Analog and digital interfaces for LCD display

Source: Internet
Author: User
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Analog and digital interfaces for LCD display

1. Analog interface

Analog and digital video sources have existed for a long time. Analog videos are often used on desktops, while digital videos are popular in laptops. Currently, a large number of computers use analog video output, which ensures that the support of analog interfaces can meet the needs of the next few years. However, does the recent advances in digital connectivity mean that analog digital interfaces can completely replace analog methods? However, medical ultrasound and X-ray applications have strict requirements on gray scale, at least these two types of applications cannot be replaced by numbers for the moment.

The analog LCD interface can be used to sample various RGB input voltages, maintain the sampling information, and then directly provide the driving transistor to each sub-pixel of the display. Theoretically, when sampling RGB information, this infinitely high precision should have an infinitely large gray level. On the contrary, the voltage provided by a digital screen must be measured in the unit of a digital addressing voltage step, and cannot be continuously changed.

The analog interface also simplifies signal transmission from the video controller to the LCD, and works on a long cable, allowing the display to be installed away from the video source. The simulated interface can run only five lines (excluding the ground line), namely red, green, blue, and hsync, and vsync ), or add the synchronous and green signals to the same line, with only four lines required (figure 2 ). This simple interface is suitable for a wide display resolution range from VGA to uxga. However, the analog LCD must accept the red, green, blue, horizontal and vertical synchronization information, and then sample the horizontal synchronous signal, using the PLL of the LCD interface) the time when the circuit is used to generate pixels to provide color information to the display. Because high resolution requires high information throughput, high resolution screens require frequent and accurate sampling. Any inaccuracy in color information or sampling points will lead to poor Video performance. Inaccurate color information may be due to impedance mismatch, reflection and excessive instantaneous disturbance on the RGB three-color line. Inaccurate sampling may be caused by clock jitter, phase, frequency drift, and other problems.

The settings of the analog screen are more complex than those of the digital screen. End users must set the total number of horizontal lines (horizontl total), horizontal and vertical positions of the analog screen, and adjust the clock phase according to the video-card feature. The digital interface does not need these adjustments. The monitor can automatically align and measure the images on the screen. However, this advantage of digital interfaces is only a small advantage, because analog interfaces only need to be adjusted when the LCD display is connected to the video source for the first time.

Obviously, all computer-based videos come from digital information. The current display card must convert a digital video into a simulated video, and in the case of a digital LCD, the monitor must convert the video back to a digital form. Like all conversion procedures, these conversions are not ideal. Because the digital interface used for digital LCD retains the original video information and avoids the conversion process, it is very attractive.

3. Digital Interface

Recent advances in connectivity, color depth, prices, and other technologies have promoted the industrialization of digital interfaces. In the PC market, reducing circuits and reducing costs is especially important to improve cost effectiveness. Converting the original digital video signal into a analog signal and then converting it back into a digital signal in the LCD monitor is obviously much more expensive than simply using a digital signal to drive the LCD.

As the price of the LCD display has dropped to a level close to that of normal computer users, the proportion of digital/analog/digital conversion interfaces in the price has become a more important factor. LCD must use low-price digital interfaces to continue impacting the desktop market.

The first generation of digital screens use 6 digits in each color. Compared with the analog screen, the color displayed on the screen is limited. With the introduction of each 8-digit color drive, the latest screen can display 16.7x106 colors, which can compete with the analog screen (see table 1 ).

Table 1 shows the number of colors in each color.

Number of digits per color Total color bits Grayscale Number of colors
6 18 64 262.144
8 24 256 16,777,216
Simulation 3. Simulation > 256 16 × 106

4. digital connection

The main goal of the digital LCD interface is to minimize the number of wires and reduce the total cost of the display subsystem. However, if a simple parallel interface is used, the number of wires becomes a problem (Figure 3 ). For example, in order to support 6-digit color display, 22 unencoded interfaces have signal lines before the shielding and/or multiple Ground Wires necessary to ensure signal integrity and anti-electromagnetic interference are added.

Signal Encoding can simplify difficult and practical interfaces. At the same time, because of the encoding, manufacturers are prompted to select an encoding scheme, and the interface is more complicated. Several companies and organizations have suggested low-voltage differential signals (low-votage ifferential signaling) (LVDS ). The basis of such schemes is to accept and process low-voltage differential signals, and the specific implementation methods of each scheme may be different.
LVDS was initially used for laptops and industrial devices, in which case the manufacturer has full control over the interface between the video signal source and the LCD. The LVDS solution reduces the number of signals to only five groups (figure 4 ). LVDS is an ideal solution for laptops, because it has low operating voltage and ensures the minimum power consumption and electromagnetic interference while achieving high-speed data transmission, and because it sends differential signals, compared to a common signal, it produces the least noise.

For many reasons, the LVDS solution is not directly used for other displays, especially for desktop displays. First, two oldest LVDS solutions from National Semiconductor Corporation and Texas Instruments Corporation are incompatible. But this is not for all reasons. This solution is optimized for the closed environment with the shortest distance from the video source to the display. The limited bandwidth only supports the resolution as high as XGA. This progress makes it difficult for LVDS to serve as a general industrial standard. This problem has been solved by introducing another interface technology. However, if you ask some companies, the technology they use may make the LVDS solution more complex. This new technology, known as the tmds solution, was developed by silicon image and used for the company's product panelinktm to the market. Similar to LVDS, tmds uses small voltage amplitude and differential signal transmission. Tmds adds a patent protocol to achieve DC balancing, and uses "exclusive or" (XOR) and "exclusive or non" (xnor) operations, reduces the number of signal conversions from high to low and from low to high.

Tmds requires less signal lines than LVDS (figure 5 ). In addition, tmds provides high torsion resistance, clock boundary independence (clock-dege independence), and easy measurement. It supports a single signal interface from VGA to uxga. Tmds uses dual-strand copper cables to reliably transmit data for several meters, and uses optical fiber cables to greatly extend the transmission distance. Genesis microchip has successfully tested its Receiver/scalar chip with a 10 m video cable.

Since the launch of the tmds solution, National semiconductor companies and Texas Instruments have developed an implementation scheme called Open LDI for desktop monitors. Other Gbit video interfaces (gvif), including Sony, were also proposed and implemented.

5. Help with standardization

Tmds has a key advantage: several research groups have standardized it. The first is the plug and display (P & D) Team of the uesa. The other two industry groups also accept the use of tmds, they are a digital graphic (DFP) team led by Compaq and a digital display Working Group (ddwg) led by Intel, Compaq, Fujitsu, HP, IBM, NEC, and silicom image ). The project name registered by ddwg is digital visual interface (DVI ). The details of these interfaces are different. They are displayed in terms of the connected device structure and the monitor or video card calibration. However, they all trust tmds so that these differences can be coordinated using adapters in most cases. Currently, DVI is widely used as a digital interface in the flat display industry. Dell has launched a plane monitor and system with DVI. Its Integrated Video subsystem has a DVI port, although the subsystem retains the RGB three-color analog interface for safety. One of the important transformations is that TI's Digital Visual Interface (DVI) has a series of multiple chips.

Another notable development of DVI is Intel's high-bandwidth Digital Content persistence (HDCP) solution. HDCP is committed to solving the problem that affects any interface between the video source and the display, that is, unprotected interfaces allow hackers to transfer video resources (such as from a DVD ). To prevent the DVI output information from being copied, HDCP provides encryption and validation programs to verify that a display device can receive protected content. The design of this solution maintains the image quality and its operations are transparent to users. With this kind of persistence, you can consider using DVI as the set-top box of the TV, digital satellite receiver, High Definition TV (HDTV), and PC interface. This interface initially supports 4.9 Gbit/sec video resources, far exceeding HDTV's requirements, while HDTV's requirements, the dual-connection version of DVI up to 9.9 Gbit/sec provides a large amount of bandwidth. (For details about DVI, visit

6. support various applications

Like most flat display manufacturers, the author's NEC company supports any interface selected on the market. NEC is the only manufacturer that produces analog and digital LCD display at the same time. This gives us a unique perspective, from which we can clearly see that different applications require different interfaces. To sum up, NEC regards DVI as an excellent solution for many applications and will support it.

One way to meet different requirements is to allow the LCD to have an "own" LVDS interface to directly support applications in closed environments. For an open environment, an interface such as DVI is required, which can be easily solved by a low-price scalar receiving/sending board.

Regardless of the application, the prospect of digital interfaces is promising. These interfaces help simplify the system design and take full advantage of the high-quality images provided by AMLCD and other high-resolution flags.

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