beat-link

A Java library for synchronizing with beats from Pioneer DJ Link equipment, and finding out details about the tracks that are playing.

See beat-link-trigger and beat-carabiner-java for examples of what you can do with this.

Installing

Beat-link is available through Maven Central, so to use it in your Maven project, all you need is to include the appropriate dependency.

Click the maven central badge above to view the repository entry for beat-link. The proper format for including the latest release as a dependency in a variety of tools, including Leiningen if you are using beat-link from Clojure, can be found in the Dependency Information section.

Beat link uses slf4j to allow you to integrate it with whatever Java logging framework your project is using, so you will need to include the appropriate slf4j binding on your class path.

It also uses the Deep Symmetry projects electro (to synchronize with the Ableton Link timeline) and crate-digger (to download and parse rekordbox database files from players).

If you want to manually install beat-link, you can download the library from the releases page and put it on your project’s class path, along with electro, crate-digger, slf4j-api.jar and the slf4j binding to the logging framework you would like to use.

You will also need Remote Tea for creating and parsing old-version NFS packets, so Maven is by far your easiest bet, because it will take care of all these libraries for you.

Compatibility

This is in no way a sanctioned implementation of the protocols. It should be clear, but:

Warning

Use at your own risk! For example, there are reports that the XDJ-RX (and XDJ-RX2) crash when BLT starts, so don’t use it with one on your network. As Pioneer themselves explain, the XDJ-RX does not actually implement the protocol:

“The LINK on the RX (and RX2) is ONLY for linking to rekordbox on your computer or a router with WiFi to connect rekordbox mobile. It can not exchange LINK data with other CDJs or DJMs.”

While these techniques appear to work for us so far, there are many gaps in our knowledge, and things could change at any time with new releases of hardware or even firmware updates from Pioneer.

Caution

You should also not expect to be able to run Beat Link, or any project like it, on the same machine that you are running rekordbox, because they will compete over access to network ports. Also, as noted above, the XDJ-RX and XDJ-RX2 do not support the full DJ Link protocol, and so will not work with software like this. And, of course, controllers which don't even have Link (Ethernet) jacks can't work either.

Tip

✅ Beat Link seems to work great with CDJ-2000 Nexus and nxs2 gear, and release 7.3.0 supports CDJ-3000 features. It works fairly well (with less information available) with older CDJ-2000s. It has also been reported to work with XDJ-1000 gear, and (starting with version 0.6.0) with the XDJ-XZ as well. If you can try it with anything else, please let us know what you learn in Beat Link Trigger's Zulip chat stream, or if you have worked out actionable details about something that could be improved, open an Issue or submit a pull request so we can all improve our understanding together.

🚧 We finally obtained the hardware, so can now truly support new features like those introduced with the CDJ-3000 and DJM-V10, including the fact that there can now be up to six devices online and on mixer and the new higher-resolution position packets.

If something isn’t working with your hardware, and you don’t yet know the details why, but are willing to learn a little and help figure it out, look at the dysentery project, which is where we are organizing the research tools and results which made programs like Beat Link possible.

Getting Help

Deep Symmetry’s projects are generously sponsored with hosting by Zulip, an open-source modern team chat app designed to keep both live and asynchronous conversations organized. Thanks to them, you can chat with our community, ask questions, get inspiration, and share your own ideas.

Usage

See the API Documentation for full details, but here is a nutshell guide:

Finding Devices

The DeviceFinder class allows you to find DJ Link devices on your network. To activate it:

import org.deepsymmetry.beatlink.DeviceFinder;

// ...

  DeviceFinder.getInstance().start();

After a second, it should have heard from all the devices, and you can obtain the list of them by calling:

  DeviceFinder.getInstance.getCurrentDevices();

This returns a list of DeviceAnnouncement objects describing the devices that were heard from. To find out immediately when a new device is noticed, or when an existing device disappears, you can use DeviceFinder.getInstance().addDeviceAnnouncementListener().

Responding to Beats

The BeatFinder class can notify you whenever any DJ Link devices on your network report the start of a new beat:

import org.deepsymmetry.beatlink.BeatFinder;

// ...

  BeatFinder.getInstance().addBeatListener(new BeatListener() {
      @Override
      public void newBeat(Beat beat) {
         // Your code here...
      }
  });

  BeatFinder.getInstance().start();

The Beat object you receive will contain useful information about the state of the device (such as tempo) at the time of the beat.

To fully understand how to respond to the beats, you will want to start VirtualCdj as described in the next section, so it can tell you important details about the states of all the players, such as which one is the current tempo master. Once that is running, you can pass the Beat object to VirtualCdj.getLatestStatusFor(beat) and get back the most recent status update received from the device reporting the beat.

With just the Beat object you can call getBpm() to learn the track BPM when the beat occurred, getPitch() to learn the current pitch (speed) of the player at that moment, and getEffectiveTempo() to find the combined effect of pitch and track BPM—the beats per minute currently being played. You can also call getBeatWithinBar() to see where this beat falls within a measure of music.

If the VirtualCdj is active, you can also call the Beat object’s isTempoMaster() method to find out if it was sent by the device that is currently in control of the master tempo. (If VirtualCdj was not started, this method will throw an IllegalStateException.)

Getting Device Details

To find some kinds of information, like which device is the tempo master, how many beats of a track have been played, or how many beats there are until the next cue point in a track, and any detailed information about the tracks themselves, you need to have beat-link create a virtual player on the network. This causes the other players to send detailed status updates directly to beat-link, so it can interpret and keep track of this information for you.

import org.deepsymmetry.beatlink.VirtualCdj;

// ...

    VirtualCdj.getInstance().start();

The Virtual player is normally created using an unused device number in the range 5–15 and the name beat-link, and announces its presence every second and a half. These values can be changed with setDeviceNumber(), setUseStandardPlayerNumber(), setDeviceName(), and setAnnounceInterval().

As soon as it is running, you can pass any of the device announcements returned by DeviceFinder.getCurrentDevices() to getLatestStatusFor(device) and get back the most recent status update received from that device. The return value will either be a MixerStatus or a CdjStatus, containing a great deal of useful information.

As described above, you can do the same thing with the Beat objects returned by BeatFinder.

In addition to asking about individual devices, you can find out which device is the current tempo master by calling getTempoMaster(), and learn the current master tempo by calling VirtualCdj.getMasterTempo().

If you want to be notified when either of these values change, or whenever the player that is the current tempo master starts a new beat, you can use addMasterListener().

If you are building an interface that wants to display as much detail as possible, you can request every device status update as soon as it is received, using addUpdateListener().

Player Remote Control

In addition to learning about what is happening on other players, the VirtualCdj can ask them to do a limited number of things. You can tell them to load a track from any media currently mounted in a player by calling sendLoadTrackCommand(). You can cause them to start playing (if they are currently at the cue position), or stop playing and return to the cue position, by calling sendFaderStartCommand().

You can tell players to turn Sync mode on or off by calling sendSyncModeCommand(), and if there is no DJM on the network you can tell them that their channels are on or off the air by sending sendOnAirCommand(). (You can do that even with a DJM present, but your command will quickly be overridden by the next one sent by the DJM.)

Requesting Media Slot Details

You can ask the VirtualCdj to find out details about the media mounted in a player's media slot by calling sendMediaQuery(). In order to receive the response, you have to previously have registered a MediaDetailsListener instance using addMediaDetailsListener(); in general it's easier to just let the MetadataFinder take care of this for you as described below.

Sending Status

As of version 0.4.0, the VirtualCdj can be configured to send status packets of its own, so that it can take over the role of tempo master, and even send beat packets to synchronize other CDJs with a musical timeline of your choice. For this to work, it must be using a standard player number in the range 1–4, configured before you start it as described above.

The only way to be able to use the features that rely on sending status is to configure the VirtualCdj to use a device number in the range 1 to 4, like an actual CDJ, using VirtualCdj.getInstance().setUseStandardPlayerNumber() as described above. You can only do that if you are using fewer than 4 CDJs, because you need to use a number that is not being used by any actual CDJ.

As long as that is true, you can turn on this feature by calling setSendingStatus().

Once it is sending status, you can adjust details about what it sends by calling setTempo(), setSynced(), setOnAir(), and jumpToBeat(). To simulate playing a track, call setPlaying(). During simulated playback, beat packets will sent at appropriate times for the current tempo, and the beat number will advance appropriately. You can find the current simulated playback time by calling getPlaybackPosition().

Appointing the Tempo Master

As long as the VirtualCdj is sending status packets, you can tell players to become Tempo Master by calling appointTempoMaster(), or have it become the Master itself by calling becomeTempoMaster(). While acting as tempo master, any tempo changes you make by calling setTempo() will be followed by any players that are in Sync mode, and if the VirtualCdj is playing (and thus sending beats) you can call adjustPlaybackPosition() to keep the players' beat grids aligned with an external sync source (this is how Beat Link Trigger aligns CDJs with Ableton Link).

In addition to responding to the above functions for setting its own Sync, On-Air, and Master states, the VirtualCdj respects commands sent to it by other players or a DJM instructing it to do these things. While it is in Sync mode, it will stay aligned with the tempo master like other players do.

Getting Track Metadata

If you want to be able to retrieve details about loaded tracks, such as the title, artist, genre, length (in seconds), key, and even artwork images, start the MetadataFinder, which will also start the VirtualCdj if it is not already running.

import org.deepsymmetry.beatlink.data.MetadataFinder;

// ...

    MetadataFinder.getInstance().start();

⚠️ The only way to reliably retrieve metadata using the MetadataFinder on its own is to configure the VirtualCdj to use a device number in the range 1 to 4, like an actual CDJ, using VirtualCdj.getInstance().setUseStandardPlayerNumber() as described above. You can only do that if you are using fewer than 4 CDJs, because you need to use a number that is not being used by any actual CDJ. If you are using 4 actual CDJs, you will need to leave the VirtualCdj using its default number of 5, but that means the MetadataFinder will often be unable to talk to the dbserver ports on the players.

As of version 0.5.0, we finally have a good solution to this problem, which is to start up the CrateDigger class, which uses a completely separate mechanism for obtaining metadata, by downloading the entire rekordbox database export and track analysis files from the NFSv2 server that is also running in the players. The NFSv2 server is stateless, does not care what player number we are using, and can be used no matter how many players are on the network. So, in addition to the above code, also do this:

import org.deepsymmetry.beatlink.data.CrateDigger;

// ...

    CrateDigger.getInstance().start();

Once the MetadataFinderand CrateDigger are running, you can access all the metadata for currently-loaded tracks by calling MetadataFinder.getInstance().getLoadedTracks(), which returns a Map from deck references (player numbers and hot cue numbers) to TrackMetadata objects describing the track currently loaded in that player slot. You can also call MetadataFinder.getLatestMetadataFor(int player) to find the metadata for the track loaded in the playback deck of the specified player. See the TrackMetadata API documentation for all the details it provides.

With the MetadataFinder running, you can also start the ArtFinder, and use its can call its getLoadedArt() or getLatestArtFor() methods to get the artwork images associated with the tracks, if there are any.

Getting Other Track Information

You can use objects in the org.deepsymmetry.beatlink.data package to get beat grids and track waveform data, and even to create Swing UI components to display the preview and detailed waveform, reflecting the current playback position.

To illustrate the kind of interface that you can now put together from the elements offered by Beat Link, here is the Player Status window from Beat Link Trigger:

Media Details

While the MetadataFinder is running, it also keeps track of which players have media mounted in their slots. You can access that information by calling getMountedMediaSlots(). If you want to know immediately when slots mount and unmount, you can register a MountListener using addMountListener(). You can also find useful information about any mounted media database by calling getMountedMediaDetails() or getMediaDetailsFor().

Loading Menus

You can explore the hierarchy of menus served by a player for one of its mounted media slots using the MenuLoader, starting with requestRootMenuFrom() to determine what menus are actually available.

⚠️ The only way to get menus is by talking to the dbserver on the players, so even if you are using the CrateDigger as described above, you will only be able to do this reliably if you are using a real player number, which means you can have no more than three actual players on the network.

Also note that in order to reliably send the correct message to obtain the root menu for a particular slot, the MenuLoader needs to know the type of database that is mounted in that slot, so the MetadataFinder should be running and tracking media details for it to check.

Recognizing Tracks

If you want to know when a particular track has been loaded on a player, independent of the specific rekordbox export media it was loaded from, you can start the SignatureFinder, and it will do most of the work for you, with the help of the MetadataFinder, WaveformFinder, and BeatGridFinder. Whenever a rekordbox track is loaded and metadata for it has been obtained, it will compute an SHA-1 hash of the track title, artist, duration, detailed waveform and beat grid. The results boil down to a 40-character string of hexadecimal digits which can be used to uniquely identify and recognize that track (for example, using it as a key in a hash map to find cues that should run when that track is playing).

You can access the signatures of all loaded rekordbox tracks by calling getSignatures(), or look up the signature for a track on a specific player with getLatestSignatureFor(). If you want to know immediately when signatures are available for loaded tracks, you can register a SignatureListener using addSignatureListener().

An Example

Here is the source for Example.java, a small class that demonstrates how to watch for changes related to the tempo master (and also shows that printing DeviceUpdate objects can be a useful diagnostic aid):

import java.util.Date;
import org.deepsymmetry.beatlink.*;

public class Example {

    public static void main(String[] args) {
        try {
            VirtualCdj.getInstance().start();
        } catch (java.net.SocketException e) {
            System.err.println("Unable to start VirtualCdj: " + e);
        }

        VirtualCdj.getInstance().addMasterListener(new MasterListener() {
                        @Override
                        public void masterChanged(DeviceUpdate update) {
                            System.out.println("Master changed at " + new Date() + ": " + update);
                        }

                        @Override
                        public void tempoChanged(double tempo) {
                            System.out.println("Tempo changed at " + new Date() + ": " + tempo);
                        }

                        @Override
                        public void newBeat(Beat beat) {
                            System.out.println("Master player beat at " + new Date() + ": " + beat);
                        }
            });

        try {
            Thread.sleep(60000);
        } catch (InterruptedException e) {
            System.out.println("Interrupted, exiting.");
        }
    }
}

Compiling and running this class (with the beat-link and slf4j jars on the class path) watches and reports on tempo master activity for one minute, producing output like this:

> java -cp .:beat-link.jar:slf4j-api-1.7.21.jar:slf4j-simple-1.7.21.jar Example
Master changed at Sun May 08 20:49:23 CDT 2016: CDJ status: Device 2,
 name: DJ-2000nexus, busy? true, pitch: +0.00%, track: 5, track BPM:
 128.0, effective BPM: 128.0, beat: 55, beat within bar: 3,
 cue: --.-, Playing? true, Master? true, Synced? true, On-Air? true
Tempo changed at Sun May 08 20:49:23 CDT 2016: 128.0
Tempo changed at Sun May 08 20:49:47 CDT 2016: 127.9359130859375
    [... lines omitted ...]
Tempo changed at Sun May 08 20:49:51 CDT 2016: 124.991943359375
Tempo changed at Sun May 08 20:49:51 CDT 2016: 124.927978515625
Master changed at Sun May 08 20:49:55 CDT 2016: CDJ status: Device 3,
 name: DJ-2000nexus, busy? true, pitch: -0.85%, track: 4, track BPM:
 126.0, effective BPM: 124.9, beat: 25, beat within bar: 1,
 cue: 31.4, Playing? true, Master? true, Synced? true, On-Air? true

Research

This project is being developed with the help of dysentery. Check that out for details of the packets and protocol, and for ways you can help figure out more. You can also view Snapshot API Documentation to see what is available in the latest preview release.

Funding

Beat Link is, and will remain, completely free and open-source. If it has helped you, taught you something, or inspired you, please let us know and share some of your discoveries and how you are using it! If you'd like to financially support its ongoing development, you are welcome (but by no means obligated) to donate to offset the hundreds of hours of research, development, and writing that have already been invested. Or perhaps to facilitate future efforts, tools, toys, and time to explore.

using Liberapay, or using PayPal

If enough people jump on board, we may even be able to get a newer CDJ to experiment with, although that's an unlikely stretch goal. :grinning:

Contributing in Other Ways

If you have ideas, discoveries, or even code you’d like to share, that’s fantastic! Please take a look at the guidelines and get in touch!

Licenses

Copyright © 2016–2023 Deep Symmetry, LLC

Distributed under the Eclipse Public License 2.0. By using this software in any fashion, you are agreeing to be bound by the terms of this license. You must not remove this notice, or any other, from this software. A copy of the license can be found in LICENSE.md within this project.

Library Licenses

Remote Tea, used for communicating with the NFSv2 servers on players, is licensed under the GNU Library General Public License, version 2.

The Kaitai Struct Java runtime, used for parsing rekordbox exports and media analysis files, is licensed under the MIT License.