Control Data Communication
As described in the previous section on control systems, there are several ways in which control data can be sent from one device to another. The most fundemental attributes are whether the signal is analogue or digital, and whether it is multiplexed. The protocols that are most likely to be found today are described in the next section: Control Protocols.
There are a large number of control protocols that have been developed over the years, and are still in use. These are wide ranging in both their operation and capabilities, but most have largely been superceded by the current standard: DMX512.
There is currently a move towards using Ethernet infrastructure to transmit and share lighting control data over a complete network. This will eventually be extended to support other building-wide performance show data, such as audio control and stage automation.
The protocols that are most likely to be found today are described in the next section: Control Protocols.
The following terms are often used when talking about control protocols and equipment. For the majority of the time, they are used when refering to the DMX512 protocol, although they are equally applicable to most protocols.
A balanced signal is a way of transmitting data without electrical interference affecting it. The original signal is transmitted along one wire, with an inverted copy on another wire, which are then twisted together (a twisted pair). Any interference that is picked up by the cable will then be picked up equally by both wires of the twisted pair. The receiver electronically subtracts the two signals from one another which removes the interference and maintains the data. DMX512 uses this method of transmission, which is why two wires and a screen are used instead of just one wire.
This is the control channel at which a device (dimmer, moving light, scroller, etc) will respond. DMX512 contains a total of 512 channels per stream (cable). If a moving light which requires 8 channels has its base address set to 21, then it will respond to channels 21 through 28. Another device could then be set to respond from channel 29, and so on.
Also known as 'bits per second'. Digital protocols such as DMX512 transfer data at a set rate which is measured in bits per second (b/s): this means that the number of electronic signal changes per second. DMX512 operates at 250,000 Baud (250 kb/s), whereas Ethernet typically operates at 100 Mb/s.
Demultiplexing consists of converting one combined signal into many individual ones. Usually this involves converting DMX512 into analogue control signals for older dimmer racks. This is the opposite action to multiplexing.
This referes to the way in which a channel is controlled. When a channel is controlled by two or more sources (for example, if two submasters on a lighting desk contain the same channel) the highest value is used. HTP channels are normally used for controlling intensity.
Also known as optical isolation or galvanic isolation, this is a method of transferring data between equipment without making an electrical connection. The signal (such as DMX512 or MIDI) is converted to light and then back to an electrical signal. Without isolation, all of the equipment is electrically connected together: this is not a problem for small rigs, but as the distances between the equipment grow problems can occur. If, for exampl, the lighting console and dimmer rack are powered from different phases, sub stations, or generators, an equipment fault could cause very large currents to flow down the control cable. Acouple of volts difference in mains earth between the console and dimmers is enough to cause serious damage. Isolatioon solves this problem by ensuring that there is no electrical connection between the equipment. Splitters are often used to introduce an isolation barrier in DMX512 signals. If all of the equipment in a system has isolated inputs, there is no need for the additional expense of providing isolated outputs.
Most DMX512 receivers will have two DMX512 connectors, allowing devices to be daisy-chained in series to the control signal. However, each receiver takes some of the power of the data signal, so care must be taken to limit the number of devices on a single DMX512 line to around thirty, otherwise problems will occur. The loop through connector on the last device of each DMX512 line must be terminated.
This refers to the way in which a lighting channel is controlled. When that channel is controlled by more than one source, the last value to change is used. LTP channels are normally used for controlling non-intensity channels such as pan & tilt and colour.
This refers to the way in which a lighting channel is controlled. When that channel is controlled by more than one source, the lowest value set is used.
Merging data is a method of allowing two lighting consoles to output onto the same DMX512 cable. This is used when two lighting consoles must control the same dimmer rack.
Multiplexing consists of converting many individual signals into one combined signal. Usually this involves converting the multiple analogue outputs of a lighting desk to DMX512. See the section on Control Systems for a more detailed description of multiplexing.
A splitter is an electronic device which is used to provide multiple DMX512 outputs from a single input. It is used when the show has a lot of dimmers or moving lights, each of which require a DMX512 input signal. It is usually a bad idea to connect more than 30 DMX512 devices to a single cable, as each receiver takes a small amount of power from the signal. Each output from the splitter will drive as many receivers as the original signal, allowing more devices to be used.
This is a male XLR connector which is plugged in to the final loop through connector of a DMX512 daisy chain of devices. It is needed to stop electronic reflections in the cable which would corrupt the data signal.