Rising to the challenge of TFT LCD tech at desktop

Electronic News, July 14, 1997 by Frank Eveleno

Many flat panel customers are now considering integrating TFT displays into desktop monitor applications in order to take advantage of their smaller size, lighter weight and lower power dissipation. Some of these applications include stockbroker trading stations, medical, CAD and point-of-sale (POS) applications. As a system designer for these applications, the engineer must design the required electrical interfaces necessary to make these flat panel desktop monitors simple and easy for their customers to integrate into their current and future computer systems.

Today, most TFT LCDs have a parallel digital RGB interface with 3, 4, 6 or 8 bits of digital information per color. This interface seems to be adequate for closed applications where a specialized graphics controller chip, providing digital RGB signals, is located close to the LCD (Fig. 1). This type of configuration is typical in laptop computers and other enclosed computer systems with LCD displays such as kiosks and rugged computers. However, for desktop monitor applications, the graphics controller is typically located inside a PC, workstation or specialized computer system that is not located near the monitor. The standard output signal these computer systems typically provide is analog RGB. For a design engineer to interface to a digital LCD module in an analog RGB application, an analog-to-digital (A/D) converter circuit must be implemented (Fig. 2). However, once this A/D converter is designed, the image resolution typically does not look desirable due to difficult timing issues and the lack of complex dithering and other mathematical algorithms which are normally implemented inside the specialized graphics controller chip used for digital LCD modules.

Analog RGB signals have long been the standard interface in desktop monitors for several reasons, including the capability to send higher-frequency signals to the monitor, longer cable lengths and lower EMI, to name a few. There are some standardization efforts now under way to specify new types of interfaces for digital LCD monitors; however, it will require the user to purchase, at the very least, a new graphics controller interface board or possibly even a new computer system that incorporates this new display interface technology.

For this new interface to become widely accepted, either computer or CRT monitor manufacturers would be required to offer two interfaces for displays (analog for CRTs and digital for LCDs). Obviously, this would add a considerable cost to the computer and the monitor.

Another issue when interfacing to desktop monitors is the requirement by the user to display multiple resolutions on a single screen while using the full display area of the screen. NEC was the first to offer multiple resolutions on the CRT through its Multisync technology. Since then, NEC has incorporated a new technology, Multiscan, that brings the ability to scan at different frequencies and different resolutions to the LCD, using the full display area.

Analog Active Matrix TFT Features

To solve many of these desktop monitor interface problems for active matrix LCDs, NEC has developed an analog display interface technology. Incorporated within the analog module is NEC's Multiscan technology, allowing the LCD to handle multiple resolutions, depending on the input source and application requirements. The images will be automatically expanded to fit the existing panel size.

This new interface technology will allow the engineer to take advantage of the standard analog interface signals and make it easier to design a simple interface for the PC or workstation without requiring the engineer to design A/D converters, digital circuitry and complex algorithms to accept these standard analog RGB signals.

To design an interface for these analog LCD modules, it is important to understand NEC's analog technology in more detail. Functionally, these technologies can be explained by focusing on two main areas: the analog interface including the analog drivers, and the NEC Multiscan technology.

Analog Interface and Analog Drivers

The analog interface solves many of the desktop monitor interface problems described above. By integrating this technology on the LCD module itself, the quality of the RGB signal is maintained by keeping the signal in its analog form at all times throughout the LCD module. It is very important to note that, unlike digital LCD modules from other manufacturers, the RGB signal never gets converted to digital information using the NEC analog display interface technology. By using analog sample and hold circuits and analog LCD drivers attached to the LCD glass (Fig. 3), there are no major timing issues or picture quality degradation as a result of color conversion.

In addition, the absence of dithering algorithms does not degrade the image quality as would likely be apparent using a digital LCD display with A/D converter circuitry.

With the analog interface, it is very important to note that the user must provide a pixel clock to the LCD module. On digital LCD modules, a pixel clock is required to enable the LCD to convert the digital RGB signals for each dot into analog information by use of the digital drivers on that module. The digital drivers perform a digital to analog conversion, which limits the number of colors and gray scales that can be reproduced by the module. NEC analog modules also require a pixel clock, which enables the LCD module to decide when to apply the sampled analog voltage to the transistor on the module. However, since the signal is already in the analog form, no D/A process is necessary and more colors can be reproduced. With this in place, NEC analog displays can reproduce millions of colors without compromising picture quality.