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

tools/bench/probe_adpcm3.lua

@@ -0,0 +1,24 @@
+-- Sweep the PPI's port C -- which is where the X68000 puts ADPCM pan and the
+-- chip's clock divider -- and feed a loud burst under each value, so that the
+-- WAV says which value un-mutes the chip. Nothing here is assumed: the segment
+-- boundaries are printed and the analysis reads the wave against them.
+M = manager.machine
+local sp = M.devices[":maincpu"].spaces["program"]
+local CTRL, DATA, PPIC = 0xE92001, 0xE92003, 0xE9A005
+local SEG = 40            -- host frames per segment
+local n = 0
+SUB = emu.add_machine_frame_notifier(function()
+  n = n + 1
+  if n <= 20 then return end
+  local k = n - 20
+  local seg = math.floor((k - 1) / SEG)
+  local off = (k - 1) % SEG
+  if seg > 15 then print("[AD] done"); M:exit(); return end
+  if off == 0 then
+    sp:write_u8(PPIC, seg)
+    sp:write_u8(CTRL, 1)
+    print(string.format("[AD] seg %2d portC=$%02X starts at host frame %d", seg, seg, n))
+  elseif off <= 30 then
+    sp:write_u8(DATA, 0x77)          -- two loud positive nibbles
+  end
+end)