Inexpensive LED Video Wall
Stephen Hitchcock and Matt Lathrop
We created a large, modular, LED video display to be used in a variety of activities from concerts, to theatrical productions, to art installations. The wall is made up of 50 4’ x 4’ panels for a total assembled size of 20’ x 40’ and an effective resolution of 200 x 100 pixels.
LED technology has always been expensive, primarily due to the high costs associated with producing batches of quality LEDs to create a uniform image. This video wall was made for roughly 1/10th the cost of a professional product with similar pixel density by using inexpensive LEDs and then imaging our panels with a dSLR to measure relative luminance. Furthermore, we used a color spectrometer to record the gamut, white point, and gamma of the LEDs. With this data we mapped the sRGB color space into the color space of the LED wall allowing us to produce content then display it on the wall while preserving the colors in the final image.
These techniques, combined with the hardware and software design, produced a professional looking video wall for a fraction of the cost of alternatives.
Background
Although LEDs were invented in 1927, when O. V. Lossev of Russia constructed the first LEDs in his paper Luminous Carborundum Detector and Detection Effect and Oscillations with Crystals, it is only within the past 25 years that they have become useful for displays. Many professionally made displays are available today; however, our constrained budget necessitated a search for affordable components adequate to accomplish our goals.
LED Display Technology
The first major LED display was unveiled by James P. Mitchell at 29th International Science and Engineering Exposition in 1978. Though only monochromatic due to the poor performance of blue LEDs at the time, the display was an important both as a prototype and as a demonstration of LED capabilities. Unlike Cathode Ray or Liquid Crystal technologies, LEDs serve as both the source and control on a per pixel level which allows their displays to be both incredibly thin and incredibly large. Furthermore, their vibrant color rendering and low power draw make them a clear candidate for large scale projects, as they are both visible at long ranges and can be reasonably powered with existing infrastructure.
To understand our methodology, it is important to note that in commercial LED display production, individual LEDs are sampled after manufacturing and matched with other, similar performing LEDs. This way, when a large number of LEDs are used in parallel, such as in a display, color rendering and luminance is consistent across the surface. As a result, the manufacturing process is extremely expensive, as the vast majority of LEDs fail to match performance and subsequently cannot be used in a commercial display.
Specifications
The unique qualities of LEDs made them the obvious choice when designing our large scale project. We knew from the outset that our display would consist of fifty 4' x 4' modular panels with a pixel pitch of 2.4 inches. This created a 20 x 20 pixel per panel resolution, allowing all fifty panels to be configured in a single 40' x 20', 200 x 100 pixel display. A commercially available product at this size and resolution would cost in the neighborhood of $100,000 and was therefore well beyond our limited budget of approximately $10,000. Therefore, we also needed to devise a strategy for building a display at 1/10th of normal cost, all while fulfilling our previous requirements.
Methods
In order to meet our specifications, we devised a number of processes and investigations that served the overall processes of sourcing, manufacturing, and controlling our display. Unfortunately, detailing all of these is beyond the scope of this article, and as such, we will be focusing primarily on our methodology for color calibration and control.
LED Sourcing
The obvious solution to reducing costs was simply to purchase inexpensive LEDs. As stated above, commercial displays rely on LEDs that are color matched after manufacturing, resulting in astronomical costs. Since we lacked the means to obtain such LEDs, our strategy from the outset was to find the best performance to cost ratio and attempt to correct inadequacies in our software.
We began sampling products from a number of different vendors internationally. To meet our specification of a 20,000 pixel display, we needed to spend approximately $0.20 per LED. We also needed LEDs with a WS2811 chipset, as that allowed us to control all three colors in a single, well documented module. With the assistance of Joyce Farrell and her lab at Stanford University, we were able to measure our candidates with a color spectrometer to assess quality and consistency. The LEDs we ended up selecting were chosen for one reason in particular; while they exhibited an expected variation in luminance across approximately fifty samples, their wavelength output was extremely consistent, making the color correction process considerably easier.
Control Architecture
In order to control our display, we developed a pipeline that captures video from a monitor, applies color correction, and distributes chunks of the image to specific panels. In order to accomplish this task cheaply and efficiently, we turned to popular Arduino based products for their low cost and well documented libraries.
We choose PJRC’s OctoWS2811 for its 8 separate data outputs; since our LEDs update sequentially, injecting data along separate strips mitigates delay issues, as 8 LEDs can be updated at a time per chip instead of just one. Each chip has two RJ45 jacks, each of which carries four of the data lines. The RJ45 jacks are paired with Cat6 Ethernet cables to carry the signals over long distances with minimal loss in fidelity. Since each adapter has two jacks, we ran a single Cat6 wire to each panel, which allows the wall to be highly modular.
The software works by presenting the user with a transparent window that can be moved around the screen. Anything under the window is captured into memory as a matrix of RGB values, allowing the wall to be independent of any particular software package. Once the software has a matrix of RGB values representing the image, we perform a series of operations for controlling color, which will be detailed in further sections. The matrix is then divided up into pieces representing the various panels and sent to the individual Teensy boards. This had to be done incredibly quickly, as there is only 1/64th of a second to transfer all data from the computer to the chips if we wanted to keep a reasonable frame rate. It is worth mentioning that for this project’s purposes, a frame rate of about 48fps seemed to give the wall the most responsive and smooth motion without inducing dropped frames. To achieve this frame rate we simultaneously outputted data for all 25 Teensy boards in parallel.
Once the data leaves the computer, the Teensy boards read the data and output it to the LEDs. One of the 25 Teensy boards is designated as a master, and once it finishes receiving its data, the master board waits for 75% of the frame rate’s time to pass before sending a pulse on frame sync wire. This pulse tells all other Teensy boards to begin sending their stored data from the computer to the wall. Once all the data had been sent out the boards began listening for new data from the computer.