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WPI Technical Theatre Handbook: Control
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Since modern lighting setups have separate dimmers and boards which are often located a hundred or more feet apart, a method of control must be used so that the board may control the dimmers.

One of the first methods used to accomplish this task was a purely analog scheme. Individual wires for each channel are used, with each carrying a small (typically zero to ten volt) analog signal representative of the position of the fader on the board. This method works reasonably well, and is in fact still used in many theatre installations. However, the multi-conductor cabling is expensive, difficult to maintain, and quite bulky. The amount of cabling required to run a large number of dimmer channels can become difficult to manage after a time.

To combat these problems, several different schemes have been developed over the years. Most rely on some form of multiplexing, the combining of several discrete signals into one by dividing time up into several slices and devoting a slice to each signal. This allows all of the multiconductor cabling to be replaced with a single piece of cable, using typically only three conductors.

Several schemes for multiplexing lighting information have been developed. Two of the most common ones AMX192 (analog multiplex, 192 channels) and DMX512 (digital multiplex, 512 channels). DMX512 is what is used at WPI for lighting control.

DMX512 can be run using regular 3-pin microphone cable over long distances. This affords much flexibility in the placement of the lighting board and dimmer racks, as they can be spaced quite far apart. Most new dimmers decode DMX512 internally. The NSI dimmers that WPI owns are prime examples of this. However, the majority of the dimmers used at WPI are older analog models that have been retrofitted through the use of DMX512 demultiplexers. These devices decode the digital signals used on a DMX line and convert them to analog signals used by the dimmers. Devices such as this give older devices an extended life, as they can be controlled by the latest computer lighting boards. Figure 5.21 shows one such demultiplexer.

Figure 5.21: DMX512/AMX192 demultiplexer, used to convert DMX or AMX signals to 0-10V analog signals used on older dimmers. The three main dimmer racks used at WPI employ DMX demultiplexers manufactured by Electronic Theatre Controls, Inc.

DMX512 is also used for the control of many automated lights. The High End TrackSpots used at WPI are controlled over DMX. Individual channels of DMX are assigned to functions such as pan, tilt and color. Appendix Q has more information about the DMX channel mapping used on the TrackSpots.

next up previous contents index
Next: Design Up: Lighting Previous: Lighting Boards   Contents   Index
Steve Richardson 2000-07-06

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