• Stage Basics: DMX

    by  • July 4, 2015 • Articles, Education, Uncategorized

    DMX is like cable television. Although your house is being pummeled with 5000 channels of garbage, you are able to safely watch the Spurs or This Old House because your TV can disregard any information it doesn’t need. Only part of the signal coming through the coaxial cable is for the station you’ve got it tuned to, and the TV only puts that part on the screen. DMX compliant equipment functions similarly in that all the fixtures, dimmers, or effects in line will only respond to the stuff they’re supposed to. This is achieved by addressing each unit and patching its address to a channel or series of channels in your system, allowing your console to communicate with each unit individually.

    Time was, all the equipment manufacturers that produced intelligent lighting and such had their own communication systems and connections. Nobody’s controller could drive another company’s fixtures. That was until 1986, when USITT developed the DMX512 protocol. Pretty much all intelligent lighting equipment is DMX compliant, although some companies produce equipment that operates on their own proprietary control standards, as well.

    Maintenance of DMX512 was transferred to the Technical Standards Program of the Entertainment Services and Technology Association in 1998. In 2011, however, ESTA merged with the similar British organization PLASA. Now when you google ESTA, you can find out how international travelers register for the Visa Waiver Program. And did you know DMX is also a rapper?

    PLASA is now responsible for keeping what became labeled the DMX512-A protocol whole. ANSI and EIA/TIA (look them up) also have contributed to the standardization of DMX. All of these organizations have helped sculpt the way we make the lights go on, spin around, flash, and change color.

    DMX is an acronym for “Digital MultipleX”. You don’t have to know what that means to know that it sounds very cool. Because it is cool. It’s probably the coolest thing that’s ever happened to live entertainment.

    The 512 refers to the number of pieces of information (DMX addresses) in a network segment known as a universe. It is also the maximum number of addresses that can be commanded on a single DMX cable. So when you run data along a pipe from unit to unit, all those instruments are in the same uni- verse. To learn everything about DMX, go to http://www.usitt.org/dmx512 or www.elationlighting.com/pdffiles/ dmx-101-handbook.pdf . You could also check out Wikipedia, but like they say, “this page has some issues.”

    The DMX system is digital, so information is dispersed in binary code. A series of 8 bits (1’s or 0’s) is created to represent a value between 0 and 255, with 255 being full, or 100%.

    Programming is done in percentages. All values are translated into a value proportional to the 0-255 range because that’s the language the units understand. So, as applied to an intensity level, 255 is full, 50% would be around 127 in this binary format, and so on.

    A single data line is capable of conveying 512 pieces of information, hence the name. These pieces of information are referred to as addresses. In its simplest form, a single unit that receives information, a dimmer, requires only one address, whereas a more complex unit may require dozens more. (A VL 4000 Spot Luminaire, in its most sophisticated control mode, requires 57 DMX addresses.)

    A dimmer only has one parameter (one aspect of all the things a unit can do) to control, that being intensity (how bright the light is). That is why it only takes up one address in its universe (the single data line of 512 addresses). Units that change color, such as LED lights, generally take up 4 spots in the universe. The additional channels are for the parameters that control the intensity of individual colors and allow the board-op to mix red, green, and blue at various levels.

    A moving light, like the VL 4000, requires more addresses because it has more non-intensity parameters (NIP) to control. There are four major parameter categories; intensity, focus, color, and beam, with each parameter within those categories requiring separate addresses. The focus (pan and tilt) requires at least two, but your unit might also have a fine pan and a fine tilt. So that’s four right there. You’re gonna need some color mixing, more than likely RGB, but you might have a unit that’s got amber and white LED’s too.  Five more addresses.

    Beam parameters are divided into three sub-categories: form (parameters that affect the size and quality of light, like edge, zoom, iris, and frost), image (gobos and effects), and shutter (framing parameters). All these functions require individual addresses. You can see how a universe can start to feel cramped, particularly with units that have many parameters.

    How a fixture’s addresses line up with its parameters is known as its channel mapping. This is the channel mapping for the Blizzard Flurry 5 in 13 channel mode:

    Channel. What It Does

    1. Pan
    2. Pan Fine
    3. Tilt
    4. Tilt Fine
    5. Dimmer
    6. Red Intensity
    7. Green Intensity
    8. Blue Intensity
    9. Amber Intensity
    10. White Intensity
    11. Color Macros
    12. Strobe
    13. Sound Active

    There’s also a 7 channel mode, but it’s not nearly as fun. You’d use it if you if you were short on DMX channels, like if your controller had a limited number of channels or you were filling up universe space.

    As for universe space, it is possible to command multiple universes in a single lighting system. The ETC ION control console, for example, has 2 DMX outputs and therefore can run 2 universes on its own. It can run even more through the use of nodes and networking. (Nodes are devices that can be installed on a network with an ethernet cable and can be configured to operate DMX addresses outside the 512 range).

    Parameters are assigned a DMX value between 0 and 255. A tilt value of 50% on the board, for instance, would be received by the unit as 127 and would tell the unit to point straight down, whereas 100% would be all the way forward and 0% all the way back, or vice-versa.

    Similarly, a parameter value for pan of full (255) will move the unit all the way to its extreme in one direction and a zero all the way in the other.

    Color can also be manipulated in a manner in which a DMX value (or range of values) corresponds with a particular color. This is how it works for scrollers and units with color wheels, anything that has only one channel reserved for color selection. This is known as the Value Mapping of a particular unit

    Typically, addresses in universe 2 would be in the range of 512-1024, and that’s what they’ll be if your plugged into the second DMX output on the board, but you can configure the nodes to begin subsequent universes with addresses such as 1001, 2001, etc., to simplify things. Addresses above 512 can be selected on the control device by actual number or by a format such as 2/129. This would be address 129 in universe 2, or actual address 641. Or address 1029 if you have universe 2 beginning with DMX address 1001 for less arithmetic.

    All of these aspects are controlled by separate DMX addresses. Not all intelligent fixtures are capable of all these things, but the ones that are occupy a lot of space in the universe.

    Lighting units in the same universe can receive data by running the DMX from the DMX ‘out’ of one to the DMX ‘in’ of the next. This linking of the units together is known as a daisy chain. The DMX standard allows for up to 32 units to be installed on a single data line at a distance of up to 4000 feet, though some manufacturers recommend that you don’t go that far without the use of an isolated optical splitter/repeater. This allows the data line to go farther and split off in multiple directions, each being capable of linking up to 32 more units. Using Y cables or T connectors won’t work. The signal needs to be regenerated and all branches optically isolated from each other.

    DMX cable typically is equipped with 5-pin XLR connectors. A positive and negative wire create a balanced signal in which any interference will effect both signals equally and therefore be eliminated. DMX cable also contains a zero voltage ground wire, but pins 4 and 5 are not necessary in standard DMX application. They are there to perform functions like communicate information about the unit, like lamp hours or operating temperatures, back to the controller. Some manufacturers also use pins 4 and 5 for applications that are specific to their own equipment, often at voltages that are potentially damaging to standard DMX compliant devices. You probably don’t have to worry about this; just use the materials you are provided with on the gig and all will go swimmingly.

    Since only 3 wires are necessary to run DMX, a 5 to 3-pin adapter can be used to incorporate 3 pin DMX cable and instrument inputs. The XLR connectors are the same as those used on audio cable, but microphone cable should not be used to transmit DMX. (XLR refers to the physical shape and characteristics of the connector; Cannon X connector with a Latch and Rubber guard). A DMX signal passes 250,000 bits of information per second. It’s way more complex than an audio signal and needs specialized protection from interference. While any 3 pin cable will probably work, especially with a low number of units and at short distances, it would be susceptible to signal deterioration. In other words, it might make your intelligent lighting act really stupid.

    So when you’re running data along a pipe from unit to unit, all those instruments are in the same universe.

    BTW: Protocol means “system of standards.”

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