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Put sound on the wire, and find three LSBs are worth 25 dB

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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
This commit is contained in:
prosolis
2026-08-25 09:11:03 -07:00
parent 6f698ca226
commit f925a1dd9a
12 changed files with 1069 additions and 6 deletions
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tools/bench/probe_adpcm.lua
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-- Ask MAME what the x68000's ADPCM device is, and where the 68000 reaches it.
-- FINDINGS 64.4 recorded that no MAME source tree is on this machine; this is
-- the way to ask the same question without one.
M = manager.machine
local done = false
SUB = emu.add_machine_frame_notifier(function()
if done then return end
done = true
print("[AD] === devices whose tag looks like an ADPCM chip ===")
for tag, dev in pairs(M.devices) do
local t = tag:lower()
if t:find("adpcm") or t:find("oki") or t:find("msm") or t:find("6258") then
print(string.format("[AD] DEV %-24s shortname=%s", tag, tostring(dev.shortname)))
end
end
local sp = M.devices[":maincpu"].spaces["program"]
print("[AD] === program map, $E90000..$EA0000 ===")
local ok, err = pcall(function()
for _, e in ipairs(sp.map.entries) do
if e.address_start >= 0xE90000 and e.address_start < 0xEA0000 then
print(string.format("[AD] MAP %08X-%08X", e.address_start, e.address_end))
end
end
end)
if not ok then print("[AD] MAP unavailable: " .. tostring(err)) end
print("[AD] done")
M:exit()
end)