Abstract: This paper discusses the problems existing in the automatic control of brightness sensing of LCD TVs, and proposes corresponding improvements and preventive measures. Through the brightness sensor, the brightness of the surrounding environment is collected; the multi-sampling automatic correction algorithm is performed to reduce the interference of the external environment brightness change, and a stable brightness is obtained; and the simulation hysteresis comparison algorithm is used to reduce the brightness change and the darkness change, setting The pulse width modulation signal of the backlight of the liquid crystal panel is stably controlled; the backlight of the liquid crystal panel is adjusted by the pulse width modulation signal to obtain the most comfortable picture brightness of the adaptive environment.
Key words: LCD TV; brightness; sensor; pulse width modulation
0 Introduction With the rapid advancement of digital technology and the increasing functionality of LCD TVs, users are able to personalize settings for channel, color, brightness, contrast, volume, and more. Brightness is an indicator of the quality of a color LCD TV. In the existing LCD TV, the user can adjust or dim the OSD menu item, but the brightness cannot be automatically adjusted according to the brightness of the current surrounding environment. Therefore, how to make individuals avoid eye fatigue when watching LCD TVs, and enjoy the program at the most comfortable brightness at any time, has become an important bottleneck affecting the further improvement of LCD TVs. When the sunlight is strong during the day or the display is reflective, if the brightness of the LCD backlight is not enough at this time, the program and operation cannot be clearly seen; if the brightness of the LCD backlight is too strong at dusk or when the light source is dark, the user will Feel the glare, the eyes are prone to fatigue. Therefore, to achieve the brightness of the LCD TV suitable for human eyes, the key is automatic control of brightness sensing.
1 Function of brightness sensing automatic control Pulse width modulation (PWM) is a very effective technique to control the analog circuit by using the digital output of the microprocessor. It is widely used in many fields of measurement, communication to power control and transformation. . PWM is a method of digitally encoding analog signal levels. With the use of a high resolution counter, the duty cycle of the square wave is modulated to encode the level of a particular analog signal. The PWM signal is still digital because at any given time, the full-scale DC supply is either fully (ON) or completely OFF. The voltage or current source is applied to the analog load in a repeating pulse sequence of ON or OFF. When the DC power is applied to the load, the power supply is disconnected when it is disconnected.
The brightness sensing automatic control is to collect the brightness of the surrounding environment through the brightness sensor. The LCD TV central processor automatically sets the PWM signal for controlling the backlight of the liquid crystal panel according to the brightness of the surrounding environment, and adjusts the backlight of the liquid crystal panel by the PWM signal. Get the most comfortable picture brightness in an adaptive environment. When the sunlight is strong during the day or when the display screen is reflective, the LCD backlight is not bright enough, and the LCD TV automatically adjusts the brightness, so that the user can clearly enjoy the program and operation; when the dusk or the light source is dark, the LCD TV automatically Turn down the brightness, then reduce the brightness of the LCD backlight, the user will not have a glare feeling, the eyes are not easy to fatigue, it is not easy to cause eye strain, and there are symptoms such as blurred vision or decreased vision.
2 Problems existing in automatic brightness control The automatic brightness control method of the existing LCD TV is not ideal. When the brightness of the surrounding environment suddenly becomes bright, the LCD TV automatically starts to brighten the brightness; when the ambient brightness suddenly becomes dark, the LCD TV automatically Start dimming the brightness. Since the brightness of the surrounding environment changes from bright to dark, the change time from dark to bright is very short, and the LCD TV also automatically adjusts from dark to bright, from bright to dark, so that the brightness changes seen on the LCD screen , greatly affecting users to enjoy the program, while increasing eye fatigue. Therefore, in order to solve the brightness can be automatically controlled, without causing the picture to flicker and dim, allowing the user to enjoy the program at the most comfortable brightness at any time, becoming an important bottleneck affecting the further improvement of the LCD TV. The problems of automatic brightness control are mainly reflected in the following aspects:
2.1 Brightness sensing automatically controls environmental instability factors (1) Changes in light. The brightness sensor of LCD TV is very sensitive to the change of ambient brightness, but the brightness of the surrounding environment is affected by many factors. The switch of ambient light, such as interior decoration lamp and fluorescent lamp, will cause the brightness to change; natural sunlight will come in. The brightness becomes brighter, and when the sunlight is blocked by dark clouds, the brightness becomes darker;
(2) Moving objects. The movement of surrounding objects, including the movement of people, produces shadows that cause changes in brightness, which in turn affects the automatic control of the brightness of the LCD TV.
2.2 Limitation of LCD TV Structure The brightness effect of the sensor for automatic brightness control is also affected by the position of the sensor mounted on the LCD TV, the aperture of the sensor plastic case, the angle of the sensor.
(1) Sensor mounting position. The general sensor receiver is mounted on the front cover of the LCD TV. If it is installed in the center of the case, the effect of receiving light will be better, but it will affect the appearance of the whole machine. In order not to affect the overall appearance, the general sensor receiver is installed in the lower right position of the front case, on the side of the LED indicator.
(2) Sensor opening aperture. The sensor receiver is mounted on the front cover of the LCD TV. The aperture size of the aperture also affects the acceptance of light. If the aperture of the aperture is large enough to improve the light receiving effect, it will affect the appearance of the whole machine. The aperture of the opening of the shell is at least the aperture size of the inductor receptacle.
(3) The sensor effectively receives the angle. Some LCD TV front frame shells, due to the shape requirements, will have some angle of the beveled edge. If the sensor receiver is installed at the oblique side position, it will affect the light sensing reception of the sensor.
2.3 Software system imperfections The rationality of the design logic architecture of the software system, the software detection side sensor method is the main factor affecting the automatic control of brightness sensing, the main factor is the time and frequency of the detection side. If the time and number of detection sides are too short, the sensor receiver is not stable enough, causing the brightness sensing automatic control to start too frequently; if the time and number of detection sides are too long, the sensor receiver may receive outdated data, not real time. Feedback changes in ambient brightness, causing the brightness sensing automatic control to start the reaction is slow. Usually on the OSD menu, design a brightness sensing automatic control to turn the optional switch on/off. If the switch is selected to be off, the brightness sensing auto-control will be disabled, ie the picture brightness will not change due to changes in ambient brightness. In addition, the LCD TV machine itself failure, component damage, will also affect the software's brightness sensing automatic control.
3 hardware circuit design The base block of brightness sensing automatic control is composed of light sensor, A/D conversion, CPU, graphics processor, brightness control module, liquid crystal panel and so on. According to the brightness condition of the surrounding environment, the light sensor outputs a DC level of a certain value, and the DC level signal is converted into a digital signal by A/D and sent to the CPU. On the one hand, the CPU outputs a PWM square wave signal with a specific period and duty cycle according to the setting of the program. After filtering and shaping, it becomes a certain DC level, as the brightness control signal of the brightness control module, and then the brightness control module. The current of the liquid crystal panel lamp is changed to change the brightness of the liquid crystal panel; on the other hand, the CPU adjusts the brightness register value of the graphic processor according to the setting of the program, thereby changing the brightness of the data output to the liquid crystal panel. The adaptive brightness data signal and the adaptive brightness control signal are output from the image processor to the brightness control module, thereby finally implementing automatic sensing control of the liquid crystal television. As shown in Figure 1.
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4 system software design
4.1 Brightness Sensing Automatic Control Software Architecture The software architecture of brightness sensing automatic control mainly includes brightness data acquisition, real-time monitoring technology, automatic correction algorithm and analog hysteresis comparison algorithm. The brightness data acquisition is to sense the brightness of the surrounding environment through the light sensor, convert the brightness signal into an analog electric signal, and then convert the signal into a digital signal, and the digital signal is sent to the CPU of the liquid crystal television for processing. The digital signal collected by the luminance data is sent to the CPU for automatic correction algorithm processing; then the analog hysteresis comparison algorithm is used to obtain a stable PWM signal; and the backlight of the liquid crystal panel is controlled by the control backlight brightness module. as shown in picture 2.
4.2 Automatic correction algorithm The steps of the automatic correction algorithm for brightness sensing automatic control: first set the time interval for collecting the brightness data and set the sampling times; when the time interval is timed, read the data of the brightness of the light sensor; when sampling After the preset number of times reaches a preset number of times, a stable brightness is output; and the CPU outputs a stable PWM to control the backlight of the liquid crystal panel. Setting the acquisition brightness data time interval 201 is the time to initialize the brightness of each time the light sensor is acquired. The time timing to 202 is the time to reach the brightness of the light sensor. The acquisition luminance data 203 is a numerical value of a digital signal that the CPU reads the luminance data of the light sensor through the ADC. The number of arrival samples 204 is the same number of times the brightness data is collected, indicating that the ambient brightness has stabilized and no longer changes. The output stable brightness 205 is a CPU output stable PWM to control the backlight of the liquid crystal panel. As shown in Figure 3, the workflow of the automatic correction algorithm for brightness sensing automatic control is shown.
4.3 Analog Hysteresis Comparison Algorithm The analog hysteresis comparison algorithm is different from a threshold voltage of ordinary auto-sensing brightness control. Two threshold voltages are set, which are called forward threshold voltage and negative threshold voltage, respectively. The input voltage that causes the PWM circuit state to change during the ambient brightness input signal from low level (dark) to high level (light) is called the forward threshold voltage, and the input signal drops from high level (bright). The input voltage that causes the state of the PWM circuit to change during the low level (dark) process is called the negative threshold voltage.
The analog hysteresis comparison algorithm of the brightness sensing automatic control divides the backlight control of the PWM into five segments and four analog hysteresis comparison algorithms. Each analog hysteresis comparator has two threshold ADC values ​​called the forward threshold ADC and the negative threshold ADC, respectively. The input ADC that changes the state of the PWM circuit during the ambient brightness input signal rising from low level (dark) to high level (light) is called the forward threshold ADC, and the input signal drops from high level (light). An input ADC that changes the state of the PWM circuit during a low level (dark) is called a negative threshold ADC. For example, ADC2 is a forward threshold ADC, and ADC1 is a negative threshold ADC. During the process of ambient luminance input signal rising from ADCI low level (dark) to ADC2 high level (lighting), the PWM state is changed from PWN5 to PWM4; The ambient brightness input signal transitions from ADC2 high (bright) to ADC1 low (dark), causing the PWM state to transition from PWN4 to PWM5. This is designed to prevent a slight change in the ambient brightness input signal (below a certain threshold ADC) and cause a change in the output PWM to ensure stable LCD panel backlight control. As shown in the graph of the analog hysteresis comparison algorithm of the brightness sensing automatic control as shown in Fig. 4.
5 Conclusion This design implements automatic brightness sensing control, which is divided into brightness data acquisition, automatic correction algorithm, analog hysteresis comparison algorithm, and control backlight brightness module. A stable brightness control signal is obtained by multi-sampling the automatic correction algorithm. A software simulation hysteresis comparison algorithm model is proposed to avoid the change of brightness and brightness of the screen due to slight changes in ambient brightness, which provides an ideal solution for brightness sensing of LCD TVs.
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