DMX 101…

Since 2000, there has been a quiet revolution in stage lighting fixtures. Relatively fragile, inefficient (high heat output), single-color (with gels) incandescent lights have been replaced by more durable, long-lasting, low-heat-output multicolor LED’s. LED lighting fixtures are becoming smarter. The earliest LED light fixtures offer only passive, one-way communication – i.e., they can listen to control sources and respond to commands, but they do not send any information back to the controller. The newer fixtures implement two-way communications in the same way that newer MIDI devices both listen to computer controllers as well as send information back to them.

In both cases, a computer communicates to lighting devices through the Digital Multiplex (DMX) software protocol and a DMX hardware interface.  The “software protocol” is how the information is sent and received, and the hardware interface converts the USB or wireless port on your computer to the right kinds of signals to operate the lighting system.

A single DMX interface services 512 “addresses” or “channels” (these two terms are often used interchangeably). A collection of 512 addresses is called a “DMX universe”, and are numbered sequentially from 1 to 512. A single lighting device requires between 1 and 30 channels (even more for some devices) to control all its built-in (i.e., “smart” device) functions. The number of channels a device has is the number of DMX addresses it will respond to.

A DMX message consists of two “codes”.  Each code is 8-bits long.  The first is the address or “channel”, the second is a value. The value most often represents intensity information, but can also be anything the manufacturer wants it to be, such as a fade-in or fade-out time, or a value that represents how fast a strobe light is flashing, or which direction a spotlight should be pointing.

In building your own lighting system, set your devices so that there are no gaps in the addresses. Gaps will result in wasted information being sent to devices that are not part of your system. It’s okay to overlap devices, especially on identical devices, and especially when the operator always wants those devices to function in unison. Combining both overlapped and unique addresses makes it possible to control a hundred devices with a single 512-channel interface – more than sufficient for the vast majority of venues that you’ll be playing.

General-purpose lighting fixtures, which provide a range of colors as well as some internal automated functions, are usually low-channel-count devices (3 to 7 channels). Special lighting devices (such as motorized or laser devices) are generally higher channel-count (more than 10 channel) devices. An exception to this might be a fog machine, which would be classified as a “special effect” device, but may only require only a single DMX channel to operate.

The number of lighting devices you can connect to a single DMX interface will vary depending on the number of channels each device requires. A complete lighting system will usually contain a mix of devices, each with its own channel count. Most will usually be fairly low-channel-count, general-purpose devices. For example, if all your lighting devices required 7 channels, then the total number of lighting devices that you can put on a single 512-channel DMX interface will be 512 divided by 7, or a little over 70 devices.

When the special effects programs found in the higher channels of some lighting devices are not used or necessary, lighting fixtures often provide for operating in more than one mode in order to save channels in larger installations that may push the DMX channel count to its 512 channel limit. One mode might be basic RGB, requiring only 3 DMX channels. Another mode might be a 7-channel mode, where the first 3 are basic RGB, but the next 4 channels might control fade-in / fade-out times, some color macros, strobe effect rates, etc. Which mode you want to use will affect how your system controller will interact with your smart light fixtures. Some lighting devices have numerous such modes, allowing the device to act as a 3-channel, 7-channel, 12-channel, or 17-channel device.  Some moving devices can take up 30 channels or more.

A typical stage lighting system for a small act can be as few as 4 or 8 lighting devices, and could be as many as 30 or 40 for larger acts. In the majority of cases, a single DMX interface is all that’s necessary for even complex lighting systems.

Refer to the Owner’s Manual for the specific lighting device provided by the device manufacturer to understand how many channels are required and how the information sent to each channel affects the operation of the device.

UTTPro makes it easy to control up to 73 7-channel devices on a single interface, more than enough for very elaborate stage lighting displays.