Automated lighting consoles are built around a basic infrastructure dependent on the DMX protocol (or map) of a fixture or device. Understanding how DMX values control specific parameters of a fixture is essential for any automated lighting programmer. However, modern consoles utilize complex pre-built libraries or profiles that alleviate the programmer from having to understand DMX functionality. One of the core concepts that should be understood is the difference between 8-bit and 16-bit DMX control.
A DMX Primer
The DMX-512 standard was originally created to control dimmers and thus, the control only had to go from 0 to 100 percent. To ensure a good resolution of control, the DMX standard was built using 256 values (or steps) to get from 0 to 100 percent. So essentially DMX values 0-255 are mapped to the standard 0 to 100 percent values displayed on the console. (See Fig. 1) This provided smooth transitions when fading across various lengths of time because there were 256 values instead of only 101.
As automated luminaires came into our industry, these same DMX values were used to control more than intensity. They were used to control all functions of a moving light. Now the 0-255 values were mapped to control items such as color mixing, gobo indexing, pan/tilt and more. It was soon discovered that having only 256 individual values per channel was not a fine-enough resolution for many parameters.
8 Bit Problems
When we use the standard 256 values to control a parameter (whether as 0 to 100 percent or open to closed iris) we are using 8-bit DMX control. This provides a decent resolution for most parameters as 256 steps allows for fairly slow movement without seeing the steps in the parameter movement. For instance, crossfading from 0 to 100 percent over four minutes equals almost one step per second. With a color wheel or dimmer flag, this will usually look smooth.
However, if you are panning a fixture from stage left to stage right, you are likely adjusting only between a small range (25 to 50 percent). If you try to do this over four minutes, you will certainly see shaky or steppy movement as the fixture pans because, with 8-bit DMX, it results in a step nearly every four seconds! You need more steps (a higher resolution) to have smooth movement.
16 Bit to the Rescue
By combining two DMX channels, we can increase the number of values to control a parameter from 256 to 65536! This is the premise of 16-bit DMX control, where a greater range is mapped to the 0 to 100 percent values of a channel. (See Fig. 2)
By applying this greater range to any parameter, we can have a much better resolution for control. For instance, when panning a fixture from stage left to stage right, you have many more values to crossfade between. Now, instead of a step every four seconds, with 16-bit DMX, the pan would use 68 steps per second! Clearly the fixture will move much more smoothly with 16-bit DMX control.
The Secret of 16-Bit
You may be wondering, “How exactly does combining two channels of DMX to control pan increase the resolution?” It is a process of breaking down each value from the first channel with values from the second channel. For example, one channel will select the coarse steps (just like 8-bit DMX), while the other channel will select from 256 steps between each of the coarse steps. This second channel is typically called the Fine control.
This finer resolution is essential with pan/tilt, gobo indexing and other parameters where 256 steps just are not enough. For example, a rotating gobo will have 360 degrees of possible positions. If you are trying to align a gobo to look straight, you don’t want to only select from 256 values within the 360 degrees. You would much rather get to select from 65536 values!
The Console Knows
Your automated lighting console must be made aware of parameters that are using 16-bit DMX control, otherwise crossfades will not work properly. Luckily the fixture library/profile will provide this awareness. Within the library/profile there is usually a specifier that alerts the desk that two channels are controlling a single parameter via 16-bit DMX. When you are actually adjusting the pan or gobo indexing, you will only see either 0 to 100 percent or real world values, depending on your desk. However, the console is sending out 65536 possible values instead of 256.
Most lighting programmers are not even aware of which parameters are using 8-bit or 16-bit DMX. Usually, the lighting manufacturer will determine which parameters they want to use 16-bit and specify this in the fixture’s DMX chart (or protocol). Some fixtures may have different modes that enable 8-bit or 16-bit DMX for specific parameters. When patching, it is important to choose the best mode needed for your production.
For instance, if you are only doing a fast rock show, then you probably don’t need a mode that uses 16-bit DMX for intensity. However, if you are lighting a cyc for a theatrical piece, you may need it. Because 16-bit DMX control uses two DMX channels, the fixture DMX count can increase quickly if there are too many 16-bit DMX controlled parameters. Be sure to choose the fixture mode wisely to balance between the production needs and the available DMX channels.
What about the Bobs?
What if you need to do a slow fade of 10 minutes and you only have 8-bit DMX control? This problem does not occur very often, but some fixtures will offer a solution. Instead of crossfading from 0 to 100 percent via the 256 DMX values, you could simply use the fixture’s timing channel to set a value equal to 10 minutes and snap from 0 to 100 percent at the console. The fixture will then use its own internal processor to calculate the best movement and resolution to dim over the specified time. Not all fixtures provide this functionality, nor is it available for all parameters, so be sure to check your fixture user manual to see what is available.
A Better Way
The future may bring us an improved communication method that is not dependent upon the original DMX 512 standard. Then larger value ranges could be possible without depending upon a 8-bit or 16-bit range of DMX channel. Until this future arrives, it is important for automated lighting programmers to have an understanding of the difference between 8-bit and 16-bit DMX control.
With this understanding, you can choose the best mode and also understand how parameters are being controlled when crossfading from one value to another. You will no longer have to ask why a change looks steppy, if you comprehend what is happening within the DMX values.
