Variable message signs and full-color displays

Historically, variable message signs and full-color video displays have utilized monochromatic through hole LEDs and required hardwired connections for manual programming and updating the messaging or images. The need for manual programming meant signs could not be easily updated to reflect current information. To create a fullcolor display, individual red, green, and blue LEDs were clustered to create a pixel. The spacing required between LEDs limited the resolution of the display, and wide binning ranges and performance differences of the LEDs made calibration for color and output uniformity challenging. Display panels were assembled with a mix of through hole and surface-mount components, increasing costs by necessitating separate assembly processes as well as introducing additional failure potential.

Recent advancements in LED and wireless technologies now enable increased functionality. Companies can now realize designs with enhanced contrast, higher resolution, elevated brightness, reduced power consumption, and improved reliability. Wireless capabilities now provide an efficient way to provide real-time updating at much lower costs. These capability improvements, combined with companies seeking alternative methods of advertising, are driving growth in the segment. The large-format display market is $15.5 billion globally today with an estimated CAGR of 6.5% over the next five years.

Features

• Enhanced resolution, contrast, and brightness
• Reduced power consumption
• Wireless connectivity for updating and monitoring
• Improved reliability
• Lower initial and long-term costs

System Block Diagram

The typical system architecture of an arrayed full-color display or variable message sign comprises three separate blocks: the controller, the receiver, and the LED tile. The controller receives the image or video file to be displayed from a computer or alternative source via standard input interfaces such as HDMI, USB, or RJ45. A controller allows the user to assign the positional arrangement of LED panels in the installation. The incoming signal is processed by an FPGA inside the controller to determine the portion of the image for which each receiver will be responsible. Multiple outputs on the controller allow for signal transmission to the receivers via RJ45 cables. When a receiver acquires data from the controller it processes the signal again to determine the information to send to specific LED tiles. Communication between the receiver and LED tiles is generally established with a HUB75 interface. Several LED tiles get connected to a single receiver by daisy-chaining the 16-pin input and output connectors on each tile with IDC16 cables.

Clocking is utilized to ensure synchronization between panels. Once the data reaches an LED tile it is decoded and sent to LED display drivers via shift registers. The LED display driver powers, controls, and updates a matrix array of LEDs in a scan-line fashion — refreshing sequential rows of LEDs at high frequencies to deliver an image output which appears uniform and complete. It also provides fault detection and thermal protection for the LED tile. It is very common for display manufacturers to create panel subassemblies which are used as larger building blocks for a display, with each panel consisting of multiple LED tiles connected to a receiver and an AC/DC power supply. The number of panels that can be supported by a single controller is dictated by the capability of the processor, the amount of memory available, and the resolution and refresh/scanning rate of the panels. Multiple controllers can be utilized to scale a display to the desired size.

System Benefits

Variable message signs and large-format displays are an assembly of interconnecting panels to form an array that can be scaled to the size needed for a given installation. These panels are commonly designed with standard dimensions and pitches from pixel to pixel. Applications require different resolutions based on its use case. Several factors influence the pitch chosen, including the size of the LED, the distance from the viewer to the display, the size of the display, the brightness target, and the needed resolution. Indoor displays have a smaller pitch between pixels compared to outdoor displays since the viewer is closer. Typical pitch for indoor applications is from 1 to 10 mm, while outdoor applications range from 5 to 25 mm. Outdoor displays require higher light output levels to enable daylight readability. The brightness of a display is measured in NITS, which is candelas/m². Indoor displays typically range from 500-1,000 NITS and outdoor displays from 7,000-15,000 NITS.

Pixel and panel consistency is important when assembling a sign or display. Light intensity and color binning at the LED level ensures that each assembled panel is well-matched. Individual panels then go through a white point calibration process to set baseline red, green, and blue output levels which further improves consistency. Matrix display controller IC’s have provisions for this calibration process. Increasing pixel density and refresh rates have increased the performance requirements of the control and driver IC’s that power and manage the LEDs on a panel. Modern drivers are highly integrated to minimize board space requirements and EMI, have low standbypower requirements and adjustable output voltages to maximize efficiency, and more memory and faster clock speeds to allow for higher refresh rates. Today, drivers are available with up to 48 current source channels — or 16 each of red, green, and blue — with excellent channel-to-channel current accuracy and ultra-fine dimming control. Features include automatic brightness adjustment based on ambient light levels, pixel deghosting, LED continuity, and thermal protection. A panel will contain multiple drivers to support the LED quantity, and the number of panels which can be daisy-chained is dictated by the controller. Multiple controllers can be utilized to scale the display size.

The latest signage and display LEDs are offered in robust, thermally stable surfacemount packages with monochromatic, RGB, and RGBW color options. These LEDs combine tight color and voltage binning, high output intensity, improved contrast, and allow for close component placement to create consistent performance from panel to panel. These features result in enhanced output and resolution, which is critical given that a 1080P display contains over 2 million pixels. A variety of package sizes, output levels, beam angles, and other applicationspecific attributes enable targeted solutions that best fit the end products’ needs. These devices are available in both indoor and IPx8 sealed outdoor versions. Lifetimes of these LEDs can exceed 50,000 hours of operation with less than a 20% degradation in light output, which exceeds the replacement cycles for most display applications.

The methods for pushing content to these displays have evolved. Hardwired, manual input and connection is no longer needed. Today, sign and display manufacturers provide software that streamlines the ability to aggregate and push content wirelessly to individual or multiple displays simultaneously. An example of such functionality is updating fuel prices at multiple stations from a single location, removing the need for a worker at each facility to perform the action and reducing the potential for errors. More critically, wireless integration of traffic signage enables real-time critical information updating based upon traffic monitoring, emergency services, and other smart city platforms.

Download Brief