Detailed explanation of the opportunities and challenges of LED backlight LCD TV regional dimming technology

Energy saving and picture quality improvement technologies have always been areas where the color TV industry continues to pursue innovation. With the popularization of LCD TVs, regional dimming technology has become one of the best technologies that combines energy saving and picture quality improvement.

Traditional CRT TVs are flat light sources, so they either light up the entire panel or dim the entire panel, and cannot achieve dimming by area. The display part of an LCD TV mainly includes a backlight source and an LCD display unit, where the backlight source mainly uses linear light source CCFL and point light source LED, which makes it possible to achieve regional dimming. The backlight of an LCD TV consumes the most energy, so the technology of adjusting the backlight brightness in various ways to achieve energy saving and improve picture quality has always been a difficult problem that the industry has been constantly tackling.

In the early days of LCD TVs, the backlight brightness was fixed or manually adjusted by the user through the menu, which was similar to the flat light source of CRT, either the whole piece became brighter or the whole piece became darker. CCFL is a linear light source, so zone adjustment is technically feasible. As LCD TVs gradually occupied the mainstream market, the backlight brightness zone adjustment technology has also developed rapidly, from 0-dimensional (0D) Dimming, 1-dimensional (1D) Dimming to the current 2-dimensional (2D) Dimming.

Backlight zone adjustment technology

The local dimming technology, also known as the backlight area adjustment technology of LCD TV, means that the LCD TV system divides the image signal into several areas, analyzes and calculates the image brightness of each area, and then automatically controls the brightness of the backlight source in each area.

0D Dimming: refers to the LCD TV system uniformly adjusting the brightness of the entire TV screen. Regardless of whether it is a CCFL backlight or an LED backlight, all CCFL lamps or LEDs have the same brightness in the same picture and are uniformly controlled by the system. When the brightness of the next picture becomes darker or brighter, the system automatically dims or brightens the backlight uniformly. The general algorithm is to use software to calculate the average brightness of the entire picture and adjust the backlight brightness according to the average brightness. For example, when the picture is completely black (as shown in Figure 1.1), the CPU automatically adjusts the backlight brightness to the darkest, and when the picture is completely white (as shown in Figure 1.2), the CPU automatically adjusts the backlight brightness to the brightest. The disadvantage of 0D Dimming is that the unified adjustment is not accurate enough. For example, when pure white and pure black pictures appear in the same picture (as shown in Figure 1.3, the upper and lower black and white pictures in the middle), the system CPU calculates the average brightness and adjusts the backlight brightness to a smaller position in the middle, which will cause the pure white area to be not bright enough, and the pure black area to be not black enough. Therefore, strictly speaking, 0D Dimming is not considered regional dimming.

1D Dimming: It is also called line dimming in English. As the name implies, it is adjusted by line. For CCFL backlight, it will be adjusted by each lamp tube, or the total number of lamp tubes will be divided into several groups according to adjacent lamp tubes and adjusted by group; for direct-type LED, it will be adjusted by each row of LEDs, or adjacent rows of LEDs will be grouped and adjusted separately by group. As shown in Figure 1.3, the CPU will calculate the average brightness of each area by area, and can dim or turn off the lamp tubes or LEDs in the upper and lower parts, and adjust the lamp tubes or LEDs in the middle to the brightest. 1D Dimming has made great technical improvements over 0D Dimming. Adjusting the backlight brightness in this way can greatly reduce the energy consumption of LCD TVs and improve the contrast of the picture. However, when it encounters a picture with black and white interlaced in the same line as shown in Figure 1.4, the brightness adjustment of the same line of the picture will be inaccurate.

2D Dimming: Local dimming means dividing the entire screen into several areas in a matrix manner. The CPU calculates the average brightness based on the distribution of each area and controls the brightness of each area independently. This can perfectly solve the problem of 1D Dimming in Figure 1.4.

However, early LCD TVs mainly used CCFL backlight, which is a linear light source. For example, 32-inch TV LCD panels require 12 CCFL lamps. If local dimming is really required, it can only be divided into 12 blocks at most. The biggest problem with CCFL light sources is that the on/off speed is not fast enough. If the on/off speed is driven too fast, the life of the lamp will be reduced. Therefore, CCFL backlights cannot achieve matrix-based brightness adjustment by area.

In 2009, the LCD TV market was swept by the LED backlight trend, that is, the backlight of LCD TVs uses LED. LED is compact and belongs to point light source, which makes it possible to realize real regional dimming technology. Taking the mainstream side backlight as an example, its LED lights are distributed around the LCD screen, and the system can control the LED lights in a matrix regional control, as shown in Figure 2.

Advantages of 2D local dimming

2D Dimming can adjust the LCD backlight to different areas and different degrees of brightness, which can significantly reduce power consumption, improve display contrast, increase the number of grayscale, reduce afterimages, and improve the image quality of LCD displays. It is the best regional dimming technology.

Why can 2D Dimming area control significantly reduce the power consumption of LCD displays? This is because regardless of whether it is a flat light source, a linear light source CCFL or an EEFL, the backlight source is generally in a full-bright state, and when displaying a dark image, it is achieved by reducing the liquid crystal transmittance, so they are not helpful in reducing power consumption. In contrast, when 2D Dimming displays a dark image, the LED brightness decreases accordingly, so the overall backlight consumption can be reduced. The University of Electro-Communications in Japan measured the power consumption of the same display screen for different types of backlight sources. The measurement results show that if the average power consumption of 0D Dimming is 100%, the average power consumption of the 2D Dimming type backlight source is only 43%.

2D Dimming regional control can not only reduce power consumption, but also improve the image quality of LCD monitors. Because 2D Dimming can independently control the brightness of each region, while traditional planar backlight sources can only light up the entire panel, 2D Dimming can greatly improve the dynamic contrast of the picture.

The fast on/off characteristics of LED light sources also greatly improve the motion smearing of LCD displays. Traditional CCFL backlight sources are continuously lit, so moving people will see a sense of shaking and smearing; when LED backlight sources simulate the pulsed light emission of CRT displays, that is, the backlight source also uses intermittent on/off mode, the LED backlight can cover the smearing image produced by fast-moving objects when it is turned off for a very short time, so the image quality presented is clearer.

Challenges and opportunities of 2D local dimming

The 2D Dimming technology mentioned above requires the CPU to analyze the brightness of multiple areas of an image at the same time, and then control the brightness of each area according to the calculation results. In essence, it is to adjust the brightness of the LED lights in each area by controlling the LED driver. The software's algorithm for image analysis is a test of CPU performance, and the LED driver timing control is also a difficult problem in design. If the timing control is improper, it is easy to cause the LED lights to burn out.

Currently, few LCD TV main chips have 2D local dimming function, so an external DSP is required when designing 2D local dimming for the entire machine. In addition, the more partitions there are, the more LED drivers are used, and the more complex the algorithm and timing control are, which greatly increases the cost of the entire machine. Therefore, relatively few LCD TVs on the market currently use 2D local dimming function.

With the development of technology, some 60Hz to 120Hz FRC chips have gradually integrated 2D local dimming algorithms and provided corresponding interfaces, which are generally SPI interfaces. Therefore, for 120Hz LED backlight LCD TVs, the implementation of 2D local dimming function only requires increasing the cost of LED drivers, thereby greatly reducing the cost of functional modules, which brings opportunities for the popularization of this technology on 120Hz LCD TVs. However, 120Hz screens are much more expensive than 60Hz screens. Due to cost reasons, the mainstream LCD TVs on the market are still 60Hz LCD TVs, and 60Hz screens do not require FRC chips, so it is still difficult to scale up the 2D local dimming function.

Fortunately, 3D TVs became popular rapidly in 2011, and shutter-type 3D TVs must use 120Hz screens, so the 2D local dimming function is expected to grow rapidly by leveraging the momentum of shutter-type 3D TVs.