Control Signals

Signals from the control desk must be conveyed to the dimmers. Early systems were analogue voltages, but these have been superseded by digital systems such as DMX, which require fewer wires; just 3 to control 512 channels. Since lighting equipment is expensive though, older systems are still commonly encountered in venues where budgets are tight.

Analogue

A separate wire (though usually bundled with others in multi-cores of six) carries a low voltage of typically between 0–10 volts (though -10–0V and 0–15V varieties exist) which corresponds to the output for each dimmer. As an example, for a 48 channel system, eight cables (each carrying six channels) must be run from the control desk to the dimmers. If other devices (e.g. smoke machine) are used, separate cables must be run from the desk to those. 8 pin din and bulgin connectors are most often used to connect the desk to the dimmers directly.

Always check the type of analogue control used. Ensure all items in the system use the same voltage and polarity. In addition, check the pin ordering is the same (different manufacturers use different pins for different signals) and the sex of the connectors.

Analogue Multiplexing (e.g. AMX)

A development of the analogue systems was to 'multiplex' (send multiple signals consequently, very quickly) these separate signals down a single wire (though other wires are required for ground and synchronisation), which greatly reduced the size of control cables and associated connectors. A number of competing protocols exist including AMX/D54. However, analogue multiplexed systems are fairly uncommonly encountered.

Digital Multiplexing (DMX)

Modern equipment uses digital signals (pulses that represent only '1' or a '0') multiplexed together in a system called DMX. DMX sends signals for up to 512 channels down a wire digitally, each with values between 0 (off) and 255 (full on). Each set of 512 channels is known as a DMX universe, with big installations requiring multiple universes. This is especially common if lots of modern fixtures are used (such as LED batterns or movers), as these can consume large numbers of channels.

While the DMX standard specifies that 5 pin XLRs should be used, only 3 pins are actually connected and consequently 3 pin XLR connectors are often found to save cost. Devices are connected in a daisy chain fashion, e.g. from the desk to the first dimmer, then from the first dimmer to second and so on. For reliable transmissions, a terminator (which is just a 120 ohm resistor across the datalines housed in an XLR plug) should be fitted to the end of the line to stop 'reflections'.

Each device along the chain has its starting address set (by setting a binary value with small DIP switches, rotary switches or through a menu). ***e.g. JAVA BINARY

A device receives a number of channels following its start address. E.g. a six channel dimmer with a start address set to channel 20, will receive channels 20, 21, 22, 23, 24 and 25 to control its six respective dimming outputs. The next dimmer would have a start address set to 26

CHANNEL MAPPING, CONNECTION PHOTO DIAGRAM

Troubleshooting DMX. Firstly, check the end of the line is terminated. The correct cable should also be used for DMX, and although tempting, microphone cable may result in erroneous transmissions. With older Martin equipment it may be worth checking if it is wired with crossed transmission lines. If so, an adaptor (known as a jumper) may be required.

Using DMX with analogue equipment

When analogue equipment is used with DMX, a demux (de-multiplexor) is used to translate the digital DMX input into the individual analogue voltages. Again care should be taken to ensure the demux's outputs are compatible with the dimmers (connectors, polarity and voltage). However, it should be noted that it is not possible to convert the other way, as analogue equipment is not used to control DMX devices. ***e.g. photo

Large DMX installations

Large DMX installations may require additional devices for DMX distribution. Splitters may be encountered to effectively allow another cable to branch off (you can't just join DMX in parallel) or mergers, which combine multiple inputs together, with either HTP or LTP. Buffers are most commonly encountered and perform two main roles:

  • remember the last value if the connection is broken (rather than it suddenly plunge into darkness)
  • boost/recover the signal for long runs. Some devices e.g. some splitters also function as buffers.

Very large installations may carry multiple DMX universes over ethernet, with protocols such as ArtNet or ACN.