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/encoder/extract_audio.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+"""Extract the audio of a Blu-ray window as mono PCM at an MSM6258 sample rate.
+
+The video side of this window is tools/encoder/extract.py; the arguments mean
+the same things and are meant to be given the same values, because an audio
+stream that is not the same seconds as the frames is not this project's audio.
+
+The disc is AC-3 5.1 at 48 kHz. The arcade original is MONO, so this downmixes
+-- ffmpeg's default matrix, dialogue from the centre channel included -- and
+resamples to the chip's rate. Nothing here shapes, gates or normalises the
+level: what the ADPCM encoder is handed is what the disc has, so that the SNR
+it reports is the codec's and not a gain stage's.
+"""
+import getpass, os, subprocess, sys
+
+BDROM = os.environ.get("DLX_BDROM") or f"/media/{getpass.getuser()}/BDROM"
+STREAM_DIR = f"{BDROM}/BDMV/STREAM"
+
+# 8 MHz / {512, 768, 1024}. The chip has no other rates and 15,625 is the one
+# every budget in this project is written against (FINDINGS 52).
+RATES = {15625: 512, 10417: 768, 7813: 1024}
+
+
+def extract(stream, out, rate=15625, start=None, dur=None):
+    if rate not in RATES:
+        raise SystemExit(f"{rate} is not an MSM6258 rate: {sorted(RATES)}")
+    src = f"{STREAM_DIR}/{stream}.m2ts"
+    cmd = ["ffmpeg", "-v", "error"]
+    if start is not None: cmd += ["-ss", str(start)]
+    if dur is not None:   cmd += ["-t", str(dur)]
+    cmd += ["-i", src, "-vn", "-ac", "1", "-ar", str(rate),
+            "-f", "s16le", "-acodec", "pcm_s16le", out, "-y"]
+    subprocess.check_call(cmd)
+    n = os.path.getsize(out) // 2
+    print(f"{stream}: {n} samples @ {rate} Hz mono = {n/rate:.3f} s -> {out}")
+    return n
+
+
+if __name__ == "__main__":
+    # extract_audio.py <stream> <out.raw> [rate] [start_s] [dur_s]
+    stream, out = sys.argv[1], sys.argv[2]
+    rate  = int(sys.argv[3])   if len(sys.argv) > 3 else 15625
+    start = float(sys.argv[4]) if len(sys.argv) > 4 else None
+    dur   = float(sys.argv[5]) if len(sys.argv) > 5 else None
+    extract(stream, out, rate, start, dur)