Comparative analysis of digital LCD interface and analog LCD interface part one

1 Analog interface

Analog and digital video sources have existed for a long time. Analog video is often used in desktop computers, while digital video is more popular in notebook computers. The analog video output used by a large number of computers currently ensures that the analog interface support can meet the needs of the next few years. But does the recent progress in digital connectivity mean that digital interfaces can completely replace analog, but the applications of medical ultrasound and X-ray have strict requirements for grayscale, at least for these two applications, digital can not replace analog for the time being.

The analog LCD interface can sample various RGB input voltages and keep the sampled information, which is then directly provided to the drive transistor of each sub-pixel in the display. In theory, when sampling RGB information, this infinitely high precision should have infinite gray levels. On the contrary, the voltage provided by the digital screen must be a voltage in units of the digital addressing voltage step length, and cannot be arbitrarily continuously changed.

The analog interface also simplifies the signal transmission from the video controller to the FSTN LCD, and is effective on very long cables, allowing the display to be installed far away from the video source. The analog interface only needs 5 wires (not including the ground wire) to run, namely red, green, blue, horizontal synchronization (Hsync), and vertical synchronization (Vsync), or the synchronization signal and the green signal are added to the same line, only four lines are required. This simple interface is suitable for a wide range of display resolutions from VGA to UXGA. However, the analog LCD must receive red, green, blue, horizontal synchronization and vertical synchronization information, and then sample the horizontal synchronization signal, and use the phase-locked loop (PLL) circuit of the LCD interface to generate pixels when providing color information to the display. Because high resolution requires high information throughput, today's high-resolution screens require more frequent and accurate sampling. Any inaccuracies in color information or sampling points will result in poor video effects. The inaccuracy of color information may be the result of impedance mismatch, reflections on the RGB three-primary color lines, and excessive instantaneous disturbance. Inaccurate sampling may be caused by problems such as clock jitter, phase and frequency drift. The setting of the analog screen is slightly more complicated than that of the digital screen. The end user must set the total horizontal line (horizontal total), horizontal and vertical position of the analog screen, and adjust the clock phase according to the video-card characteristics. The digital interface does not require these adjustments, and the monitor can automatically center and measure the image on the screen. But this advantage of the digital interface is only a small advantage, because the analog interface only requires the user to make some adjustments when the LCD display is connected to the video source for the first time. Obviously, all computer-based video is derived from digital information. Current display cards must convert digital video to analog video, and in the case of digital LCD, the monitor must convert the video back to digital form. As with all conversion processes, 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.