DMX

DMX controller

DMX stands for Digital Multiplex. It’s a digital control technology (like MIDI) that was developed in the late 1980s to allow computerized control of stage lights.

Multiplex: “…a system for transmitting or receiving simultaneously two or more messages or signals over a common circuit…” (Webster’s 1982).

DMX allows a very low-power digital signal to control many very high-power theatrical lighting fixtures. Now that LEDs are bright enough to use for this sort of lighting, the power needed to run such lights has gone way down, and so has the cost of the fixtures. So I got a used one to get some mass on how this system works.

DMX fixture

The DMX system design is very simple. The controller regularly sends out a signal that can contain data for up to 512 different parameters, known as “channels.” A parameter can have any value between 0 and 255. A lighting fixture can be controlled by as little as one parameter (how bright or dim it is) up to many that can control its color, where it’s pointing and other functions that fixture is capable of.

For example, the fixture I bought has three colors in it (red, green and blue). So each color is controlled by one channel, and a fourth channel controls the overall brightness of the entire fixture. When setting up such commands with a computer, all sorts of intricate automated sequences can be created.

Fixtures have separate cables for power and control signals. To save cabling, most fixtures have “in” and “out” connectors so that each fixture can be connected to the one closest to it, and they will all get the control signal in “daisy chain” fashion. For some reason, probably because of their sturdiness and availability, the same connectors that are used for professional audio are also used for DMX signals. Technically, though, since the digital signals contain much higher frequencies, normal audio cable cannot be used for DMX; special digital cable must be used.

DMX fixture

More about MIDI, DMX and possibly other digital control schemes in future articles…

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