For displaying graphic images on a cathode ray tube display monitor, personal computer systems generally include a video adapter circuit board housed within the computer system case. The video adapter circuit board typically includes a bus interface circuit, a dedicated processor, dual-port random access memory (VRAM), and a digital-to-analog converter. The video adapter circuit board is typically controlled by the central processing unit (CPU) of the computer system and receives digital data representative of an image to be displayed from the computer system memory or from an input device coupled to the computer system bus.
Typically, video adapter circuit boards generate red, green and blue (RGB) analog component video signals in addition to horizontal (HSYNC) and vertical (VSYNC) synchronizing signals. These signals are generally formatted according to a selected one of several commonly utilized computer scanning formats. These formats determine the resolution of the video image and include video graphics array (VGA), super video graphics array (SVGA), color graphics adapter (CGA), extended graphics adapter (EGA), and extended graphics array (XGA) formats. A cathode ray tube (CRT) adapted to the selected scanning format receives the analog RGB component video signals and synchronizing signals and provides a display of the image.
Due to concerted efforts of display manufacturers, flat panel display monitors are becoming increasingly available. Flat panel display monitors offer several advantages over CRT display monitors. For example, flat panel display monitors are typically lighter in weight, occupy less space, consume less power, emit less radiation and have less glare than do their CRT counterparts. Commercially available flat panel display monitors are commonly liquid crystal displays that utilize active-matrix addressing. These include thin film transistor (TFT) and supertwisted-nematic (STN) flat panel display monitors.
TFT and STN flat panel display monitors operate on markedly different principles than do CRT display monitors and, therefore, require distinctly different driving signals. TFT and STN flat panel display monitors utilize the optical properties of liquid crystal which are different when measured in a direction parallel to the axis of the crystals in comparison to when measured in a direction perpendicular to the axis of the crystals. Liquid crystal is sandwiched between two polarizing filters having polarizing directions oriented 90 degrees apart. The inside surface of the display adjacent to the liquid crystal is treated to cause the liquid crystals to align such that they appear transparent. When an electric field is applied to an area of the display, liquid crystals in that area are reoriented by the electric field such that they appear opaque. By adding a backlight and color filters, a color image can be displayed. In active matrix displays, a transistor is dedicated to control the electric field for each pixel. TFT displays are characterized in that under an applied electric field, the crystals are reoriented by twisting up to 90 degrees. In an STN display, the crystals are reoriented by twisting up to 270 degrees under the applied electric field. STN displays generally have higher contrast and greater viewing angles that TFT displays.
Because a CRT accepts RGB signals, whereas a flat panel display accepts signals utilizing a digital protocol, these devices cannot merely be interchanged in a computer system. Rather, the computer must typically be disassembled and appropriate video card drivers are exchanged. It is anticipated, however, that a user of an existing personal computer system will wish to utilize a flat panel display monitor with the personal computer system that is configured for a CRT. It is further anticipated that such a user will be discouraged from so doing if hardware and/or software modifications to the personal computer are required or if considerable technical knowledge is required on the part of the user.
Therefore, what is needed is a circuit for adapting a personal computer system for driving a flat panel display monitor without requiring modification of the personal computer system hardware or software. Further, what is needed is a circuit that can determine the scanning format of analog RGB signals generated by a personal computer and, based upon the determination, convert the analog RGB signals into signals appropriate to driving a flat panel display monitor. What is also needed is an ability to adjust display parameters for the flat panel display monitor, such as brightness and contrast, without requiring modification of the personal computer system.