grayscale on LCD and LCD display

How to achieve grayscale on LCD

As we all know, the display effect of the liquid crystal is determined by the effective voltage applied to the liquid crystal. There are two ways to achieve grayscale (color), namely PWM (pulse width modulation) and FRC (frame rate control).

PWM is divided into several time slices in one scan time, such as 16 gray scales, it is divided into 16 time slices. If 5/16 gray scale is displayed, then there is driving voltage in only 5/16 time (for the same For one point), the final equivalent voltage is only 5/16 of total black; FRC is similar to PWM, except that each time slice becomes one frame. If 16 levels of gray are displayed, then 16 frames are required. , Displays the gray scale of 5/16, only 5 of 16 frames have driving voltage (for the same point), the final equivalent voltage is only 5/16 of the total black. PWM can produce the highest degree of color expression, and because of the higher switching frequency, more power must be consumed. FRC consumes low power, but it will produce visible jitter under certain special screens, which makes users feel uncomfortable.

FRC uses several frames of pictures as a time unit to control the number of frames of display pixel strobe to achieve gray scale control. It combines several frames into a large unit, but this method will cause grayscale flicker. To solve this problem, the frame frequency must be increased, and the response speed of the liquid crystal is not very high, so this grayscale modulation method is used Can not display higher levels of gray and faster moving images. In the PWM mode, a gray-scale modulation pulse is drawn in the data pulse. The width of this pulse width can be divided into multiple levels. Different pulse widths represent different gray-scale information, so that the selected pixels can achieve different gray-scales. level. Since liquid crystals cannot respond to too narrow pulses, they generally cannot be used to generate higher gray levels. From the above analysis, it can be seen that neither of these two methods can produce higher gray levels when used alone, so a combination of the two is used when the gray level of the entire chip is realized. Generally, for gray levels above 4, the combination of PWM+FRC is adopted. Because the higher the gray level, the higher the frequency required to use PWM, such as 16 gray levels, 320 lines, and a refresh rate of 60HZ, which requires 16x320x60=307200Hz. The higher the frequency, the more complex the structure of the IC, the worse the stability, and the greater the power consumption. With FRC, the more gray levels, the more frames are required for a cycle, for example, 16 gray levels require 16 frames. When the refresh rate is 60Hz, there are less than 4 cycles per second, so it seems that there will be flicker, so you have to increase the refresh rate, which also needs to increase the frequency, increase the power consumption, and increase the response speed of the liquid crystal. The reaction speed is always limited, and increasing the speed will greatly increase the cost of the liquid crystal. If you use PWM+FRC, you can use 2-bit (4 levels) PWM and 2-bit (4 frames) FRC or 3-bit (8 levels) PWM and 1-bit (2 frames) FRC, which can be solved very well. These problems make up for their shortcomings.

Basic knowledge of LCD

1. High display quality

Since each point of the LCD display keeps that color and brightness after receiving the signal, it emits constant light, unlike the cathode ray tube display (CRT) that needs to constantly refresh the bright spot. Therefore, the LCD display has high image quality and will never flicker, minimizing eye fatigue.

2. No electromagnetic radiation

The display material of the traditional display is phosphor, which is displayed by the electron beam hitting the phosphor. The electron beam will generate strong electromagnetic radiation at the moment when it hits the phosphor. Although there are many display products that have been compared in dealing with radiation problems. Effective treatment can minimize the amount of radiation as much as possible, but it is difficult to completely eliminate it. Relatively speaking, liquid crystal displays have inherent advantages in preventing radiation, because there is no radiation at all. In the prevention of electromagnetic waves, liquid crystal displays also have their own unique advantages. It uses strict sealing technology to enclose a small amount of electromagnetic waves from the drive circuit in the display. In order to dissipate heat, ordinary displays must let the internal circuit as much as possible. In contact with the air, the electromagnetic waves generated by the internal circuit will "leak" to the outside.

The liquid crystal display also has the advantages of large viewing area, wide application range, good picture effect, digital interface, low power consumption, uniform and compact "body".