Virtual refresh.TM. architecture for a video-graphics controller

Abstract

A single-chip video-graphics controller displays a color image stored in an external display memory simultaneously on two devices having different refresh and display requirements. The controller includes a first display processor which fetches the color image from the external memory and displays the image on a first device, such as a color CRT monitor or TV. The controller reduces the bandwidth of the fetched color image on-the-fly and stores a copy of the reduced bandwidth image in another part of the external display memory. A second display processor fetches the reduced bandwidth copy at a rate compatible with a second display device, such as a DSTN flat-panel device using dithering for color depth enhancement. The two display processes are independent of each other, controlled only by the refresh requirements of the two display devices. In a specific embodiment of the invention, the controller includes a clock control circuit which alternatively slows or stops clock signals which operate the first display processor and the bandwidth reduction function when the first display device is shut off and when the color image is not changing over time. The controller can be programmed to access two buffer areas of the external display memory permitting different images to be displayed on the two display devices. This feature is useful in presentations when a TV projector displays material for an audience and a flat-panel is used to display a speaker's notes.

Dependencies are inferred from the claim text.