Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
2002-03-22
2004-06-29
Tran, Henry N. (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S087000, C345S531000, C345S545000
Reexamination Certificate
active
06756976
ABSTRACT:
BACKGROUND OF THE INVENTION
The well-known cathode ray tube (CRT) is widely used for television (TV) and computer displays. Other display technologies such as the transmissive liquid crystal display (LCD) panel are widely used in certain specialized applications such as displays for portable computers and video projectors.
Market demand is continuously increasing for video displays with higher resolution, greater brightness, lower power, lighter weight, and more compact size. But, as these requirements become more and more stringent, the limitations of conventional CRTs and LCDs become apparent. Microdisplays the size of a silicon chip offer advantages over conventional technologies in resolution, brightness, power, and size. Such microdisplays are often referred to as spatial light modulators (SLMs) since, in many applications, (for example, video projection) they are not viewed directly but instead are used to modulate an incident light beam which forms an image projected on a screen. In other applications such as ultraportable or head-mounted displays, an image on the surface of the SLM may in fact be viewed by the user directly or through magnification optics.
CRTs currently dominate the market for desktop monitors and consumer TVs. But large CRTs are very bulky and expensive. LCD panels are much lighter and thinner than CRTs, but are prohibitively expensive to manufacture in sizes competitive with large CRTs. SLM microdisplays enable cost-effective and compact mid-sized projection displays, reducing the bulk and cost of large desktop monitors and TVs. Desktop computer monitors that would be unreasonably bulky using CRTs and too expensive using LCDs will be cost-effective and compact using SLMs.
Transmissive LCD microdisplays are currently the technology of choice for video projection systems. But, one disadvantage of LCDs is that they require a source of polarized light. LCDs are therefore optically inefficient. Without expensive polarization conversion optics, LCDs are limited to less than 50%-efficient use of an unpolarized light source. Unlike LCDs, micromirror-based SLM displays can use unpolarized light. Using unpolarized light allows projection displays using micromirror SLMs to achieve greater brightness than LCD-based projectors with the same light source, or equivalent brightness with a smaller, lower-power, cheaper light source.
The general operation and architecture of SLMs and SLM-based displays is well known in the industry as shown, for example, in U.S. Pat. Nos. 6,046,840, 5,835,256, 5,311,360, 4,566,935, and 4,367,924, the disclosures of which are each hereby incorporated by reference.
FIG. 1
shows the optical design of a typical micromirror SLM-based projection display system. A light source
200
and associated optical system, comprising optical elements
202
a
,
202
b
, and
202
c
, focus a light beam
206
onto the SLM
204
. The pixels of the SLM are individually controllable and an image is formed by modulating the incident light beam
206
as desired at each pixel. Micromirror-based projection displays typically modulate the direction of the incident light. For example, to produce a bright pixel in the projected image, the state of the SLM pixel may be set such that the light from that pixel is directed into the projection lens
208
. To produce a dark pixel in the projected image, the state of the SLM pixel is set such that the light is directed away from the projection lens
208
. Other technologies, such as reflective and transmissive LCDs, use other modulation techniques such as techniques in which the polarization or intensity of the light is modulated.
Modulated light from each SLM pixel passes through a projection lens
208
and is projected on a viewing screen
210
, which shows an image composed of bright and dark pixels corresponding to the image data loaded into the SLM
204
.
A ‘field-sequential color’ (FSC) color display may be generated by temporally interleaving separate images in different colors, typically the additive primaries red, green, and blue. This may be accomplished as described in the prior art using a color filter wheel
212
as shown in FIG.
1
. As color wheel
212
rotates rapidly, the color of the projected image cycles rapidly between the desired colors. The image on the SLM is synchronized to the wheel such that the different color fields of the full-color image are displayed in sequence. When the color of the light source is varied rapidly enough, the human eye perceives the sequential color fields as a single full-color image.
Other illumination methods may be used to produce a field-sequential color display. For example, in an ultraportable display, colored LEDs could be used for the light source. Instead of using a color wheel, the LEDs may simply be switched on and off as desired.
An additional color technique is to use more than one SLM, typically one per color, and combine their images optically. This solution is bulkier and more expensive than a single-SLM solution, but allows the highest brightness levels for digital cinema and high-end video projection.
In a CRT or conventional LCD panel the brightness of any pixel is an analog value, continuously variable between light and dark. In fast SLMs, such as those based on micromirrors or ferroelectric LCDs, one can operate the pixels in a digital manner. That is, pixels of these devices are driven to one of two states: fully on (bright) or fully off (dark).
To produce the perception of a grayscale or full-color image using such a digital SLM, it is necessary to rapidly modulate the pixels of the display between on and off states such that the average of their modulated brightness waveforms corresponds to the desired ‘analog’ brightness for each pixel. This technique is generally referred to as pulse-width modulation (PWM). Above a certain modulation frequency, the human eye and brain integrate a pixel's rapidly varying brightness (and color, in a field-sequential color display) and perceive a brightness (and color) determined by the pixel's average illumination over a video frame.
FIG. 2
a
illustrates a typical display system including an SLM
204
and associated control circuitry
300
. A video signal source
301
, such as a television tuner, MPEG decoder, video disc player, video tape player, PC graphics card, or the like, provides a video signal
304
in any standard format. If necessary, a conversion circuit
302
performs any necessary conversion operations, such as analog to digital conversion, decompression, or luminance/chrominance decoding, in order to convert the provided video signal into digital RGB pixel data
306
.
A display controller
308
accepts the incoming pixel data
306
, converts it to bit-plane format, and stores it in a frame buffer
310
. Display controller
308
retrieves stored bit-plane-formatted data from the frame buffer and provides it to SLM
204
over a data bus
312
according to a predetermined algorithm, such that each pixel displays data from each bit-plane for a duration proportional to that bit-plane's desired PWM weighting, thereby producing a grayscale or color image. Addressing and control signals
404
control which SLM pixels are updated with each write operation.
An alternative display system architecture is shown in
FIG. 2
b
. In a standalone application such as in a video-camera or still-camera viewfinder, personal digital assistant (PDA), or a next-generation mobile phone, display controller
308
presents a RAM-like interface
315
to the system's microprocessor
314
. Display controller
308
interleaves the microprocessor's frame-buffer read and write operations with the steady stream of read operations moving data from the frame buffer
310
to SLM
204
. In another implementation, display controller
308
shares the frame buffer
310
with the system's microprocessor
314
as shown in
FIG. 2
c.
Depending on the application, display controller
308
, frame buffer
310
, and SLM
204
may be separate devices. Alternatively, two or more of these system components may be integ
Muir Gregory R.
Reflectivity, INC
Tran Henry N.
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