Dynamic LED Backlight Dimming Technology And Application In LCD TVs

After more than 30 years development, liquid crystal display (LCD) has become a mainstream technology for flat panel displays. LCs don’t emit lights; therefore backlight units (BLUs) are necessary. Caused by light leakage of LC cell, low contrast ratio (CR) is one big drawback for LCDs. Another shortcoming of LCDs is power efficiency. The luminance of a BLU is constant, which means the power is consumed even displaying a completely dark image. Based on the display image, the backlight dimming technology can dynamically dim the backlight luminance and simultaneously change the LC pixels grayscales to, which can increase the contrast and reduce power consumption.The thesis introduces the dynamic dimming technology and analyzes the pros and cons of the nowadays dimming algorithms. The S-curve-based tow-step dimming algorithm for global dimming is proposed. The local dimming algorithm for sparse distributed LED backlight is introduced. The hardware design scheme for both the global and local dimming algorithms is then introduced. FPGA dimming board and LED backlight driving board are produced. The 2 prototypes are developed.As the fact that nowadays global dimming algorithms present the disadvantages of low power saving ratio and overflow distortion, the proposed algorithm is based on S curve, containing 2 steps. The grayscales of all the pixels are firstly linearly amplified, based on the max value of the pixels. The parameters of S curve are then decided on the modified image. The dimmed backlight level and the coefficient to compensate the LC pixels are confirmed.To increase the versatility of the circuit design, the hardware circuit is designed suitable both for global and local dimming algorithm. The FPGA main board is based on image storage. Several function modules are created and described on RTL level, which can reasonably balance the hardware resource efficiency and processing speed.Two prototypes to realize the proposed global and local dimming algorithms are developed and measured. The measurement results show that the ratios of power saving are 18.4% and 25.2%, respectively for the global dimming LCD TV of 50 inch and the local dimming TV of 32 inch. Both the 2 prototypes work well and present better display quality than the no-dimming LCD TVs.