Grayscale display scheme design for high-density LED display

Information display is one of the important links of information science and technology. Photoelectric integrated display has been used more and more widely as a bridge between man and machine. For a long time, information display technology remained in the era of cathode ray tube (CRT), but as a vacuum device, CRT has some inherent shortcomings that limit its application in certain fields. In order to solve these problems brought by CRT displays, both China and foreign countries are stepping up the research and development of flat panel displays (FDP). Among the many FDPs, light-emitting diode (LED) displays have some inherent advantages: low working power supply, fast response speed, wide operating temperature range, low power consumption of all-solid devices, small size, impact resistance, high reliability, long life, etc. At the same time, it is a semiconductor device itself, so it is fully compatible with IC circuits, and the control and drive circuits are easily integrated in flat panel displays, further reducing the size of the display, and it is also convenient to realize the multi-channel transmission of signals. Based on an in-depth study of the luminous characteristics of LED devices, a solution for realizing grayscale display on high-position and high-density LED flat-panel displays is introduced, and the solution is experimentally verified. The experiment shows that the LED grayscale display realized by this solution can take into account both the brightness and grayscale of the display.

1 LED dot matrix grayscale generation principle

Each pixel of the LED dot matrix is ​​composed of red (R), green (G), and blue (B) LEDs, corresponding to a pixel of the video image. When displaying synchronously with the computer CRT, if the luminous brightness of the red, green, and blue LEDs of each pixel of the LED dot matrix changes with the R, G, and B signals of the corresponding pixel of the CRT, the corresponding CRT image can be displayed synchronously. If grayscale is used to describe the brightness change of a monochrome LED, the more grayscales there are, the more colors the image has and the richer the layers. The forward volt-ampere characteristics of LEDs are roughly the same as those of ordinary diodes. There is no current before the voltage turn-on point. Once the voltage exceeds the turn-on point, it shows the conduction characteristics. At this time, the relationship between the forward current I and the forward voltage U is as follows:

Where m is the recombination factor, I0 is the reverse saturation current, UT = kT/e is called the temperature voltage equivalent. At the thermodynamic temperature T = 300 K, UT = 26 mV1. In wide bandgap semiconductors, when I <0.1 mA, the spatial recombination current through the deep energy levels in the junction plays a dominant role, and m = 2 at this time; when the current increases and the diffusion current dominates, m = 1. U is the applied voltage.

Figure 1 is the volt-ampere characteristic diagram of LED forward bias voltage

As can be seen from Figure 1, the volt-ampere characteristic of an LED is generally linear from the time it is turned on until the maximum current at which it does not burn out. In this linear region, the luminous intensity of an LED is basically proportional to its current intensity. There are two ways to achieve LED brightness control:

(1) By adjusting the forward current of the LED, the brightness of the LED can be modulated. If the forward current of the LED is adjusted in a certain step, its luminous brightness can be divided into several gray levels. However, the driving circuit required for this method is too complicated and is not feasible in practical applications. It will not be discussed here.

(2) Control the on-time of the LED per unit time. LED has a fast time response characteristic, which can reach up to tens of megahertz. It can be driven to emit light by pulses. For example, using a pulse of 1 MHz, a duty cycle of 0.25%, and a peak current of 1 A to drive the LED has the same luminous brightness as using a 25 mA DC drive. Obviously, by adjusting the duty cycle of the driving pulse, different gray levels can be obtained. If the discrete image data of each pixel of the CRT image signal is used to control the on-time of the corresponding LED, a multi-grayscale display image can be obtained. Figure 2 shows that within the cycle time T, pulses with duty cycles of 1, 4P7, 2P7, and 1P7 are used to drive the LED. Obviously, the brightness ratio is 7:4:2:1, so the highest gray level is 7, and a total of 8 gray levels can be obtained.

Figure 2a only shows the conduction conditions of four grayscale LEDs, 7, 4, 2, and 1. This method of directly adjusting the drive pulse width to obtain the grayscale of the displayed image is generally not easy to implement. The only indirect method is to make the drive pulse duty cycle of the LED subject to the image data to obtain the grayscale of the displayed image.

Figure 2b shows that a pulse is used to drive the LED within the T time. The brightness of the LED can be controlled by controlling the number of pulses within the time. When the number of driving pulses within each T time is 7, 4, 2, and 1, the brightness ratio is 7:4:2:1, and 8 gray levels are also obtained. However, it is not difficult to see that with this method, the duty cycle of the driving pulse must be reduced, which means that the brightness of the LED at each gray level must be reduced. Figure 2b is driven by a symmetrical method, so compared with Figure 2a, the conduction time is shortened by half, and the brightness must be reduced by half at the same forward conduction peak current. The brightness loss can be compensated by increasing the peak current, but it is important that this method of modulating the number of pulses to obtain image grayscale is also difficult to implement in the circuit.

Figure 2c is another method that has been applied in plasma display. When the brightness of the captured image is converted into eight levels, it can be represented by a three-bit binary code "a3 a2 a1". The highest brightness is represented by "111" and the lowest brightness is "000". Then, the period T can be divided into three small time intervals t. In the time t, the binary codes a3, a2, and a1 are used to control the conduction state of the LED. "1" represents conduction and "0" represents disconnection. At the same time, a3, a2, and a1 are weighted codes, and the corresponding weights are 4, 2, and 1 respectively. If a3, a2, and a1 not only control the conduction of the LED within the time t, but also control the conduction time, the length of the conduction time depends on the corresponding weight, and there can be 8 gray levels. As shown in Figure 2c, the first cycle T represents the highest brightness, and its binary code is "111". Then, in each t time, the LED is turned on, and the ratio of its on-time is 4:2:1. The second cycle T represents the highest brightness of 4P7, and its binary code is "100". Obviously, the LED is turned on only in the first t time. Compared with Figure 2a, it is not difficult to find that the LED on-time is still reduced in T time, so the brightness of each gray level is reduced under the same peak current. Therefore, this method of sacrificing the brightness of the displayed image is not good for LED dot matrix display.

Figure 2d shows that if each cycle is divided into 7 small time intervals, the level of each time interval determines the on and off of the LED, then 8 gray levels can be obtained. For example, when the level of 7 time intervals is all high, the LED brightness is the highest. When the level of 6 time intervals is high and the other is low, the highest brightness of 6P7 is obtained. By analogy, when the level of 7 time intervals is all low, it is completely black. Figure 2d also shows that the brightness ratio is 7:4:2:1. Compared with Figure 2a, the corresponding conduction time is equal, so there is no loss in the brightness of the displayed image. In the experiment, the author used this method. The specific implementation is as follows.

2 LED grayscale implementation

CRT images are refreshed at a frame rate. Each frame of the image can be represented by a matrix with M rows and N columns, corresponding to a video image of M × N pixels. Each image is different, and the values ​​of the matrix elements will change accordingly. The matrix expression is:

Each element in the matrix (2) represents the image brightness information of the corresponding pixel in the M × N point CRT image, and the position of the element in the matrix is ​​exactly the position of the pixel in the CRT image. Among them, A, B, and C represent the three primary colors R, G, and B with unit brightness. The coefficients aij, bij, and cij are zero and positive integers, which determine the brightness share of the three colors R, G, and B required to mix the color of the pixel. If the brightness of the three primary colors R, G, and B contained in the white level brightness of the image is divided into N levels, then each brightness is A, B, and C, that is, unit brightness. The values ​​of aij, bij, and cij are from 0 to N, respectively, indicating that aij parts of unit brightness red, bij parts of unit brightness green, and cij parts of unit brightness blue can be mixed to form the color of the pixel corresponding to the element. According to the matrix operation rules, the matrix (2) can also be expressed as

It means that an image can be decomposed into R, G, B monochrome images. Similarly, by superimposing R, G, B monochrome images in space or time, the corresponding color image can be restored.

Each item in formula (3) represents red, green, and blue monochrome images, which have several gray levels. If the values ​​of aij, bij, and cij are from 0 to N, then the gray level of each monochrome image is N + 1. According to the matrix operation rules,

Where aij ( n) = 0 or 1, and aij = aij (1) + aij (2) + ...+ aij ( n).

From formula (4), we can see that a monochrome image data matrix can be decomposed into the sum of several binary matrices (each element in the matrix is ​​0 or 1), and each binary matrix represents a monochrome binary image with unit brightness. Then the meaning of formula (4) is: a monochrome image with (N + 1) gray levels can be composed of several N monochrome binary images with unit brightness superimposed in time. Obviously, superposition in space is unrealistic. This means that a monochrome video image of a TV field can be divided into several monochrome binary images, and then these monochrome binary images are displayed in sequence. According to the integration effect of the human eye, the original monochrome video image can be reproduced. In the same way, the R, G, B monochrome video images can be restored at the same time, and then they can be superimposed in space to obtain a color video display image.

Specifically, if the number of gray levels of each monochrome pixel is N+1, the gray levels are 0, 1, ..., N from low to high, and a string of N-bit '0' and '1' control codes corresponding to each gray level is specified, and in each control code, the total number of '1' is equal to the number of its corresponding gray level, that is, the higher the gray level, the more '1's there are in the control code.

Each display cycle of the LED is divided into N equally spaced segments, and each segment uses a control code to control the on and off of the LED. '1' controls the LED on, and 0' controls the LED off. Since the number of '1's in different gray levels is different, the LED is turned on for different times in one cycle, and the difference in brightness is produced through the integration effect of human vision.

By analogy, a frame of monochrome picture displayed by CRT is divided into N fields and displayed on the LED dot matrix screen. The display time of each field is TPN. In each field, each pixel is controlled by its corresponding control code to control the on and off of the LED. Through the visual integration effect, the grayscale effect of the entire picture is produced.

3 Hardware Implementation of LED Control Board

The hardware design block diagram of the LED board is shown in Figure 3.

The red signal of CRT is sampled and quantized to form a red 4-bit binary number, so the quantized brightness has 16 levels. For example, a frame of CRT image is divided into 15 fields for display on the LED dot matrix screen, and different brightness levels are formed by controlling the number of times the LED pixels are lit in the 15 fields. For example, if a certain pixel in the 15 fields is not lit at all, it is black. It is lit once, and the image brightness is just higher than black; it is lit twice, and the brightness is one level higher; all are lit, which is the brightest. In the actual circuit, considering the complexity of the circuit, the LED screen is divided into 16 fields for display, and the first field is always not lit. This will of course lose some brightness, but simplify the design.

4 Conclusion

After the development of the test system was completed, some performance parameters of the high-density LED matrix display were tested. The experiment showed that the LED grayscale display achieved by this scheme can take into account both the brightness and grayscale of the display.