# SLOOP turns the M-VAVE FM-1 into a four-track groovebox

> SLOOP is GPL-3.0 firmware for the M-VAVE FM-1 that replaces the hardware's fixed functions with three synths, a drum machine, twelve synthesis engines and 153 sounds, plus MIDI, USB audio and a browser-based editor. It is still labelled beta, it is written in C, and it is built on Felucca by Leo Kuroshita.

**isod89/sloop-fm1** — A live groovebox firmware for the M-VAVE FM-1

- Repository: https://github.com/isod89/sloop-fm1
- Stars: 620 · Forks: 85
- Language: C
- License: GPL-3.0
- Published: 2026-10-22 · Updated: 2026-10-22 · Language: en
- Canonical page: https://hysenlabs.com/projects/isod89-sloop-fm1

## What the firmware replaces

SLOOP turns the FM-1 into a four-track groovebox you play live: three synths and a drum machine with 16 sounds on the white keys, twelve synthesis engines, 153 sounds and 37 drum kits.

The device underneath is the M-VAVE FM-1, a small hardware groovebox, and SLOOP is replacement firmware for it rather than a companion app. The README's framing is that it does not pick a style for you: house, techno, hip-hop, trap, drum and bass, amapiano, synthwave, lo-fi, ambient and chiptune are all listed as things it can do, and the point of the list is that none of them is a preset mode you select. The line that captures the philosophy is No factory patterns, nothing to load: everything you hear, you play.

That is a real design position and it has consequences. A device with factory patterns can be picked up and played by anyone. A device where everything is played from the keyboard needs a person who knows what a sound is doing. SLOOP is therefore a firmware for a player, and the hundreds of sounds in it are there to be chosen from rather than loaded as songs.

The base it is built on matters too. SLOOP is based on Felucca by Leo Kuroshita of Hügelton Instruments, and the version history repeatedly credits Felucca releases as the source of new capability: the fifteen arp modes came from Felucca 1.5.1, the INSERT effects and the ANALOG filter from Felucca 1.4 and 1.5, and the safer handling of the second core follows Felucca 1.6.1. So the firmware is a layer on top of an existing engine, and the two projects move together.

The licence is GPL-3.0, the primary language is C, and the last push came a few days after the v2.7.1 release.

## Two cores and a crash worth reading about

The 2.7 release is the one with the most interesting engineering story, and the 2.7.1 fix is worth understanding because it explains a behaviour you would otherwise find confusing.

The FM-1 has two processor cores. Since 2.7 the sound runs on both of them, which the README describes as about twice the room, so many voices and INSERT effects no longer crackle. It is on by default and can be turned off from the HOME menu under SYSTEM and 2ND CORE.

Then 2.7.1 fixed a crash that the feature created. When the second core did not finish its share of the sound in time, SLOOP stops it and carries on on one core, but stopping it raised a false alarm on the chip, and the FM-1 showed a red FELUCCA CRASH screen and restarted. In 2.7.1 it holds the second core the safe way, after Felucca 1.6.1, so the result is a short glitch and the music goes on.

The crash screen also changed. It now says more, including the stack and the second core's state, and it stays on screen for ten seconds, long enough for a photo. That is a small thing that tells you something about the project: a crash you cannot photograph is a crash you cannot report, and the debugging posture is aimed at users rather than at a lab.

The repository has a DUALCORE.md at the top level, which is where the fuller account of this two-core arrangement would live, along with RAM.md and SLOOP.md for the manual and BUILDING.md for the build. Having per-topic documents like that, rather than one long README, is a sign the project has grown past a single page of documentation.

## The synthesis engines and what 2.7 added

The engine list is where the musical scope sits, and the recent releases are what give it breadth.

2.7 brought physical models, covering guitars, sitar, bells and hand drums, and a noise engine. It also brought six-operator FM with DX7 patches, which is the closest thing to a Yamaha DX7 emulation you will find on a device of this size, and parameter locks plus micro timing, which is how you nudge a single hit off the grid without moving the whole pattern.

Earlier additions stack up fast. Version 2.6 added 27 punch-in effects, eleven of them on the black keys, including KILL LOW, MID, HIGH, REWIND, OCT UP, FLANGER, AUTO-PAN, FREEZE, LOOP 8T, MONO and FADE OUT. It also added an INSERT effect on every track with a choice of SOFT, HARD, FOLD, FUZZ, CRUSH, PHASR, FLANG and CHOR, each with dry and wet, plus a second LFO that can be tempo-synced or free-running, ENV SYNC so that attack, decay and release become note values, an ANALOG filter in LP, BP or HP, a note's length in steps, per-step velocity, and MIDI LEARN.

The physical models are the part that changes what the device feels like, because a guitar or a bell model responds to velocity and to the note length in a way an FM patch does not. That, combined with per-step velocity and the punch-in effects on the black keys, is what turns a sound into something you can perform with rather than select.

Two smaller 2.7 details are worth knowing because they affect the hardware rather than the software. The synthesised kits' kicks were a 50 Hz sine, which the FM-1's own speaker, a phone or earbuds cannot play, so only a quiet click was left; they now have the drive and pitch sweep that carry them into that range. And the supplies at M-VAVE's own levels are steadier on two cores, following X0X 1.0.9, which is the kind of detail that only appears when someone has run the firmware on real hardware for a long time.

## Playing it: sections, arpeggios and the hardware controls

The live playing model is built around four sections and a lot of keyboard shortcuts, and it is worth understanding before you install it.

You play sounds from the white keys, 16 of them, and the black keys carry the punch-in effects when you hold them. There is a KBD HOLD feature, added in 2.7, where HOLD with the arp off holds your chord, and ARP SWG now works stopped too. Fifteen arp modes came in from Felucca 1.5.1, including REPEAT, DNUP, UP+8, CONV, DIVG, PINKY, THUMB, WALK and CHORD, and the arp keeps running when you change the sound underneath it.

The arp ordering rule is a small thing that shows how carefully this was designed: ORD in the order you pressed. The arpeggiator follows the order in which you pressed the keys rather than a fixed pattern, so a phrase you play by hand becomes the sequence the arp repeats.

Sections are the song structure mechanism, and there are four of them, A through D. The 2.7 release renamed the save slots from PROJECT 1 to 4 to SECTION A to D, because the FM-1 holds one song, so keeping several means keeping them as editor backups. The editor's LIVE page can then play a whole show from a computer, tablet or phone, with sections A to D queued on the next bar, a mixer with level, mute, solo, filter, pan, sends and sound per track, the punch-in effects as pads, fills, and computer-key shortcuts.

Drums get their own treatment in 2.7. Every drum sound has a tune and a decay, adjustable on the DRUMS screen under the new SOUND page, and the setting is kept with each section. Live drum mutes arrived in 2.6: EDIT plus a black key, then the sounds' keys. There is also a MUTE mode that stays after EDIT, which came in 2.7 and was a reported issue at #126.

The manual is SLOOP.md, there is a quick start guide in GUIDE.md, a French quick start in DEMARRAGE-RAPIDE-FR.md, and INSTALL-SLOOP.bat at the root for the Windows install path.

## MIDI, USB audio and the browser editor

The connection story got a substantial upgrade in 2.7, and it is the part that turns the device into a piece of studio gear rather than a standalone toy.

The GLO to MIDI 3 mapping covers punch-in effects from MIDI on the FX channel, drums from any of the 16 notes via DR MAP 1 to 16, the 14 buttons over MIDI on REMOTE, and CLOCK OUT on USB at issue #106. MIDI 3 also brings a CC 74 sweep of each engine's own cutoff, credited to @renebohne at issue #71.

MIDI itself is complete enough for a live rig. Version 2.6 added MIDI LEARN, sustain pedal and pitch bend from a MIDI keyboard, with the bend range set from GLO to MIDI 2, and per-track MIDI channels with GLO to MIDI offering CH1, CH2, CH3 and OTHER. The sequencer can also send MIDI out, so the FM-1 can drive another instrument. QUANT can be turned off on the REC screen so your timing stays as nudges rather than being quantised, and overdub works over held notes.

USB audio is there for capture, with a USB RATE option of 44.1 or 48 kHz for phones. The editor can record the set as a WAV file.

The web editor is the second half of the project and it runs in the browser, with the install page, the editor and a theme page that also offers light themes, alongside screen colours set from MENU to COLOR. The editor also has a Samples page for your own samples, including samples that loop while held with LOOP, LOOP+TAIL, zero-crossing loop points, or a WAV's own loop. OPEN-EDITOR.bat at the repository root is the Windows launcher for it.

What makes the editor interesting is the breadth of the LIVE page in 2.7: scenes, of which there are eight and which can be morphed over bars with an A/B crossfader, a PERFORM card that exposes every knob of the track you are playing and its steps, chains, rolls, throws, a MIDI lock, playlists, and a question mark on every card that explains it.

## Building it, licensing it and installing it

You do not build this to use it, but the build path is documented, and it is the same path a contributor would use.

The repository root holds build.sh and build-sloop.ps1, so the build is a shell script on Unix-like systems and a PowerShell script on Windows, with BUILDING.md explaining the details. The tree is organised as firmware/ for the code, web/ for the editor, tools/ for supporting scripts, tests/ for the test suite, docs/ for documentation and assets/ for images. There is also a LICENSE file, a LICENSES/ directory and a LICENSING.md.

The licensing deserves a mention because it is more layered than a single GPL file suggests. The project itself is GPL-3.0, and the LICENSING.md and LICENSES/ files exist to account for the third-party code that Felucca and the browser editor bring in, some of which carries different terms. Since Felucca is a separate upstream project by Leo Kuroshita and the README describes SLOOP as free and open source under GPL-3.0, anyone shipping the firmware or a derivative needs to read LICENSING.md rather than assume the single LICENSE file covers everything.

For a user, installation is a browser download rather than a build. The README's link row points to an install from the browser page, alongside the complete guide, the manual, the French guide, the web editor and the releases. INSTALL-SLOOP.bat in the repository root is the local installer for Windows, and the device has a USB port that the firmware is written to.

The update story is the reassurance that matters for a beta. The README says your projects, presets, samples and settings are kept when you update, and you can go back at any time, with a Going back section explaining how. It also says projects, autosaves and backups from 2.6 load as they are, while a project saved in 2.7 opens in 2.6 without the 2.7 features. That backward compatibility is deliberate and it is what makes it reasonable to try a beta on a device you use.

## Where SLOOP fits, and where it stops

The comparison worth making is with the stock firmware on the FM-1, and with a general purpose groovebox.

Against the stock firmware, the difference is that the stock device does a much smaller set of things well and predictably, while SLOOP gives you a synthesiser, a drum machine and a sampler with a much wider engine list, at the cost of a beta firmware and a nontrivial learning curve of keyboard shortcuts. If you want the FM-1 to be a simple instrument you can trust, the stock firmware is the safer choice. If you want it to be the centre of a live set with dozens of sounds and per-hit editing, there is no other way to get there.

Against a general purpose groovebox, the difference is the web editor. SLOOP pairs a hardware instrument with a browser-based editor that can act as a live show controller from a phone or tablet, with scenes, playlists, a mixer and a recorded WAV of the set. Few hardware grooveboxes have that combination, and the editor also works as a configuration surface for sounds, samples and drum tuning.

The limits are concrete. The project targets exactly one device, the M-VAVE FM-1, so none of this transfers to other hardware. The README states plainly that it is still a beta, install at your own risk, and asks you to report what you find. The crash screen in 2.7.1 was a regression introduced by the two-core feature in the same release, which tells you the code is moving fast enough to introduce and fix faults in days rather than months. And the audio engine is the weak point on cheap hardware: the README's own account of the 50 Hz kicks, which the FM-1's speaker, a phone or earbuds could not play, is a reminder that a small device cannot do everything.

For an FM-1 owner who plays live and is willing to install beta firmware on the device they perform with, this is the project that makes the hardware worth more than its price. For anyone else, it is a well-documented piece of firmware for a specific piece of hardware, released roughly every few days and worked on in public.

## Conclusion

SLOOP suits an owner of an M-VAVE FM-1 who wants to play it rather than patch it, since the whole point of the project is that everything you hear you play, with no factory patterns and no style imposed on the user. It is the wrong tool if you do not own an FM-1, since the firmware targets one specific device, and it is the wrong tool if you need a finished instrument you can rely on at a gig, because the README states plainly that it is still a beta and asks you to install at your own risk. Before updating, read the Going back section, because the README says your projects, presets, samples and settings are kept when you update and you can go back at any time, and note that a project saved in 2.7 opens in 2.6 without the 2.7 features.

## FAQ

### What is SLOOP?

SLOOP is free and open source firmware, licensed GPL-3.0, for the M-VAVE FM-1. It turns the device into a four-track groovebox with three synths and a drum machine, twelve synthesis engines, 153 sounds, 37 drum kits, your own samples, a song mode, USB audio, MIDI in and MIDI clock. It is based on Felucca by Leo Kuroshita of Hügelton Instruments.

### Is SLOOP stable enough to install on my FM-1?

The README says plainly that it is still a beta and asks you to install at your own risk and report what you find. It also notes that your projects, presets, samples and settings are kept when you update, and that you can go back at any time, and that a project saved in 2.7 opens in 2.6 without the 2.7 features.

### What did SLOOP 2.7 add?

Version 2.7 runs the sound on both of the FM-1's two cores so many voices and INSERT effects no longer crackle, adds fifteen arp modes from Felucca 1.5.1, KBD HOLD, drum tune and decay, favourite sounds, looping samples, a QUANT off option, MIDI 3 with clock out on USB, your own screen colours, a LIVE page in the editor, USB RATE selection and SECTION A to D save slots. Version 2.7.1 then fixed the crash screen that the two-core feature could trigger.

### How do I install SLOOP on the M-VAVE FM-1?

The README points to an install from the browser page for the device, and the repository contains INSTALL-SLOOP.bat for the Windows install path and OPEN-EDITOR.bat for launching the web editor. The manual is SLOOP.md, with a complete guide in GUIDE.md and a French quick start in DEMARRAGE-RAPIDE-FR.md.

### What synthesis engines does SLOOP offer?

The README lists twelve synthesis engines, and recent versions added physical models for guitars, sitar, bells and hand drums, a noise engine, and six-operator FM with DX7 patches. Version 2.6 added an INSERT effect on every track, a second LFO, ENV SYNC, an ANALOG filter as LP, BP or HP, per-step velocity and MIDI LEARN.

### Can I use SLOOP as a MIDI controller or with USB audio?

Yes. The 2.7 release maps punch-in effects to MIDI on the FX channel, drums from any 16 notes through DR MAP, the 14 buttons over MIDI on REMOTE, and adds CLOCK OUT on USB. Each track can have its own MIDI channel, the sequencer can send MIDI out, and USB audio supports 44.1 and 48 kHz rates with the set recordable as a WAV file.

## Sources

- [isod89/sloop-fm1 on GitHub](https://github.com/isod89/sloop-fm1)
- [Issues](https://github.com/isod89/sloop-fm1/issues)
- [License: GPL-3.0](https://github.com/isod89/sloop-fm1/blob/main/LICENSE)
- [README](https://github.com/isod89/sloop-fm1/blob/main/README.md)
- [Releases](https://github.com/isod89/sloop-fm1/releases)

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Hysen Labs editorial analysis, written from the project's own repository and release notes. Cite the canonical page: https://hysenlabs.com/projects/isod89-sloop-fm1