Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
1999-11-18
2002-08-27
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
Intensity or color driving control
C345S099000, C345S100000, C341S141000
Reexamination Certificate
active
06441829
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a circuit and method for driving a pixel with a pixel drive signal whose duty cycle is defined by a digital input value.
BACKGROUND OF THE INVENTION
A substantial need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. In particular, wearable display devices could be used instead of the conventional displays of laptop and other portable computers, and portable Digital Versatile Disk (DVD) players. Potentially, wearable display devices can provide greater brightness, better resolution, larger apparent size, greater privacy, substantially less power consumption and longer battery life than conventional active matrix or double-scan liquid crystal-based displays. Other potential applications of wearable display devices are in personal video monitors, in video games and in virtual reality systems.
Recently, a miniature video display based on a light valve that uses a ferroelectric liquid crystal material was described in U.S. patent applications Ser. Nos. 09/070,487 and 09/070,669, assigned to the assignee of this disclosure and incorporated herein by reference. Such a miniature video display can form part of a wearable eyeglass display that can be used to display computer graphics when connected to the video output of a computer, especially a laptop computer, and can be used to display video when connected to the video output of a TV receiver, a video cassette player or a DVD player, especially a portable DVD player.
One embodiment of the light valve of such a miniature video display includes an array of 1024×768 pixels, each including a reflective electrode driven by a respective pixel driver. The pixel driver converts an analog sample derived from an analog video signal into a two-state drive signal having a duty cycle that defines the apparent brightness of the pixel. Sequentially illuminating with light of two or more different colors and setting each pixel to an apparent brightness associated with each color during the respective illumination period enables a color frame to be displayed. A similar pixel driver can be used in video displays based on other binary electro-optical transducers, such as solid-state or organic light-emitting materials, in which duty cycle of the drive signal coupled to the electro-optical transducer determines the apparent brightness of the pixel.
When the miniature video display just described is driven by a conventional analog video signal, analog samples are derived from each line of the analog video signal and are distributed via column busses to the pixel circuits in each row of the array. Recently, however, it has been proposed to use the video display just described as the viewfinder of a digital camera that generates a digital video signal. Moreover, many other video applications generate a digital video signal composed of parallel red, green and blue pixel values. To drive the above-mentioned analog video display, a digital-to-analog converter must be used to convert the digital video signal generated by the camera to an analog signal. A parallel digital-to-analog converter suitable for this purpose is described in U.S. patent application Ser. No. 09/249,600, assigned to the assignee of this application.
Using a digital-to-analog converter to convert the digital video signal to an analog signal suitable for driving the above-described analog-based miniature video display requires considerable additional circuitry and increases the power consumption of the display. Power consumption is an important consideration since the miniature video display is especially intended for use as the display for laptop computers and portable DVD players. Moreover, the analog circuitry of the miniature video display presents significant challenges when the highest picture quality is desired. Another important shortcoming is that new analog samples must be obtained from the video signal and be distributed to the pixels constituting the display after each display period, which is typically one video frame period. When the frame being displayed is relatively static, as in a computer display or the display of an electronic book, this needlessly increases the power consumption.
Miniature video displays that incorporate liquid crystal-based light valves and are indirectly driven by a digital video signal are known. In these, the digital video signal is converted into a grey scale binary-weighted, time-multiplexed, time domain binary-weighted drive signal to drive each pixel. The time domain weighting of the drive signal creates tremendous inefficiencies in the link between the converter and the display since the link is idle during the ON time of the high order bits. Including an image buffer in the display removes this inefficiency, but significantly increases the cost and power consumption of the display. The bitwise binary-weighted time domain drive signal also imposes a significant burden on the bandwidth of the liquid crystal material itself due to the switching speed necessary to display the low-order bits, which have a very short duration. Current ferroelectric liquid crystal materials do not have sufficient switching speed to display a full 24-bit (eight bits per color) color palette. This problem is further exacerbated by the desire to move process technology to lower and lower voltages, since the switching speed of ferroelectric liquid crystal material depends on the strength of the applied field, and therefore on the voltage of the drive signal.
An additional complexity is the bit reordering that must be applied to the digital video signal. Most digital video signals are composed of sets of RGB pixel values in raster scan order. Bitwise imaging requires buffering of the RGB pixel values and then reordering them into a bit-plane sequential data stream in which the lowest-order bits (for example) of all the pixels are presented first, followed by the next-but-lowest order bits of all the pixels, and so on until the highest-order bits of all the pixels are presented. Re-ordering the digital video signal requires a buffer memory that has significant bandwidth requirements, and power consumption, when the display has a high resolution.
Accordingly, what is needed is a pixel driver capable of directly receiving a pixel value constituting part of a conventional digital video signal and of generating, in response to the pixel value, a drive signal having a duty cycle that, in a monochrome display, determines the apparent brightness of the pixel and, in a color display, determines the apparent brightness of the pixel for each of two or more color components. The pixel driver should be simple, so that the pixel can be made sufficiently small to allow a high-resolution display composed of hundreds of thousands, or even millions, of pixels to be formed on a semiconductor chip having dimensions of the order of 10 mm×10 mm. The pixel driver should have low power consumption to enable it to be used in portable, battery-powered applications. Finally, when the digital video signal is relatively static, the pixel driver should be capable of operating in a mode that does not require the pixel values to be re-loaded into the pixels after each display period to reduce power consumption.
SUMMARY OF THE INVENTION
The invention provides a digital pixel driver that operates in response to an M-bit digital input value defining the apparent brightness of the pixel. The pixel driver generates a pixel drive signal having a duty cycle that sets the apparent brightness of the pixel. The pixel driver comprises a memory, a digital sequence generator and a comparator. The memory receives and stores an N-bit word that represents the digital input value. The digital sequence gene
Blalock Travis N.
Nishimura Ken
Agilent Technologie,s Inc.
Hardcastle Ian
Piziali Jeff
Shalwala Bipin
LandOfFree
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