Basics

Background

Theater lighting goes back several hundred years.  From candles, to oil, to gas and electric fixtures, all with the purpose of setting the mood on the stage.  The modern lighting system consists of various types of fixtures (for various purposes), power and control signal cabling, a controller and possibly dimmers.  There are other elements such as supporting piping, or trusses, clamps and safety cables that most likely would be required as well. We'll discuss each of these and some design issues below.


In a typical system the controller sends a control signal through a low voltage cable either directly to a dmmer (or a fixture with built in dimming - like most LED or Moving Head fixtures), which allows the user to change the brightness of a fixture (i.e. in a range from off to full on) by altering voltage waveform.  Dimmers are created for a set number of channels that they can control and provide power for those channels.   


DMX512

A common control signal protocol is DMX512 which stands for Digital Multiplex (also known as simply DMX), as is an industry standard.  This is a form of communication which sends packets out to devices (through a DMX cable) that receive the packets and interpret them. The transmission side is known as Multiplexing and the receiving side as DeMultiplexing.   It allows for up to 512 different channels to be controlled (per universe).  Diimmers and/or intelligent fixtures (with other capabilities) can be daisy chained together, if they have both an IN and OUT connector.  This requires the user to set addresses for each dimmer and/or fixture with dimming capabilities in the chain.


Each DMX channel can have a value from 0 to 255 (256 steps).  These are usually changed by moving or setting values with a fader (usually a sliding potentiometer) on a controller (or software equivalent).  For dimming, 0 is off, and 255 is full on, with increasing degrees of brightness towards 255.  Note that the controller may or may not indicate these values, in that case, top is full on, and bottom is off. 


Intelligent fixtures usually use several channels, each with a different function.  For example, an LED PAR fixture may use channel 1 for dimming, 2 for Red, 3 Green, 4 Blue, 5 White, 6 for various preset color mixes, and so on.  When you assign an address, it works similar to the dimmer example, where the fixture now consumes a number of channels.  For example, an LED PAR that uses 8 channels, will consume channels 21 through 28, when its address is set to channel 21.


Address

Addresses are used to allow the DMX signal to control the correct fixture or dimmer.  In a single DMX universe, you can have up to 512 total channels that fixtures or dimmers can consume.  You set the address on a device either by dip-switches or by some type of interface (i.e. buttons and LCD/LED screen).  And you will usually only be setting the starting address (more on that in the Intelligent lighting section).


For example, let's say that you have two dimmers and each dimmer can control six channels.  You might want to set the address of the first dimmer at channel 1.  It would then consume channels 1 through 6.  For your second dimmer you might want to set the address at channel 7, which would consume channels 7 through 12. 


However, the addresses don't have to be consecutive, but you also don't want to overlap.  For example, you could set the second dimmer to start at channel 11, which would consume channels 11 through 16.  However, you normally wouldn't want to set the second dimmer's address to start at channel 4 (4-9), since channels 4, 5 and 6  are already being used by the first dimmer.  Essentially, you'll be controlling 3 channels in the first dimmer and 3 channels in the second dimmer with the same 3 faders in the controller (4, 5 and 6).  Some unexpected things can happen with this if you're not careful, but it can also be used to control several fixtures from one set of faders.


DMX Cable

DMX Cable is how the signal gets from a controller (transmitting source) out to dimmers and/or fixtures.  However, wireless DMX transmitters and receivers are available today as well.  The DMX512 cable standard uses shielded twisted pair cable, with a characteristic impedance of 120 Ohms resistance.  Dimmers and fixtures are daisy chained together, with a terminating resistor required at the end of the chain, to reduce signal reflections (echoing) on the line.


The DMX electrical specification requires two pairs, but only one is defined.  It also requires earth ground.  Many in the industry ignore this second pair of wires, and hence, you will see some DMX cable with 3 conductors and some with 5 conductors.  The electrical spec is for 5 conductors and 5-pin XLR type connectors (for portable installations - permanent installations can use other connectors).  Many manufacturers however will use the 3-pin XLR.  You can get converter plugs that will switch 5-pin to a 3-pin and vise versa.


Controllers

Like the name indicates, controllers are used to control the dimming of lights. The controller can be a physical console or can be a physical interface with computer software, or some combination of both. The controller sends a DMX signal (or some other low DC voltage control signal, such as ArtNet) out to dimmers and intelligent lights.  The Dimmer or Intelligent light fixture then reads the signal.


Most Intelligent fixtures such as LED and Moving Head fixtures have their own dimming circuitry and can use the DMX signal directly.  The intelligent fixture will use several DMX channels to control not only dimming, but usually other attributes of the fixture, such as color mixing, or in the case of moving head fixtures, Pan and Tilt.