Illuminati Creative Technology, Colchester UK
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Stage Lighting: A basic technology manual.
How to talk to your dimmer - and get some sensible answers!
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INTERFACE between dimmer and controller
The signal between the control desk and the dimmer rack can be one of four things:
1) Analogue Voltage Control
This stands for Digital Multiplex, and is a variant of the normal protocol by which computers talk to each other. The idea is that each channel in the control desk sends its information in turn down a single cable to suitably equipped dimmer racks or other equipment such as scrollers, moving lights or other DMX-compatible items. This process is called time division multiplex or, colloquially, MUX.
The first piece of information that is sent is the channel destination address, which alerts the particular device to expect instructions. It is then followed by the instruction itself, which could be dimmer level, fade or movement information and finally some checks and a stop instruction.
At the receiver, a device called a demultiplexer or demux decodes the serial signal into up to 512 separate channels and distributes the signals to the correct destination. Each receiver will be set to a number which is its identity, and will then respond only to instructions sent to that number. A dimmer rack or moving light using more than one control channel will usually have its own demux which is set to the correct start number, the remainder of the numbers in sequence being automatically distributed. Individual receivers such as smoke machines may be equipped with their own built in demux.
Start numbers are set either digitally or by means of thumb switches. Sometimes you have to set the numbers in binary code.
Many demux units on the market also contain backup memory systems, so that in the event of failure of the DMX signal, the existing lighting state is retained in memory. In some units, additional memories are available.
Emulators are available, to simulate any output required for test purposes, which can also act as DMX cable testers.
CONNECTION: DMX cables must be suitable for E1A485 (used to be called RS485) with one or more low-capacitance twisted pairs with overall braid and foils shielding.
The preferred connector type is an XLR-5 pin: The pin wiring details are as follows:
Some early equipment may be fitted with XLR-3 connectors. This is not standard and does not form part of the official specification for DMX. In this case the pinout is the same as the first three pins of the XLR-5.
Normally, all items are daisy-chained, with the console at one end of the line and the last item in the chain terminated with a propriety terminator to avoid unwanted digital reflections. This consists of a 110
If more than 32 devices are required on one line or splitter leg, then a repeater amplifier may be used. It is important to note that these are amplifiers and will only amplify what is there, and will not re-create missing data.
Unfortunately there is more than one variant of the DMX protocol, and they are not fully compatible. Variations include:
PROBLEMS WITH DMX:
The signal can become degraded and system faults can occur if:
a) A very long daisy chain is used,
A fault means that everything grinds to a halt. However, the use of a network can usually confine the problem to an individual fixture.More about DMX
The ETHERNET SYSTEM is widely employed in the computer industry to create these networks, and is rapidly coming into use in the theatre. Individual network stations, which can consist of any suitably-equipped lighting desk or moving light system, etc. are connected via "nodes." Nodes are connected by coaxial cables, optical fibres, or twisted pair cables. Networks that are set up for individual functions and purposes are called local-area networks, or LANs. Wider networks are possible, however, and use telephone lines to connect to other systems. and are known as WANs (Wide area networks). Local-area networks are looped around the building and are simply ‘teed off’ into an Ethernet Node as required. The vastly increased speed and bandwidth has made possible a huge increase in the amount of Data, (at present more than 1536 separate DMX signals), which can be sent down a single cable and also increased the distance over which the signals may be reliably distributed.
Co-axial links ( legacy wiring - out of date now) may be used using 50 Ohm coax cable. Standard 75 ohm video cable is not suitable.
Twisted Pair links require the use of a network hub which is basically a splitter module. The hub is connected directly to the lighting desk and the outputs from this are taken singly to the individual nodes, or further hubs.
The correct cable is Category 5 4 pair UTP
Twisted pair should not be run parallel to lighting load cables.
Modern Theatres have an extensive networked sytem. The Royal Opera House, for example, a network of Personal computers linking the entire premises, from the box-office to the fly-floor, dealing with all the day-to day work of a busy theatre office, as well as holding flying, lighting, sound and hanging plots, rehearsal schedules and calls, as well as keeping track by means of light pen and bar code of scenery and props. For the historian, 35 years of archive are available. The network allows access to various parts of the system only by authorised persons. At present there are no plans to link the system to the stage lighting board, but...watch this space
The sequencer can also simultaneously record information from other sources, for example a sound track or a MIDI equipped projector controller. Once recorded onto a sequencer, the show may be edited on or off-line using a graphical digital editor supporting cut-and-paste, etc.
MIDI can also be used to extend a desk by allowing its masters to operate a slave desk. It can also be used to make one desk slavishly follow the actions of another, allowing an instant backup system in case of desk breakdown.
Any MIDI keyboard or other MIDI-equipped device can operate a lighting desk. Alternatively, a lighting desk can play music!
Midi can enable a fully integrated show to happen very easily, incorporating any MIDI-capable item to be interfaced, such as slide and movie projectors, sound desks, moving light and scenery controllers and of course synthesisers etc. This is a serial signal, so only one event can happen at a time, but events can be spaced apart as little as one twentieth of a second.
MIDI instructions are sent as a series of of bits. The transmission rate is 31.25K, with each data word consisting of 10 bits. Each group of bits making up an individual MIDI code is called a “Message”
MIDI-in for receiving MIDI from other devices
MIDI cables are connected with 5-way screened cable joining like-numbered pins, with the screen and one core on PIN 1 and the others connected to the remaining pins. Standard Hi-fi cables work perfectly well, but a limit of 40 feet is advised. Check that it is “straight wired” not mirror image wired.
MIDI relay boxes are available to allow a MIDI signal to operate, or be operated by external pieces of equipment. For example, using the M+M MIDI Box, a door switch or pessure mat can be made to operate a sound or lighting cue. For example, a microswitch on a fake gun can operate an infra-red beam which is connected to MIDI receiver triggering a gunfire effect. thus ensuring perfect self-operating synchronisation. more about MIDI
The latest development is MSC (MIDI Show Control). This simplifies the codes to allow more information in a smaller time space, transmitting complete instructions rather than individual key presses. In the example above, the string would be replaced by “GO Q34”
Midi is not directly compatible with DMX, but can be used in conjunction with it using a specialist interface unit.
Unfortunately there is little cross-manufacturer compatibility. For the complete MIDI show control protocol and standard click here
5) SMPTE TIMECODE
Strictly, this is not a control protocol, but is included as a method of using one piece of equipment to control another.
SMPTE (pronounced ‘simpty’) stands for Society of Motion Picture Theater Engineers This is a system whereby a numerical code is inserted onto a film track or onto recording tape or stored as a digital datastream
It is simply an electronic count which is produced by one of the pieces of equipment or from a stand-alone synchroniser, producing a continuous stream of numbers starting from any number you care to set it at and increasing by one every thirtieth (USA) or 25th (UK) of a second. This number is chosen to represent the frames of a cine camera, and each sequential number is therefore known as a frame.
It is normal to use an 80-bit system: The first eight bits represent Hours: Minutes: Seconds: and Frame No: it takes the form of say 06: 42: 20: 19 or six Hours 42 minutes, 20 seconds and 19 frames. The maximum count is 23: 59:59:23
This can be used as a synchronisation system, whereby each device, be it a lighting desk, a sound source, camera, motion control system, laser system or pop-up toaster, anything fitted with a decoder and relay set, can be made to operate at a specific time. The signal is simply sent digitally around the system and tapped off where required. It enables live shows to be synchronised as well as easing pre and post-production editing. The decoder simply waits for the correct Frame Numbers to which it has been preset to come along and operates a relay or similar device.
On a lighting desk, it would be normal to set the playbacks to auto-sequence and allow the SMPTE signal to initiate the GO button
It makes planning a show very simple as the whole thing can be mapped out on paper or spreadsheet first and then use to create lists of events which can be sent out well in advance to the various effects contractors
THE INTEGRATED SYSTEM