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Control Systems

To understand how luminiares are controlled from the lighting desk, it is necessary to look first at the dimming system. Working back from the luminaire to the operator, the complete control system comprises:

Dimming system

In almost all situations, luminiares are not used constantly at full power. They are generally required to fade in and out, and to be used at different brightnesses, or intensities, at different times. A device is required to regulate the amount of electrical voltage sent to each luminaire, thereby allowing the intensity of the light to be varied: this is a dimmer.

Each dimmer regulates one lighting circuit, or channel, allowing the electrical supply sent to the attached lumainire to vary between 0 and the mains voltage (230V). The action of the dimmer itself is controlled by the lighting desk, via low-voltage signals: this is typically no more than around 10 volts.

Each dimmer is designed to work up to a maximum electrical load, called its capacity. Any number of luminaires can be connected to a dimmer, until the capacity is reached. For example, a 2000W dimmer could be loaded with two 1000W spotlights, or up to four 500W luminaires. Common capacities are 2.2kW or 3kW, and on higher-end systems 5kW and even 10kW.

A dimmer pack comprises a number of individual dimmer modules, housed together for convenience. Modern, high quality dimmer packs can control many lighting circuits, yet still remain extremely compact and lightweight.

Control desk

The lighting console, control desk, or simply the desk, is the front-end of the lighting control system, and provides an interface between the dimmers and the operator. The state of each dimmer can be changed from the desk, therby controlling the output from the luminaires. Scenes can be created from the desk by adjusting the level of each dimmer, and thus directly controlling the intensity of each luminaire: this procedure is called plotting. Each change in the lighting state is termed a cue, and a sequence of cues makes up the show.

Simple stage lighting can be performed by a manual control desk, which will have a single level control (a fader) to relate to each dimmer channel. More advanced manual desks provide two faders for each channel, allowing the operator to set the next lighting cue before changing (crossfading) from the current lighting state. These are called two-preset desks.

The crossfade may be performed manually by using a master fader for each preset; however some desks provide a timed fade control. Whichever type is used, the operator must still set each cue manually during every performance, which can be difficult when the time between each cue is short.

More sophisticated lighting can be achieved by using a memory control desk. These allow a scene to be created and then stored in the desk's memory, and can be recalled at any point. Memory control desks are ideal for complicated shows, and also when there will be many performances. Due to the computerised nature of these desks, fade times can be recorded with each cue, and details of the next cue to activate saved.

Memory desks also offer effects sections, which can be used to perform more advanced sequences. As the complexity of the desk increases, more and more features and control possibilities become available. Advanced desks offer the possibility to control scrollers and moving lights directly, speeding up the plotting procedure.

Control Protocol

The control desk must communicate with the dimmers in order for the changes made by the operator to take effect on the stage. As with most things, there are a number of ways in which this can be done, however the most significant factor is whether the control system is analogue or digital.

Analogue systems use a control signal that varies in voltage or current in direct relation to the required intensity. The dimmer uses this signal level to 'scale' the electrical supply sent to the channel output. Analogue signals typically use values from zero to ten volts, with ten volts corresponding to 100%, or full intensity. Analogue control signals are generally no longer used.

Digital systems use control signals that send the required control level numerically for each channel. That is, instead of being set at 4 volts for 40% intensity, the digital signal will send the code for 40 repeatedly. Digital systems are more reliable than the analogue equivalent, and are faster.

The control information could be communicated to the dimmers by using one control wire for each channel: this is called multicore control. However, as the number of channels controlled by the lighting desk increases, so does the number of cores required. This eventually becomes unfeasible and, to some extent, unreliable.

Alternatively, each control channel can be instantaneously measured, and the resulting values sent down a single control wire to the dimmer in sequence. If this is repeated quickly enough, rapid changes in the control level will be followed smoothly. This procedure is called multiplexing. A multiplex signal is demultiplexed at the receiver back into the original individual channels.

By far the most common control protocol in use today is DMX512, which is a digital multiplexed system. Modern lighting desks and dimmers, as well as moving lights, scrollers, and effects equipment all use this protocol. The many types of control protocol are listed in the Communication Protocols section, whilst common terms used are given in the next section: Communication Systems.

See also:

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