MIDI Clock to JACK Transport Bridge

A lightweight C++ program that bridges MIDI clock signals to JACK transport, allowing hardware sequencers, drum machines and other MIDI-clock-capable devices to control tempo in JACK/PipeWire-based audio environments.

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Overview

This program listens to incoming MIDI clock messages (via ALSA) and translates them into JACK transport tempo and BBT (Bar/Beat/Tick) information. It acts as a JACK timebase master, providing stable tempo synchronization for JACK-aware applications. The tool was specifically created to work seamlessly with Carla as a primary use case, ensuring tight synchronization when using external MIDI-clock hardware with Carla-hosted instruments and plugins.

┌─────────────────────────────────────────────────────────────────┐
│                    🎹 EXTERNAL CLOCK SOURCE                     │
│               MIDI Clock Source (HW/DAW)                        │
│               • Sends F8 (clock ticks at 24 PPQN)               │
│               • Optional: FA/FB/FC transport messages           │
└────────────────────────────┬────────────────────────────────────┘
                             │ MIDI Protocol (ALSA)
                             ▼
┌─────────────────────────────────────────────────────────────────┐
│                🔄 MIDI CLOCK SYNC PROGRAM                       │
│                     ** BPM TRANSLATOR **                        │
├─────────────────────────────────────────────────────────────────┤
│  ✓ Opens ALSA MIDI input port                                   │
│  ✓ Listens to MIDI Clock ticks (F8)                             │
│  ✓ Measures time between ticks                                  │
│  ✓ Calculates current BPM from tick intervals                   │
│  ✓ Continuously updates JACK transport BPM                      │
│  ✓ Provides BBT position as JACK timebase master                │
└────────────────────────────┬────────────────────────────────────┘
                             │ Writes BPM to JACK Transport
                             ▼
┌─────────────────────────────────────────────────────────────────┐
│                   🎚️  JACK / PIPEWIRE ENGINE                    │
│  • Current BPM ← continuously updated                           │
│  • BBT time information                                         │
│  • Transport state (rolling/stopped)                            │
└────────────────────────────┬────────────────────────────────────┘
                             │ All clients read shared BPM
                             ▼
┌─────────────────────────────────────────────────────────────────┐
│                        🎛️  CARLA / ARDOUR                       │
│  • Registers as JACK client                                     │
│  • Reads current BPM from JACK transport                        │
│  • Plugins stay in sync with external MIDI clock                │
└─────────────────────────────────────────────────────────────────┘

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Features

• MIDI Clock Detection: Listens for 24 PPQN MIDI clock
• Adaptive BPM Smoothing: Intelligent smoothing & stability detection
• JACK Timebase Master: Provides tempo, time signature (4/4), BBT
• Transport Control: Responds to Start/Stop/Continue messages
• Auto-start: Begins JACK transport on first received MIDI clock
• Realtime Status Reports: View status using SIGUSR1
• PipeWire Compatible: Works via pw-jack

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Requirements

Runtime

• ALSA
• JACK or PipeWire (with JACK compatibility)
• MIDI clock source (hardware or software)

Build Dependencies

sudo apt-get install \
    build-essential \
    libasound2-dev \
    libjack-jackd2-dev \
    pipewire-jack

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Installation

1. Clone Repo

git clone https://github.com/ariel10aguero/midi-clock-jack-bridge
cd midi-clock-jack-bridge

2. Build

chmod +x build.sh
./build.sh

Build settings:

• C++17
• -O3 optimization
• Links ALSA, JACK, pthread, atomic

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Usage

1. Identify ALSA MIDI Port

aconnect -l

Example:

client 24: 'Scarlett 2i4 USB'
    0 'Scarlett 2i4 USB MIDI 1'

2. Run (PipeWire)

pw-jack ./midi_clock_sync 24:0

Replace 24:0 with your MIDI source port.

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Using with Carla

For stable tempo sync, use Multiple Clients mode:

1. Open Carla
2. Configure Carla → Engine
3. Set Process Mode: Multiple Clients
4. Enable Use JACK Transport
5. Restart engine
6. Then run:

pw-jack ./midi_clock_sync <client>:<port>

Recommended Startup Order

1. PipeWire running
2. Run midi_clock_sync
3. Launch Carla

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Checking Status

Send a SIGUSR1:

kill -USR1 $(pidof midi_clock_sync)

Example:

╔════════════════════════════════════════╗
║          MIDI CLOCK SYNC STATUS        ║
╠════════════════════════════════════════╣
║ Transport State: ▶  PLAYING            ║
║ JACK BPM:        86.00                 ║
║ Position:        Bar 4, Beat 1         ║
║ Frame:           416768                ║
║ Time Signature:  4.00/4.00             ║
║ Detected BPM:    86.00                 ║
║ Measurements:    16                    ║
║ Current Pos:     4:1:854               ║
╚════════════════════════════════════════╝

Carla Configuration Note

For proper JACK transport syncing, ensure Carla’s config file contains:

nano ~/.config/falkTX/Carla2.conf

Set:

TransportMode=2

Press “R” in the terminal → Resets the bar counter to 1