06Why your CDJs are on a network

The short version: Almost every system at a modern show talks over the same ordinary Ethernet used in offices. The DJ players use it to share one USB library and beat timing; the lighting desk uses it to carry hundreds of channels of control data; the audio and video systems use it to replace heavy copper cables. Most show-network failures are boring, and so are the fixes: a separate switch for each system, wired connections, and a sensible power-up order.

Ethernet in plain words

Ethernet moves data in small addressed envelopes called frames. Each frame starts with the hardware (MAC) address of where it is going and where it came from, says what kind of data it carries, holds a payload of typically 46 to 1,500 bytes, and ends with a checksum that detects corruption in transit¹⁵⁴. An IP packet - the thing the internet runs on - rides inside that payload¹⁵⁴.

A hub, the old way, repeats every frame out of every port, like someone shouting every letter to the whole office. A switch learns which device sits on which port and sends each frame only where it needs to go, like a mailroom that knows every desk¹⁵⁵. Switches still pass broadcasts to everyone, and they use the Spanning Tree Protocol to stop accidental cable loops from flooding the network¹⁵⁵.

Every device also needs an IP address. Usually a DHCP server - the front desk, often built into a router - hands them out on a lease¹⁵⁶. If nobody is at the front desk, devices give themselves a link-local address in the 169.254.x.x range: they pick a number at random and ask around (with ARP probes) to make sure nobody else has it¹⁵⁷. Pioneer players do exactly this when there is no DHCP server, which is why a laptop has to be in the same range to see them¹⁵⁸.

The physical rules are plain too. Copper Ethernet runs a maximum of 100 meters per cable; gigabit needs Cat5 or better (AlphaTheta specifies Cat5e), on the 8P8C connector most people call RJ45^159160161. Longer runs go on fiber: gigabit multimode reaches 220 to 550 meters (further with better fiber), and single-mode goes to 5 or 10 kilometers¹⁶⁰. Power over Ethernet sends power down the same cable, from 15.4 W per port in the 2003 standard up to 90 W in the 2018 one¹⁶². VLANs carve one physical switch into several isolated networks, so audio, lighting and DJ traffic can share hardware without stepping on each other¹⁶³¹⁶⁴.

PRO DJ LINK: the answer to "why does a CDJ need Ethernet?"

PRO DJ LINK was announced with the CDJ-2000 in September 2009. MusicRadar's report described an "industry-first" feature letting up to four players and rekordbox connect "by LAN connection" so that one USB or SD card could supply music to every linked player¹¹⁹.

Here is the problem it solved, in our words (AlphaTheta has never published its design rationale). A DJ wants to plug one stick into one player and load any track on any deck. Each deck also needs to know where the others are in the beat, so sync, quantize and tempo-synced effects work. That means moving whole files and metadata between machines, many-to-many, plus a steady stream of tiny timing messages. MIDI cannot carry files, and USB is a one-host-to-one-device connection. Ethernet was cheap, standard, two-way, worked with any switch and laptop, and needed no setup thanks to link-local addressing. The protocol's actual traffic matches that reading: small broadcast packets for beats and status, and larger requests for files and database lookups^165166167.

What LINK does, per AlphaTheta's own setup guide: it connects compatible players and mixers with rekordbox on a computer or phone "via wireless LAN or LAN cable", and enables beat sync, an on-air display, quantize, key sync, library sharing, and PRO DJ LINK Lighting, which sends phrase data to lights through a DMX interface¹⁶¹. Up to six players are supported, though players released before 2019 are limited to four¹⁶¹. What it does not do: "It isn't audio over ethernet," as DJ TechTools puts it - the sound still goes to the mixer over audio cables - and it does not turn a player into a MIDI controller¹⁶⁸.

Why you need a switch. Two players can share one cable. A third needs a switch; DJ TechTools' advice, written for earlier players, is that a cheap 10/100 switch with five or more ports is enough¹⁶⁸, while CDJ-3000-generation gear supports a gigabit link¹¹⁸. AlphaTheta's guidance is to power the switch or router first and let it finish booting before connecting DJ gear, to use a separate router for each LINK system in large venues, to avoid loops, and to use Cat5e or better¹⁶¹.

How the players find each other. Under the hood, devices announce themselves by broadcasting to UDP port 50000 and negotiate player numbers; plugged into a mixer channel port, a player accepts the number the mixer assigns¹⁶⁵. Mixers send keep-alives about every 1.5 seconds and CDJs about every 2 seconds¹⁶⁵. Beat packets go out on UDP 50001, one per beat, carrying tempo, pitch and the position within the bar; CDJ-3000s add absolute-position packets every 30 milliseconds¹⁶⁶. Track metadata, artwork, beatgrids and waveforms come from a small database server on each player, queried over TCP¹⁶⁷.

The famous problems.

• Player Conflict. Two players with the same number. The one that joined last complains, and the number cannot be changed while media is inserted¹⁶⁸. LINK is only established when the player number on screen is lit white¹⁶¹.
• Boot order. On a network with DHCP, if the server is not up before the players power on, they fall back to 169.254 addresses and a laptop on the venue range cannot see them. Power-cycling a player after the server is up usually fixes it¹⁵⁸.
• Wi-Fi. Deep Symmetry, whose tools depend on this network working, is blunt: "Wi-Fi has latency and collision issues that destroy the ability to reliably interact with DJ gear"¹⁵⁸.
• Old or odd models. The CDJ-850 and CDJ-350 have no Ethernet, and some all-in-one units cannot exchange LINK data with CDJs¹⁶⁸¹⁶⁹.

The people who documented it. AlphaTheta has never published the protocol. In summer 2015 James Elliott of Deep Symmetry wanted to sync his light shows to CDJs: "I broke out Wireshark and after staring at packet captures over a weekend..."¹⁷⁰. The resulting analysis became the de facto public documentation, and a whole ecosystem of third-party tools grew on it - Beat Link, Beat Link Trigger and libraries in Python, Go, C++ and TypeScript¹⁷⁰¹⁶⁹. Beat Link's own disclaimer: "This is in no way a sanctioned implementation of the protocols ... Use at your own risk!"¹⁶⁹

The official answer. AlphaTheta's rekordbox Lighting mode shipped in February 2018, using phrase analysis to build light shows automatically and outputting one DMX universe through the RB-DMX1 interface - which uses a 3-pin XLR, a connector the DMX standard technically prohibits¹⁷¹¹⁷². Its PRO DJ LINK Bridge software sends real-time track and playback data from the players to lighting, video and effects systems, with partners including Avolites, ChamSys, Resolume, SoundSwitch and TC Supply¹⁷³. In September 2026 rekordbox and Bridge added official SoundSwitch support, replacing an older integration that relied on Ableton Link tempo sync¹⁴⁵. TC Supply's ShowKontrol is the other established bridge from CDJs to lighting desks¹⁷⁴.

Denon has its own equivalent, StageLinQ, whose public documentation is likewise community-built¹⁷⁵. And Ableton Link (2016) syncs beat, phase and tempo between apps over wired or wireless networks with no master: "No single participant is responsible for maintaining the shared session tempo"¹⁷⁶¹⁷⁷.

DMX, RDM, Art-Net and sACN: how the lights get their orders

DMX512 is the language lights speak. Created in 1986 by USITT and now the ANSI E1.11 standard (current edition 2024 per ESTA), it sends 512 channels - a universe - one way down a daisy-chained cable, each channel a number from 0 to 255¹⁷²¹⁷⁸. A moving light might use a dozen or more channels: one for pan, one for tilt, one for dimmer, one for the gobo wheel, and so on. The physical rules: 250 kbit/s on RS-485 signaling, five-pin XLR (the standard prohibits three-pin, which is widely used anyway), no more than 32 devices (unit loads) per line, no more than 400 meters, and a 120-ohm terminator at the end. A full universe refreshes about 44 times a second¹⁷².

RDM (ANSI E1.20) adds two-way traffic on the same cable, so a console can find fixtures, set their addresses and read back their status remotely¹⁷²¹⁷⁸.

The arithmetic explains why lighting moved onto Ethernet (this is our reasoning from the cited limits). One DMX cable carries 512 channels to at most 32 devices. A festival rig of moving lights, LED fixtures and pixel-mapped strips can need dozens or hundreds of universes. A single Ethernet cable carries many universes, runs over fiber, uses commodity switches and can talk back. The usual chain is console to Ethernet to a node, which converts back to DMX cable for the last daisy chain to the fixtures.

Two protocols do the carrying:

ENTTEC's advice: either is fine for a small rig; choose sACN for console takeover or big managed networks, and Art-Net for RDM over the network or lots of universes on unmanaged switches¹⁸².

Timecode and clock: "where" versus "how fast"

Timecode says where you are in a show (hours:minutes:seconds:frames). Clock says how fast to go. SMPTE LTC carries timecode as an audio signal - 80 bits per frame at 24, 25 or 30 frames per second - so it can travel over any audio cable¹⁸³. MIDI arrived in 1983¹⁸⁴; MIDI Timecode (1986) packs the same timing into small quarter-frame MIDI messages¹⁸⁵, and MIDI beat clock sends 24 pulses per quarter note plus start, stop and continue messages¹⁸⁶. OSC, proposed at CNMAT in Berkeley in 1997 and published as spec 1.0 in 2002, is the flexible networked successor that many visual and lighting tools use¹⁸⁷.

In practice (our reading of how the tools are used), a fixed headline show with lights, video and pyro cued to a playback track runs on timecode. A DJ set with no fixed timeline drives lighting and video from beat data instead: PRO DJ LINK, StageLinQ, Ableton Link or MIDI clock. Bridges like ShowKontrol can generate SMPTE timecode synced to the decks, which blurs the line¹⁸⁸.

Audio and video over IP: Dante, AES67, AVB, NDI

Dante, developed in 2006 by Sydney's Audinate, carries uncompressed multichannel audio over ordinary IP networks - up to 1,024 channels per link, with latency as low as 150 microseconds¹⁸⁹. Yamaha's design guide spells out the switch rules: non-blocking gigabit, quality-of-service priority for clock sync then audio, IGMP snooping for multicast, managed switches, and Energy Efficient Ethernet turned off, because "it can be said that EEE is incompatible with real time applications"¹⁶⁴. AES67 (2013) is the interoperability bridge between Dante and rival audio networks¹⁹⁰. AVB, a set of IEEE standards, and its pro-audio certification Milan (launched 2018, backed by L-Acoustics, Meyer Sound, DiGiCo and others) are the alternative favored by some loudspeaker makers¹⁹¹¹⁹². NDI, revealed by NewTek in 2015 and now published by Vizrt, moves video between computers and cameras over the network at about 100 Mbit/s for HD, or 8-20 Mbit/s in its compressed HX form¹⁹³¹⁹⁴.

Why it matters at a show (our summary): one fiber pair replaces a multicore snake, the same audio can feed front of house, monitors, broadcast and recording without splitters, and VJ laptops, media servers and cameras can swap feeds without new cable runs.

The boring checklist that saves shows

Distilled from vendor guidance (we found no measured data on failure rates):

• Give each system - DJ link, lighting, audio, video - its own switch or VLAN¹⁶¹¹⁶⁴.
• Keep show traffic off the venue LAN and off Wi-Fi¹⁵⁸.
• Power the switch first, then the gear; on DHCP networks, the server before the players¹⁶¹¹⁵⁸.
• Check player numbers before doors¹⁶¹.
• Only one LINK program per network adapter; give a laptop a second Ethernet adapter if it must join¹⁵⁸.
• No loops¹⁶¹.
• Energy Efficient Ethernet off; IGMP snooping on for multicast¹⁶⁴¹⁸².
• Confirm Art-Net universe numbering between vendors¹⁸².
• DMX lines: terminate them, stay under 32 devices and 400 meters¹⁷².
• Copper over 100 meters becomes fiber¹⁵⁹.
• Carry a spare switch of the same model, and keep firmware current¹⁶⁴¹⁶⁸.

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