Basics 2. Channel and value: how a fixture reads a command
In lesson one we established that 512 numbers run down a DMX cable, each one a command on its own «lane». Today we put that lane under the microscope. The channel and its value are the entire alphabet of DMX — there is nothing else in it. Everything else you will ever see in lighting software is an elegant layer built on top of the two ideas in this lesson.
A channel is one knob
A channel controls one parameter of one fixture. Not the whole fixture — one single ability of it.
An ordinary wash light does not have many abilities: overall intensity, red, green, blue. So it takes four channels, one per ability. A moving head has far more: pan, tilt, intensity, shutter, colour wheel, gobo wheel, gobo rotation, prism, focus, zoom… every ability gets its own channel, which is why heads come with 14, 20 or even 32 of them.
A good analogy is a sound engineer’s mixing desk, where every parameter has its own knob. DMX works the same way, except the knobs are virtual and there are exactly 512 of them per line.
The value: from 0 to 255
What travels down a channel is not «on» and not «a bit more», but a number from 0 to 255. Zero means off, minimum, the start of the travel. 255 means maximum, fully open, the end of the travel. Everything in between is a smooth intermediate state.
Why 255 and not 100 or 1000? Because each channel in the packet gets one byte — the smallest portion of data, with exactly 256 states: 0 to 255. It is a technical detail, but worth accepting as a fact, because it explains the beginner’s biggest confusion.
The confusion: values are not percentages. Software usually shows intensity as a percentage, while raw values go into the cable. They line up like this:
| Percent | DMX value | What you see on the fixture |
|---|---|---|
| 0 % | 0 | Dark |
| 25 % | 64 | A quarter of full |
| 50 % | 128 | Half |
| 75 % | 191 | Three quarters |
| 100 % | 255 | Full intensity |
One channel, several different functions
Here comes the part that surprises beginners most. Channels are scarce and fixtures have many abilities, so manufacturers economise: one channel splits the 0–255 range into bands, and does something different in each band.
The classic example is the shutter (or strobe) channel on a fixture:
| Value | What the fixture does |
|---|---|
| 0–7 | Shutter closed — no light at all |
| 8–134 | Shutter open — steady light |
| 135–239 | Strobe: the higher the number, the faster the flashes |
| 240–255 | Random flashes |
So the very same channel is both an «on/off switch» and a «flash rate dial». Colour wheels (a narrow band per colour), gobo wheels, prisms and built-in auto programs are arranged the same way.
Seeing the channels with your own eyes
The easiest way to grasp channels is to see them all at once. Every lighting program has a mode showing all 512 channels as individual faders — raw access to the line, with no fixtures and no scenes in the way.
This is also the best test tool there is: push a single channel to 255 and watch what happens in the room. The right fixture responded with the right parameter — address and profile are fine. Its neighbour responded — the addresses are mixed up. Nothing responded — the problem is the cable, the interface or the fixture itself.
When 255 steps are not enough: 16-bit channels
Some parameters make 256 positions painfully coarse. The prime example is the pan of a moving head. Spread 540 degrees of travel over 256 steps and one step is about two degrees. At twenty metres that shifts the beam by a good metre: slow movement looks less like gliding and more like a series of jerks.
The fix is simple: two channels are assigned to one parameter. The first is called «coarse», the second «fine». The coarse channel moves in big steps; the fine one divides each of those steps into 256 more. Together that gives 256 × 256 = 65,536 positions, and movement becomes genuinely smooth.
An analogy: tuning an old radio, which had a big dial for finding the station quickly and a small one for fine adjustment.
In a fixture manual such channels appear in pairs, labelled roughly like this:
| Channel | Name | Meaning |
|---|---|---|
| 1 | Pan | Rotation, coarse |
| 2 | Pan Fine | Rotation, fine adjustment |
| 3 | Tilt | Tilt, coarse |
| 4 | Tilt Fine | Tilt, fine adjustment |
16 bits shows up most often on pan and tilt, less often on the dimmer (for the very slow fades used in theatre).
Key points
- A channel is one knob for one parameter of one fixture; a line holds 512 of them.
- A channel’s value is a number from 0 to 255; 50 % intensity is 128, not 50.
- One channel often splits its range into bands, doing something different in each (open / strobe / random flashes).
- A closed shutter is the most common reason a fixture «won’t light».
- 16-bit (fine) channels are a coarse + fine pair for one parameter, 65,536 steps instead of 256; above all they are what makes head movement smooth.
- Direct channel access is the number one tool for testing hardware.
Coming next: we know what a channel is and what number travels down it. What remains is understanding how a command finds the right fixture — that is the job of the address. The next lesson covers addressing: how it is calculated, how it is set on the fixture, and what happens in the room when addresses overlap.