How Do We Control? 

How Do We Control?

A Basic Theatrical Lighting System

In order for artists in the theatre to begin to manipulate light, they have come up with different lighting systems. A lighting system can be as simple as a lamp being plugged into the wall or a light being switched on by a singular light switch. However, if you want to be able to control all of the properties above you will need something a little more sophisticated. Let us look at what comprises a basic theatrical lighting system. Remember, that since there has been a long evolution of equipment in the past few decades each theatre will be different. Theatres are usually under budgetary stress and cannot upgrade every time a better piece of technology comes out.

Most lighting systems will include:

The Hanging Location

Where you locate the light, hanging or sitting. This establishes what direction the light beam is coming from. In established theatres these are generally permanent locations. In other venues you may very well setup your hanging location on the spot and where every you need and have access to. For more information, see Where to Put the Lights.

The Lighting Unit

The actual instrument that produces light. Here you can change the color, shape and sharpness. Most fixtures accommodate color filters to change the color. The reflector creates the shape of the light. The shape can be adjusted via shutters, irises and barn doors. The sharpness can be changed with different relationships between lenses and reflectors. For more information, see Lighting Units.

Power Distribution

The light needs power. This is how the electricity gets from the dimmer to the light. It can be as simple as a lighting cable (heavy-duty extension cord) from the dimmer to the light, or as complicated as a patch bay that assigns circuits to chosen dimmers. A power distribution system may also include raceways, which permanently place circuitry throughout the theatre. For more information, see Lighting Cable & Electrical Distribution.

Intensity Control – Dimmers for Conventional lights and Built In Intensity Control for LED units.

The dimmers are controlled by the control board (light board) and are the physical units that either let all of the electricity get to the light, causing it to go to full, or restrict the electricity to the light, causing it to dim. Dimmers are used for conventional lighting units, not LED units. For LED units the intensity control is built into the unit. You send a control cable with the control signal from the light board to the LED fixture. With more and more LED units being used, it is very common to split the control signal with data splitters. This way, the cabling doesn’t get too crazy. It is also very common to send a signal line to each hanging location, and then have the control signal daisy chain (hold hands) from unit to unit. For more information, see Dimmers.

Control

The controller is the brains of the system. The light board, whether manual or memory, is controlled by the human. Once the control board knows what should happen it sends a signal through control wires, or wireless signals, to the dimmers and other controlled devices. Controllers (consoles/light boards) can be as simple a one slider box to a PC that can do just about anything you can imagine to huge stand alone lighting desks that can control an entire theme park.  Where it used to be a very simple device it has now gone into the world of computing where speed and complexity of programming become very important.

Control Language

In order for the controller to communicate with the lighting device there must be communication.  I may speak English and you may speak Italian we do need to find a common language for us to understand each other.  In todays stage lighting world that language is called DMX-512.  DMX 512 is short for Digital Multiplex 512.  It sends out electrical impulses to the devices it is speaking to at an incredibly fast rate.  They receive this information from the controller, through either wireless signal or a control cable, to the first device.  The first device can then send an out and continue the "daisy chain" to a second device and so forth.

Here are some important things to keep in mind about daisy chaining.

• Technically there is a limit of 1800 feet and 32 devices per a DMX line.  This however is not a hard and fast rule.  Each device will have a certain amount of resistance and therefore degrade the signal.  The less devices you have and the shorter the distance the more consistent your signal will be.  Certainly consider this if the units you are sending that signal to are hard to get to.
• You can have many lines of DMX being sent by using a splitter.  A splitter, of which there are a few varieties, can turn one signal into many by bolstering the signal and giving you more ports into which you can send out multiple lines of DMX.  Some devices can even adjust the speed of the DMX you are sending.  Some advanced lighting units can handle very fast DMX while some cheaper less advanced units may need slower speeds.
• While you may need the ability to split signals, it is not uncommon to have to go the opposite way and "merge" signals from two different controllers.  For this we have devices called mergers.  There are generally two settings you will need to set on the merger.  You will either set it for HTP (Highest signal Takes Precedence), or LTP (Lowest signal Takes Precedence)
• At the end of the daisy chain line please use a DMX terminator.  This absorbs the final DMX signal and stops it from bouncing back in to the DMX chain which would really confuse the receiving units.

Each DMX line can handle up to 512 addresses.  A channel typically refers to a digital address that has 256 steps.  In the most simplistic form a dimmer that is assigned to channel 1 has no output when the channel is outputting a 0.  It is at full intensity when the output is at 255.  Zero is counted as a step which brings it to the 256 steps.  The graduations in the middle are what gives you the ability to have different intensities of light.

Here are some important things to keep in mind about those 512 addresses.

• Each pack of 512 DMX addresses is called a Universe.
• Universe one is 1-512, universe two is 513-1024.
• Not every controller has multiple universes.  If it has one output than safe to say it only supports one universe.  If it has two outputs then it is plausible that it has two universes.  
• Some controllers can handle tons of universes but that we will speak about separately.

The information must go from point A to B.  Simplistically, the digital signal must get from the controller to the device.  We can do this with wires or without wires.  To do it without wires you must have a transceiver and a receiver.  They need to "see" each other, in other words nothing can be in the way to block the signal.  Blue tooth is just beginning to hit the market.  The old fashioned way is to use a hard wired cable.  DMX cables come in two basic varieties, 3 pin and 5 pin.  The information passed can be handled with 3 pins (2 wires and a ground), so why the 5 pin variety.  The reason is that some systems use the other wires for other information.  An example would be a console and a dimmer rack talking back and forth if a “load” has changed which may indicate a blown lamp.  It is typical to use XLR connectors for DMX 512.  The 3 pin looks a lot like microphone cable.  It is different.  Microphone cables have the wires lying side by side.  DMX cables have the wires twisted together.  This keeps the signal more consistent.  A circumstance where this would matter is if the control cable happens to cross a power cable and there is a power surge.  If the control cable wires aren’t twisted then it is possible that only one of the wires would be effected and then your data signal will go all hay wire.  If they are twisted they would be effected equally and it there would be a better possibility that the control signal would remain consistent.  

8 Bit vs. 16 Bit control

Now this was genius.  When DMX first came out it was mostly only controlling dimmers.  The 256 steps gave you a very smooth transition.  As moving lights came into play we started controlling things like gobo rotation and pan and tilt.  Here the 256 steps just didn't seem to give you enough control.  So here is where they came up with this choice of either setting it to 8 Bit which would be limiting the address to 256 steps or going to 16 bit control which meant there would be two addresses set to that function.  Lets use pan as the example.  There would be Pan (course) and Pan Fine.  So you could set a value for Pan at lets say 65 and it would bring you to that location.  Then you could set the second channel (Pan Fine) to let us say 124.  All of a sudden the location was 65.124.  There are now 256 steps between the initial 65 and 66.  What an improvement in smooth movement.

The Flow of a Lighting System

For a conventional light with intensity controlled by a dimmer…

Control board > Control signal transporter > Dimmer > Electrical cable > Lighting unit

For a LED light…

Control board > Control signal transporter> LED unit (with constant power provided)

For a mixed system with conventional and LED units together…

Control board > Control signal transporter > Control signal splitter sending a signal to dimmers and then also to the LED unit > then continue on to the lights as listed above.