With the rapid advancement of digital technology and the increasingly complete functions of LCD TVs, users can personalize settings such as channel, color, brightness, contrast, and volume. Brightness is an indicator of the quality of color LCD TVs. In existing LCD TVs, users can adjust the OSD menu items to adjust the brightness or dimming, but they cannot automatically adjust the brightness according to the brightness and darkness of the current surrounding environment. Therefore, how to enable individuals to avoid eye fatigue when watching LCD TVs and to enjoy programs at the most comfortable brightness at any time has become an important bottleneck affecting the further improvement of LCD TVs. In the daytime when the sunlight is strong or the display screen is reflective, if the brightness of the LCD backlight is not enough at this time, you cannot clearly enjoy the program and operation; at dusk or when the light source is dim, if the brightness of the LCD backlight is too strong, the user will Feeling dazzling, eyes are easily fatigued. Therefore, the key to realizing the brightness of the LCD TV suitable for human eyes is the automatic control of brightness sensing.
1 Brightness sensor automatic control function
Pulse Width Modulation (PWM) is a very effective technology that uses the digital output of a microprocessor to control an analog circuit. It is widely used in many fields from measurement and communication to power control and conversion. PWM is a method of digitally encoding analog signal levels. Through the use of a high-resolution counter, the duty cycle of the square wave is modulated to encode the level of a specific analog signal. The PWM signal is still digital, because at any given moment, the full-amplitude DC power 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 it is on, it is when the DC power supply is added to the load, and when it is off, it is when the power supply is disconnected.
The brightness sensor automatic control is to collect the brightness of the surrounding environment through the brightness sensor. The LCD TV central processor automatically sets a stable PWM signal to control the backlight of the LCD panel according to the brightness of the surrounding environment. The PWM signal adjusts the backlight of the LCD panel. Get the most comfortable picture brightness in adaptive environment. In the daytime when the sunlight is strong or the display screen has reflective phenomena, the brightness of the LCD backlight panel is not enough, the LCD TV automatically adjusts the brightness, so that users can clearly enjoy the program and operation; at dusk or when the light source is dark, the LCD TV automatically If the brightness is lowered, the brightness of the LCD backlight panel is reduced, and the user will not feel dazzling. The eyes are not easily fatigued, and it is not easy to cause eye strain, blurred vision or decreased vision.
2 Problems in automatic control of brightness sensor
The existing method for automatically controlling the brightness of the LCD TV is not ideal. When the brightness of the surrounding environment suddenly becomes bright, the LCD TV automatically starts to adjust the brightness; when the brightness of the surrounding environment suddenly becomes dark, the LCD TV automatically starts to adjust the brightness. Since the brightness of the surrounding environment changes from bright to dark, and the change time from dark to bright is very short, the LCD TV also automatically adjusts to change from dark to bright, and bright to dark, so that the sudden light and dark changes seen on the LCD screen , Greatly affecting users to enjoy the program, while increasing eye fatigue. Therefore, in order to solve the problem that the brightness can be automatically controlled without causing the screen to flicker and dim, so that users can enjoy the program at the most comfortable brightness at any time, which has become an important bottleneck affecting the further improvement of the LCD TV. The problems in the automatic control of brightness sensor are mainly manifested in the following aspects:
2.1 Brightness sensor automatically controls environmental instability factors
(1) Changes in light. The brightness sensor of the LCD TV is very sensitive to changes in the brightness of the surrounding environment, but the brightness of the surrounding environment is affected by many factors. The switch of the ambient light, such as interior decoration lamps and fluorescent lamps, will cause changes in brightness; natural sunlight rays come in , The brightness becomes brighter, when the sunlight is blocked by dark clouds, the brightness becomes darker;
(2) Moving objects. The movement of objects in the surrounding environment, 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 Limitations of LCD TV structure
The light-sensing effect of the sensor for automatic control of brightness sensing is also affected by the position where the sensor is installed on the LCD TV, the aperture of the plastic case where the sensor is installed, and the angle at which the sensor is installed.
(1) Sensor installation location. Generally, the sensor receiver is installed on the front shell of the LCD TV. If it is installed in the center of the shell, 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 at the lower right foot of the front case, next to the LED indicator.
(2) Aperture of sensor opening. The sensor receiver is installed on the front shell of the LCD TV. The aperture size of the opening will also affect the light reception. If the aperture is large enough to improve the effect of receiving light, it will affect the appearance of the whole machine, so it is generally The aperture of the opening of the shell is at least the size of the aperture of the sensor receiver.
(3) The effective receiving angle of the sensor. The front frame of some LCD TVs will have a beveled edge angle due to modeling requirements. If the sensor receiver is installed at the position of the beveled edge, it will affect the light receiving of the sensor.
2.3 Imperfect software system
The rationality of the design logic structure of the software system, the method of the software detection side sensor is the main factor that affects the automatic control of the brightness sensor, and the main factor is the time and number of detection side. If the time and frequency of the detection side are too short, the sensor receiver is not stable enough, which causes the automatic control of the brightness sensor to start too frequently; if the time and frequency of the detection side is too long, the sensor is connected.
The receiver may receive outdated data, which cannot feedback the change of ambient brightness in real time, resulting in a sluggish response to the automatic start of brightness sensing. Usually on the OSD menu, design a brightness sensor automatic control on / off optional switch. If the switch is selected to be off, the automatic control of brightness sensing will be prohibited, that is, the brightness of the screen will not change due to changes in ambient brightness. In addition, the failure of the LCD TV machine itself and damage to components will also affect the automatic control of the software's brightness sensor.
3 hardware circuit design
The base block of automatic brightness control is composed of light sensor, A / D conversion, CPU, graphics processor, brightness control module, LCD 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, which is used as the brightness control signal of the brightness control module, and then the brightness control module Change the current of the LCD panel lamp to change the brightness of the LCD panel; on the other hand, the CPU adjusts the brightness register value of the graphics processor according to the program settings, thereby changing the brightness of the data output to the LCD 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 realizing the automatic control of the induction of the LCD TV. As shown in Figure 1.
4 System software design
4.1 Brightness sensor automatic control software architecture
The software architecture of brightness sensor automatic control mainly includes brightness data acquisition, real-time monitoring technology, automatic correction algorithm and analog hysteresis comparison algorithm. Brightness data collection is to sense the brightness of the surrounding environment through a light sensor, convert the brightness signal into an analog electrical signal, and then convert it into a digital signal from the ADC, and this digital signal is sent to the CPU of the LCD TV for processing. The digital signal collected through the brightness data is sent to the CPU for automatic correction algorithm processing; and then the analog hysteresis comparison algorithm is used to obtain a stable PWM signal; the backlight brightness module controls the backlight of the liquid crystal panel. as shown in picture 2.
4.2 Automatic correction algorithm
The steps of the automatic correction algorithm for brightness sensor automatic control: first set the time interval for collecting brightness data and set the number of sampling; when the time interval expires, read the data of the light sensor brightness; when the number of sampling reaches the preset number , Output stable brightness; then the CPU outputs stable PWM to control the backlight of the LCD panel. The set interval 201 for collecting brightness data is the time to initialize the brightness of the light sensor every time it is collected. Time to 202 is the time to reach the brightness of the light sensor. The collected brightness data 203 is the value of the digital signal that the CPU reads the brightness data of the light sensor and passes through the ADC. The number of samplings reached 204 is the same number of times to reach the collected brightness data, indicating that the ambient brightness has stabilized and no longer changes. The stable output brightness 205 is that the CPU outputs stable PWM to control the backlight of the liquid crystal panel. As shown in Fig. 3, the workflow of the automatic correction algorithm of the brightness sensor automatic control is shown.
4.3 Analog hysteresis comparison algorithm
The analog hysteresis comparison algorithm is different from the one threshold voltage of the ordinary automatic induction brightness control, and two threshold voltages are set, which are respectively called a positive threshold voltage and a negative threshold voltage. The input voltage that changes the state of the PWM circuit when the ambient brightness input signal rises from low level (dark) to high level (bright) is called the forward threshold voltage, and the input signal falls from high level (bright) The input voltage that changes the state of the PWM circuit during the low level (dark) is called the negative threshold voltage.
The analog hysteresis comparison algorithm of the brightness control automatic control divides the backlight control of the PWM into five sections, and the four analog hysteresis comparison algorithms control. Each analog hysteresis comparator has two threshold ADC values, called positive threshold ADC and negative threshold ADC, respectively. The input ADC that changes the state of the PWM circuit when the ambient brightness input signal rises from low level (dark) to high level (bright) is called a positive threshold ADC, and the input signal falls from high level (bright) The input ADC that changes the state of the PWM circuit during the low-level (dark) process is called a negative threshold ADC. For example, ADC2 is a positive threshold ADC, and ADC1 is a negative threshold ADC. When the ambient brightness input signal rises from ADCI low level (dark) to ADC2 high level (bright), the PWM state changes from PWN5 to PWM4; When the ambient brightness input signal drops from ADC2 high level (bright) to ADC1 low level (dark), the PWM state changes from PWN4 to PWM5. This is designed to prevent small changes in the ambient brightness input signal (below a certain threshold ADC) to cause changes in the output PWM, ensuring stable backlight control of the LCD panel. As shown in the graph of the analog hysteresis comparison algorithm of the brightness control automatic control in FIG. 4.
5 Conclusion
This design realizes the automatic control of brightness sensing, which is divided into brightness data collection, automatic correction algorithm, analog hysteresis comparison algorithm, and backlight brightness control module. Through multiple sampling automatic correction algorithm, a stable brightness control signal is obtained. A software simulation hysteresis comparison algorithm model is proposed to avoid the sudden change of the brightness of the screen caused by small changes in the ambient brightness, which provides an ideal solution for the brightness sensing processing of LCD TVs.
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