Stage Lighting 101, Part 2: Understanding DMX

Date:2020/3/4 10:21:37 Hits:

We covered the basics of lighting fixtures in our article Stage Lighting 101, Part 1: Understanding the Basics about what fixtures are and how to place them, so now how do we control them? How do they work? What problems or issues might we run into? This article will examine how we direct the lights to accomplish a great look onstage. Most lighting fixtures on the market communicate using a protocol called DMX (Digital Multiplexing). While there are some other lighting standards as well, we will save those for a different article.

First, what is DMX? DMX (officially titled USITT DMX512) is a unidirectional serial data protocol, meaning it leaves the controller (computer or lighting board) and travels through all lighting fixtures in a daisy-chain. If the chain is extremely long or has a lot of fixtures, you may need a terminator at the end of the chain, plugged into the last fixture in the chain. Otherwise, most fixtures will automatically terminate the chain if nothing is plugged into their output. DMX networks typically only have one master device on the network, usually the controller, and many slave devices — the lights. In more advanced systems, there can be multiple controllers; however, there is still only one master device. To have multiple controllers on a network, there are other devices needed to merge these signals together and to set priorities.

In the infancy of digital lighting control, many lighting manufacturers used their own proprietary protocols and connectors. When DMX was finally standardized in 1986, these manufacturers made firmware updates to their fixtures and made the 5-pin XLR the standard connector. The reason for five pins is that pin 1 would be the ground, pins 2 and 3 would be data link 1, and pins 4 and 5 were reserved for data link 2 and/or proprietary data.

Over the years, the second pair of pins (pins 4 & 5) on the connector stopped being used, since 3-pin DMX proved to be very reliable. This is why you may sometimes see fixtures with a 3-pin, 5-pin, or both connectors on the fixture. While 3-pin is far more common (until you get into semi-pro and pro fixtures), you can use 3- and 5-pin configurations interchangeably because DMX only needs 3 pins to work, and a simple 3-pin to 5-pin adapter can interface between the two. However, it is recommended to use 5-pin cables as much as you can to distinguish them from audio cables that also use 3-pin XLR connectors. One of the biggest justifications for this is to prevent a lighting cable from being inadvertently connected to an audio console that has phantom power (+48V) enabled. If this happens, it could damage the internal electronics of the lighting fixture.

The copper inside of DMX cables is also different from standard audio or mic cables. DMX cables use roughly 110-ohm cable, where microphone cables are typically around 45 ohms. The different impedance between these cables matters with lighting networks and can cause your lights to either not respond or respond sporadically. In addition, with DMX cables the total length of a DMX run must be less than ~3,900 feet, which is easily attainable. You also cannot have more than 32 devices connected on a single chain. If you have more than 32 devices, you would need to use an Opto-Splitter. A splitter like the Chauvet DJ Data Stream 4 will allow you to have 32 devices connected to each DMX output connector. You cannot use Y-cables, as this approach does not electrically isolate the DMX lines and would cause data reflections.

DMX also has a limitation to how much data can be sent down a single line. A DMX line is limited to a total of 512 channels, which is also called a universe. Each lighting fixture you have uses a number of DMX channels depending on how many parameters the fixture has. Lights can also have multiple personalities, or profiles, depending on how much or how little control you want. Note that the 512-channel limit is independent of the 32-device limit. Let’s say you have 40 lighting fixtures that use three channels each: you are only using 120 channels total. You can fit these all in the same universe of control, however, if you have more than 32 devices. You would implement an Opto-Splitter and split your devices up among the outputs in whatever configuration you would like, as long as each DMX leg has less than 32 devices on it.

Now let’s see how lights use DMX. For example, let’s look at the Chauvet DJ SlimPAR Pro H. This light has three different personalities, or profiles. It can be used in a 6-, 7-, and 10-channel mode, and again the more channels a fixture uses, the more control you have. Let’s look at 7-channel mode:

Channel 1: Dimmer
Channel 2: Red
Channel 3: Green
Channel 4: Blue
Channel 5: Amber
Channel 6: White

Channel 7: UV

Each DMX parameter on a fixture operates independently, so we can make changes that are very subtle or impactful. Say I was to make this fixture a magenta color. I would turn up channel 2 (Red) and channel 4 (Blue) until I got my desired shade of magenta. However, turning up just these channels on the fixture (2 & 4) would not put out any light. I would also need to turn up channel 1, which is my dimmer that controls overall intensity. On moving fixtures, this control becomes even more complex, because there are other parameters available, such as Pan and Tilt or gobos, again all independent.

When setting up a lighting rig for the first time, our lights are “dumb,” meaning that they need to be told what to do (personality), as well as what their starting address is. If I have four of the same fixture mentioned above in the same personality, their addresses would be: 1, 8, 15, and 22. The fixture already knows, “Hey, I’m using up 7 channels, but where do I start??” Remember, all 512 channels of data flow through every fixture in a DMX lighting chain. So we have to tell the fixture which channels it should “listen to.” You may be wondering, “Can I put two fixtures on the same address to save channel count?” You sure can; however, there are two caveats: each fixture must be the same make and model, and every fixture that is set to the same DMX address will ALWAYS react the same exact way. If you don’t want this result, then each fixture must have its own starting address.

What happens if you need more than 512 channels of control? Then, quoting from Jaws, “You’re gonna need a bigger boat,” e.g., a larger controller. That means getting a larger console and/or software that can output multiple universes. For example, the Avolites Titan Mobile, when paired with a computer, can output 16 universes of control (allowing 512 channels each) or 8,192 channels for you math nerds.

Now that you understand the basics of DMX, what if you don’t want to use it or don’t need that degree of programmability. Well, good news! There are a few ways around it. Lighting technology in recent years has come a long way and added some features to make lights more plug and play, depending on the application. Many lights have auto-programs, or even static color store in them. You can access these programs, or static looks, with a built-in menu on the fixture. Let’s say you’re a wedding DJ and want to have up-lighting around the room matching the bride and groom’s color scheme. You can go around to each light and set a static color and simply walk away — that fixture will stay that color. FYI, the Chauvet DJ Freedom PARs are awesome for this application.

Now if you want a bit more control but still not at a controller level, you can make one light a master control and designate the rest as slaves. This will only work with like fixtures, but the results can be impressive. For example, if you have four Chauvet Pro R1X spots, you can set the first fixture to be the master and have it run a program with movement and color changes. Then by setting the remaining fixtures to slave mode and daisy-chaining them with DMX cables, all the fixtures will act together in unison. You could have any group of fixtures running their own independent light programs daisy-chained together to make a decent show.

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