 frames — 1.66 B/frame, 20 B/s**
against a 110 KB/s budget. Until `encode.py` does it, `prep_dlx.py` realigns at
load time.

### 28.4 The measurements agree with hand-derived MC68000 timings
As in FINDINGS 24, each figure was derived from the instruction timing tables
before being believed. A V1 block, summing dispatch, index decode, the indexed
`movem.l` load and four `movem.l` stores, plus its quarter share of the header
loop: **298.5 cycles derived against 299.9 measured — 0.5%.** RAW derives to
396 against 400.4 measured (1%). V4 derives to 415 against 448 (7%, the gap
being in the indexed two-register `movem.l`, the mode this decoder uses most
heavily). So these are 68000 cycles, not a MAME artefact.

### 28.5 A full frame does not fit at 12fps in ANY mode
An all-V1 frame — the cheapest possible way to redraw all 3,072 blocks — costs
**921,187 cycles, 110.5% of the budget**. All-V4 is 165.2% and all-RAW 147.6%.

So the ceiling is structural, not a tuning problem: **at 12fps on a 10MHz 68000
no more than ~88% of the screen can change in one frame**, however cheaply it is
coded. Scene cuts change 100% of it. Either a cut gets one late frame (the
outgoing content is unrelated, so this may be free to the eye), or cuts have to
be spread across two frame times, or the framerate has to come down — at 10fps
the budget is 1,000,000 cycles and an all-V1 frame fits.

### 28.6 What this does not measure
One 10 s window of one stream at one profile, and MAME still models no GVRAM
wait states. The `scsi` profile will be worse: FINDINGS 25.5 has it collapsing
to RAW under stress, and RAW is 1.93x the old model's block. Nothing here has
been run on `00020` or on quiet content, where the median frame is far cheaper.