Analysis of display effects of handheld mobile devices

introduction

Handheld mobile terminal Display From the initial monochrome LCD display screen, to STN, CSTN display technology, to TFT, OLED display screen, and even the concept of Super OLED proposed by Samsung.

At present, the discussion about the display color of handheld mobile devices is getting more and more intense. In the parameter description of each handheld device, the display color has changed from the original 4,096 colors to 65K colors and then to 262K colors, and even the 16M display colors that appear on most high-end handheld mobile devices now, but the meaning is often confusing. This article takes the commonly used LCD display as an example to explain in detail the display color realization principle, and compares the actual data measurement and display effect to explain the impact of different color depths on the display effect of handheld mobile devices.

1 Theoretical Analysis

1.1 Color Depth

Color depth simply means the maximum number of colors supported by the display, which is usually described in "bits", so it is also called bit depth. Generally speaking, the more colors that can be displayed, the more complex the image can be displayed, and the richer the layers of the picture. Each pixel of the LCD is composed of 3 sub-pixels, namely red, green and blue.

By strictly controlling and changing the voltage applied to the liquid crystal, each sub-pixel presents a different grayscale. After combining these three sub-pixels, a display screen that can display different colors can be obtained.

For example:

R4*G4*B4, that is, 2^4*2^4*2^4=4,096

R5*G6*B5, 2^5*2^6*2^5=65K

R6*G6*B6, 2^6*2^6*2^6=262K

R8*G8*B8, that is, 2^8*2^8*2^8=16M

The 16 million colors mentioned in the market are 24 bits, 260,000 colors are 18 bits, and 65K colors are 16 bits. Figure 1 shows the colors of some mobile phones on the market. It can be seen that most mobile phone manufacturers, especially large mobile phone manufacturers, currently choose 24 bits, which is 16 million colors.

Figure 1 Screen parameters of common mobile phones on the market

1.2 Implementation of color depth of mobile phone display

The picture displayed on the mobile phone is transmitted to the LCD control chip through the CPU interface in 12-bit, 16-bit, 18-bit or 24-bit format through the LCD interface according to timing control. After a frame of picture is temporarily stored in GRAM, the data drives the liquid crystal to twist in the form of different voltages to achieve different grayscales, thereby realizing rich color display.

Figure 2 shows a typical master control connection method. CS, RS, WRB, and RDB are control types, which are chip select signal, data or command, and write/read signal respectively. Data can be transmitted according to a certain timing control and the picture can be displayed normally.

Figure 2: Connection method of the host terminal

Figure 3 shows the process of driving the display screen to twist the liquid crystal to achieve different grayscales. After the image information passes through the GRAM, each pixel is divided into three parts: RGB, and its grayscale information is also converted into a digital signal. The driver side corresponds to different driving voltages to control the twisting of the liquid crystal, so that different light passes through the liquid crystal panel, thereby achieving different brightness. After passing through the filter, the image to be displayed is presented.

Figure 3 Display screen driven to achieve different grayscales by twisting the liquid crystal

Figure 4 shows how the image data is grayscaled. The first is a typical 24-bit display interface, where the data is converted into 8-bit Red, 8-bit Green, and 8-bit Blue after passing through GRAM. Similarly, the 18-bit display interface converts the data into RGB 666, and the 16-bit display interface converts the data into RGB 565, and then mixes the colors into different grayscales from 0 to 255.

Figure 4 Grayscale implementation of image data

2 Experimental verification

For LCD displays of handheld mobile devices with different bit depths, the brightness difference of LCD displays from 240 grayscale to 255 grayscale is shown in Figure 5. For LCD displays with a bit depth of 24bit, the change of 1 grayscale can be accurate, and the curve shows a slow and uniform change. For LCD displays with a bit depth of 16bit, for 16 uniform grayscales (240~255), only two brightness changes can be presented, that is, there is only one brightness change for a span of 8 grayscales. For LCD displays with a bit depth of 18bit, between 16bit and 24bit, there is one brightness change for a span of 4 grayscales. In other words, a display with 16 million colors can reflect subtle color differences and provide users with a more delicate and rich color experience.