Control signal (lighting technology)
Due to better visibility, lower losses in the load cables , technical necessity (e.g. scanner , head-moving headlights , color changers ), noise development and fire protection requirements, control devices ( light controls ) are usually set up separately from the dimmers and headlights in event technology . In order to transport the control information from the control unit to the receivers, various control signals have been developed over time.
The control information is transmitted as a voltage between the signal wire and a common ground . Depending on the manufacturer, 0-10 volts and -10-0 volts are used (the latter mainly due to the beach), currents were also used as a signal to reduce losses on longer transmission distances. The relative intensity of the channel is proportional to the voltage / current.
The advantage of this control method is the relatively low cost for small systems, the disadvantage is the rapidly increasing cost for larger systems and the sensitivity to interference.
In order to avoid the necessity of the increasingly thick, expensive, unwieldy and error-prone multicores, analog multiplexing was introduced. Here the signals of the individual channels of the lighting system are sent one after the other via a pair of conductors. With the dimmers, the signal was broken down into individual channels and fed to the dimmers. Well-known names for analog multiplex formats are AMX (ADB) and D 54 (Strand).
Although this saved the multicore, the systems became more susceptible to failure and the signal voltages often had to be readjusted. With the introduction of DMX that followed soon, analog multiplexing never really caught on.
The advantage of digital control is obvious: the signal is transmitted serially , so the number of conductors in a cable is low. The susceptibility to interference is low, the required update rate is very low due to the inertia of the eye, which only requires low transmission rates .
The development of digital multiplexing compensates for the disadvantages of analog multiplexing, the systems have become less sensitive to interference, readjustments are no longer necessary. The elimination of expensive DAC and ADC and the use of simpler circuits for distribution, processing and evaluation of the signals made the development of transmitters and receivers possible without any problems.In the transition phase, demultiplexers were (and are) often used that convert the digital control signal into analog information and thus allow the operation of analog load parts on digital controls.
The variety of different protocols (ADB 62.5, AVAB, CMX, VMX, DMX512) was quickly reduced by the standardization of DMX512 / 1990 by the USITT .
In the meantime, DMX512 / 1990 has become the undisputed standard in lighting technology.
Disadvantages of the system are the limitation of the transmittable channels by the update rate (this can be circumvented by setting up several universes , which in turn requires several cables), the fixation of the system on a transmitter with several receivers and the lack of verification of the data, which is why it is used in makes safety-relevant areas ( pyrotechnics , stage mechanics such as trains , elevators, etc.) impossible.
The concept behind digital multiplexing (the transmission of different information on one transmission medium) is continued by network systems. The larger lighting controls usually work with a built-in or external PC , hardware for distributing the signal is inexpensive due to its widespread use, and the transmission is reliable. The parallel use of several transmitters (e.g. main desk, emergency desk, rehearsal desk, stage management desk, manual control for illumination ) is possible without any problems. The most widespread is Art-Net, behind which the Artistic License company stands.
Due to the small amount of circuitry and the widespread use of DMX receivers, the last route to dimmers and spotlights is again carried out via DMX, but extended functions (e.g. automatic end device detection or direct addressing of individual channels and devices), as required by protocols such as ACN or PSI provided in this way can be used only with difficulty or not at all.
A connection of different media on one network has not been possible up to now in network form. As a future network protocol, ACN is designed to bridge this gap, but until then the various systems must be synchronized via time code or trigger signals such as SMPTE or MIDI .
- Michael Ebner: Lighting technology for stage and disco. A handbook for practitioners. Elektor-Verlag, Aachen 2001, ISBN 3-89576-108-7 .
- Michael Ebner: Lighting technology for stage and disco. A handbook for do-it-yourselfers. Elektor-Verlag, Aachen 1992, ISBN 3-928051-12-1 (6th edition, ibid 1996).
- Wilhelm Gerster: Modern lighting systems for indoors and outdoors. The practical reference work for do-it-yourselfers. Compact, Munich, 1997, ISBN 3-8174-2395-0 .
- Artistic License
- Reference book for sound and light - event technology
- The little dictionary of light
- Basics of signal technology in event technology