What Is DMX Merging? HTP and LTP Explained
What happens when two sources send data to the same DMX universe? The answer is not “they mix” — it depends entirely on the merge rule. Choose the wrong one and your moving heads will point at the ceiling the moment the backup console takes over.
In short
- Merging combines two sources feeding the same universe into one output.
- HTP: the larger value per channel wins — correct for intensity.
- LTP: the most recent source to change wins — correct for attributes.
- Merging moving-head channels with HTP produces meaningless positions.
- On a network, the sACN priority field gives far cleaner redundancy than merging.
What merging is
Merging combines two (sometimes more) DMX sources into a single output. Typical situations:
- Main console plus backup console
- Main console plus an architectural or wall-panel controller
- Main console plus an effects generator or media server
- Different playbacks inside one console touching the same channel
The device doing the work might be a DMX merger, a node, or the console itself. The rule is one of two options: HTP or LTP.
HTP — highest takes precedence
The merger compares each channel individually and passes through the larger value. If source A sends 100 on channel 1 and source B sends 180, the output is 180.
| Channel | Source A | Source B | HTP output |
|---|---|---|---|
| 1 | 100 | 180 | 180 |
| 2 | 255 | 0 | 255 |
| 3 | 0 | 0 | 0 |
| 4 | 60 | 45 | 60 |
HTP is the right choice for intensity channels. The logic is intuitive: if either source wants that light on, it comes on. This is the default behaviour of classic dimmer systems.
LTP — latest takes precedence
Here the size of the value is irrelevant; what matters is which source changed most recently. The instant a source alters a channel it takes ownership and keeps it until another source changes that channel.
LTP is the right choice for attribute channels: position, colour, gobo, prism.
Apply HTP to a moving head and…
If source A sends 40 on pan and source B sends 200, HTP passes 200. But 200 is not an average or a “better” position — it is simply the bigger number. The result is a position neither source asked for. The same mistake on a colour wheel produces a colour nobody selected. Never merge attribute channels with HTP.
What about a mixed universe?
A real universe contains both dimmer and moving-head channels. That is why serious mergers and consoles let you define the rule per channel range: HTP on 1–96, LTP on 97–512, for example. Cheap mergers apply one rule to the whole universe, which inevitably breaks something in a mixed rig.
Merging on the network: sACN priority
If your data travels over Ethernet there is a cleaner route. sACN (E1.31) carries a priority value from 0 to 200 in every packet. Receivers follow the highest-priority source, and the merge-rule debate simply disappears.
A typical redundancy setup: the main console transmits at priority 100, the backup at 90. If the main console dies its stream stops, and after a timeout receivers fall over to the backup. When the main console returns it takes control again. No channel is merged and no attribute is corrupted.
Practical checklist
- HTP for intensity, LTP for attributes.
- Can your merger set rules per channel range?
- If the goal is redundancy, consider sACN priority instead of merging.
- Rehearse the failover. Actually pull the plug on the main console and watch.
- The merger itself is a single point of failure; carry a spare on critical systems.
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