fm1-sampler

A sampler + audio looper (8 tracks) for Android with the M-VAVE FM-1 as its hardware front end. The phone does all the sound; the FM-1 is the control surface (keys, buttons, 7 encoders), the second screen (240x240, drawn with SysEx commands) and the audio output (USB audio). All DSP is original code in engine/dsp (no third-party audio engine); the only library is KissFFT (BSD) for the Paulstretch mode.

Screenshots

FM-1 second screen (240x240, from the host simulator app_sim: sample, edit, envelope, LFO, arp, tape, mixer, loop length, key, mic waiting, rec armed, loop playing):

Phone app (sample with voice positions and the whole-loop lane, voices, LOOP page, LENGTH page, mic recording, library):

Downloads

APK and the flash page of the matching firmware are attached to the releases of this repository. Flash the firmware first (SEL = loop length page needs it), then install the APK.

FM-1 controls

Control	Does
SELECT (encoder 0)	track 1..8
PRESETS (encoder 6)	browses the sample library; after 250 ms of rest the entry is loaded on the selected track and played (audition)
ALGORITHM (encoder 1)	MELODIC, ONE-SHOT, LOOPED, KIT/SLICE, GRANULAR, PAULSTRETCH (the Sampler's MODE; KIT slices on transients, equal parts if there are none)
K1..K4 (encoders 2..5)	the four parameters of the current page (below)
HOME	sample: START, END, SPEED (time stretch 0.25..4x), PITCH (+-24 st). Speed and pitch are offline stretches, applied 350 ms after the knob rests
FX	REVERB SIZE, REVERB D/W (return level), DELAY TIME, DELAY FEEDBACK (global sends; per-track send levels are on MIXER)
ENV	ATTACK, DECAY, SUSTAIN, RELEASE of the selected track
LFO	RATE (0.1..20 Hz), DEPTH, TARGET (PITCH, FILTER, VOLUME, PAN), SHAPE (SINE, TRI, SQUARE, S&H)
ARP	RATE (OFF, 1..64 with triplets), PATTERN, OCTAVES, LENGTH (own arpeggiator on the tempo grid, free running)
SEQ ("TAPE")	LOOP SPEED (0.25..2x tape-style varispeed of the loop, GLOBAL for all tracks), EXCITER (cassette saturation), LO-FI (bit crush + hiss + dark tone), WOBBLE
EDIT	mixer of the selected track: VOLUME, PAN, REVERB SEND, DELAY SEND
GLO	key and scale: SCALE, ROOT, OCTAVE, MODE (KEY/CHORD). Drives the firmware's key mapping (SysEx 06)
SAVE	page LOOP. Tap = save the loop of the selected track into the library, hold 0.7 s = clear ALL loops (every track), also the phone CLEAR button. Knobs: MIC SAMPLE, LOOP SOURCE, SAVE LOOP, MIC THRESHOLD (-46..-6 dB, level that starts a mic take; used by mic sampling and mic loop source)
PLAY	play / stop
REC	clean looper: arms the first take (it starts with the first key or sound and sets the loop length and tempo; REC again ends it). Loop exists: toggles overdub on the selected track
SEL (the button after FX; index 3, the protocol and the firmware call it SCL)	page LENGTH: K1 LENGTH (turn up = loop x2 with its content repeated, down = /2), K2 DUPLICATE, K3 HALVE, K4 bars/bpm (read-only). Needs the firmware built here (firmware/FM1-Sampler-Workbench.html): it forwards SCL as button 3 and has no scale screen; the stock/fm1-daw firmware keeps SCL local
OCT -/+	handled by the firmware (octave)
keys	KIT algorithm: all 27 keys are pads (note 60 + key, 16 slices repeat over the keys), every key lit, tonality and octave ignored. Otherwise the firmware is kept on a plain chromatic keyboard (scale OFF, SysEx 06 always sends 7; notes 53+key) so keys outside the tonality still play; the app adds the octave, builds triads itself in CHORD mode and lights ONLY keys that belong to the scale (root of each octave blinks; scale OFF: only roots); a sounding key goes dark.

Sample sources

• Bundled library (samples/, packed into the APK assets, copied to files/samples once): Bass, Drums (loaded as KIT), Keys, Pads.
• Microphone: phone button "RECORD FROM MIC" or LOOP page K1. Waits for sound above the trigger level, trims and normalises, saves sample_<ms>.wav in the data dir, loads it on the track it was started on.
• Disk: phone button "LOAD FILE FROM DISK" -> system picker -> AudioImport.java converts to a 16-bit WAV (sample_<name>.wav in files/): WAV 8/16/24/32/float directly, anything else through MediaCodec (mp3, aac, ogg, flac). Max 90 s.
• Saved loops and the two above all live in the "User" group of the library (sample_*.wav in the data dir). Browsing replaces the sample of the selected track; a new sampler keeps the track's settings (applyAll after sampleVersion bumps). At start: Tape Key on track 1, Acoustic bass on 2, Shiny on 3, TrapKit (KIT) on 4.

Looper details

• Loop source (LOOP page K2, or the label in the phone top bar): SOUND = what the track plays (keys, arp, sample, filter, before the cassette) or MIC. Default SOUND.
• One clip per track; all tracks share the loop length of the first take. All loops are read (and recorded) through ONE tape pointer (Session::tapePos), so LOOP SPEED moves them together. At 1.0x the pointer is locked to the transport.
• Recording into the loop works at any speed like a tape head: below 1x the input is spread over the two neighbouring cells weighted by the speed, above 1x the skipped cells are filled by interpolating the input.
• A free take records audio only (no notes), so nothing in the loop runs at a speed of its own.
• Exciter / lo-fi / wobble are global: a per-track cassette (saturation, tone, wow and flutter, hiss, mix) followed by a bit crusher.
• Reverb and delay run 100 % wet as send returns; D/W is the return gain (Session::setRetLevel).
• The whole-loop overview in the phone UI (Session::loopPeaks) is refreshed by the audio thread, 8 of 400 bins per block.

Engine

• engine/dsp/Sampler.*: the instrument. Up to 8 voices; MELODIC, ONE-SHOT, LOOPED (crossfaded loop), KIT (16 slices cut at the hits by Slicer.h, pad = note - 60), GRANULAR (Hann grains at random positions in the window), PAUL (Paul.*: windowed spectra with random phases, overlap-add, at most 4 voices). Adsr.h, Filter.h (state variable low-pass, used by the LFO filter target).
• engine/dsp/Stretch.*: offline time stretch (WSOLA) and pitch shift (stretch + resample) of the sample window; built on the worker thread 350 ms after the SPEED / PITCH / START / END knob rests, then swapped in.
• engine/dsp/Reverb.h (8 combs + 4 allpasses per side), TapeDelay.h, TapeInsert.h, Arp.h (UP, DOWN, UPDN, CONV, RAND over 1..4 octaves, sample accurate gate), Wav.*.
• engine/session/Session.*: tracks, mixer, LFO (once per block, ~170 Hz: pitch, filter, volume or pan), looper, mic recorder, library listing, command queue to the audio thread, worker thread. status() is lock-free.
• engine/app/SamplerApp.*: all behaviour: knob pages (knobs(Page) is the single source for FM-1 and phone), per-track state, library, looper, FM-1 screen drawing and LEDs, SysEx pacing and loss recovery.
• engine/ui/SamplerUI.*: the phone UI as draw ops (left tracks, centre sample/waveform with draggable START/END, voice markers, whole-loop lane, right page tabs + 4 sliders, library sheet). ui/UiOp.h, Theme.h (paper/ink/amber, square corners).
• android/: Backend (FM-1 USB MIDI + USB audio routing, Oboe, 30 Hz tick), SamplerActivity (file picker, mic permission), AudioImport, SamplerView (replays ops), native-lib.cpp. Package com.ferluht.fm1sampler, lib fm1sampler.
• firmware/: the FM-1 firmware (M-VAVE's open firmware with the control-surface protocol of PROTOCOL.md). firmware/FM1-Sampler-Workbench.html is the flash page; tools/build_fw.py firmware ~/fm1/wine/drive_c/JL/pi32 rebuilds it (JieLi toolchain under wine, ~30 s). Changes in app/ctrl.c: SEL (BTN_SCL) forwarded to the app (no on-device scale screen), splash "SAMPLER".

Build and test

• Host: make -C engine -j4, then make $PWD/engine/build/<t> with t = test_app (checks: every algorithm, LFO targets, stretch and pitch, tape, arp, loop, speed, overdub, loop length, voices, KIT keys, save/clear, mic, disk import, audition, DSP units), app_sim (virtual FM-1, writes app_NN_*.ppm screenshots of its screen), phone_render + python3 engine/tools/render_ui.py ui_ops.json outdir 1.5 (phone screenshots), probe (what the default project loads). Paths in the tools assume ~/fm1-sampler/samples. The tools sleep a little per audio block so the worker thread (sample loading) keeps up with the simulated time.
• Android: cd android && ANDROID_HOME=~/android-sdk ~/gradle/gradle-8.9/bin/gradle assembleDebug -q (~4 min first, never edit engine sources while it compiles: clang then reports "source file is not valid UTF-8"). Output app/build/outputs/apk/debug/app-debug.apk.

Status

Host-tested only (test_app all green, screenshots of both screens checked). Not run on a phone or an FM-1. Open questions: MediaCodec import on a real phone, offline stretch time on the phone CPU, Paulstretch / granular CPU on the phone, arp is free running (not locked to the loop), loop clips are not saved with a project (no project save/load).