Application of point-by-point correction technology on large LED screen

The development of led pixel screen display technology is becoming more and more mature, and point-by-point correction technology is one of the rising technologies in recent years, and it will surely become a must-have technology in the industry. Due to the light decay of LEDs during use, the uniformity of the screen will decrease after the screen is installed and applied. Therefore, on-site recalibration technology has become another important technology that LED display manufacturers should master.

1. Origin of the concept of point-by-point correction technology

The current status of the LED chip production process determines that even LED chips produced in the same batch still have considerable differences in luminous intensity and dominant wavelength between individuals. For LED display applications, this difference will seriously affect the display quality. It must first be classified and screened through spectroscopic and color separation for indicators such as luminosity, chromaticity, and electrical parameters before it can be applied to the same display.

However, using the method of spectroscopic and color separation to solve the problem of inconsistency in individual chip luminosity and chromaticity cannot achieve perfect image quality due to factors such as insufficient precision, the influence of the subsequent process flow, and inconsistent light attenuation during the aging process. In addition, the display quality that has been used for a period of time will also decline due to factors such as inconsistent light attenuation, and "flower screen" will appear, which is beyond the reach of light and color separation.

Therefore, the industry tries to solve this problem by starting from the last process of display manufacturing and adopting differential driving for different LED light points, which is point-by-point calibration.

In the late 1990s, the theoretical prototype of point-by-point correction appeared at home and abroad, and the practical exploration of this technology was started. However, due to factors such as the lack of applicable general data collection tools and technical barriers, the research on this technology has been in a discontinuous and unsystematic state for a long time, and it lacks communication. Point-by-point correction also lacks a recognized definition. Here I try to put forward my own understanding. Point-by-point correction is defined as follows: that is, by collecting the brightness (and chromaticity) data of each pixel (or each primary color sub-pixel) area on the LED display, each The correction coefficient of the primary color sub-pixel or the correction coefficient matrix of each pixel is fed back to the control system of the display screen, and the correction coefficient is applied by the control system to realize the differential driving of each pixel (or each primary color sub-pixel), This improves the brightness uniformity and color fidelity of the display.

2. Technical composition of point-by-point correction

As can be seen from the above definition, the point-by-point correction technology can be decomposed into four parts: ① raw data acquisition; ② correction data generation; ③ drive control; ④ maintenance after correction. These four aspects are analyzed and elaborated in the follow

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(1) Raw data collec

on

Raw data acquisition is the first step in point-by-point correction, the most basic step, and the slowest and most difficult step in development. According to the collection parameters, it can be divided into brightness data and chromaticity data; according to the collection objects, it can be divided into module-level collection, cabinet-level collection and full-screen sub-area collection; according to the collection environment, it can be divided into factory mode collection with live mode acquisit

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From the perspective of collection technology routes and tools, it can be roughly divided into the following directi

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(1) Mechanical device + photometric probe. That is, the mechanical transmission device is used to control the photometric probe to collect the data of each lamp point one by one. In the early experimental device, the screen body was placed vertically on the ground, and the luminance meter was moved point by point with a rack equidistant. Later, it gradually developed into a machine form, with modules or unit boards placed horizontally and probes collecting data vertically. In order to improve efficiency, a single machine can be equipped with multiple probes to collect data in units of cabin

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The advantage of this collection method lies in its high precision, but it also has a fatal flaw: low efficiency, making it difficult to achieve large-scale industrial applications. Furthermore, on-site corrections cannot be achieved. In recent years, with the advancement of technology, this machine-based acquisition method is gradually fading out of the historical st

e.

(2) Digital cameras. It can be said that it is currently the cheapest acquisition solution to realize point-by-point correction by using the imaging grayscale data of the light point by the digital camera. Since 2008, several major display control system manufacturers have successively invested heavily in research and development, developed their own camera acquisition systems, and carried out the practice of point-by-point correction, which greatly promoted the promotion and popularization of point-by-point correction technol

y.

The advantage of the digital camera solution is that the equipment is relatively cheap, but the disadvantages are low precision, poor stability, and large differences in consistency between individuals, which makes it difficult to meet the needs of large-scale industrial production. In addition, digital camera solutions are mostly independently developed by control system manufacturers in combination with their own systems, and are incompatible with each ot

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(3) CCD-based planar luminance/chromaticity distribution measuring instrument. The research and development of this type of instrument is accompanied by the rapid growth of the global flat panel display industry. Using the principle of imaging luminance measurement, it can efficiently obtain the luminance/chromaticity value of any area on the imaging pl

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This type of equipment has high precision, good stability, and good correction effect, but the price is relatively expens

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(4) Industrial CCD acquisition scheme. In addition to the above-mentioned directions, there are also some solutions based on industrial cameras, most of which are developed by display manufacturers themselves for internal use o

y.

If a worker wants to do a good job, he must first sharpen his tools. With the improvement of the efficiency and function of the collection tools, the data collection of point-by-point correction has a wider space and possibility, extending from the factory to the field, from new screens to old screens, from flat screens to curved screens and even special-shaped screens Scr
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(2) Generation of correction

ta

The generation of correction data can be decomposed into three parts, one is the original data correction processing, the other is the setting of the correction target value, and the third is the calculation and generation of the correction data. The most important technological breakthrough lies in "raw data correction processing", especially the data correction in the field correction environm

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1) Raw data correction proces

ng

The simplest case of on-site calibration is: flat screen, select the best audience area of ​​the display screen as a single data acquisition camera position, and sequentially collect data from the sub-areas of the full screen. The data collected in this way must have systematic errors introduced by different viewing angles. The collected data shows that the brightness in the vertical normal direction is high, and the brightness in the direction away from the normal line decreases. The larger the deviation angle is, the lower the brightness

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If it is not corrected, the corrected display screen will be dark at the bottom and bright at the top; the vertical direction of the camera position will be dark, and both sides will be bri

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However, when the screen body is curved or the site environment is limited, multiple camera positions are required to complete the acquisition. Since different camera positions have different acquisition angles, if no correction is made, there will be obvious dividing lines at the se

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The above problems make many screens unable to perform on-site calibration. Recently, some digital camera solutions adopt the method of comparing and feedbacking neighboring areas, and some devices adopt the method of taking full-screen images as a reference for correction. Introduced in 2010, the solution for post-statistical analysis and processing of the original collected data to realize data correction completely eliminates the link of repeated collect

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In addition, there are occlusion problems such as on-site trees, wires and even traffic signal settings, incomplete images, and how to deal with missing light point data? These were once insurmountable obstacles for on-site calibration, and now there are mature, simple and practical soluti

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2) Setting of calibration target v

ue

The setting of the calibration target value is also a part worthy of in-depth discussion of the point-by-point calibration technology. As we all know, brightness correction loses brightness, and chroma correction loses both brightness and color gamut space and color saturation. So how to set reasonable calibration target brightness and chromaticity values, combined with customer needs, to find the best balance between brightness, color gamut and uniform

y?

At present, many digital camera calibration schemes put the setting of the target value before the acquisition because of the lack of intermediate data. However, different display screens have different optimal balance points, especially for chromaticity calibration. If it is unreasonable, it will directly lead to the failure of calibration! A reasonable target value setting depends on the statistical analysis of the collected data. Therefore, it should be a more reasonable solution to set the target value after the collection is completed, and provide various auxiliary parameters and graphs to help users adjust the target va

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(3) Drive con

ol

With the correction data, the correct application of the control system is also required to achieve point-by-point correct

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There are two ways to realize the drive control: one is current amplitude control, and the other is pulse width control (PWM mode). Since the current amplitude and brightness are not strictly linear, and the increase or decrease of the current will cause the shift of the dominant wavelength of the LED chip, the current control is used less and less. pulse wi

h.

Major domestic control system suppliers have already achieved point-by-point differential drive control of LED lights, but due to the lack of general acquisition equipment, until 2008, point-by-point calibration was still the exclusive technical advantage of a few industry leaders with their own control systems . With the breakthrough progress of acquisition equipment, most of them remained in the promotional selling point in 2008, and the point-by-point correction function of the control system that could not be practical has gradually become a necessary weapon for the control system to enter the market by 2010. As of today, there are very few full-color display control systems on the market that do not have the ability to correct the brightness point by po

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However, there are still areas to be improved in terms of drive control for point-by-point correction, which are reflected in the following aspe

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(1) The corrected low brightness and linear performance need to be impro

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(2) At present, there are not many systems with chroma correction funct

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(3) The number of loading points after correction needs to be expan

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In addition, in addition to using the control system to achieve drive control, there is another technical idea to realize correction from the level of the signal source through real-time processing of the front-end video stream. It can be divided into hardware implementation and software implementation. Hardware implementation is to add a signal processor between the video signal source and the control system, internally store the correction data, apply the correction data to the input video stream signal for real-time calculation, and then output it to the control system. The software implementation is to intercept the display data stream of the computer as the signal source, and then output it to the DVI port after correcting the data calculat

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Compared with the control system to achieve correction, since the DVI signal is only 8 bits, this method of using the front-end video processor to achieve correction will seriously lose the gray scale, and its low brightness and poor linear performance will be the inevitable result, and the application of chroma When correcting, the effect is not ideal due to insufficient precis

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(4) Maintenance after correc

on

After the point-by-point calibration is completed, the follow-up maintenance of the display faces new problems, such as replacing the receiving card, updating the data after replacing the module, and adjusting the target brightness and color gamut of the disp

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At present, in many point-by-point calibration solutions, there is a lack of original data and intermediate data, and it is impossible to reproduce the parameter settings and calibration target values ​​during calibration. After calibration, only calibration data files are saved for subsequent maintenance. This mode can cope with the replacement of the receiving card, but it can’t do anything for other maintenance needs such as module replacement. It can only be re-collected on site, or even full-screen collect

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There is also an imported calibration system that solves the data update after module replacement by providing a light-shielding tube-type on-site module measurement device that covers the newly replaced module at close range and compares it with the surrounding module data to obtain the correction coefficient of the new mod

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The above-mentioned methods all require maintenance personnel to visit the site in person, and the end user of the display screen cannot complete the maintenance work by himself. In 2010, the system added the functions of project management and module data management. Whether receiving card replacement, module replacement, or color gamut conversion, all can be easily realized by remotely sending a new set of data to customers. In this technical system, since the original image, original luminance data, corrected luminance data and other intermediate data as well as the system parameters at the time of acquisition are all saved, the follow-up maintenance work can be traced and evidence-based, and flexible data fine-tuning is provided Tool to help customers solve the problem of bright block patches on the newly replaced spare modules due to the inconsistency between the light decay and the original screen, and realize "repair the old as o
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3. Correct the interactive mode of each link point by p

nt

Before 2009, the collection system was mostly developed by the control system itself and used in conjunction with its own control system. Therefore, whether it is module-level, cabinet-level or full-screen level calibration, the interaction between the acquisition system and the control system is mostly completed using a custom control interface proto

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This combination of control interface protocols has a high degree of automation, and the process of writing data does not require manual intervention. For some digital camera calibration schemes, due to the accuracy and stability of the measurement equipment, it is necessary to repeatedly collect and compare neighboring areas to ensure the consistency between regions. It is often required to closely interact with the collection and display control. The combination of this control interface protocol is only choice. However, this also caused the acquisition system and the control system to be bundled and incompatible with each ot

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LED screen manufacturers introduce imported acquisition equipment and combine their own control systems. There are two situations. One is to modify the control system in accordance with the control interface protocol requirements of the acquisition system, and use the software functions of the acquisition system to complete the calibration; the second is to develop software by itself to realize Display control, acquisition system acquisition and correction data generation and writing. Both of these situations mean that technology introduction is difficult and costly, and similarly, it is incompatible and cannot be combined with the control system commonly used in the market. In 2009, the data acquisition was decomposed and independent from the point-by-point correction process, and the combination of the acquisition system and the control system was created: the data interface proto

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Because the acquisition device only needs one data acquisition to complete the calibration, the part of display control becomes very simple and can be completed without interactive communication with the control system. The step of writing to the control system can be in the form of a data file, and the control system can complete the work of reading and writing to the relevant storage area by its

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In this way, the general control system does not need to do any modification, and does not need to disclose any control interface commands. It can realize point-by-point correction by reading the correction data file in the public format. The workload of system docking is minimized, and the acquisition system is also minimized It realizes the greatest compatibility with the endless control systems. http://www.szadled

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The combination of data interface protocols is simple, flexible, and compatible, but the degree of automation is low. In the future, a standard point-by-point correction control interface protocol will be formed in the industry, or a dedicated control interface protocol will be formed between the acquisition system manufacturer and the control system manufacturer. It will be the ultimate solution to this l

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4. The future development trend of point-by-point correction techno

gy

Point-by-point correction has experienced a long budding period of nearly ten years from the end of the 1990s to 2008, and a three-year rapid growth period from 2008 to 2010. In the process of accumulation of theory and practice, the division of labor will be further refined and the process will be standardized. , protocol unification, evaluation standardization process, gradually entered its matur

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(1) Further refinement and specialization of technical division of l

or

Under the general trend of industrial division of labor becoming more and more refined and specialized, the point-by-point correction acquisition equipment also presents a trend of separation from the control system and increasing specializat

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The pursuit of high quality and the promotion of market competition have led more and more companies to adopt professional point-by-point calibration acquisition equipment, and solidify point-by-point calibration into a standard process, not just an emergency after product quality problems Policy. Only professional subdivision can achieve extensive compatibility, and can also enable professional teams to continuously and intensively cultivate each subdivision field, complement each other with resources, and promote technological progr

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(2) Faster, easier, automated and standard

ed

The point-by-point calibration technology has developed from the initial one-month calibration of a large screen on a machine to the correction of a large screen of nearly 200 square meters in one night, which has achieved epoch-making progress. In the future, point-by-point calibration will continue along the route of high efficiency, simplicity, automation, and standardization, which is an inevitable trend determined by industry demand. On the way to achieve this goal, in addition to the technological innovation of acquisition equipment, the cooperation and support of industry control system partners, the construction and promotion of industry standards, and the experience accumulation and exchange of point-by-point calibration practitioners are needed. http://www.szadled

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(3) Data storage to the mo

le

The production mode of the LED display manufacturing industry has always been order-based production. Nowadays, many large manufacturers have begun to explore the planned production mode. On the one hand, it can shorten the supply cycle to generate competitive advantages. On the other hand, standardized production can improve scale and management. benefit and competitive advantage. Planned production means module or standard box stock, which means the combination diversity at the time of shipm

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To meet the requirements of planned production, the best way to store point-by-point correction data is on the module, rather than on the receiving card of the control system as it is now. After the module or cabinet is assembled into a display screen, the control system reads back the calibration data on the module, or reads back the original data to generate a calibration coefficient based on the calibration target value and apply it. Such a technical structure will make data maintenance after module replacement easy. In fact, international and domestic well-known LED screen manufacturers mostly use this correction data storage met

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However, to achieve this goal, a unified data communication protocol between the control system and the modules has yet to be formed in the indus

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LED display is gradually accepted by the market due to its energy saving, environmental protection and high brightness. Among them, it has been widely used in the advertising rental market, exhibitions, entertainment performances and other fields. LED display technology has not only been applied in municipal traffic guidance signs, but has also been developed into car light sources and lighting source direction screens. In layman's terms, it is a more energy-saving and more environmentally friendly "service-oriented" technical product. In order to promote the development and application of point-by-point correction technology, it will add the finishing touch to the LED display industry!