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mdj-900-tools

Gemini Sound MDJ CDJ DJ Media Player tools

Introduction

I wanted to learn how to DJ using 'CDJ's but the market monopoly of Pioneer DJ means that genuine CDJ equipment is extremely expensive, especially when two units are involved. Instead, I chose a copycat device from Gemini Sound called the MDJ-900. These are based on Linux 2.6 kernel and implement a similar network approach that Pioneer DJ units do, i.e. you can connect them to their own network switch and share a single USB stick across all of the MDJ players. Additionally you can define one player to provide 'master sync' and any client players will follow the tempo.

Since I love tinkering with systems, I wanted to achieve three things:

  • Remove the need to use a USB stick - instead allow a computer to act as a music server
  • The MDJ supports AAC/MP3/WAV/AIFF files. Most of my collection is FLAC, so I want the music server to transcode FLAC -> WAV
  • Support the native 'Playlist' format of the MDJ series so it's easier to work with industry standard tools like Serato

Protocol

These are brief notes to aid my memory rather than a full write-up.

  • Each MDJ will get a random MAC address at boot
  • No DHCP address is requested - each MDJ will choose a random autoconf address in 169.254.x.x/16
  • Each MDJ sends out two UDP heartbeat packets to 169.254.255.255 port 6996 every 2-3 seconds. The SOURCE PORT of these broadcasts is mathematically linked to the source IP address, via a currently-unknown mechanism.
    • e.g. IP 169.254.175.49 MUST use source port 35756 or MDJ's in the network will not acknowledge the server's existence
  • If it's been eight seconds since the last heartbeat, the MDJ will go into 'offline' / emergency loop mode
  • The contents of those heartbeat packets are mostly blank with a few magic values, and some reverse-engineered values
  • The MDJ's are Linux-based and use NFS v3 to share the USB stick - the mountpoint is typically '/tmp/0' but can be changed provided it's not longer than seven (!) characters

Usage

I use a Mac, so I needed to set up the NFS server. This was mercifully simple.

  • Manually set the IP address of your Mac to be 169.254.175.49 with netmask 255.255.0.0 - it must be precisely this IP address + netmask!
    • You might want to use an extra USB network adapter so you don't lose Internet access
  • Copy a bunch of MP3s into a new directory /opt/0
  • Create /etc/exports with the following single line /opt/0 -mapall=501
  • run nfsd start
  • run showmount -e to verify that path is being exported
  • run python server.py
  • Power up an MDJ unit and press the 'Link' button on the right. It will stay at 'CONNECTING' for about 10 seconds before showing a waveform.

Issues

  • The gemini sqlite database is not being read correctly - per-track BPM is not set!
  • Takes much longer to join the Link group then when using two MDJ's on a private network

Todo

  • Virtual NFS server
  • Transcode file formats
  • Serato Playlists / Crate conversion

find ../*mp3 -print0 | xargs -0 -n 1 python3 ~/mdj-900-tools/update_geminidb.py