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Raise both quality profiles; rule out entropy coding on CPU grounds

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The profiles shipped in e4062ed were set far too low. 45 KB/s (sasi) and
75 KB/s (scsi) are 12% and 7% of the respective folklore bus figures. They had
been read off the knee of the rate-distortion curve and then presented as
though bandwidth-derived, which they were not.

Raised to sasi 110 KB/s (lam=60) and scsi 280 KB/s (lam=10) -- 35% and 28%
utilisation. scsi is now within 0.52 dB of the palette ceiling on scene 00020.

Checking the CPU side, which nobody had done for the decode path, produces a
second and more important result. Against the 833k cycle/frame budget at 12fps:

  full-frame blit, every frame     319k   38%   affordable
  LZ4/LZSS decode ~30KB/frame      450k   54%
  deflate decode  ~30KB/frame     1800k  216%   infeasible

So raising the VQ bitrate is nearly free -- RAW, the mode that dominates at
high rate, is the cheapest mode to blit -- but entropy coding is not viable at
all. That demotes the "247 KB/s lossless changed-spans+deflate" figure from
FINDINGS 8 to a compression upper bound rather than a shippable design, and
removes entropy coding from the roadmap. VQ is the right architecture precisely
because its decode is a table copy.

Also confirms the architecture unifies: the hybrid at lam=0 lands within 3% of
the purpose-built lossless coder, so there is no separate lossless path.

Consequence for planning: the blocked disk benchmark is now critical-path, not
optional. If SCSI sustains >=800 KB/s the correct scsi profile is lam=0 --
pixel-exact video at ~450 KB/s and 38% CPU. Whether this port ships transparent
or lossy on SCSI is waiting on one measurement.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
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prosolis
2026-08-23 12:06:52 -07:00
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@@ -370,3 +370,83 @@ Both checked this session:
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
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.