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When looking at LED displays, rarely do we pay attention to the mechanisms behind them. However, the importance of what goes behind a display cannot be underestimated.
Refers to embedded systems; also called LED display offline control system or offline card. This system does not need to be controlled by a computer; user will enter the visual information into the LED display memory, and display will be shown one by one and looped.
Brightness shows the lighting intensity of the surface area of the LED display, and this is the most important parameter in any form of display equipment. The primary unit of the lighting intensity is called “candela” and shown as “CD.” A singular LED lighting intensity frequently employs the unit “MCD,” which refers to one-thousandth of CD. If you sum up the lighting unit in 1 square meter of LED, you can get the surface area lighting intensity, which is shown by “Nit.” One Nit＝1 CD/m2.
The scope of the brightness depends on applications, and the brightness level of outdoor displays usually falls from 5,000 to 10,000 Nits. The brightness level of indoor displays is usually between 50 to 1,200 Nits (data as of November 2015).
The best solution to adjust brightness without losing grayscale is the current gain feature of driver ICs. But because LEDs emitting different spectral characteristics under different current, attaining white balance under different brightness means that the driver IC’s parameters must be optimized accordingly.
The brightness adjustment modules allow manual adjustments of driver ICs current gain under different brightness with the aid of controllers and ensure the display’s white balance under variable brightness. The module also stores these parameters in the control system process. When the brightness of display changes, users can directly choose optimized parameters, which make utilization much less complex. For more information, please refer to the technical paper: Analyzing Methods for Adjusting LED Display Brightness.
Calibration is based on the principles of chrominance compensation, and the brightness and color in each pixel can be adjusted through dither compensation so that every single pixel in the display can achieve high conformity in both brightness and color. This enhances the uniformity and the useful life of the display.
Calibration system is frequently utilized during production line and onsite calibration. Production line calibration refers to the adjustments conducted on each cabinet of displays before shipment, which ensures that any on-the-spot mix and matching will achieve the same visual effects. Currently, many LED display manufacturers will undertake production line calibration prior to shipping out new products. Alternatively, another large scale application is rental firms, who wish for displays that were manufactured in different times and in different batches to achieve uniform results for optimized stage effects.
Onsite calibration is conducted at the site of the display. This produces the best effects, but can only be achieved on fixed displays, and may not work under certain conditions.
In studies of color cognition, CIE1931 XYZ color space is one of the first color space to be defined mathematically, and was established in 1931 by the International Commission on Illumination (CIE).
It means quantity of different brightness levels. As the depths of the primary colors of red, green, and blue will determine the capacity to demonstrate the vividness of colors in a display. The quantity of colors displayed in an LED display is determined by the grayscale of RGB colors, or in other words, the number of orders shown in binary RGB grayscale. In the case of Macroblock’s driver IC, ideally a combination of 16-bit grayscale and RGB colors can create 216x216x 216= 281,474,976,710,656 color variations.
The color temperature of a lighting source is defined as the absolute temperature of a black body radiator that has the same lighting color as the source, the corresponding point of this temperature can be found on the Planckian locus in the color spectrum. The higher the temperature of the black body, the higher the amount of blue color in the light spectrum would be emitted. However, at lower temperatures, red light is emitted. For example, a regular incandescent bulb that emits warm, white light has a color temperature of 2,700K and the color temperature of a daylight fluorescent light is 6,000K. Unit: Absolute temperature (Kelvin, K)
Contrast ration is the ratio of the highest LED display brightness to the background brightness under a consistent environmental luminance. The condition of a LED screen’s contract ratio can be observed by verifying whether shades from the darker parts to the brighter parts of an image are smooth or not. If the display shows color patches or if the darkest part or brightest part of the images cannot be distinguished, then the contract ratio is poor.
Also known as LED display controller or LED display control card. It is the core component of a LED display, which facilitates the functions of driver ICs and controls the individual lights of a LED display.
The functions of driver ICs are becoming more and more complicated. In order to achieve the designed features, it must be closely integrated with the controllers. Therefore, Macroblock will periodically update controller support charts for high-level driver ICs as the following for user references:
Current skew is mainly caused by the differences in primary passive components built by semiconductor manufacturing, and the difference can result in current skew. For LED displays, current skew will result in non-uniform brightness across the display, especially in the field of fine-pitch displays, which is even more sensitive to current skew because of lower current use and can easily result in poor uniformity.
There are two types of current skew, including: Channel current skew - the skew bias between the different channels of the same LED driver IC IC current skew - the current offset between different driver ICs.
A common method of adjusting the brightness of LED display is to change the current through the LEDs through the current gain feature of driver ICs. The luminous intensity of LEDs is determined by the current; thus, increasing or decreasing the currents through the LEDs can achieve display brightness adjustment. Traditionally, the current gain feature is often used to adjust the color temperature of display displays. However, because using current gain to adjust brightness has the advantage of not losing grayscale details, this feature is widely used in various LED displays that require brightness adjustment support.
Dead pixel isolation refers to the driver IC function for isolating and limiting the effect on overall display quality of a single dead pixel to a single display pixel when dead pixels occur on LED displays.
This is a technique for continuous conduction of videos on LED driver IC. Based on traditional on-off design, D-PWM technique interpolates low grayscale bit data into high grayscale bit data to improve refresh rate and utilization rate, and is considered a mid-level IC technique.
Functions of a driver IC can be distinguished as LED open circuit detection, LED short circuit detection, LED current leakage detection (often caused by pins being penetrated by static electricity and resulting in leaking current and an always on state), and LED overheating detection (detected temperatures over 150 degrees is considered overheating), which, coupled with controllers, can quickly determine if a display has malfunctions.
A frame is a completely displayed image. Fast display for consecutive frames forms videos or animations, such as television images.
B. Frame Rate
This is the number of frames displayed by the screen per second. Frame rate describes the frequency that the displayed image is updated in terms of frames/sec (fps, frames for second). In other words, the number of image frames transmitted in one second, or, how many times the image processor can refresh the image. Each frame is a still image and the appearance of movement is done by rapidly and continuously displaying frames. Higher frame rate can achieve smoother and more realistic animation. The higher the fps, the more fluid the movement displayed will seem.
The brightness spectrum of a single color, which can be perceived as the resolution of brightness of each LED on the display. For example, the 2-bit grayscale in the white LED below has 22 = 4 steps.
Hawkeye is an integrated solution introduced by Macroblock to target indoor fine-pitch displays. The concept is to connect the important components of LED displays, (including LED driver ICs and peripheral ICs), collaborate with controller manufacturers, realize IC functionalities, elevate visual performance and specification, and improve user experience. Choosing the Hawkeye Solution Platform is not only about component selections, but it also symbolizes that you are opting for unique characteristics of LED displays as well as the best visual effects.
LED is the acronym for light emitting diode.
This is a function that exists mainly in lower power driver ICs. Using LED displays with low knee voltage supported ICs can reduce the voltage power required and conserve energy.
Macroblock’s MBI5035, MBI5045, and MBI5155 are all equipped with this feature.
Out of control pixel refers to a pixel that has a lighting state inconsistent with the state requested by the controller. Dead pixel is one such phenomenon. According to the General Specification for LED displays, the strictest standard is to have an out of control pixel rate of lower than 1 in 10 thousand for both indoor and outdoor use. For fine-pitch products that have a standard of 1 in 10 thousand, the high pixel density can cause poor visual performance. Out of control pixel rate has to be kept under 1 in 100 thousand or even 1 in a million to satisfy this application need. This strict out of control pixel rate is a challenge to all suppliers of the supply chain, and requires even higher standards to be applies to manufacturing techniques.
Note: “The General Specification for LED display” JS/T1141-2012, released by the Ministry of Industry and Information Technology of the People’s Republic of China.
This is the rate in which the display information is completely rendered on a LED display every second (Hz). This is different from the definition of frame rate.
Digital image resolution stands for the number of pixels that make up the digital image, and is generally written as width x height. The higher the resolution, the finer the image.
Display resolution is the ability of a display to show detailed images. This is also quoted as width x height. If the resolution of the display is lower than the image resolution, it results in the loss of clarify in the displayed image.
The resolution of LED displays depends on its pitch, or in other words, depends on pixel count per unit area. In a constant display area, displays composed of modules with smaller pitch will result in higher resolution, in comparison with displays composed with modules of larger pitch.
Note: Full HD stands for full high definition image, which is 1920*1080 pixels; UHD (ultra-high redefinition) refers to anything from 4K=3840*2160 and 8K = 7680*4320.
There are 1/2 duty cycle, 1/4 duty cycle, 1/8 duty cycle, 1/16 duty cycle, and 1/32 duty cycle. Higher duty cycles are often used for the smaller pitch display, which makes better image quality but more challenging to produce, and may result in related issues such as ghosting.
Static driving refers to the “point-to-point” control from the driver IC’s output pins to the pixels, all the LEDs will simultaneously display grayscale data, and differs only in the brightness and how long it is being lit.
This is mainly used in displaying objects such as videos, images, texts, and announcements in real time, and is often used with full color displays. Synchronous architecture refers to “computer – (video process) – sender card – communication circuits – receiver card – display” architecture. This architecture is invented by Nanjing Lopu Co., Ltd. In the late 80s, and is named “synchronized” because the displayed content mirrors to the image or part of the image displayed on the computer monitor.
This is a technique for continuously conducting videos on LED driver IC and is often used in mid and high level ICs. S-PWM is primarily used to increase the refresh rate of the overall display by scrambling video conduction time into several shorter conduction periods. S-PWM technique can be used to evenly scramble an input image into 64 equal parts. Thus, the refresh rate can be improved by 64 times.
Driver ICs with built-in memory, primarily used in time-multiplexing display, are the highest level ICs today. Driver IC with built-in SRAM can greatly improve display refresh rate and utilization rate without damaging grayscale performance, and is the driver IC used in mainstream time-multiplexing display in the market today.
Scan driving refers to the “point-to-row” control from the driver IC pins to the pixel rows. Each LED row displays the grayscale data sequentially, but with different LED brightness and length of time being lit.
The quality of the entire image is related to the design and manufacturing of the display, and the performance of the driver IC. However, the most crucial factor is the uniformity of LEDs: this includes brightness uniformity and color uniformity.
“Utilization rate” refers to the percentage of the times that LEDs are effectively lit in a duty cycle. Using a wave diagram observing OE pin as an example (the low parts represent the time that LED is lit), LED utilization rate = T(LED lit)/T(Total time). A LED utilization rate of 63% means only 63% of effect luminous intensity (measured in Nits) of the LED is utilized.
Video processor is high performance image processing and control equipment made for full-color LED displays. This equipment is used between the signal source and the display end (receiving end). It can be seen as equipment for converting external (e.g. high resolution cameras, Blu-ray DVD, computer, laptop computers, and HD video players) image signals to the signals that the LED displays can accept (DVI) and to send them to the controller.
A viewer can see the LED’s highest brightness when facing the LED panel at the optic axis. When the viewer moves to the left or right, the perceivable brightness will be lower. The sum of the two angles on each side of the display, at which the perceivable brightness is half of the highest brightness, is called horizontal viewing angle. Vertical viewing angle can be measured using the same method. Normally display suppliers will provide the measurements of viewing angles.
Viewing angle determines the size of active display area and the optimal angles for viewing the display. If viewing angle is too small, then the display may look discolored when the viewer is not directly facing the display.
For any kind of display apparatus, the best viewing distance should be the shortest distance at which the human eye cannot distinguish the distance between the pixels. Particular to LED displays, each supplier has different recommendations, and there hasn’t been an established standard in the industry; hence, users need to adjust according to his/her requirements.
Listed below is the recommended viewing distance calculation formula provided by 3 different suppliers:
1. Barco: Optimal viewing distance (M) = pitch * 2, the smallest viewing distance (M) = pitch * 1. For a P3 display, the optimal viewing distance is 3*2 = 6 meters (M). The smallest viewing distance would be 3 meters (M).
2. Christie: Optimal viewing distance (M) = pitch * 2.5. For a P3 display, the optimal viewing distance is 3*2.5 = 7.5 meters (M).
3. Planar: Simulation can be shown according to the type and size of the place the panel is used, by visiting their website: http://directlightcalculator.planar.com/#/directlight
This is also known as pixel sharing or dynamic pixels, and is commonly achieved by using images have 4X the physical pixels of the LED panel and dividing them into 4 parts by odd and even rows and columns, then rapidly sending these parts to display on the physical pixels. The resulting effect is that the pitch will appear to be half of the actual pitch. The cost of this method is basically the same as the more traditional methods, but it can deliver 4 times of the original resolution.
Color coordinate is a coordinate that represents the RGB brightness. The brightness of red, green, and blue colors must be balanced in order to reproduce realistic colors. In other words, the white color displayed by the LED must be truly white, and not tinted pink. If all the RGB lights are at their highest brightness, the mixed color is usually not truly white. In order to obtain white (usually with a color temperature of 6500K), 1 or 2 colors of the RGB must be tuned to a lower brightness. To accurately achieve white color, the brightness must be calibrated repeatedly, and this process is called white balance.
This is a function of LED driver IC, and is often seen in traffic signs (VMS). Since traffic signs do not need to be fully lit at all times, voltage can be adjusted so the LED enters sleep mode when it is not lit. Thus, we can simulate a switched-off circuit to conserve power. For example, if applied to the speed displays on highways, the displays will automatically enter 0-Power mode when there are no cars passing by to conserve power as well as to increase usable lifespan of the displays.