Backspn

Free, open source and made for any controller
macOS · Windows · Linux

Mix with sync and keylock, split any track into stems on your own machine and play 74 effects on the beat. Backspn is written in Rust, runs on macOS, Windows and Linux, and is released under the GPL.

Status: public beta. Every feature below is finished. Download a build for macOS (.dmg), Windows (.exe) or Linux (.deb, AppImage) from Releases, or build it from source (see Building). Please report what breaks in Issues; testers with a controller are especially welcome.

The beta builds are not notarised by Apple or signed for Windows yet (see Installing on macOS). On Windows, choose More info ▸ Run anyway in SmartScreen.

Installing on macOS

While Backspn is in beta, the macOS build is not notarised: that needs a paid Apple developer certificate, which will come once the app has more users. Until then, macOS blocks it the first time you open it:

1. Drag Backspn to Applications and open it. macOS shows "Backspn" Not Opened. Press Done (not Move to Bin).

   

2. Open System Settings ▸ Privacy & Security, scroll down to Security and press Open Anyway next to "Backspn" was blocked to protect your Mac. Confirm with your password or Touch ID.

   

3. Open Backspn again and choose Open. You only need to do this once per version.

Features

• Decks and mixer
• Stems
• Effects
• Drum Machine
• Sample decks
• Loop recorder
• Mix recorder
• Library
• Layouts
• Themes

Decks and mixer

Two or four decks. Sync locks every deck to the master clock, half and double time included, and keylock keeps the pitch while the tempo moves. Each deck has eight hotcues, loops and beat jump from 1/32 to 64 beats, slip and reverse. The four-channel mixer has an isolator EQ, a filter (standard or a resonant club filter), a crossfader, cue on a second output and a limiter on the master.

Stems

Split any track into drums, bass, other and vocals on your own CPU. You don't need a GPU, and once the separation model is downloaded you don't need a connection either. Load the track as a stem deck and each part gets its own volume, filter and FX send. Results are saved, so the next load is instant. .stem.mp4 files load as stems too.

Effects

74 effects timed to the beat of their deck. There are two or four FX units, each in group mode (three effects) or single mode (one effect with all its parameters), with insert, post-fader or send routing.

Kind	Effects
Delay and echo	Delay, Delay II, Tape Delay, Beat Delay, Reverse Delay, Multi Tap Delay, Ramp Delay, Echo, Ping Pong, Pitch Echo, Coil Echo, Low Cut Echo, Low Cut Pitch Echo, Rising Echo, Release Echo
Reverb	Reverb, Reverb II, Glass Reverb, Beat Reverb
Filter	Filter, Beat Filter, Formant Filter, Peak Filter, Ladder LFO, Ladder Pulse, SVF Filter, SVF LFO, SVF Pulse
Modulation	Flanger, Flanger Pulse, Flanger Hold, Beat Flanger, Phaser, Phaser Pulse, Phaser Hold, Beat Phaser, Barber Pole, Shepard Saw, Shepard Tri, Shepard Jet, Ringmod, Pan
Rhythm and slicing	Gater, Beat Chopper, Slicer, Trans, Roll, Slip Roll, Reverse Roll, Step Stutter, Auto Step Stutter
Pitch and time	Pitch, Transpose Stretch, Stretch, Reverse Grain, Turntable, Vinyl Brake, Release Brake, Back Spin
Distortion	LoFi, Drive
Macros	Tunnel, Zap Cut, Grain Sweep, Bass Machine, Stretch Fast, Stretch Slow, Polar, Horizon, Zap, Dark, Flight
Effect modifiers	Filter, Decay

Drum Machine

Any FX unit can turn into a step sequencer that plays in time with the master clock. Pick a kit and a sound, switch between the kit's eight patterns, change pitch and decay from the knobs, and turn on duck so each hit dips the deck.

Two kits come with the app, Analog Kit and Club Kit, each with eight sounds and eight patterns. Their sounds are synthesized by Backspn itself, so they are as free as the rest of the code. Build more in Preferences ▸ Drum Machine:

• New, Duplicate and Delete kits, and give each one a name and an author.
• Add sounds… loads your own samples in any supported audio format. Rename each sound, set its level, preview it in the headphones or remove it.
• Write your own patterns: eight per kit (P1 to P8), with a step row for every sound. Each row has its own step size: 1/4, 1/8 or down to 1/16 of a beat (16, 32 or 64 steps per bar), so a straight kick can run under fast hat rolls in the same pattern.
• Choose the decay, pitch, pattern and volume a kit starts with when it is picked.
• Export… a kit as one .bskit file with its sounds inside, and Import… kits other people share.

_{One pattern, three step sizes: kick in 1/4 of a beat, snare in 1/8, hats and percussion in 1/16.}

Sample decks

Any deck can become a sample deck with four slots. Each slot holds a stack of cells. Drag loops and one-shots from the browser into the cells, then fire them on the beat. Every slot has loop or one-shot playback, gate or latch triggers, keylock, punch-in, size ÷2 and ×2, reverse, and its own volume, filter and FX send. Sample sets are saved in the library.

Loop recorder

Record a loop of 4, 8, 16 or 32 beats from the master, the cue, or an external or aux input. It plays back in time with the clock, and you can overdub layers and undo them. Save the loop to the Loops list in the browser, or drag it straight onto a sample deck cell.

Loops from a deck. Any active loop on a track deck can become a sample too. Right-click the loop on the waveform and choose Save as Loop: it goes to the Loops list in the browser, named after the track and the time. You can also drag the loop straight onto a sample deck cell, or onto Loops.

Mix recorder

Record the whole mix to WAV (16-bit, 24-bit or 32-bit float), split by file size if you want, with its own gain. Recordings show up in the browser under Audio Recordings.

Library

Every track is analysed when it loads: BPM, beatgrid, key and EBU R128 loudness. The library has search (with filters such as techno $BPM ~ 128), playlists, smartlists, ratings, history, configurable columns and keyboard navigation, and plays MP3, AAC/M4A, ALAC, FLAC, WAV, AIFF and Ogg Vorbis. Backspn imports the collections of other DJ applications (NML collections, keyboard and controller mappings) and reads Serato, VirtualDJ and Engine DJ libraries. Everything it writes uses its own open formats.

Layouts

Each deck can be Micro, Essential, Full or Advanced, or show its waveform in a parallel lane with the other decks. Hide the browser or the FX units when you need room. Save your setups as layouts and switch between them from the top bar.

_{Parallel waveforms for four decks.}

_{Four Essential decks with the browser.}

Themes

Dark, Light, Darcula, Solar, Purple and Ocean, from Preferences ▸ Global Settings.

_Light.

_{Darcula, Solar, Purple and Ocean.}

Controllers

Kind	Brand	Models
Native drivers (HID)	Native Instruments	Kontrol S4 MK3*, S4 MK2, S8, S5, D2, S2 MK3, S2 MK2, S3, MX2, X1 MK2, X1 MK3, F1, Z1, Z2
MIDI profiles	AlphaTheta	DDJ-FLX2, DDJ-FLX4, DDJ-GRV6
	Pioneer DJ	DDJ-200, DDJ-400, DDJ-800, DDJ-1000, DDJ-ERGO, DDJ-FLX6, DDJ-RB, DDJ-REV5, DDJ-RR, DDJ-RX, DDJ-RZ, DDJ-RZX, DDJ-SB, DDJ-SB2, DDJ-SP1, DDJ-SR, DDJ-SX, DDJ-SX2, DDJ-SZ, DDJ-WeGO, DDJ-WeGO2, DDJ-WeGO3, DDJ-WeGO4, DDJ-XP1, DDJ-XP2
	Denon DJ	MC7000
	Hercules	DJControl Inpulse 200, 300, 500
	Numark	Mixtrack Pro FX, Mixtrack Platinum FX
	Reloop	Buddy, Ready
Anything else		MIDI learn and keyboard mappings

_{* Only the Kontrol S4 MK3 is tested on hardware so far. The others should work, but they have not been tried on the real device yet and may have bugs. If you own one, please report what works and what doesn't.}

The drivers draw their own screens. On the 320 × 240 screens of the S4 MK3 you pick Standard, Classic, Waveform or Big Numbers (below), or one of the booth layouts: Phrase (beat, bar and phrase, bars to the next cue and to the end), Mix (both decks stacked with tempo, phase and key against each other), Cues (every hotcue and the bars until it) and Stems. The 480 × 272 screens of the S8 and D2 add Wide Deck, Wide Waveform and Wide Mix, which also name what the faders, knobs and buttons under the screen are doing.

Profiles live in crates/control/profiles/. Several of them are adapted from Mixxx mappings (see MIXXX-NOTICE.md). Controller names are trademarks of their owners and are used only to identify the hardware.

S4 MK3 screens: the manufacturer's background services (NIHardwareAgent and NTKDaemon) hold the screens, so Backspn can't draw on them while they run. Stop them before opening Backspn. They start again with the manufacturer's apps or after a restart, so repeat this if the screens stay blank.

• macOS: open Activity Monitor, search for NIHardwareAgent, select it and press the ✕ button (Force Quit). Do the same for NTKDaemon. Or, in Terminal: killall NIHardwareAgent NTKDaemon
• Windows: open Task Manager (Ctrl+Shift+Esc), go to the Details tab, right-click NIHardwareAgent.exe and choose End task. Do the same for NTKDaemon.exe.
• Linux: the manufacturer ships no services for Linux, so there is nothing to stop.

Linux: to reach HID controllers and the S4 MK3 screens without sudo, install the udev rules and plug the controller in again:

sudo cp packaging/linux/99-backspn.rules /etc/udev/rules.d/
sudo udevadm control --reload && sudo udevadm trigger

Windows: buttons, lights and audio work with the manufacturer's driver. The S4 MK3 screens use raw USB and need the WinUSB driver on the device's interface 4 (for example, with Zadig), installed with the manufacturer's hardware service closed. Untested.

Building

You need stable Rust (rustup; the toolchain is pinned in rust-toolchain.toml), CMake and a C/C++ compiler with libclang.

• macOS: Xcode command line tools and brew install cmake.
• Windows: Visual Studio Build Tools (C++), CMake and LLVM.
• Linux (Debian/Ubuntu):

  sudo apt install build-essential cmake pkg-config libclang-dev libasound2-dev \
    libudev-dev libxkbcommon-dev libwayland-dev libgl1-mesa-dev libx11-dev

Then:

git clone https://github.com/suitux/Backspn
cd Backspn
cargo run --release -p backspn-app             # optionally pass up to 4 audio files to load

Settings live in settings.json in your system's data folder (override with BACKSPN_SETTINGS): ~/Library/Application Support/Backspn on macOS, %APPDATA%\Backspn on Windows and ~/.local/share/Backspn on Linux.

Anonymous usage statistics

Backspn can send an anonymous summary once a day so its developer knows how it is used. The aggregated results are public at backspn.com/insights. Nothing is sent until you answer the notice shown on first launch.

What is sent: a random install ID (a new one after Reset All Settings), the version, the system and processor, the interface language, the sample rate, buffer size and channel count, whether headphones cue is set up, the models of the supported controllers you connect, the number of tracks, analysed tracks and playlists with a count per file format, a few settings (number of decks and FX units, quantize, snap, auto gain, external mixer, Link, MIDI clock) and counts of what happened since the last report: sessions, time open, playing and recording, tracks loaded and played, mixes, recordings, stems separated, sync, hotcues, loops, beat jumps, samples, auto fades, Link sessions, effects switched on (per effect), imports (per kind), presses from a controller or the keyboard, crashes and unexpected closes.

What is never sent: track, artist or playlist names, file paths, tags, names of audio or MIDI devices, the content of logs or crash reports, and your IP address (the server does not store it).

See it and switch it off: Preferences ▸ Global Settings ▸ Anonymous Usage Statistics shows the exact report, when the last one was sent, and a switch. BACKSPN_INSIGHTS=0 turns it off for good. Development builds never send.

The collector is a Cloudflare Worker in insights/.

Contributing

Bug reports, controller profiles and pull requests are welcome. The workspace layout, coding practices, tests and release process are described in CONTRIBUTING.md, and the visual rules of the interface in docs/design.md.

License

Copyright (C) 2026 Xavier Rincón.

Backspn is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version (GPL-3.0-or-later). It is distributed without any warranty. Full text in LICENSE.

The Backspn name and logo (crates/app/icon/) identify this project and are not covered by the code license. Third-party components keep their own licenses: the Inter and Geist Mono fonts are under the SIL Open Font License 1.1 (crates/app/fonts/OFL-*.txt), and every Rust dependency is under a license compatible with the GPL-3.0 (MIT, Apache-2.0, MPL-2.0, BSD, ISC, Zlib, Unicode-3.0…).

All other product names mentioned here are trademarks of their respective owners. Backspn is not affiliated with or endorsed by any of them.

Technical reference

Files and folders

Data folder: ~/Library/Application Support/Backspn on macOS, %APPDATA%\Backspn on Windows, ~/.local/share/Backspn on Linux. Three environment variables move it: BACKSPN_SETTINGS points to another settings.json (the window state, kits, model and logs go next to it), BACKSPN_LIBRARY to another library.sqlite (covers and backups go next to it) and BACKSPN_MAPPINGS to another mappings.json.

Path	Contents
settings.json	Preferences and the session state (decks, mixer, FX units)
ui-state.ron	Window size and panel state
mappings.json	Controller and keyboard mappings
library.sqlite	The library: tracks, analysis, cues, playlists, history and sample sets
Kits/<kit>/	Drum Machine kits: a kit.json plus the kit's sounds
Covers/	Imported cover art
models/htdemucs_fp16weights.onnx	The stem separation model, downloaded on first use
Backup/	Library backups and .bak copies made before settings are rewritten
Logs/Backspn.log	The session log
Crashlogs/last-crash.txt	The report of the last unclean exit
insights.json	The random install ID and the usage counts not yet sent

Music folder: ~/Music/Backspn/ (each folder can be changed in Preferences).

Folder	Contents
Stems/	Generated stems (.bstem, Opus audio)
Samples/	Captured samples and loops
Sample Sets/	Exported sample sets, one folder with a set.json each
Recordings/	Mix recordings (WAV)

Formats Backspn writes

Extension	Contents
.backspn	A collection, playlist or history
.bsmap	A controller mapping
.bskit	A Drum Machine kit with its sounds
.bstem	The four stems of a track
set.json	A sample set, inside its folder
.m3u / .m3u8	Standard playlists
.wav	Mix recordings

Formats Backspn only reads

Audio: MP3, AAC/M4A, ALAC, FLAC, WAV, AIFF, Ogg Vorbis and .stem.mp4. Imports from other DJ applications: NML collections, the transient cache, .tsi keyboard and controller mappings, and Serato, VirtualDJ and Engine DJ libraries. Backspn never writes these formats, and it refuses to save into another DJ application's data folder.