Session 2 reversed several of its own conclusions. The docs are append-only, so
a reader could land on a superseded section and act on it. This pass makes the
repo internally consistent.
Defects found and fixed in STATUS.md:
- claimed "Hybrid VQ with k=1024: no" as the answer to the linework question,
directly contradicting FINDINGS 14, which rejected k=1024. Both profiles are
k=256.
- malformed profile table (six column separators, five columns).
- next-steps list had two items numbered 3 and listed the full-disc survey
twice.
- the disk-benchmark section still read CRITICAL-PATH with "if SCSI sustains
>=800 KB/s, ship pixel-exact". That was written while the bandwidth figure
was misread as 4 MB/s. At 4 Mbps pixel-exact needs 92-97% of the pipe and is
not available, and the ring-buffer result means the design no longer hangs on
the benchmark at all. Rewritten with what it IS still worth doing: confirming
the 4 Mbps provenance, and confirming DMA is used rather than PIO.
FINDINGS now carries supersession blockquotes on 5, 8, 11, 17 and 18 pointing
at the sections that correct them. 18 is the dangerous one -- its peak-vs-
sustained test is reversed by 21 -- so it is marked DO NOT ACT ON THIS SECTION
while noting the per-frame data itself remains valid.
profile_gen.py had the same problem in code: it defaulted to the superseded
peak sizing and returned lam=25 where the docs say lam=10. The buffered test is
now the default and peak sizing is behind --size-for-peak as a bound only. A
tool that contradicts the findings is worse than no tool.
Also preserves the five measurement scripts that produced this session's
numbers as tools/analysis/05-09, following the session 1 precedent, and adds an
"explicitly abandoned -- do not re-propose" list to STATUS covering entropy
coding, k=1024 codebooks and flat 4x4 VQ.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
User clarified the bandwidth figure is 4 Mbps (488 KB/s), not 4 MB/s -- ~8x
tighter than the previous commit reasoned against. Two consequences, plus a
correction to session 1.
1. The scsi profile committed in f0f2f80 DOES NOT FIT. Its mean is a
comfortable 52% of the pipe but it PEAKS at 96.4% (470.8 KB/s on scene
00020), and a frame that arrives late is a dropped frame, not a slow one.
Peak/mean is 1.4-1.9x even on 1.2-1.7s clips. Sizing a real-time stream on
the mean was the error. Flagged in STATUS rather than silently retuned,
because the fix is rate control, not a lower lam.
This promotes ratectl.py -- written in session 2, never wired into
encode.py -- from a loose end to the highest-value work in the repo. It is
worth a full step on the quality ladder (lam=25 -> lam=10, +0.7/+1.2 dB)
because it allows sizing for the mean instead of the peak.
2. Pixel-exact is off the table at this bandwidth: lam=0 needs 92-97% of the
pipe. The previous commit's "if SCSI sustains >=800 KB/s, ship transparent"
conclusion only applies at roughly double the user's figure.
3. FINDINGS 5 said that because transfers are DMA, streaming "costs essentially
no CPU" and the 68000 is "nearly idle". That is wrong. The HD63450 steals
~8 clocks per 16-bit word: 10-20% of the machine at the rates the profiles
now use, on top of a 38% full-frame blit. Bandwidth and CPU are one budget.
Adds tools/encoder/profile_gen.py, which derives lam FROM a bandwidth figure
(accounting for audio, peak/mean and DMA steal) instead of reading it off the
knee of the RD curve, and docs/BENCHMARK.md covering how to actually measure
the subsystem -- including why MAME cannot answer the bandwidth question and
would be the same class of error as the FINDINGS 4 traps.
The 4 Mbps figure is user-supplied and its provenance is not recorded; every
profile now hangs off it.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
prosolis