Put sound on the wire, and find three LSBs are worth 25 dB
ROADMAP P6, everything in the item except the bus half session 20 closed. tools/encoder/adpcm.py is an MSM6258 codec, tools/encoder/extract_audio.py takes the same seconds of the same stream the frames come from, tools/bench/verify_adpcm.py is the gate, tools/analysis/32_audio_wire.py the container arithmetic. There is no reference encoder -- ffmpeg has a decoder for this format and none the other way -- so what is gated is the decoder the encoder runs INSIDE its own nibble search, sample-exact against ffmpeg's over 4,268 nibbles. An encoder that agrees with its own wrong decoder is what that catches. The Singe window: 156,250 samples -> 78,125 B at 21.97 dB, which is 7,812.5 B/s to the byte. Normalising the disc's -13.4 dBFS level moves the SNR 21.97 -> 21.97, so the level is not a lever. And the two published delta formulas are not the same codec. They differ by at most 3 in 12-bit units; encode for one and decode on the other and the SNR goes 21.97 -> -2.88 dB, the noise louder than the signal, because ADPCM is recursive the way the video codec is temporally recursive. Which one the chip runs is now P6a and it is a precondition on shipping any audio. And audio is the first thing the packed branch's simplification has cost anything for. A record has no index BY DESIGN, so audio cannot be per-record without making records variable; it rides a fixed cadence (F, A), the obvious F=1 wastes 57.3% of every audio sector, and the pick is F=11 A=14 -- 0.09% padding, 14,336 B held, wire 582.0 -> 589.6 KB/s. The codec container, which kept its index, pays zero. The MAME experiment did not work and 65.5 says so: :okim6258 is there at $E92001/$E92003, read out of the machine's own program map, and feeding it from Lua recorded silence across control 0..3 x port C 0..15. The register semantics were not guessed at further. FINDINGS 65. check.sh ALL GREEN before and after, with a new stage. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
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# Status & next-session handoff — end of session 32 (2026-08-25)
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# Status & next-session handoff — end of session 33 (2026-08-25)
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## Session 33: audio gets an encoder, and the packed container's best property gets a bill
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**Green light first and last: `./tools/bench/check.sh` was ALL GREEN before any
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of this (`tmp/check_s33_start.log`) and ALL GREEN after** — the same stages, plus
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one new one.
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**FINDINGS 65. ROADMAP P6 is three-quarters done and has grown two sub-items.**
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`tools/encoder/adpcm.py`, `tools/encoder/extract_audio.py`,
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`tools/bench/verify_adpcm.py`, `tools/analysis/32_audio_wire.py`, and three
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throwaway MAME probes kept because their failure is a finding
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(`tools/bench/probe_adpcm*.lua`).
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**Everything below is HOST ARITHMETIC plus one MAME introspection.** No board
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ran, and the one MAME *experiment* attempted did not work — 65.5 says so.
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**0. SESSION 32'S WORK WAS UNCOMMITTED AND ITS STATUS BLOCK HAD NO HANDOFF.**
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Both fixed before anything new was written: the tree was re-gated ALL GREEN,
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the handoff written, and the whole of session 32 committed as one change.
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**1. THE ENCODER, AND THE GATE IT NEEDED INSTEAD.** ffmpeg has a DECODER for
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this format (`adpcm_ima_oki`) and **no encoder**, so there is nothing to diff
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against. What is gated instead is the decoder the encoder runs **inside its own
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nibble search** — sample-exact against ffmpeg's over 4,268 nibbles. An encoder
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that agrees with its own wrong decoder is what that catches.
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| | |
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|---|---|
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| the window | 00223 @539.4 s, 10.000 s — **the same seconds as `tmp/fr_singe`** |
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| encoded | 156,250 samples → **78,125 B, SNR 21.97 dB** |
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| the rate | 78,125 B / 10 s = **7,812.5 B/s to the byte** — 52's figure, from the other end |
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| nibble order | **HIGH FIRST, measured** — low-first mismatches ffmpeg on 3,285 of 4,268, and that is the gate's negative control |
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| the level | disc peaks at **−13.4 dBFS**; normalising ×4.7 moves the SNR **21.97 → 21.97**. Not a lever |
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**2. THE HEADLINE, AND IT IS A THREE-LSB DIFFERENCE THAT COSTS 25 dB.** The two
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published delta formulas — ffmpeg's `((2*(n&7)+1)*step)>>3` and the OKI
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datasheet's per-term truncation — differ **by at most 3 in 12-bit units**.
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Encode for one, decode on the other:
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| encoded | decoded | SNR |
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|---|---|---:|
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| `shift` | `shift` | **21.97 dB** |
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| `shift` | `terms` | **−2.88 dB** |
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**The noise comes out louder than the signal** (mean disagreement 78.2 against a
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source RMS of 74.4). **ADPCM is recursive**, so a rounding difference does not
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stay where it happens — the same property, one dimension down, that let 64.1's
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gate audit 120 frames by comparing one. **Which formula the chip runs is
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therefore a PRECONDITION on shipping any audio**, and it is ROADMAP P6a.
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**3. AND THE PACKED CONTAINER'S BEST PROPERTY IS WHAT MAKES AUDIO COST.** A
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packed record is 97 sectors and its address is `LBA0 + i*97` because a literal
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frame's length is geometry — **no index, and none can be needed** (63, 64.1).
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Audio at 15,625 Hz is **651.0417 B a slot**, and the `.0417` is the same
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remainder the frame clock carries (54). Per-record audio makes records variable
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length, which needs an index, which ends the format. So audio rides a **fixed
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cadence** — every `F` frames, `A` whole sectors — and `(F, A)` is a rational
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approximation to 15625/12288 from above:
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| F | A | lump | padding | wire adds | held |
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|---:|---:|---:|---:|---:|---:|
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| **1 — the obvious one** | 2 | 1,024 B | **57.29%** | 12.00 KB/s | 2,048 B |
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| **11 — the pick** | **14** | **7,168 B** | **0.09%** | **7.64 KB/s** | **14,336 B** |
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| 81 — the floor | 103 | 52,736 B | 0.003% | 7.63 KB/s | 105,472 B |
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**Wire: 582.0 KB/s silent → 589.6 KB/s with sound (+1.31%).** F=11 buys 57.2
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points of padding for 12,288 B of RAM; the floor buys the last 0.09 of a point
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for 91,136 B more, and K4 already wants 99,328 B.
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**4. THE CODEC CONTAINER PAYS NONE OF IT.** It already has an index and already
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has variable records, so it puts exactly 651.0417 B in record *i* and pads to
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the sector it was padding to anyway: **zero audio padding**, 440.4 → 448.1 KB/s.
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**That is the first price anyone has found for the packed branch's own
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simplification.** 64's risk list said a deletion that large usually hides
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something; this is the first thing it hid, and it is small — a cadence, a
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padding fraction and 14,336 B.
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**5. THE EXPERIMENT THAT DID NOT WORK, and the facts it did leave.** MAME's
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x68000 **has the chip** — `:okim6258`, reached at **`$E92001` and `$E92003`**,
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both read out of the machine's own program map rather than from folklore, with
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the PPI at `$E9A000-$E9BFFF`. **Feeding it from Lua produced silence**: control
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0..3 against port C 0..15, `-wavwrite` capture **0 of 567,360 samples non-zero**.
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The gap is register semantics and it was **not guessed at further**. The way to
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close it is from 68000 code with the IPL ROM's own channel-3 configuration,
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which `21_iplrom_dmac.py` already reads out of the ROM.
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**RISKS IN THIS SESSION'S RESULT, stated rather than left to be found:**
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- **The encoder is greedy, not optimal.** Each nibble is chosen by exhaustive
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search over all sixteen minimising THIS sample's error; a nibble also moves
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the step index, so a locally worse choice can pay later. 21.97 dB is a floor
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for this format, not its ceiling, and no lookahead was tried.
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- **21.97 dB is against the 12-bit word**, not against the disc's 16-bit PCM,
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and the source was already resampled and downmixed to mono by ffmpeg. The
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downmix matrix is ffmpeg's default and was not chosen.
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- **P6a is open and it is worth 25 dB**, so every byte `adpcm.py` has produced
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is provisional.
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- **Nothing has played.** No audio has left an emulated machine, let alone a
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board.
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## HANDOFF — start here
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**THE TREE IS ALL GREEN**, session 33's stage included.
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### The work, in the order it should be done
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**1. P6a — WHICH DELTA FORMULA, and it needs no board.** It is worth **25 dB**
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(65.2), so every byte the encoder has produced is provisional until it is
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answered, and it is the cheapest open item in the project by a distance. MAME's
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x68000 has the chip and this session found where it lives — `:okim6258` at
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**`$E92001`/`$E92003`**, PPI at `$E9A000-$E9BFFF`, read out of the machine's own
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program map (65.5).
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**Do NOT retry the Lua feed.** It was swept — control 0..3 × port C 0..15 —
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and recorded silence, and the gap is register semantics that nobody here should
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be guessing at. **Do it from 68000 code with the IPL ROM's own channel-3 DMAC
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configuration**, which `tools/analysis/21_iplrom_dmac.py` already reads out of
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the ROM: dual address, 8-bit port, cycle steal without hold, external request.
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That is the one ADPCM path in this machine that is known-correct because Sharp
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wrote it, and it is also the real design — so it is not scaffolding, it is P6b's
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transport arriving early.
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The discriminating stream is already worked out: **twelve loud nibbles to climb
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the step index, then every nibble in turn**, whose two reconstructions differ
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first at sample 3 (523 against 522). Capture with `-wavwrite` and compare against
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`adpcm.decode(nibs, 'shift')` and `adpcm.decode(nibs, 'terms')`. **Watch the
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clipping** — the sequence above saturates at 2,047 within six samples, which
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destroys discrimination; build one that alternates sign to hold the signal
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mid-range while the step index climbs.
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**2. THEN P6b — DLXP2, a container with sound in it.** 65.3 is all the
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arithmetic: cadence **F=11, A=14**, `LBA(i) = LBA0 + i*97 + floor(i/11)*14`,
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14,336 B held, wire 589.6 KB/s. It waits on P6a, because a container full of
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audio encoded against the wrong formula is 25 dB of work to redo.
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**3. WHAT IS LEFT OF P6 AFTER THAT** is the fourth quarter: 51.3's refill climb
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with a second consumer through a real branch point. The slack table is in
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`32_audio_wire.py`; nothing has been run.
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### What is still BLOCKED, so it is not picked up by mistake
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**K4 — the packed player that is on screen — is conditional on B2**, a board
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question. If buffer mode does NOT blank, K3's player is already on screen the
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whole slot and K4's paint is 27.3% of a frame spent on nothing. **E7, E4 and C1**
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are parked (61.8), and **P4a's wiring** is parked with the ring K3 deleted.
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**The hardware list is unchanged and is the user's**: B1 (sustained AND the
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data-phase BURST rate, 64.2), B2 (blanking — the five-minute half), B3 (`#EXREQ`),
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B4 (a byte write to a palette register).
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### Risks that are OURS, not hardware
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1. **P6a is open and it is worth 25 dB.** Everything in `adpcm.py`'s output is
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provisional.
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2. **The encoder is greedy.** Exhaustive per-sample search, no lookahead;
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21.97 dB is this format's floor here, not its ceiling.
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3. **Nothing has played.** No audio has left an emulated machine.
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4. **The packed branch's simplification has now cost something once** (65.4).
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It was predicted in the abstract and it was small. It may not be the only one.
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### Reproducing this session
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./tools/bench/check.sh # ALL GREEN
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python3 tools/encoder/extract_audio.py 00223 tmp/au_singe.raw 15625 539.4 10.0
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python3 tools/bench/verify_adpcm.py tmp/au_singe.raw
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python3 tools/analysis/32_audio_wire.py tmp/packed_singe.dlxp
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**WHAT IS NEXT.** P6a: ask the chip which formula it runs, from 68000 code.
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---
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## Session 32: the packed player runs, and the write window turns out to be the frame
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