Point by Point Correction Technology of LED Display Screen

1 Concept of point by point correction technologyStarting from the last process of display manufacturing, this problem is solved by using differential driving for differential LED light points, which is point by point correction.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 the lack of applicable general data acquisition tools and technical barriers, the research of this technology has been discontinuous and unsystematic for a long time, lack of communication, and lack of a recognized definition of point-to-point correction.

At present, the more reasonable definition is: point by point correction, that is, the correction coefficient (or coefficient matrix for each pixel) for each light point is obtained by collecting the brightness (and chroma) data of each light point area on the LED screen, which is fed back to the control system of the display screen, and the control system uses the correction coefficient to realize the differential driving of each light point, Thus, the pixel brightness (chroma) uniformity of the display screen is greatly improved.2 Composition of point by point correction technologyAs can be seen from the above definition, the point by point correction technology can be divided into the following four parts:

1. Original data collection;2. Generation of correction data;3. Drive control;

4. Combination of acquisition system and control system;These four aspects are analyzed and described below.2.1 original data acquisition

Raw data acquisition is the first step of point-by-point correction, the most basic step, and the slowest and most difficult step of development. According to the acquisition parameters, it can be divided into brightness data and chroma data; According to the acquisition object, it can be divided into module level acquisition, box level acquisition and full screen sub area acquisition; According to the acquisition environment, it can be divided into factory mode acquisition and field mode acquisition; From the perspective of the technical route and tools of collection, it can be roughly divided into the following directions:

1. Mechanical device photometric probe: that is, use the mechanical transmission device to control the photometric probe to collect the number of each light point one by one. The early experimental device used to place the screen perpendicular to the ground and measure it point by point with a frame moving luminance meter with equal spacing. 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. The author has seen that the maximum number of probes in a single machine is 16, which is collected in the unit of box.

The advantage of this acquisition method is high precision, but it also has a fatal defect: low efficiency. It is difficult to realize large-scale industrial application. In addition, field correction cannot be achieved. In recent years, with the progress of technology, this desktop acquisition method is gradually fading out of the historical stage.2. Digital camera: it is the cheapest acquisition solution to realize point by point correction by using the imaging gray data of light points by digital camera. Since 2008, several major display control system manufacturers have invested heavily in R & D, developed their own camera acquisition system and carried out the practice of point-to-point correction, which has greatly promoted the promotion and popularization of point-to-point correction technology.The advantage of digital camera scheme is that the equipment is relatively cheap, but the disadvantage is that the accuracy is low, the stability is poor, and the consistency between individuals is also very different, which is difficult to meet the needs of large-scale industrial production. In addition, most digital camera schemes are independently developed by control system manufacturers combined with their own systems, which are incompatible with each other.

3. CCD based plane luminance / chroma distribution measuring instrument: with the rapid growth of the global flat panel display industry, the R & D of this kind of instrument can efficiently obtain the luminance / chroma value of any area on the imaging plane by using the imaging luminance measurement principle. Since 2006, relevant products have been produced in Japan, the United States, Denmark, France, Germany and China, but few can meet the special requirements of LED point by point correction.This kind of equipment has high precision, good stability and good correction effect, but the price is relatively expensive.4. Industrial CCD acquisition scheme: in addition to the above directions, there are some solutions based on industrial cameras, such as the industrial camera module correction pipeline scheme developed by Barco company; Another example is Changchun Sida, the pioneer of point by point correction. Their independently developed and continuously improved industrial CCD correction scheme is the first bright color integrated correction solution in China.

Sharp tools make good work. With the improvement of the efficiency and function of the acquisition tools, the data acquisition of point-to-point correction has a wider space and possibility, extending from the factory to the site, from the new screen to the old screen, from the plane screen to the arc screen and even the special-shaped screen.2.2 generation of correction dataThe generation of correction data can be divided into three parts: the first is the correction processing of original data, the second is the setting of correction target value, and the third is the calculation and generation of correction data. The most important technical breakthrough is the "original data correction processing", especially the data correction in the field correction environment.

2.2.1 original data correction processing

The simplest case of on-site correction is: flat screen, select the best audience area of the display screen as a single data acquisition stand, and carry out data acquisition in turn in different areas of the full screen. The data collected in this way must have systematic errors caused by different observation angles. The collected data shows that the brightness in the vertical normal direction is high, the brightness deviates from the normal direction decreases, and the greater the deviation angle is, the lower the brightness is. If it is not corrected, the corrected display screen will be dark at the lower part and bright at the upper part; The vertical direction of the stand is dark and both sides are bright; Distortion of light and shade occurs when viewing away from the correction point.

When the screen body is an outer arc or the field environment is limited, and the acquisition can be completed only by multiple positions, due to the different acquisition angles of different positions, if it is not corrected, there will be an obvious boundary at the joint.The above problems make many screens unable to be corrected on site. Recently, some digital camera schemes adopt the method of adjacent area contrast feedback, and some equipment adopt the method of shooting full screen images as reference for correction.2.2.2 setting of correction target value

The setting of correction target value is also a part of point by point correction technology worthy of further discussion. As we all know, brightness correction loses brightness, and chroma correction will lose both brightness and gamut space and color saturation. So how to set a reasonable correction target brightness and chroma value, combined with customer needs, and find the best balance between brightness, color gamut and uniformity?At present, due to the lack of intermediate data, many digital camera correction schemes put the setting link of target value before acquisition. However, different display screens have different optimal balance points, especially chromaticity correction. The unreasonable setting of target value will directly lead to the failure of correction! Reasonable target value setting depends on the statistical analysis of collected data. Therefore, we set the target value after the collection is completed, and provide various auxiliary parameters and graphs to help users adjust the target value.2.3 drive control

With the correction data, it also needs the correct application of the control system to realize point by point correction.There are two ways to realize drive control: one is current amplitude control, the other is pulse width control (PWM mode). Because the current amplitude and brightness are not strictly linear, and the increase or decrease of current will cause the offset of the main wavelength of LED chip, current control is less and less applied. At present, the main way to realize point-by-point correction drive control is to adjust the pulse width.

Major domestic control system suppliers have already realized the point-to-point differential driving control of LED light points, but due to the lack of general acquisition equipment, point-to-point correction is still the unique technical advantage of a few industry leaders with their own control systems until 2008. With the breakthrough progress of acquisition equipment, most of them remained in the publicity selling points in 2008, and the point-to-point correction function of the control system that can not be applied has gradually become a necessary weapon for the control system to enter the market by 2010. Today, few full-color display control systems on the market do not have the ability of brightness point by point correction.

However, the driving control of point by point correction still needs to be improved, which is reflected in the following aspects:1. The corrected low brightness and linearity need to be improved;2. At present, there are few systems with chromaticity correction function;

3. The number of load points needs to be expanded after correction;

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

Compared with the correction realized by the control system, since the DVI signal is only 8 bits, this method of correction realized by the front-end video processor will seriously lose the gray level, and its low brightness and poor linear performance will be the inevitable result, and the effect will not be ideal due to insufficient accuracy when applying chromaticity correction.2.4 combination of acquisition system and control systemIn the process of point by point correction, the following three steps are required: control the system to control the screen body and display the red, green and blue pictures in the designated area; The acquisition system completes the acquisition respectively; The correction data is generated and written into the control system.

Before 2009, the acquisition system was mostly developed by the control system and used with its own control system. LED screen manufacturers introduce imported acquisition equipment combined with their own control system. There are two situations: one is to transform the control system according to the requirements of the control interface protocol of the acquisition system and use the software function of the acquisition system to complete the correction; The second is to develop software to realize the generation and writing of display control, acquisition system acquisition and correction data. These two situations mean that the technology introduction is difficult and costly. Similarly, it is incompatible and cannot be combined with the general control system in the market.

Because the acquisition equipment is professional and stable, only one data acquisition is needed to complete the correction, so the display control part becomes very simple and can be completed without interactive communication with the control system. For the step of writing to the control system, the control system can read and write the relevant storage area by itself in the form of data document.In this way, the general control system can realize point-to-point correction by reading the correction data document in the open format of zhongkeweiyou without any modification or disclosure of any control interface command. The workload of system docking is reduced to the minimum, and the acquisition system realizes the maximum compatibility with the emerging control systems at the minimum cost.