Touch Audio

Touch Audio analysis contributed by @nudge.

The ability to stream audio from players to compatible mixers was added with the introduction of the CDJ-3000 in late 2020.

As of September 2020, the CDJ-3000 has the ability to stream Touch Cue and Touch Preview audio data to DJM-900NXS2 and DJM-V10 mixers. With the analysis of this protocol, we now have an understanding of how audio is handled.

Touch Cue/Preview Protocol

After startup, the mixer continuously sends timing packets (20) to each supported player every 7ms. In order for a player to receive timing packets, bit 5 of byte cd of the CDJ status packet (type 0a, port 50002) must be set.

The timing packets contain a flag E which indicates that the "Link Cue" button on the mixer is on or off.

Audio packets are carried over UDP port 50004.

In the below description, the phrase "touch audio is engaged" means that a DJ is actively scrubbing on a waveform to engage the Touch Cue or Touch Preview feature. The phrase "touch audio is disengaged" means that the DJ stops scrubbing on a waveform.

When touch audio is engaged on a player (the initiator player):

  • The initiator player sends a zero-filled audio packet (1e) to the mixer.

  • If the initiator player is currently not elected, the mixer sends a handover packet (1f) to the initiator player. The next timing packet will set T to the player number that initiated touch audio. If the player is already the elected player, this step is skipped.

  • If E is 00 and touch audio is engaged:

    • The elected player sends zero-filled[1] audio packets (1e) to the mixer for every second <audio-timing,timing packet>> received.

  • If E is 01 and touch audio is engaged:

    • The elected player sends audio packets (1e) to mixer with uncompressed PCM signed 16-bit, little endian, 48000Hz, stereo data for every timing packet received.

    • If touch audio is disengaged, the elected player continues to send zero-filled audio packets for every timing packet received.

  • The elected player stops sending audio packets (1e) to the mixer when both E is 00 and touch audio is disengaged OR the timing packet (20) specifies that another player has taken over. The mixer continues to send timing packets with T set as the last elected player.

Audio Timing Packets

Audio Timing packets are sent from the mixer to supported players every 7ms.

0123456789abcdef5173707431576d4a4f4c200010Device Name (padded with 00)012000DlenrCET
Audio Timing packet.

D indicates the number of the mixer sending the packet.

lenr reports the length of the rest of the packet, in other words, the number of bytes which come after lenr. Currently seen as 06.

C is a 32-bit counter that increments on every timing packet sent. Equates to the number of 7ms periods passed since mixer turn on (DJM-900NXS2 starts emitting timing packets roughly after 19 seconds from turn-on, i.e. around 0a73).

E specifies that the "Link Cue" button is enabled on the mixer, 01 for active, 00 for inactive.

T indicates the player number elected as the audio sender. 0 if no player is elected.

Audio Data Packets

Audio Data packets are sent from players to the mixer in response to Audio Timing packets.

Irrespective of the lenp field, this packet is always 56c bytes long.
0123456789abcdef5173707431576d4a4f4c1e0010Device Name (padded with 00)012006DlenpI000000C30PCM Data40i+00
Audio Data packet.

D indicates the number of the player sending the packet.

lenp specifies the length of valid data of the entire packet, either 056c (header plus audio data) or 002c (header only). 056c packets are sent in response to a timing packet in which E is set to 01. 002c packets are sent in response to a timing packet in which E is set to 00. Note for 002c packets , the entire packet is still 56c bytes long.

I specifies that the packet is in response to touch audio being initially engaged. A value of 1 is sent on packets where touch audio is initially engaged. Once engaged, subsequent audio data packets send 0 until touch audio is disengaged.

C specifies the value of counter received from the mixer that the player is responding to.

PCM Data 540 bytes (336 samples) of uncompressed PCM, signed 16-bit, little endian, 48000Hz, stereo data, or zeros.

Audio Handover Packets

Audio Handover packets are sent from the mixer to the initiator player to indicate that the player should start sending Audio Data packets.

0123456789abcdef5173707431576d4a4f4c1f0010Device Name (padded with 00)012000D0000
Audio Handover packet.

D indicates the number of the mixer sending the packet.

1. If touch audio is engaged but E is toggled from 01 to 00, instead of the player sending zero-filled audio packets (1e), the player will repeatedly send the last frame of audio data previously sent to the mixer when E was 01.