Even if you are brand new to the stage lighting industry, you will have almost definitely have heard the terms DMX, ArtNet and RDM. For those new to the technology, these can often be confusing concepts. In this article, we’ll go right from the basics of these terms through to an in-depth look at their technical aspects.
Let’s start with DMX (Digital Multiplex). This a term associated most often with lighting consoles and fixtures. It refers to the digital protcol that allows a controller to speak with fixtures, giving them various commands such as colour, position and beam attributes. The DMX512 protocol was orignally developed in 1986, with subsequent revisions in 1990, leading to the USITT (United States Institue for Theater Technologies) standard, referred to as DMX512/1990. In 2004, DMX grew again in to DMX512/A. This standard was set by another orgasation called ANSI (American National Standards Institute). Soon after DMX512/A was established, another group called ESTA (Entertainment Services and Technology Association) developed RDM (Remote Device Management) and officially termed it as ANSI E1.20.
Remote Device Management offers an enhancement to the DMX protocol by adding bi-directional communication between lighting (or system) controllers and attached RDM compliant devices. All of this can still be done over a standard DMX512 cable, providing that all five wires are connected at both ends of a DMX 512 cable. Ordinarily, DMX512 only sends commands from a controller to the source. RDM, on the other hand, allows source fixtures to talk back to the controller. This is particularly useful for the setting of addresses, as well as many more functions.
When driving a car, we don’t necessarily care how our vehicle works, we just need to know the rules of the road and techniques that allow us to safely drive the vehicle. Similarly, we don’t need to know exactly how DMX works, but there are certain rules and techniques that help insure a properly executed lighting setup. Below is a detailed technical explanation of what a DMX512 signal comprises of. But first, we will begin here with what we need to understand the basics of DMX512. This protocol has developed to what we call the industry standard. Prior to the implementation of DMX, each manufacturer tended to have their own signal protocols, many similar to DMX512, but that unfortunately could differ vastly from one other. On old consoles, you might come across terms such as Muliplex, or other similar language. You can still find old hardware that uses the original protocols for control signals, however in this case you would need some form of converter to translates the old signal format into DMX, or vice versa.
DMX512 came to offer a common standard that both console and fixture manufacturers were able to implement into their products, providing consumers with a predictable platform that worked industry wide.
DMX512 is structured in segments known as UNIVERSES. When we use a console, we need to PATCH our fixtures. We typically layout and organise fixtures within these universes.
What is a DMX universe? A DMX512 universe is a set of channels (512 to be precise) on which DMX signals will operate. If more than one universe is required for a set up, then this will require a desk that provides control for multiple universes, therefore offering more channels to patch our fixtures to. This also where ArtNet enters the picture.
ArtNet is a protocol which was created in 1989 and is owned by Artistic License. It was developed as way to overcome the channel restrictions that standard DMX512 posed. It allowed for multiple universes to be transported over a signal Cat5 cable. It provided a solution that operates on DMX512 protocol, but is transmitted using Ethernet.
Of course, we lso need some type hardware to receive our DMX512 signal and respond to our commands. Lighting fixtures are the main item we use for this purpose.
Let’s look at how all of this is applied. We start with a cable to carry our signal from from the controller to the source. We will typically use DMX512 cable, which has XLR connectors on both ends. 5 wire cable is the standard, although we, for the most part, still only use 3 of the 5 wires. Often you will still see people using 3 pin XLR cables (which are easily found in the audio cable box) which work but are not often specified for professional fixtures. There are rules to follow when using these cables, such as not wanting to use Y-Type splitters and similar items that will degrade our signal. Degradation of signal can also occur if we have systems that contain cable runs which are too long. We also want to ensure that signal terminators are used at the end of all cable runs. This ensures that our signal flows properly and doesn’t bounce back onto to itself at the end of our cable runs.
So, our cable is running from our lighting console and is plugged into the first fixture, into its input. From here, we daisy chain from the output and interlink into other lights in our layout. Here is an example of a typical lighting setup:
If we look at the back of almost any stage lighting controller, we will see some type of OUTPUT. If a console is able to output DMX512, then there will usually be XLR connectors mounted onto the back of the desk. This is where we plug a physical cable into the desk and run it out to the fixtures we plan to control.
Now we must address our lights. When addressing our lights, we give them a DMX starting channel. This means the channel at which the command signal will begin to be sent for that individual fixture. Each fixture will require a certain amount of channels to operate, so depending on the amount of channels a fixture uses, we will then know where the next light’s address should begin. For example, many LED lights can take up 3 channels. If we are using these, and our first light starts at 001, then we know our next fixture should begin on channel 004 or above. There are often on board digital LED or LCD window displays that allow us to adjust items like our fixture’s address.
Some fixtures will feature a series of switches, which allow for the fixture’s starting address to programmed using binary steps.
Now that the fixtures are patched, given their proper addresses and with cables running to all of our fixtures, we are now ready to begin to design the lighting scenes that will form the structure of our show’s lighting design. This is what is known as Lighting Programming, and this is a subject in itself which we will discuss further in future articles.
For now, we can at least say that we have a better understanding regarding these basic terms, along with a knowledge of how DMX512 is utilised in our everyday practice.
You can find a more in-depth technical explanation of DMX512 on Wikipedia
Below is a definition of the process of a DMX data stream, as provided by the University of Aberdeen:
“DMX512 slots are transmitted sequentially in asynchronous serial format at a baud rate of 250kbps, using the DMX physical layer.
The content of a frame comprises a set of serial bytes (slots) sent with eight bits of data, one start bit, two stop bits. The frame starts with slot 0 and ending with the last implemented slot, with a maximum total of 513 slots.”
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