midi-sequencer

A browser-based MIDI step sequencer with a C ALSA backend for accurate timing and a Node.js/Express web UI.

Architecture

Browser (app.mjs)
    ↕  HTTP + SSE
Node server (server.mjs, port 3000)
    ↕  Unix socket (/tmp/midi-sequencer.sock)
C backend (midi-sequencer binary)
    ↕  ALSA sequencer API
MIDI output

• protocol.yaml — single source of truth for the binary protocol; drives codegen for both C and Node
• codegen/gen.mjs — generates c-backend/generated/protocol.{h,c} and node-server/src/generated/protocol.mjs
• c-backend/ — C11, drift-free tick thread using clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME), links libasound + libpthread
• node-server/ — Express 5 web app, serves the UI and bridges the browser to the C backend

Features

• Multi-track playback — up to 16 patterns playing simultaneously, each looping at its own independent length
• Live track control — add/remove patterns from the playing set without stopping; mute and solo per track
• GM percussion editor — channel 10 patterns show GM drum names in drum-machine layout with family colour coding
• Auto-save — step grid edits are sent to the backend immediately on each toggle
• Preview notes — per-row ▶ button auditions a note without affecting sequencer state
• Click-to-rename — note and percussion labels are editable inline
• Sub-patterns — patterns can reference other patterns as sub-sequences at any step
• BPM — tempo settable to 0.1 BPM resolution (stored as bpm_x10 uint16, no floats in protocol)

Protocol

Binary frames: [record_type: u8][payload_length: u16le][payload…]

ID	Name	Direction	Description
0x01	HELLO	node→C	Version handshake
0x02	DEFINE_PATTERN	node→C	Create/redefine a pattern
0x03	CLEAR_PATTERN	node→C	Remove all notes from a pattern
0x04	ADD_NOTE	node→C	Add a note event at a step
0x05	ADD_SUB_PATTERN	node→C	Schedule a sub-pattern within a pattern
0x06	PLAY	node→C	Single-pattern play (backward compat)
0x07	STOP	node→C	Stop all playback
0x08	SET_TEMPO	node→C	Set BPM (as bpm×10)
0x09	PREVIEW_NOTE	node→C	Play a single note immediately
0x0A	ADD_TRACK	node→C	Add a pattern to the multi-track set
0x0B	REMOVE_TRACK	node→C	Remove a pattern from the multi-track set
0x0C	PLAY_TRACKS	node→C	Start the multi-track engine
0x0D	SET_TRACK_MUTE	node→C	Mute/unmute a track
0x81	ACK	C→node	Acknowledge a command
0x82	ERROR	C→node	Report an error
0x83	BEAT_TICK	C→node	Step notification (one per track per tick)
0x84	PATTERN_END	C→node	Pattern completed one full cycle

Build

Requires: GCC, make, libasound2-dev, Node.js ≥ 18

# Full build (codegen + npm install + C binary)
make

# C backend only
cd c-backend && make

# Run the Node server
cd node-server && make start
# → http://localhost:3000

The C backend runs separately and is started independently:

./c-backend/midi-sequencer [socket_path] [alsa_client_name]
# defaults: /tmp/midi-sequencer.sock  midi-sequencer

Usage

1. Start the C backend
2. Start the Node server (make start in node-server/)
3. Open http://localhost:3000
4. Create melodic or drum patterns with + Melodic / 🥁 Drums
5. Click the ● dot next to each pattern to add it to the playback set
6. Press ▶ Play to start — all active patterns loop independently

Timing

One step = one 16th note. Step duration: 150,000,000,000 ns / bpm_x10

At 120.0 BPM: 125 ms/step. The tick thread uses an absolute deadline so tempo drift does not accumulate.