DT-FM for Digitakt

DT-FM is a four-operator FM synth machine for the original Digitakt (Mk1), OS 1.53 and 1.54. It replaces the earlier Sophie-based voice with a dedicated FM engine and keeps Digitakt's regular AMP, filter, mixer and effects path. The source, not a modified Elektron OS, is what this repository distributes.

Changelog

• 1.2.1:
  • Added support for parameter locks on SRC page (#1)
  • Fixed an issue with visual feedback of operator parameters (#2)
• 1.2.0: Compatibility changes for cooperation with other Elekloader mods
• 1.1.0: Added support for OS version 1.54
• 1.0.1: Fixed operator values changing after a power cycle
• 1.0.0: Initial release. Replaced the Sophie voice code with a dedicated 4-op FM machine

USE AT YOUR OWN RISK. This modifies your instrument's firmware; back up your projects and sounds, and keep a stock OS file for recovery.

In hardware testing, four DT-FM tracks playing together used about 95% CPU without FAST AUDIO and about 85% with it. Eight-track operation is not claimed.

AI Disclaimer

This fork of digisophie has been created with the assistance of LLM agents. I don't know how digisophie was made, but pretty much all commits after 961c39ce have been "vibe-coded". That is to say, a human provided the prompts, design decisions, and performed testing on the hardware, but all of the heavy lifting, DSP coding, assembly hacking, etc., was done by an AI. Heck, all but this disclaimer have been generated and/or modified by AI. I make no promises to proofread or improve anything, so use this modified firmware at entirely your own risk. I am aware of the ethical implications of using AI and fully deserve any criticism related to that. However, I already am basically forced to use AI at work, so my hands are already dirty, so to speak. I am not proud of outsourcing my thinking and learning to an LLM, but if it were not for these tools, stuff like this probably just wouldn't get made. My idea with releasing this is to minimize the damage caused by this foolish endeavour by sharing my results with others, so that they don't have to go and do the same thing.

I have the utmost respect for people that do low-level hacking and write audio code - you are probably some of the smartest people out there. I am not one of you. I'm just an idiot who wanted a digitone but couldn't afford one at the moment, and seeing the newly-released custom firmware made for the MK1, wondered if it might be possible to make an FM synth on the DT.

Controls

Each sound keeps all four operators: the one OP shows in its normal parameters, the other three in parameter slots the stock OS leaves unused. Copying a sound, track or pattern brings all four, while pasting only the SRC page brings just the one OP shows. They are saved with the project, in a part of each pattern's stored kit that the stock OS neither writes nor reads, and survive a power cycle like any unsaved edit. The sound pool keeps only the operator OP shows: a pool sound's other three start at defaults, as in a project saved without DT-FM. A project from the earlier build that kept them per track gives every pattern its tracks' old values. Every SRC control can be parameter-locked. A trig's RATIO, LEVEL, ATTK and DECAY locks change the operator OP selects on that trig: lock OP as well to pick it, or they change the operator OP shows when the trig plays. A track switched to DT-FM starts with every operator at its default. The normal AMP page controls the note envelope: set HOLD to NOTE for TRIG LEN to determine when the release begins. Use a finite DEC to hear that release. DEC INF can keep the sound going indefinitely. Retriggering chokes the previous voice on that track, with a brief transition to suppress a click.

Algorithms

ALGO's knob draws the selected algorithm instead of a dial. Each square is an operator, and modulation flows downwards: an operator modulates the one it is joined to below it. The operators standing on the bar along the bottom are the carriers, the ones you hear. The others are modulators, whose LEVEL sets modulation depth rather than volume. A shared line joins several modulators into one operator, as in algorithm 4, or spreads one modulator to several, as in algorithm 6.

The screen does not number the squares. These diagrams show the same layouts with operator numbers; > means "modulates":

1  4>3>2>1     2  (3+4)>2>1   3  (2+4>3)>1   4  (2+3+4)>1

    4
    │
    3            3   4              4
    │            └─┬─┘              │
    2              2            2   3          2 3 4
    │              │            └─┬─┘          └─┼─┘
    1              1              1              1
  ═════          ═════          ═════          ═════

5  2>1, 4>3    6  4>(1,2,3)   7  1,2,4>3     8  1,2,3,4

  2   4            4                4
  │   │          ┌─┼─┐              │
  1   3          1 2 3          1 2 3          1 2 3 4
  ═════          ═════          ═════          ═══════

Operator 4 also has feedback, set by CHAR; the diagrams do not draw it.

Install: no compiler required

You need only the prebuilt DT-FM mod for your OS, dt-fm-1.2.1-os1.53.elemod or dt-fm-1.2.1-os1.54.elemod from the 1.2.1 release, elekloader (0.4.0 or later for OS 1.54), and your own original Digitakt Mk1 OS 1.53 or 1.54 .syx. DT-FM is also in elekloader's shop, so on elekloader's web page you can add it from the library instead of downloading it. The .elemod contains this project's code, not Elektron's firmware. You do not need ColdFire tools, Python, or a source checkout to install it.

1. Open elekloader and select your stock OS using Add its stock OS file.
2. Choose Add from the library and add DT-FM, or choose Add your own .elemod and select the .elemod file for your OS. elekloader refuses the file made for the other OS. Enable DT-FM. elekloader's built-in core 3.0 should enable with it. If your elekloader has an older core or shows a dependency error, update elekloader before building.
3. Optional but highly recommended: install and enable the bundled digihealth diagnostic for your OS too. This adds the FAST AUDIO feature available from the settings menu. Enabling this allows several DT-FM tracks to play simultaneously without freezing the Digitakt.
4. Wait for elekloader's Ready to build check, then choose Build Firmware. Save the generated .syx on your computer.
5. Send that .syx to the Digitakt with Elektron Transfer

Elekloader builds and verifies the OS; Elektron Transfer does the actual upload to the instrument. Do not power off during the update. Neither the stock nor modified OS file belongs in this repository.

Build the mods from source (developers only)

ColdFire tools are needed only to change or recompile the mod. You need Python 3.9+, a source checkout of elekloader, a ColdFire cross-toolchain (m68k-linux-gnu- or m68k-elf- assembler, GCC and linker), and your own stock OS 1.53 or 1.54 file. For 1.54, use elekloader 0.4.0 or later. Git ignores firmware/, a convenient place for the stock file. From this repository:

ELEKLOADER_CROSS=m68k-elf- sh scripts/build.sh \
  firmware/Digitakt_OS1.54.syx /path/to/elekloader

This builds core 3.0, machine-pages 1.1, DT-FM and the optional diagnostic from source for the OS of the file you give it, lints the combination, and writes the verified custom OS to out/Digitakt_OS1.54_DT-FM_S034.syx. Your elekloader checkout must have core 3.0 and machine-pages 1.1; the compatible source tree is tagged kit-v0.5.0. This is how to run 1.2.0 before an elekloader release can. The mods are named for the OS too: out/dt-fm-1.2.0-os1.54.elemod and diagnostics/digihealth/out/digihealth-1.0.1-os1.54.elemod (1.53 for a 1.53 file). To test DSP alone, run make test; make cross-check additionally compiles for ColdFire. Optional emulator probes in tests/ require digiemu.

On Windows with Docker Desktop, the container provides the compiler and elekloader checkout. Provide your own stock OS file; it is mounted read-only and is not included in the image:

.\scripts\docker-build.ps1 .\firmware\Digitakt_OS1.54.syx

The verified custom OS and .elemod files are written under out/ and diagnostics/digihealth/out/ in this checkout. Docker must be running.

Use and recovery

This changes firmware on the instrument. Back up projects and sounds first, check that your stock OS is OS 1.53 or 1.54 for the original Digitakt, and keep that stock file for recovery. If you install the optional diagnostic, FAST AUDIO is off by default; SYSTEM INFO and FAST AUDIO can be enabled separately in SETTINGS. See diagnostics for the monitor.

DT-FM is independent of, and not endorsed by, Elektron. It contains no Elektron firmware or samples.

Source layout

The Digitakt addresses DT-FM uses are kept apart from the code that uses them: src/os153.inc / src/os154.inc for glue.s, src/os153.h / src/os154.h for digitakt.c. The OS 1.54 build defines OS154 (src/mod.json, under ports), which picks the 1.54 files and patch sites. The bundled digihealth is split the same way.

Source and licenses

The DT-FM implementation is MIT licensed. Third-party attribution and the separate GPL-2.0-or-later digihealth source are documented in THIRD_PARTY.md. The digihealth license is also included in its source directory.

The DSP reference records the parts of the stock OS that DT-FM uses and the renderer's design.