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/check.sh

@@ -735,4 +735,22 @@ else
   echo "  SKIPPED: no x68000 romset -- the player was not run"
 fi
 
+echo "--- session 33: AUDIO -- the encoder, and what it does to the wire (FINDINGS 65) ---"
+# ROADMAP P6, everything in it except the bus half session 20 closed. The audio
+# is the SAME WINDOW as the frames -- 00223 from 539.4 s for 10 s -- because an
+# audio stream that is not the same seconds as the picture is not this project's
+# audio, and a gate that lets the two drift apart would never say so.
+[ -f tmp/au_singe.raw ] || python3 tools/encoder/extract_audio.py 00223 tmp/au_singe.raw 15625 539.4 10.0
+# There is NO ffmpeg encoder for this format -- adpcm_ima_oki is decode-only --
+# so the encoder cannot be checked against a reference. What is checked is that
+# the decoder our encoder runs in its own loop IS ffmpeg's, sample for sample.
+# An encoder that agrees with its own wrong decoder is the failure this catches.
+python3 tools/bench/verify_adpcm.py tmp/au_singe.raw || exit 1
+# And the container arithmetic. The interesting line is the padding: a packed
+# record has no index BY DESIGN, so audio has to ride a fixed cadence, and the
+# obvious cadence throws away a third of every audio sector.
+python3 tools/analysis/32_audio_wire.py tmp/packed_singe.dlxp \
+  > tmp/audio_wire.log 2>&1 || { cat tmp/audio_wire.log; exit 1; }
+grep -aE "^  (   1|  11|  81) |THE FLOOR|F=1 |F=11|is ZERO|SOUND IS WHAT" tmp/audio_wire.log
+
 echo "ALL GREEN"