CHAPTER 5. LIGHTING 74
Most boards also allow some form of soft patching, meaning assigning more
than one individual dimmer channel to a board channel. This is advantageous
because each dimmer channel is rated for a maximum power handling capability.
Soft patching allows more instruments to be controlled as a logical unit than a
single dimmer channel allows. Older lighting boards use a matrix of diode pins
to electrically connect board channels to dimmer channels. Modern boards tend
to accomplish the same task through the use of software.
Advanced lighting boards usually provide all of the same features as an X-
Y board, but add the ability to store scenes, record a sequence of scenes in a
cue stack, and record light chases. Scene storage is an especially handy feature
because it becomes increasingly difficult to set an entire scene when the number
of channels on a board is high. With a standard X-Y board, every fader in a
scene has to be set to a level recorded on a piece of paper. Scene storage boards
remove this hassle by allowing the operator to record a scene and assign it to
a single fader. More advanced boards allow these recorded scenes to be strung
together into what is commonly known as a cue stack. This enables the operator
to simply push a “go” button to advance to the next lighting cue. Lastly, some
boards allow repeating sequences of scenes to be run. Most are outfitted with
a speed control that allows the operator to adjust how quickly the sequence
progresses. Some boards even provide an input for an audio synchronization
signal, such that lighting chases may be synchronized with an audio track. See
figures 5.19 and 5.20 for examples of advanced lighting boards.
5.8 Control
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
atime.
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.