JWD Deep Dive - LED Pixel Tape
Pixel lighting is the latest evolution of LED lighting to be incorporated into scenic elements. Most often it is used in the form of linear tape lighting, or clusters of between 1 and 6 LED chips formed into some type of round or square form factor, often referred to as a bullet, chip, or node. Designers are making it a popular choice because it offers the ability to control light in much smaller increments, making lighting effects more dynamic and exciting. Below we will cover all the information you need to know to get familiar with LED pixels.
If you haven't read our article JWD Deep Dive: City Theatrical QolorFLEX HiQ 5-in-1 RGB +2700K+6500K, you may want to read that first and then come back to this article. Many of the basic concepts and terminology we will use below are introduced there.
Here are the topics we will cover in this article:
What is pixel tape and how is it different from LED tape?
Different types of pixels? What is a pixel IC?
How to dim pixel tape smoothly
What is pixel tape and how is it different from LED tape?
Pixel tape looks a lot like LED tape. It is typically sold in reels of 2M to 5M long, each reel contains multiple LED chips fixed to a flexible circuit board, and it is usually the same width (10-12mm) as LED tape. But once you look beyond the physical qualities the similarities end quickly. Pixel tape uses a completely different method of wiring and control from LED tape, and it's capability as a lighting tool is vastly different.
So, what is a pixel?
For the purposes of this article, a pixel represents a single control group. Pixels can be found in many different configurations. Just like LED tape, you can find multiple color combinations in the chipset like RGB, RGBA, RGBW, etc. Depending on the type of tape, a pixel may be one or several chips grouped together. The important thing to know is that the term "pixel" represents one control group - be it a single LED chip or 5 in succession. A pixel will take up a number of control addresses based on it's configuration. An RGB pixel will use 3 addresses (Red, Green & Blue), RGBA or RGBW will take up 4. At the moment, all pixel tape is 8-bit control, so the number of control channels will be equal to the number of colors on the chip. We will cover this more below in our section on pixel resolution.
What else makes pixel tape different from LED tape?
The biggest difference is in the way pixel tape is controlled. With a traditional strip of LED tape all of the LED chips are wired together, making the entire length of tape a single control group. With pixel tape the LED's are broken down into many small control groups (pixels) along the length of the tape. So a 5M length of RGB LED tape will have 1 control group and use up 3 control channels, but a 5M length of pixel tape can have as many as 240 pixels and use up as many as 720 control channels. That means that over the same 5M length you can have 240 unique sections in different colors and intensities. It's easy to imagine how you could use pixel tape to make a lighting effect far more dynamic with that much extra control.
Wait, if I have 720 control channels does that mean I need a 720 channel dimmer pack?
Thankfully, no. Pixel tape works on completely different control technology from LED tape. Whereas LED tape requires a dimmer that can physically adjust DC voltage going into the tape to control the light level of each color, pixel tape does not. In the case of pixel tape the voltage supplied stays constant. Instead of dimming voltage a control signal is sent that tells each pixel what to do.
To learn more about this, let's start by looking at how pixel tape is wired. Pixel tape can be found in either 3 or 4 wire configurations, depending on the type of pixels used. The 3 wire version will have one wire for positive DC voltage (v+), one wire for negative DC voltage (v-) and the third wire supplies the data signal. In the case of 4 wires there are 2 possible options depending on the pixel type. The first 3 wires are the same as the 3 wire version (V+,V-,Data) and the 4th wire is either a "clock" signal or a backup data wire. Systems that use a clock signal have the ability to change the refresh rate of the pixels, allowing for either longer more stable runs or a faster more responsive refresh rate.
DC voltage is typically supplied in either 12 or 5 volts to the tape. 12v tape will suffer less from voltage drop and can usually be run for longer distances without needing to re-inject power.
If the voltage isn't being dimmed, how does the pixel dim and change color? Is this magic?
It certainly seems magical, but it all comes down to that data line we spoke about earlier. The data line is sending a digital pulse to all the pixels in the chain. Each pulse is made up of packets of data with instructions for either 3 control channels (RGB) or 4 (RGBA/RGBW). Here's the "magical" part - each pixel is designed to use up the first 3 (or 4) pieces of data it receives and then send all the rest down the chain to the next pixel. This is what is known as cascade. Say we have 4 RGB pixels in a chain. That would be 12 control channels, 3 (RGB) for each pixel. The data stream for this example would then contain 12 pieces of data. The first pixel would get all the data (1-12) and use up the first 3 (data 1-3), passing 4-12 down the line. Pixel 2 would use up data 4-6 and pass 7-12 down the line, pixel 3 would use up 7-9 and pass data 10-12 down the line. When pixel 4 receives data 10-12 there is no extra data to pass on so it just uses them up and the data chain stops there.
I've seen the term pixel IC used to describe pixel tape, what is a pixel IC?
Pixel IC can be thought of as the control language which a particular LED chip is programmed to respond to. Pixel IC's delineate pixels of differing voltages and control resolutions. It's always important to make sure that the controller you use can output the IC of the pixels in your project, otherwise you won't be able to send the pixels data they can understand. Many pixel controllers will output a variety of IC's, but often they can only output one IC type at a time. So, if your project uses pixels with differing IC's it will require extra thought during the engineering phase.
Working with pixel tape
Pixel tape opens up a wide range of design options that are not possible with regular LED tape. Fantastic high resolution light chases, low resolution video effects, brilliant sparkles and carefully timed sequences are all easy to achieve with some simple programming on a lighting controller. All of those new possibilities do come with a few compromises as well, and we will cover those in more detail below.
Pixel tape is hungry and it wants to eat up all your data outputs
As mentioned above, a 5M section of pixel tape can use up as many as 720 control channels. For this reason, many pixel tapes will gang 3-5 LED chips together to make a larger size pixel, thus reducing the total number of pixels per meter and lowering the overall control channels required. Certain pixel controllers will also allow you to group pixels within the software, allowing for even further reductions in total data outputs required - but resulting in a larger size pixel. This balance needs to be determined based on the desired resolution of the design and the physical limitations of the control equipment. In most cases more data outputs means more money, so you need to find the right solution to balance both design and budget concerns.
I want a smooth neon like line of light, but all I see are hundreds of little dots! Help!
This is where a professionally designed LED solution starts to become the most evident. There are literally hundreds of companies claiming to produce various diffusers that will turn your pixel tape into pixel neon. Many of them do provide some help, but very few actually homogenize the light to the point where it becomes a smooth even line. There are several factors at play, and all of them need to be considered in order to successfully achieve a smooth neon look. JWD has extensive experience creating smooth linear light, and many of our techniques are so effective that we choose to keep them as a trade secret. If you have a need for smooth linear pixel neon, we would be happy to help.
How to dim pixel tape smoothly
There are 2 common complaints with pixel systems, particularly in a theatrical situation. The first is that there is a visual "pop" at the bottom of the dimming curve, second is that the dimming is not as smooth as other sources such as LED tape. These concerns are valid and solving them is tricky. As we learned earlier, all pixel tape is currently 8 bit. This is really the crux of the issues above. Because 8 bit dimming is limited to 256 control steps, the highest possible resolution is still far less than most designers would like to have. This causes dimming curves to appear choppy, and small changes in control level to seem like bigger jumps than desired. Which leads us to the reason for the "pop" at the bottom of the dimming curve - with so few control increments in use, pixels can often seem to jump up from zero because the difference in light output between zero and 5% is non-linear. Often, users will see that the pixel stays off between 0% and 4% and then watch it pop on at 5%. This particular challenge can be helped notably using a combination of quality pixel controllers and special software that helps to drive the pixel to a low level as early as .05%.
Again, we are treading into trade secret territory here, but for now know that there are ways to help smooth out these problems, and JWD would be happy to help you with that ;)
Pixel lighting is a great technology that is still in its early years of use. In the coming years, you can expect to see the technology behind this great lighting tool expand and improve, with an eye towards making even bigger and better designs possible. JWD is working with several manufacturers of pixel lighting and control in the building of systems that represent the leading edge of what's possible with the current technology. As new opportunities emerge, we will continue to bring the latest in what is possible to each production we work on.