Before you flash · SLOOP guide

1.1Basics

What SLOOP is

SLOOP replaces the FM-1’s own firmware with a groovebox you play live. Instead of one FM synth, you get four tracks: three synth tracks and a drum track, each with its own sound, pattern and colour. Everything is played in by hand — there are no factory patterns to load.

What’s inside

Three synth tracks that share eight voices, and a drum track with 16 sounds on the white keys (Four tracks, four colours).
Twelve synthesis engines behind 153 factory sounds, from analogue-style and six-operator FM with DX7 patches to samples, physical models and noise (The twelve engines).
37 drum kits, plus kits and instruments made from your own samples (The 37 drum kits, and yours, Your own samples).
A 64-step sequencer per track with parameter locks, micro timing and fills (How the sequencer works).
Layers: hold a button and the keys and knobs change job — punch-in effects, erase, note repeat, mutes, song sections (Tap for pages, hold for a layer).
MIDI in on the jack and over USB, MIDI clock, USB audio and a browser-based editor (Where MIDI comes in, The web editor).

SLOOP is free and open source under GPL-3.0, written by SLOOP’s developer on top of Felucca by Leo Kuroshita / Hügelton Instruments, which supplies the engines, sequencer, editor and installer it grew from. It is an independent project: not made, supported or endorsed by M-VAVE. The code, releases and issue tracker live at github.com/isod89/sloop-fm1.

This guide describes SLOOP 2.8.5, released on 4 November 2026. For the official firmware, and for other custom firmwares people run on the FM-1, see the FM-1 guide’s custom-firmware topic.

From the community

Sloop - Firmware for the M-VAVE FM-1 Complete tour
3dSam
A seven-minute tour of SLOOP 2.2 from 3dSam, the channel behind SLOOP, played on SLOOP itself. Much has been added since, but the layers it shows work the same way.

1.2BasicsGUIDE.md §3, §26

Reading this guide

Almost everything in SLOOP is “hold one button, then press a key or turn a knob”. This guide writes that with a plus: [FX] + [KEY 3] means hold FX, and while it is still down press white key 3. [GLO] + ((KNOB 2)) means hold GLO and turn KNOB 2. Where a combination only works in one place, the place is given alongside it — on the drum track, while stopped, on the NOTES page.

Tap: Press a button and let go quickly — within about half a second — without touching anything else. A tap opens the button’s pages.
Hold: Keep the button down. On seven buttons this opens a layer for as long as you hold it (Tap for pages, hold for a layer).
Turn: A knob, shown like PRESETS or KNOB 1–4, the four dials under the screen.
GO value: A setting that does something rather than storing a number: LOAD, SAVE, ERASE, NEW, CLRSQ, INIT. One click shows AGAIN: … on the title line; a second click within about 1.5 s carries it out. Wait and nothing happens (Pages, the SELECT knob and GO values).
Screen names: Pages, parameters and messages appear as they read on the FM-1: SWING, GLO → MIDI 2, AGAIN: SAVE.

The white keys by number

Row of tiles	First	Second	Third	Fourth
Row 1	`KEY 1` F3	`KEY 2` G3	`KEY 3` A3	`KEY 4` B3
Row 2	`KEY 5` C4	`KEY 6` D4	`KEY 7` E4	`KEY 8` F4
Row 3	`KEY 9` G4	`KEY 10` A4	`KEY 11` B4	`KEY 12` C5
Row 4	`KEY 13` D5	`KEY 14` E5	`KEY 15` F5	`KEY 16` G5

The 16 white keys are numbered from the lowest, F3, to the highest, G5. Inside a layer they become 16 tiles in four rows of four, so the rows above are also how the screen lays them out. The 11 black keys keep their note names: F#3 G#3 A#3 C#4 D#4 F#4 G#4 A#4 C#5 D#5 F#5. “The first four black keys” are F#3 to C#4; in chord mode they are named without an octave, as F# G# A# C# D# (CHORD+: the black keys change the chord).

From the community

SLOOP 2.8.5 — The Complete Guide (GUIDE.md)isod89 — SLOOP’s own reference in 26 chapters, with a one-page cheat sheet at the end. Every section here names the chapters it was written from.

1.3BasicsGUIDE.md §21, §25

What you give up while SLOOP is installed

Installing SLOOP doesn’t add to M-VAVE’s firmware — it takes its place. Know what goes with it before you flash.

The official firmware is gone while SLOOP runs. Its FM sound engine, its presets and the features V16 added are not available until you go back to stock.
No Bluetooth MIDI. SLOOP never switches the FM-1’s radio on, so wireless MIDI doesn’t work. MIDI comes in over USB and the 3.5 mm jack instead (Where MIDI comes in).
No USB keyboard plugged straight in. The FM-1 and a USB keyboard are both USB devices, and a USB link needs a host — a computer, phone or USB MIDI host box — between them. A keyboard with a MIDI output works on the jack.
Your SLOOP work stays in SLOOP. The official firmware can’t open SLOOP’s projects, as SLOOP’s developer points out in issue #186. Save a backup in the The web editor before you go back, and restore it when you reinstall SLOOP (Backup and restore).

Not everything FM is lost: SLOOP’s FM6 engine is six-operator FM and imports DX7 patches and banks as SysEx (DX7 patches and the FM6 editor).

1.4BasicsGUIDE.md §1, §21, §22

What you need

SLOOP’s installer and editor are web pages that talk to the FM-1 over Web MIDI with SysEx, so the browser matters more than the operating system.

You need	Details
A browser	Chrome or Edge, on Windows, macOS or Linux — the browsers SLOOP supports for Web MIDI with SysEx.
A cable	A USB data cable, plugged directly into the computer, with no hub in between.
For USB audio	Any computer, phone or tablet that accepts a class-compliant USB audio input; no driver needed (Recording over USB audio).
For the MIDI IN jack	A TRS-to-DIN MIDI adapter for the 3.5 mm jack. If nothing plays, try the other type, A or B (Where MIDI comes in).
Not supported	Bluetooth MIDI, and a USB keyboard plugged straight into the FM-1 (What you give up while SLOOP is installed).

The installer steps in SLOOP’s README assume a computer. The editor can also run as an installed app on Android (The web editor).

From the community

1.5BasicsGUIDE.md §21, §25

Known risks and owner reports

These are the risks on record as of November 2026, from SLOOP’s own documents and its GitHub issues. The issues are reports from individual owners, not confirmed faults: each one links to the thread so you can read it, and any later replies, yourself.

No Bluetooth MIDI. SLOOP never turns on the radio, so wireless MIDI is unavailable while it is installed. Source: the README’s compatibility table and SLOOP’s guide, chapter 21.
A new unit that went dark mid-install — issue #185, opened 20 October 2026, open. An owner installing from Chrome on Windows onto a new FM-1 shipped direct from China reported that the screen went black after Verify 2 and Windows no longer saw the unit; holding OCT- at power-on didn’t help. SLOOP’s developer replied that the install had stopped before anything was written, so SLOOP was never on the unit and its rescue modes couldn’t work; the reply pointed at the stock firmware or bootloader on that unit and suggested checking Device Manager and trying another cable, port and PC. Another commenter linked a transporter unbrick guide.
Return to stock stalled — issue #186, 20 October 2026, open. With SLOOP 2.7.1 on Windows 11 and Chrome, the in-browser return to stock V15 stopped at Verify 75 (“the update loader did not appear”) and the unit restarted into SLOOP. The developer said it was not bricked and recommended M-VAVE’s M-UPGRADE. In the same thread another owner reported a unit that stopped responding after an install from a phone was interrupted when the phone went to sleep. SLOOP 2.8 then removed the in-browser return to stock.
Install fails on a Mac — issue #239, 2 November 2026, open, no reply yet. Installing over stock V16 from a Mac, in Chrome and in Edge, failed every time with “the loader was disconnected after 3 requests”. After each failure the FM-1 started normally on stock V16.
M-UPGRADE error going back — issue #249, 8 November 2026, open, no reply yet. Going back from 2.8.5 to the official V16 with M-UPGRADE failed with “error 258”, a MIDI receive timeout while preparing.
The UC25HQ80 flash chip — fixed in 2.8.5. Earlier versions couldn’t save or update on units with this chip (What you need; source: SLOOP.md).
Last resort. If an FM-1 no longer starts at all, SLOOP’s README points to FM-1-transporter, a community tool that reads and writes the FM-1’s flash through a small RP2040 or RP2350 board wired to its USB lines. It needs that board, three wires and some care.

From the community

1.6BasicsGUIDE.md §1

Installing SLOOP

The recommended route is SLOOP’s browser installer. Nothing to download or compile: the page carries the firmware and sends it over USB.

If SLOOP is already on the FM-1, save a backup in the editor first (Backup and restore).
Close every other app or browser tab that uses MIDI — a DAW, M-UPGRADE, another editor tab.
Open the installer in Chrome or Edge on a computer.
Connect the FM-1 with a USB data cable, straight into the computer — no hub.
Press INSTALL and allow MIDI access when the browser asks.
Wait for Done. The FM-1 restarts and shows the SLOOP logo.
Unplug the FM-1 and plug it back in once, so the computer finds its USB audio input.

Other ways

Python: download the .fwsc file from a release and install it with the tools/fm1_install.py script from SLOOP’s source; it needs the mido and python-rtmidi packages.
Build it yourself: with the JieLi toolchain and files from the AC79 SDK, ./build.sh on Linux or macOS, or INSTALL-SLOOP.bat on Windows with WSL, builds the firmware and serves the installer and editor locally on http://localhost:8766. Keep the build window open until the install is done.

Two checks guard every install. The installer verifies the package’s SHA-256 before writing anything, and SLOOP’s update loader checks its CRC again before starting the new firmware; a damaged package is refused before anything is written. Since 2.8 the installer recognises the FM-1’s update loader by its handshake, so it still finds it when Windows renames it to a generic MIDI port.

From the community

Using the M-VAVE FM-1 with SLOOP Custom Firmware | First Go
Darren Lock Music
A first session from early October: jamming with SLOOP, first impressions and a look at the browser editor. By its own description, a first go rather than a full review.

SLOOP installerisod89 — Installs and updates SLOOP from Chrome or Edge over USB; nothing to download. Also offers the offline editor.

1.7BasicsGUIDE.md §1, §25

Updating and changing versions

Updating SLOOP is the same as installing it: open the installer and press INSTALL. Your projects, user presets, samples, drum kits and settings stay on the FM-1. 2.8 made no change to the project format: 2.7 projects and backups load unchanged, and 2.8 projects still open in 2.7.

Back up first, every time. Save a backup in the editor before every update and before going back to an older version or to stock (Backup and restore).
Going back to an older SLOOP: install the .fwsc file from that version’s release page. Samples, user presets and settings stay.
2.3 and older can’t read newer projects. From 2.4, projects use the FUN5 format, which holds locks, nudges and fills. 2.3 sees them as empty, and its autosave can overwrite the music in progress. A backup made with 2.4 or later only restores into 2.4 or later, so keep it and restore it when you come back.
Lights or SYNC back to OFF and INT after going back to an earlier version: those versions don’t keep them, so set them again.
An install stopped half-way: the FM-1 waits in update mode. Press INSTALL again and it finishes.

From the community

1.8IntermediateSLOOP 2.8+GUIDE.md §5, §25

USB rescue and power-on combinations

Three combinations are built into SLOOP for when things go wrong. Two work as you switch the FM-1 on; the third works while the transport is stopped.

Key combinations

Hold OCT- while switching on: Starts USB rescue (SLOOP USB RESCUE), so you can install again from the browser.
Hold OCT- + OCT+ while switching on: Starts hardware calibration: press each button and turn each knob as the screen asks.
Hold OCT- + OCT+ for 5 s: Enters update mode (UBOOT) after a countdown; let go to cancel. Your work is saved first.While stopped

If SLOOP won’t start

Switch the FM-1 off.
Hold OCT- on its own and switch the FM-1 on. The screen shows SLOOP USB RESCUE.
Connect it and press INSTALL on the installer page.

Calibration fixes buttons or knobs that do the wrong thing. It is also in the menu, under SYSTEM (The HOME menu), and if you leave it 30 s without input it cancels and keeps the old setup. Update mode is mainly for developers; the countdown reads UPDATE MODE IN 3….

From the community

1.9IntermediateSLOOP 2.8+GUIDE.md §25

Going back to the official firmware

From 2.8, SLOOP no longer has its own button for going back to stock — it wasn’t reliable on every system. Use M-VAVE’s own updater instead.

Save a backup with the editor (Backup and restore). The official firmware can’t open SLOOP’s work, so this is how you get it back later.
Download M-UPGRADE from m-vave.com → Downloads → PC Software, and the official FM-1 firmware from PC Firmware → FM-1: version V16, the file FM-1.fwsc.
Close every other app or tab that uses MIDI.
Run M-UPGRADE with the FM-1 connected and install FM-1.fwsc.

With SLOOP on it, the FM-1 shows up under SLOOP’s name, and on some systems M-UPGRADE then doesn’t list it. In that case open the Clipper installer page and use Return to the official firmware with the same file, V16 or V15. It works with SLOOP on the unit — you don’t need to install Clipper — and it checks the file before writing anything.

If the official firmware then starts with strange settings or a scrambled display, nothing is damaged: it is reading settings SLOOP left in memory. A factory reset in the official firmware, or M-UPGRADE’s full restore, clears them. To return to SLOOP, install it again and restore your backup.

From the community

One owner reports that installing V16 over SLOOP and other custom firmwares stopped half-way and left the FM-1 looking bricked, and that installing V15 through a Web MIDI installer instead brought it back. Their report, not a tested procedure. via oktatracker on Elektronauts

1.10BasicsGUIDE.md §3, §17

Specifications

The numbers behind SLOOP 2.8.5, gathered from its README. Each row links to the part of this guide that explains it.

Area	SLOOP 2.8.5
Tracks	Three synth tracks sharing 8 voices, plus a drum track with 16 sounds and 6 voices (Four tracks, four colours).
Sounds	153 factory sounds on 12 engines, browsed by kind and matched in level; 32 user presets; 8 sampled sets (CC0) (The sound bank).
FM	A six-operator engine with DX7 SysEx import and a bank of 27 patches (FM6: six-operator FM).
Polyphony	One engine playing alone: 8 voices on ANALOG, DIGITAL, PHASE, LOFI, SAMPLE, TRIO, WHEEL and NOISE; 6 on FM6; 4 on VOICE; 3 on GRAIN and PHYS (2 on PHYS’s SYMP model). MONO and LEGATO play one note at a time (Polyphony and voice modes).
Drum kits	37 kits of 16 sounds — 5 sampled, 32 synthesised — plus your own in USR1–USR4 and the 15-second USR3+4 (The 37 drum kits, and yours).
Your samples	Four slots, USR1–USR4, of about 7.4 s each (Your own samples).
Sequencer	Up to 64 steps per track, every track with an independent length and a division from 1/32 to two bars; chords, ties and slides; 24 parameter locks a track; micro timing of ±½ step in 1/64 steps; fill conditions; swing from 0, straight, to 100, the MPC’s 75 % (How the sequencer works).
Recording	Live, quantised as you hear it with latency compensation, overdub; a free take that sets the tempo, or a set tempo; start on the first note or after a one-bar count-in (How recording works).
Effects	27 punch-in effects; DUST, DUCK, a DJ filter and a limiter on the master; drive, slicer, filter and an INSERT effect per track; sends to a stereo chorus, a tempo delay and a stereo reverb (The signal path).
Memory	Autosave, undo and redo, one song of four sections A–D, 32 user presets, 27 FM6 patches, and full backup and restore from the editor (Sections and the one song).
Audio	44.1 kHz fixed-point processing; a class-compliant USB audio input carrying the master output, 16-bit stereo at 44.1 or 48 kHz (Recording over USB audio).
MIDI	USB MIDI in and out, 3.5 mm TRS MIDI in, MIDI clock in over USB or TRS and clock out over USB (Where MIDI comes in).
Lights	Button backlight at three levels, glowing C keys, white keys or all keys, and played notes (Lights and knob feel).
Updates	Over USB from the browser with SHA-256 and CRC checks, USB rescue, and a way back to stock with M-UPGRADE and V16 (Installing SLOOP).

From the community

SLOOP on GitHubisod89 — The project page: what SLOOP does, the specifications, compatibility, installing, rescue and the issue tracker. 767 stars in November 2026.