Previous: Lighting Boards
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.
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.
Previous: Lighting Boards