Public copy of MrScruff's firmware modifications for the Analog Rytm MKII, redistributed with permission. Released as-is, responses to issues should not be expected.

rytm-mods

Firmware modifications for the Elektron Analog Rytm MKII, OS 1.73.

Every build starts from the pinned stock image and rebuilds from source. The .syx is an artifact; the patch sources, the symbol map and the allocation registry are the project.

Same conventions as octatrack-mods, same registry and verification discipline, different device. Nothing is shared between the two trees.

Layout

path	what
`stock/`	checksum of the pinned stock firmware, which you supply. Never written to.
`re/`	cumulative reverse-engineering knowledge: `symbols.toml`, subsystem notes, Ghidra bootstrap
`mods/NNNN-slug/`	one modification: manifest, stub source, design rationale
`requirements.txt`	Python packages for the mod 0006 and 0013 calibration and measurement tools
`registry/`	`allocations.toml` - every claim on cave space and detour sites
`tools/`	build, layout validation, verification, symbol generation, cave scan; capture off the device (`mix_capture.py`); mod 0013 measurement (`voice_dac_*.py`)
`vendor/`	third-party code that is executed (elektron-firmware-tool), fetched by `make setup`
`docs/`	user manual, flashing, hazards, calibration
`build/`	artifacts, gitignored

Build

Prerequisites:

Python 3.11 or newer (build and verify use only the standard library)
git, make and a C compiler, for the vendored container tool
m68k-elf-binutils (brew install m68k-elf-binutils on macOS)
a Ghidra install, only for make ghidra

The stock firmware is not included. Download Analog Rytm MKII OS 1.73 from Elektron's support site and place the .syx at stock/Analog-Rytm_MKII_OS1.73.syx. The build refuses any file whose checksum differs from stock/SHA256SUMS.

make setup     # venv + Python deps, clone and build the vendor tool at its pinned commit
make verify    # build + assert the result

Produces build/Analog-Rytm_MKII_OS1.73_<tag>.syx, the only delivery format this device has - there is no CF-card .bin equivalent. The default build's tag is 0000_0002_0003_0004_0006_0008_0009_0010_0012_0013_0014_0015. See docs/FLASHING.md to install it.

Build a subset for hardware bisection:

.venv/bin/python tools/build.py --mods 0000-shared 0002-euclid-accents

The container

Unpacks and repacks with the vendored tool, all checksums verified, five sections:

id	name	size	load base	we touch it
5	meta	15 B raw	-	no
2	bootstrap	42,302	0x07020000	no - see `docs/HAZARDS.md`
1	FPGA	149,516	0x00000000	no
3	MAIN OS	3,005,432	0x40000400	yes, this is the whole project
8	?	159,948	0x00000000	no

MAIN OS is ColdFire big-endian, as on the Octatrack, but it is a C++ build with its Itanium-ABI symbol names left in (ParameterPageView::changeParameterLockValue, AnalogRytm::logicalParamID_t, Kit::updateMirror). Most of the map can be recovered by demangling rather than by guessing.

Section 8 is new in 1.73, little-endian ARM Thumb rather than ColdFire, and carries {"version": "1.00E", "release": "0018"}. 1.73's only listed improvement is "[MKII] Added Outbox 8 configuration support", so it is almost certainly the Outbox 8's own firmware riding along - inferred, not proven.

make verify asserts that sections 5, 2, 1 and 8 come back out of our rebuilt .syx byte-identical to stock, so a mistake can only ever reach MAIN OS.

Mods

The default build enables all of these. Retracted mods and one-off hardware probes are not part of this package; the pool notes in registry/allocations.toml still record what those probes established.

id	title	status
0000	The shared runtime other mods call into	built
0002	Euclidean generator places accents instead of trigs	hardware-verified
0003	Per-track random velocity offset	built
0004	Two per-sound random modifiers on a second LFO page	built
0006	Scrolling compressor gain-reduction estimate	hardware-verified
0008	Filter frequency shown in Hz	hardware-verified
0009	Main output RMS indicator on the compressor VOL label	built
0010	NO + PLAY toggles between the edited kit and the saved one	hardware-verified
0012	SLICE LOOP in the track menu: sixteen trigs slicing a one-bar loop	hardware-verified
0013	Voices to MAIN through the DAC, with a per-sound filter	hardware-verified
0014	CTRL ALL: TRK + knob turns that parameter on every sound track	hardware-verified
0015	ENV RESET reaches AMP and AMP+FILT	hardware-verified

Status ladder: re-only -> built -> hardware-verified, plus retracted. Only hardware-verified counts.

Adding a mod

Add any new addresses to re/symbols.toml with an honest status.
Claim cave space and detour sites in registry/allocations.toml, recording the stock bytes at each detour host in expect. A mod may hold several cave claims, each naming the section it holds (.text by default): the pools are separate regions of the image and can never merge, so anything larger than the biggest free run has to be split, and the split belongs in the registry rather than being implied by sizes.
Write mods/NNNN-slug/{mod.toml,stub.s,design.md}.
make verify, then flash that mod alone.

The expect bytes are the load-bearing guard: if an address is wrong or two mods collide, the build aborts instead of quietly patching the wrong place.

Before the first mod ships, the cave pool has to be earned: make cavescan reports the candidates, registry/allocations.toml explains why the current one was chosen, and it stays status = "candidate" until it has been proven untouched at runtime on hardware. octatrack-mods inherited a pool it had not checked and lost a round trip to it.