What is sACN (E1.31)? Multicast DMX for beginners

The one-sentence answer

sACN is Streaming ACN, standardised as ANSI E1.31. It is another way to send DMX universes over Ethernet. Receivers join a multicast group for the universes they care about, so the switch does not have to flood every packet to every port the way a naive Art-Net broadcast does.

The payload is still DMX: up to 512 channels per universe. If you can patch a fixture, you can use sACN. The new ideas are multicast, universe numbers, and optional priority so two senders can coexist without a fistfight.

Multicast, in human terms

Unicast is a letter to one address. Broadcast is shouting in a room. Multicast is a club: devices that joined the club hear the traffic; devices that did not join should not. sACN maps each universe to a multicast address. A node that needs universe 3 joins that group. A camera on the same switch that never joined should stay quiet.

Cheap or misconfigured switches treat multicast like broadcast. IGMP snooping is the feature that keeps multicast contained. On a five-port unmanaged show switch with only lighting devices, you may never notice. On a venue backbone, ask the IT person whether IGMP is on before you dump forty universes onto the corporate LAN.

Universe numbers and priority

sACN universe numbers start at 1 in the standard (universe 0 is not used the way some Art-Net stacks use it). Confirm what your sender and your node call “universe 1.” Off-by-one errors are as common here as they are in Art-Net.

Priority is a number in the packet. If two controllers send the same universe, receivers typically use the higher priority. This is how a house system can hold a look while a guest desk takes over, then fall back when the guest stops sending. It is not magic merge on every channel unless the receiver documents HTP/LTP behaviour.

Sync and preview packets exist for more advanced consoles. You can ignore them on a first club install. Get unicast or multicast streaming, matching universe numbers, and a wired path working first.

When sACN is the easier pick

If you are building a dedicated lighting LAN and the nodes speak sACN, multicast is often cleaner than Art-Net broadcast. Many modern nodes speak both. Festivals and rental houses often standardise on sACN for shared backbones.

If the venue’s nodes are Art-Net only, send Art-Net. Converting at the last hop is cheaper than replacing hardware. The fixtures do not care which Ethernet dialect carried their 512 bytes.

The same rules as any show network

Wired Ethernet. Same subnet as the nodes, or a carefully designed L3 setup you actually understand. Firewalls open for the sACN UDP ports your stack uses (commonly 5568 for the data stream). No guest Wi-Fi. No “I’ll just plug into the office switch.”

Test with a DMX viewer or a known-good node before the doors open. A visualiser receiving sACN proves the sender is alive. It does not prove the hardware node joined the right group on the right NIC.

Multicast gotchas you will actually hit

A receiver that never joins the group hears silence. Some nodes need you to enable sACN per port and type the universe. “It speaks sACN” on the spec sheet is not the same as “port 2 is subscribed to universe 7.”

Two senders at the same priority can produce a flicker war. If the house system and a guest laptop both stream universe 1 at priority 100, pick a winner: raise one, or stop one. Priority is a tie-break, not a mixer.

Laptop power-saving sometimes sleeps the NIC. Disable USB selective suspend on a show machine. A node that “works in soundcheck and dies at 23:00” is often a dock that napped, not a protocol bug.

A first sACN night

Set the sender to universe 1, unicast or multicast as the node manual prefers. Set the node port to universe 1. Put one fixture at address 1. Full the first channel. If nothing happens, you have an sACN problem, not a show-file problem. Only then add more fixtures.

Write the multicast address the node joined, if the display shows it. When IT later asks “what are you spraying on the LAN,” you can answer with a number instead of a shrug. That conversation goes better when you already isolated lighting onto its own switch.

Related

• What is DMX? →
• Art-Net vs sACN →
• DMX over Ethernet →