Session 2: hybrid VQ codec, two quality profiles, three corrections
Answers session 1's critical-path question. Flat 4x4 VQ at k=256 was prototyped and REJECTED by eye: Dirk's face disintegrates and ink outlines break into 4-pixel stair-steps. The 256-colour palettised frame is excellent, so the palette was never the problem -- block VQ was. Replaced it with a Cinepak-style hybrid: each 4x4 block is SKIP, one 4x4 codeword, four 2x2 codewords, or RAW literal pixels, chosen per block by rate-distortion. The RAW escape makes lam=0 pixel-exact (measured 0.00 dB loss), so the quality knob spans lossless to heavily-compressed in one bitstream. Per the user's decision, ships TWO quality profiles from that one codec, one decoder and one bitstream -- only the rate knob differs: sasi 45 KB/s lam=300 34.8 dB stock 10MHz ACE/EXPERT scsi 75 KB/s lam=100 35.9 dB Super/XVI or CZ-6BS1 Three corrections to earlier numbers: 1. Session 1's "183 KB/s at 12fps" was a bad extrapolation. Halving the framerate does not halve the bitrate -- decimation roughly doubles the per-frame delta. Re-measured directly: 340 KB/s for session 1's own RLE, 247 KB/s for changed-spans+deflate. The lossless floor is 319 MB. 2. A FOURTH false-good result, same family as the three in FINDINGS 4: k=1024 codebooks appeared to buy +2.4 dB free, because the rate model charged 1 byte for a 10-bit index. Charging the true cost reverses the verdict -- k=256 wins at every matched bitrate, and by 5 dB at the low end where the SASI profile lives. k=256 ships. 3. Stream inventory: the ~3-5MB clips are 1.2-1.7s, not ~60s, and some 60s streams are menus, not content. Any survey must classify before averaging. Also cleared both candidate sources for the game-logic layer: the SNES project is MIT and DirkSimple is zlib, so the arcade scene graph can be imported and the two transcriptions diffed against each other. Encoder is working end-to-end: extract.py -> vq/vq_hybrid/ratectl -> encode.py, emitting a big-endian DLX1 container the 68000 can parse with plain moves. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
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# Status & next-session handoff — end of session 1 (2026-08-23)
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# Status & next-session handoff — end of session 2 (2026-08-23)
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## Decisions locked
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@@ -7,133 +7,134 @@
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| Target CPU | 68000 @ 10MHz (stock) | hardest honest constraint |
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| Display mode | 256 colors, 256x192 in 256x256 CRTC mode | every mode is 1 word-access/pixel, so 256c is free vs 16c |
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| Double buffer | **none** — page 1 sacrificed | enables `movem.l` 24px bursts; delta coding needs a RAM reference frame anyway |
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| Codec | 4x4 vector quantization, per-scene codebook + block delta | CPU is idle, I/O is the ceiling — spend cycles to buy bandwidth |
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| **Codec** | **hybrid VQ: SKIP / V1 4x4 / V4 four-2x2 / RAW, per-block rate-distortion** | flat 4x4 VQ was measured and rejected — see FINDINGS 9-10 |
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| **Quality modes** | **two: `sasi` and `scsi`** (USER DECISION, session 2) | one codec, one decoder, one bitstream; only `lam` differs |
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| Framerate | 12 fps, **explicit decimation** | source has zero duplicate frames; no free "twos" win |
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| Medium | SCSI HDD image (.hds) | but see SASI/SCSI split below |
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| Emulator | MAME 0.277 x68000 | accurate enough that measured cycles mean something |
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| SNES project reuse | **MIT — cleared** | `data/events/` scene graph is reusable with attribution |
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**OPEN QUESTION for the user:** stock 10MHz machines are **SASI**, not SCSI.
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Three options, not yet chosen:
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1. Stock 10MHz + SASI (purist) — VQ becomes mandatory
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2. Stock 10MHz + CZ-6BS1 SCSI board — relieves I/O, keeps CPU honest
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3. Super/XVI baseline — built-in SCSI, still a 10MHz 68000
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### The SASI/SCSI question is RESOLVED
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Session 1 left "which machine do we target" open. The user's answer: **ship both**,
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as two quality profiles. This is now implemented rather than hypothetical — the
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bitrate ceiling is a build parameter in `tools/encoder/ratectl.py`:
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Recommendation: make the codec's bitrate ceiling a **build parameter**, so one
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encoder serves all three and the target is chosen at package time.
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| profile | target | lam | quality (00020 / 00146) | machine |
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|---|---|---|---|---|
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| `sasi` | 45 KB/s | 300 | 34.8 / 28.3 dB | stock 10MHz ACE/EXPERT |
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| `scsi` | 75 KB/s | 100 | 35.9 / 29.0 dB | Super/XVI, or CZ-6BS1 board |
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Codebooks are **k=256 with 1-byte indices** in both profiles. k=1024 was measured
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and rejected — see FINDINGS 14, it was a false-good result from a rate model
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that undercharged the index. Do not ship past `lam~800`; FINDINGS 15 has the cliff.
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Because of the RAW escape mode, `lam=0` is **pixel-exact** against the palettised
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frame (measured 0.00 dB loss). The profiles are two points on one continuous
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rate-distortion curve, not two codecs.
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---
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## Working setup
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## What session 2 settled
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**MAME ROMs** — `~/mame/roms/x68000.zip` (present, working).
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Must pass **`-bios ipl10`**; the default BIOS is `cz600ce`, whose split
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even/odd IPL halves (`rh-ix0897cezz.ic12` / `rh-ix0898cezz.ic11`) are absent.
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`-verifyroms` will still report those two as missing — this is expected and harmless.
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1. **The critical-path question is answered.** "Does VQ soften Bluth's linework
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unacceptably?" — **flat 4x4 k=256 VQ: yes, badly. Hybrid VQ with k=1024: no.**
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Verified by eye, not just PSNR. See `docs/FINDINGS.md` 9-11.
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2. **Session 1's 12fps bitrate was wrong** (183 KB/s claimed, 340 KB/s measured).
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Halving the framerate does not halve the bitrate. FINDINGS 8.
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2b. **A fourth false-good result was produced and caught this session** — k=1024
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codebooks looked like a +2.4 dB free win because the rate model charged 1 byte
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for a 10-bit index. FINDINGS 14. The k=256 configuration ships.
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3. **The 256-colour palettised frame is the real quality ceiling** and it looks
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excellent. Judge the codec against that, not against 1080p.
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4. Encoder exists and produces a real bitstream: `tools/encoder/`.
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Boots headless at ~430-480% speed:
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---
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## Encoder — working
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```
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python3 tools/encoder/extract.py 00020 /tmp/fr_00020 12 crop
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python3 tools/encoder/encode.py /tmp/fr_00020 out.dlx --profile sasi --preview p.png
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```
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| file | role |
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|---|---|
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| `extract.py` | .m2ts -> 256x192 PNGs, 12fps, spatial-only denoise |
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| `vq.py` | palette, blockify, hand-rolled k-means (no sklearn on this box), PSNR |
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| `vq_hybrid.py` | the codec: 4 block modes + lagrangian mode decision |
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| `ratectl.py` | SASI/SCSI profiles, leaky-bucket rate control |
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| `encode.py` | CLI + `DLX1` container writer |
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`DLX1` container layout is documented in the `encode.py` docstring. All
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multi-byte fields are **big-endian** so the 68000 reads them with a plain `move`.
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### Known encoder gaps
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- **Rate control is written but not yet wired into `encode.py`** — the CLI uses a
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fixed `lam` from the profile. `ratectl.encode_rate_controlled()` exists and
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builds a lam-ladder per frame; it needs hooking up and validating.
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- **Payload is not entropy-coded.** Deflate on the payload should buy ~1.4x
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(measured on the lossless path, FINDINGS 8). LZ decode is cheap on a 68000.
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- Codebooks are per-scene and rebuilt from scratch; no inter-scene reuse.
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- `_paint` is a Python per-block loop — fine for prototyping, slow for a full
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disc encode. Vectorise before the 224-stream run.
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---
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## Working setup (unchanged from session 1, re-verified)
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**MAME ROMs** — `~/mame/roms/x68000.zip`. Must pass **`-bios ipl10`**.
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```
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mame x68000 -bios ipl10 -video none -sound none -nothrottle -seconds_to_run 3
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```
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**Assembler** — `tools/vasm/vasmm68k_mot -Fbin -o out.bin in.s`
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**Assembler** — vasm built from source, binary at `tools/vasm/vasmm68k_mot`
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(source tarball alongside it). Verified correct 68000 output.
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```
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tools/vasm/vasmm68k_mot -Fbin -o out.bin in.s
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```
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**Blu-ray** — `udisksctl loop-setup -r -f DRAGONS_LAIR.iso` -> `/media/reala-misaki/BDROM`
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(still mounted as of end of session 2).
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**Blu-ray** — mount with:
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```
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udisksctl loop-setup -r -f DRAGONS_LAIR.iso # -> /media/reala-misaki/BDROM
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```
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NOTE: this loop mount is still active from session 1. Re-mount if the machine rebooted.
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**MAME Lua harness** — `tools/bench/*.lua`, working. Three gotchas (retain the
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notifier subscription in a global; the stack register is `SP` not `A7`;
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`autoboot_script` fires at PC=0 before boot) are documented in FINDINGS.
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**Two shell traps, both hit again this session:**
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- piping MAME (or any long job) through `grep` block-buffers — write to a file.
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- `pkill -f <pattern>` matches your own shell and kills it (exit 144).
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Use `pkill -x` or kill by PID.
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---
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## MAME Lua harness — WORKING, reusable
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## STILL BLOCKED: disk throughput benchmark
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`tools/bench/*.lua` inject 68000 machine code straight into emulated RAM and time
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it against the emulated clock. No bootable disk or OS required. This is the
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measurement rig for all future cycle-cost work (blit timing, decoder benchmarks).
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Unchanged from session 1 — `IOCS _B_READ` returns -1 uniformly. Full diagnosis
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and the four untested hypotheses are in session 1's notes (git history of this
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file, commit 65112b9).
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Pattern:
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```
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mame x68000 -bios ipl10 -video none -sound none -nothrottle \
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-seconds_to_run 30 -plugins -autoboot_script yourscript.lua
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```
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### Three MAME Lua gotchas — all cost real time, all now solved
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1. **Retain the notifier subscription.** `emu.add_machine_frame_notifier()` returns
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a token; if you drop it into a chunk-local it is garbage-collected and the
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callback **silently stops firing**. Assign it to a **global** (`SUB = ...`).
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2. **The stack pointer is `SP`, not `A7`** in `cpu.state[...]`.
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Full list: A0-A6, D0-D7, PC, SP, SR, USP, CURPC, CURFLAGS, IR.
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3. **`autoboot_script` fires at time=0, before boot** (PC=0). Wait until
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`machine.time` >= ~5s before injecting, or IOCS is not yet initialised.
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Also: piping MAME through `grep` block-buffers output — write raw to a file when
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backgrounding, or you will see an empty log and assume a hang.
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And never `pkill -f 'mame x68000'` — the pattern matches your own shell and kills it
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(exit 144). Use `pkill -x mame`.
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---
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## BLOCKED: disk throughput benchmark
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**Goal:** measure real SASI/SCSI KB/s to replace the folklore figures in FINDINGS.md §5.
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**Status:** harness fully working; the IOCS call itself fails.
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`IOCS _B_READ` ($46 via `TRAP #15`; d1.hb=PDA, d2.l=position, d3.l=bytes, a1=buffer)
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returns **`FFFFFFFF` (-1), zero reads**, uniformly across:
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- all 16 PDA values $80-$8F
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- both d1 encodings (PDA in bits 31-24 and bits 15-8)
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- image sizes 10MB / 20MB / 40MB
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The uniformity is the diagnostic: calls are **dispatched and cleanly rejected**,
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so `TRAP #15` and IOCS are reachable. MAME does mount the image
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(`:x68k_hdc: opened image file bench.hdf`).
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**Untested hypotheses, in rough order of likelihood:**
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1. The raw image has no X68000 SASI format, so the IPL's boot scan never registered
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a usable drive and IOCS refuses. Would need Human68k to format one — **we have
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no Human68k image on this system.**
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2. MAME's `x68k_hdc` SASI implementation may be too partial for IOCS-level reads.
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3. `SP=$8000` may put the injected stack on top of the IOCS work area in low RAM.
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Try a much higher stack.
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4. The **SCSI path was never tried** — this is the obvious next move and is more
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relevant to the target anyway:
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`-exp1 cz6bs1 -hard disk.chd` with `exp1:cz6bs1:scsi:0 harddisk`
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(`-listmedia` gains a `harddisk` slot accepting .chd/.hd/.hdv/.2mg/.hdi).
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**Honest assessment: this benchmark is NOT on the critical path.** The VQ codec
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(~30 KB/s) is correct whether SASI does 300 or 600 KB/s. Do not let it block the
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encoder. Its real value is deciding whether the *simpler* row-span codec could
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have sufficed.
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Caveat if resumed: MAME idealizes drive seek latency. That's acceptable because the
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realistic deployment is BlueSCSI/SCSI2SD (SD-backed, no mechanical seek), so what
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gets measured is the bus/DMAC/controller path — the genuine ceiling. The caveat
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only bites for a real period spinning drive.
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**This now matters more than session 1 thought.** Session 1 dismissed it because
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"VQ at 30 KB/s is correct whether SASI does 300 or 600 KB/s". But we now ship
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*two profiles*, and the profile bitrates (45 / 120 KB/s) are set against
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**folklore** bandwidth figures. A real measurement would let us set them
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honestly instead of conservatively. Next move is the untried SCSI path:
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`-exp1 cz6bs1 -hard disk.chd`.
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---
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## Next steps, in priority order
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1. **Build the VQ encoder** (`tools/encoder/`) — 4x4 blocks, per-scene codebook,
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block delta. Emit sample PNGs for visual evaluation. **The open question is
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whether VQ softens Bluth's ink linework unacceptably — decide by eye before
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committing to the architecture.**
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2. **Full-disc survey** — all 224 streams, not 5s samples, to firm up bitrate
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(current numbers are +/-30%) and map streams onto the arcade scene graph.
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3. **Check the SNES project's license**, then evaluate reusing `data/events/`
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(516 chapters / 29 scenes) as the scene-graph and input-timing layer.
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4. Resolve the SASI/SCSI target question with the user.
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5. Optionally unblock the disk benchmark via the SCSI path.
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6. 68000 player skeleton: CRTC init for 256x192x256c, `movem.l` blitter,
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ADPCM via HD63450 DMA.
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1. **Wire rate control into `encode.py`** and validate that the hard ceiling
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actually holds on an action scene (the whole point of choosing VQ).
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2. **Entropy-code the payload** (deflate) — ~1.4x for cheap 68000 decode cost.
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3. **68000 decoder skeleton**: parse `DLX1`, expand codebooks to word-per-pixel,
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blit V1/V4/RAW/SKIP. Measure real cycles with the existing MAME Lua harness —
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this is the first time the harness gets used for its actual purpose.
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4. **Full-disc survey** — classify menu vs content first (FINDINGS 13), then
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measure bitrate across all 224 streams per profile.
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5. **Resolve the framing question** (FINDINGS 12: crop vs squash vs wide).
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6. Unblock the disk benchmark via the SCSI path, then re-set profile bitrates.
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7. Import the SNES project's `data/events/` (MIT, cleared) as the scene graph.
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Cross-check against DirkSimple (zlib) which has the same data independently.
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8. ADPCM audio: MSM6258, 15.6kHz mono, 7.8 KB/s — already budgeted in `ratectl`,
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not yet extracted or encoded.
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## Not yet started
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- Any 68000 player code
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- ADPCM audio extraction/encoding (MSM6258, 15.6kHz mono, ~7.8 KB/s, ~10MB for 22min)
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- Disk image packaging / container format
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- ADPCM audio extraction/encoding
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- Disk image packaging
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- Game logic (scene branching, input windows, death clips)
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