Patent Application
20120075320
Some types of electronic displays require that input image data, when supplied by a standard video signal, be reformatted, re-ordered, or re-sequenced prior to display. Examples include sequential-color displays and displays, like plasma displays, that use certain kinds of digital gray scale. The reformatting or conversion allows the display to operate in the simplest way while maintaining compatibility with legacy video standards. However, the data reformatting or conversion results in a need to pass a great deal of data to the display in a very short period of time if video image quality is to be maintained. The image data may typically have been stored in a frame buffer external to the display. Passing such large amounts of data to the display has numerous practical disadvantages. High data rates necessitate display electronic interconnection with high I/O pin counts that in turn increase display system production cost. Further, high data rates result in undesirably high display power dissipation. It would be desirable therefore to be able to display high-quality video images, even on displays that best operate on input image data in an order different than that of current video standards, without having to pass large amounts of data at high rates through the conversion or reformatting system and on to the display. These concerns about display system power consumption, interconnect size, bandwidth, and cost are heightened in many applications that use microdisplays, since the very nature of the application often stresses portability, compactness, and battery life. A “microdisplay” is a display that is magnified for viewing (whether by projection of an image larger than the microdisplay onto a more or less distant screen, or by the production of virtual image viewed with the display near to the eye), particularly when implemented on an integrated-circuit backplane utilizing semiconductor substrates or thin films.
To date, most “digital” displays (displays that vary some variation of a temporal characteristic of a digital signal driving or controlling a pixel's optical modulation or light-emitting means to achieve variation of the gray shade displayed by that pixel) have either had a very minimum amount of data storage at each pixel (for example 1 or 2 bits), or, if they utilized more storage per pixel, have still relied on data processing external to the pixel to such a degree that high bandwidth, high-power-consumption data transfer to and across the microdisplay was still required. On the other hand, many inventors and engineers have described more sophisticated hypothetical microdisplay architectures that have not yet found commercial application that rely on in-pixel circuitry so complex that the resulting pixel would be so large that a high-resolution microdisplay could be made only with a silicon backplane of prohibitive cost.
Dynamic random access memory (DRAM) has found only limited use to store image data in microdisplays. One reason for this is that DRAM registers only retain their data for a short, finite time. The amount of time varies from register to register or cell to cell due to inevitable variations in the silicon fabrication process. Cells that are unable to retain the data therein beyond some specified retention time may be considered to be defective. Because a DRAM memory requires periodic refresh and because it will typically have a significant, non-zero number of defective cells, such a memory architecture has heretofore been considered undesirable for storing of image data to be displayed.
Another difference between most digital displays and their historical antecedents is their gamma characteristic, which is the exponent of a power-law relationship between display brightness and input image value. Cathode ray tube (CRT) displays typically have a characteristic with a gamma value of 2 or a bit more. Digital displays, on the other hand, to date have typically been characterized by values of gamma (γ) essentially equal to 1. Providing a display with gamma values close to those of historical displays is important for a number of reasons. First, standard video cameras continue to have gamma values around 0.45, ensuring compatibility with the installed base of video displays. Second, legacy image and video recordings, whether analog or digital, require displays with γ≈2 for proper replay. Third, in the case of digital or quantized video signals and image representations, it turns out that a gamma characteristic with γ≈2 better matches the characteristics of human perception than does a gamma characteristic with γ≈1. It is desirable for the brightness steps in a display that result from numerically adjacent input data to have a constant perceptual spacing. Unfortunately, for displays having γ≈1, the perceived brightness steps are small at the high-brightness end of the grayscale but large at the low-brightness end, which produces perceptible and objectionable contouring of brightness gradients in the dark parts of displayed scenes. For displays having γ≈2, the perceived brightness steps are much closer to equal across the gray scale, and the contouring is greatly reduced. In some commercial digital displays this undesirable characteristic has been compensated for with extra bits of data. For example, standard eight-bit input image data can be mapped to the 10-bit values of a γ≈1 gray scale that are closest to the originally desired output value. Two to four extra bits of gray-scale data per color to make 10-12 bits/color is generally thought to provide an image on a display having a gamma characteristic of 1 that is roughly equivalent to an 8-bit/color image displayed on a display with a gamma characteristic of 2. However, the use of extra bits increases the amount of data storage registers needed to make a frame buffer, and it increases the needed bandwidth to transport the image data onto the microdisplay.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
The following embodiments and aspects of thereof are described and illustrated in conjunction with systems, tools, and methods which are meant to be exemplary and illustrative, and not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.
A display includes an array of pixels that can be driven to different optical states and a clock that generates a signal that is used to control the optical state of each pixel in the array of pixels, wherein the signal is varied to achieve a gamma characteristic different than 1.
The display may further include a light source to illuminate the array of pixels, wherein the light source is not varied in intensity to achieve a non-unity gamma characteristic. The achieved gamma characteristic may be greater than 1. The achieved gamma characteristic may be approximately 2. The achieved gamma characteristic may be programmable.
A display includes an array of pixels that can be driven to different optical states and a light source to illuminate the array of pixels. The display panel provides a gamma characteristic different than 1 without varying the intensity of the light source to achieve a gamma characteristic different than 1.
The display further includes a clock that generates a signal that is used to control the optical state of each pixel in the array of pixels to drive the pixels, wherein the signal is varied to achieve a gamma characteristic greater than 1.
A digital display includes an array of pixels, each having a selectable optical state and a plurality of logic circuits that each receive a pair of digital inputs and provide an output signal based on the digital inputs, wherein the optical state of each pixel is based at least in part on the output signal, wherein each such logic circuit is shared by a number of pixels, the number being between and including 1 and 24.
One of the digital inputs may be representative of a ramp value. One of the digital inputs may be representative of a pixel value.
The digital display may further include other logic circuits that are shared by more than 24 pixels. The array of pixels may include significantly more rows of pixels than 48. Each pixel may include no more than 700 transistors, no more than 500 transistors, no more than 300 transistors, no more than 200 transistors, or no more than 150 transistors.
Each pixel may store more than 2 bits of image data, more than 8 bits of image data, more than 24 bits of image data, or 48 bits of image data.
A digital display includes an array of pixels and a frame buffer that stores image data for the pixels therein.
The display may include memory registers therein that indicate the rows within the frame buffer that have a defect therein. The display may arrange for relatively lower significant bits of the image data to be stored in the rows within the frame buffer that have defects. The display may arrange for portions of the frame buffer with defective cells to contain data for less easily perceived color than green. The frame buffer may be tested to determine the rows within the frame buffer that have a defect therein and information indicative of those rows is stored in the memory registers. The polarity of the stored image data may be selected to be such that a defect causes a pixel to provide less light than would be displayed by the pixel if there were no defect.
A method of operating a digital display includes providing a display have an array of pixels and a frame buffer; identifying the rows within the frame buffer that have one or more defects; storing information indicative of which rows have the defects; using the stored information to place relatively lower significant bits of image data in the rows within the frame buffer that have defects.
The method may further include selecting the polarity of the stored image data to be such that a defect causes a pixel to provide less light than would be displayed by the pixel if there were no defect.
A digital display includes an array of pixels having M columns of pixels and N rows of pixels and a clock that generates a clock signal that is provided to the array of pixels to drive the pixels, wherein the rate of the clock signal is no greater than (equation that is a function of M,N).
The clock rate may be kept relatively low by writing data to each pixel only once for each frame of data to be displayed.
A digital display includes an array of pixels having M columns and N rows, the pixels including circuitry therein that converts stored data representative of the optical state to be displayed by the pixel into a drive signal for the pixel, wherein M is at least 400 and N is at least 250.
A digital display includes an array of pixels having M columns and N rows, the pixels storing data therein that is representative of the optical state to be displayed by the pixel, wherein each pixel includes no more than 700 transistors, wherein M is at least 400 and N is at least 250.
In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions.
Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein be considered illustrative rather than limiting.
Reference will now be made to the accompanying drawings, which assist in illustrating the various pertinent features of the present invention. Although the present invention will now be described primarily in conjunction with a reflective ferroelectric liquid crystal (FLC) microdisplay, it should be expressly understood that the present invention may be applicable to other digital display applications such as plasma display panels (PDPs), micromechanical display panels and microdisplays, organic LED display panels and microdisplays, and digitally-driven, analog-responding nematic displays and microdisplays and/or to other applications where it is desired to produce a digital gray-scale drive waveform or to utilize frame buffers or memory registers storing image data which may be susceptible to failure. In this regard, the following description of a reflective FLC microdisplay is presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the following teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention.
In the case of displays generating field-sequential color images, current commercially available products typically include a separate interface chip upstream of the microdisplay to convert the incoming standard video image data into an acceptable format for the display. For example, a standard digital video image signal may first provide red data, green data, and blue data for a first pixel (picture element). This will be followed by red, green, and blue data (RGB data) for the next pixel and so forth. This is continued for each of the pixels in a particular line in the image, followed by the next line in the image, and so forth. The data is typically delivered at an almost even rate throughout the time allotted for the display of a frame, except for short horizontal blanking intervals at the end of each line and a short vertical blanking period at the end of each frame. For example, in the CCIR 601 and CCIR 656 standard video signals, the horizontal blanking occupies approximately 17% of the time allotted to each line (which time is on the order of 60 μs), while the vertical blanking occupies approximately 8% of the frame time. The remainder of the time, data is being delivered for display. Field sequential color displays, on the other hand, typically require first the red data for each of the pixels in the image, followed by the green data for each of the pixels in the image, followed by the blue data for each of the pixels in the image. In the simplest sequential-color display illumination schemes the entire display is illuminated with a single color primary at one time. In this case, all the data corresponding to a given primary color is best written to the pixels before the illumination commences, which further aggravates the data-supply problem, requiring that the data be provided to the display at a high rate for a short interval of time to avoid unduly reducing the illumination duty factor. For these reasons, field-sequential color display systems require additional circuitry to receive the data in one format and supply it to the display in a different format. This format conversion or data re-sequencing necessarily requires a considerable amount of buffer memory—at least the substantial fraction of a buffer capable of storing all the red, green, and blue data for all the pixels in the displayed image. With moving images, additional buffer memory is required to prevent the “tearing” artifact resulting from the display being refreshed from a single frame buffer that is simultaneously being updated with a new incoming frame. A depicted object may be moving (horizontally, for example), which causes its position to change from frame to frame. Since the image on the display is changed at a rate that is different (i.e. three or more times higher) than the rate at which new video frames come in, these two operations cannot be entirely synchronized, and it is therefore unavoidable that portions of the image data corresponding to a present frame and to a previous frame appear simultaneously on different regions of the display. Horizontal lines along which there is a mismatch in the position of the displayed object separate these regions. The object's details or texture will appear to be “torn” along these lines. This artifact is quite obvious and objectionable to the average viewer. Avoiding it requires double buffering the image data, i.e., using one buffer memory to store and display the previous frame, while a second buffer memory is updated with incoming image data. The role of the two buffers can be reversed between incoming frames.
In many digital gray scale and sequential-color schemes the average rate at which data is read out from the frame buffer exceeds the input rate. For sequential-color systems, displaying color fields at just three times the standard video frame rate (i.e. at 150 Hz for a 50 Hz frame rate or at 180 Hz for a 60 Hz frame rate) produces color break up. This can be dramatically reduced by increasing the color field rate. Typical color sequential systems that rely on a color wheel today utilize field rates 2, 4, or even 6 times higher than the 150-180 Hz minimum. Bit-plane-type gray scale schemes, used in plasma displays and in the Texas Instruments DLP displays, produce an artifact called dynamic false contouring. This artifact can be overcome by “splitting” the display of the more significant bit planes into multiple non-contiguous intervals which are distributed throughput the video field time. For example, in the color-sequential bit-plane gray-scale scheme taught by Akimoto and Hashimoto in “A 0.9-in UXGA/HDTV FLC Microdisplay,” published in the 2000 SID International Symposium Digest of Technical Papers, Jay Morreale, editor (Society for Information Display, San Jose, Calif., 2000) pp. 194-197, each pixel is addressed 108 times during the display of one video frame to achieve display of three colors of 8-bit/color standard input data. This requires a readout rate 4.5 times higher than the input data rate.
One way to provide the needed additional data reformatting or reordering and image buffer circuitry practiced in the art is to supply it on semiconductor chips separate from the display. A disadvantage of this separate interface chip approach is the increased cost due to the need for the display system to have additional chips, for example one extra chip for the data format conversion and another dedicated to image buffering memory. Another disadvantage is the increased size of a multiple-chip display system. A further disadvantage is that the need to support higher bandwidth between the frame buffer and the display means that the display must have a larger number of connections or “pins” that it otherwise would. Finally, off-display buffering further requires high-bandwidth communication between the buffer chip and the display, which invariably produces increased power consumption.
In the case of a microdisplay, an alternative location for the needed circuitry and buffer memory is on the microdisplay backplane itself, perhaps within the pixel array. However, the large amount of backplane circuitry required to effect image buffering limits practical implementations, since it tends to make the resulting backplane large and hence expensive. If the frame buffer were simply a memory block separate from the pixels, but still on the microdisplay backplane, the ratio of pixel array area to total backplane area would be undesirably reduced, since it would be impractical for the pixels to cover the memory block area. Alternatively, the circuit architecture of the microdisplay pixels could be designed so that the needed buffer memory for a given pixel was part of the circuitry physically associated with and underneath that pixel. Although this does not solve the overall backplane size problem, it does avoid the unfavorable active-area ratio problem of a separate memory block, since the pixels now cover the memory circuits. However, this benefit comes at the price of introducing another substantial problem. The failure of any of the memory registers produces visible pixel defects. Redundancy techniques used in the semiconductor memory art to improve yield by “mapping” around the address of defective registers cannot easily be used to compensate for such pixel failures, since a defective pixel at one location cannot be replaced by a functioning pixel at a different location.
The impracticality of prior-art techniques for providing the desired fully digital sequential-color format conversion entirely within a microdisplay backplane can best be illustrated by examples. For purposes of illustration, consider a microdisplay capable of displaying full color, in a field-sequential mode, with eight bits of gray scale per color. Consider further that the microdisplay utilizes a double image buffer, with the buffer circuitry located within the pixel, to eliminate visual artifacts and to allow high color field rates. Although the layout size of an arbitrary pixel circuit cannot be determined exactly without carrying out a complete design, its lower bound can be estimated by assuming that its transistors are laid out with the same density as transistors in a standard six-transistor SRAM cell. Given that the design rules and layout for standard SRAM cells are highly optimized, it is very unlikely that arbitrary pixel circuits could be laid out with higher transistor density. In a survey of leading CMOS silicon foundries performed by the applicant, it was found that the area of optimized six-transistor SRAM cells offered by the foundries was generally larger than 130f 2, where f designates the CMOS process ground rule (usually the finest feasible half-pitch for polysilicon lines in the specified process). For example, in a 0.35 μm CMOS process, six-transistor SRAM cells generally had areas of about 16 μm2. The formula a=130f2 produces an estimate of SRAM area a slightly larger than that estimated for future processes and future years in the “International Technology Roadmap for Semiconductors 2002 Update”, sponsored by (among others) the United States' Semiconductor Industry Association.
In-pixel buffering and re-ordering of image data could conveniently be accomplished with shift registers, as is known in the sequential-color display art. Standard static CMOS shift register cells comprising two static latches (each latch further comprising four transistors in the form of cross-coupled inverters) and two transmission gates (each transmission gate comprising two transistors) require twelve transistors per stored bit. Thus, double-buffering 24 bits of image information requires 48×12=576 transistors. If these transistors could be laid out with a density matching that of the highly optimized standard SRAM cells, they would occupy 1536 μm2 in a 0.35 μm CMOS process. Thus, just the transistors associated with the image buffer would limit the minimum achievable pitch of square microdisplay pixels to 39.2 μm for this candidate CMOS process. It is known in the sequential color display art that a stored digital image value can be converted to a pixel-duration signal (in effect, a PWM drive signal) by using a down counter. Each stage of the counter can be conventionally implemented using a half-adder and a master/slave flip-flop, with a NAND gate to detect the zero condition. The half-adder includes an eight-transistor XOR gate plus a four-transistor AND gate, the master stage includes four transistors arranged as cross-coupled inverters plus a load transistor and an enable transistor; the slave stage is the same, minus the load transistor. The NAND gate requires two transistors per input. Thus, the counter requires 25 transistors per bit, which, for an eight-bit gray scale translates into a total of 196 transistors, after four transistors in the unused AND gate at the zeroth stage of the counter are discarded. In total, then, this double-buffered PWM implementation of 24-bit color display requires 576+196=772 transistors per pixel. This estimate ignores miscellaneous transistors needed for pixel selection, and so on. In the aforementioned 0.35 μm CMOS process, this 772-transistor pixel would require more than 2050 μm2, which would make the smallest achievable square-pixel pitch 45 μm.
Simpler implementations that use standard SRAM cells for the frame buffer are still problematic. To fit the 48 registers needed to double-buffer standard color video data under a 12 μm pixel would require that each register occupy no more than 3 μm2. According to the aforementioned survey of silicon foundry capability, a standard SRAM cell occupies an area of about 130f2. Thus, obtaining a register with area less than 3 μm2 would require a CMOS process finer than 0.15 μm. To make provision for other needed circuitry such as sense amplifiers and pixel drive circuitry would necessitate further reducing the area allotted to memory registers at the expense of a still-finer CMOS process. Dropping to a 0.13 μm process would probably not be sufficient: a 90 nm or finer process would likely be required. Such fine processes have high associated design and manufacturing costs, resulting in undesirably expensive microdisplay backplanes. Although DRAM registers have implementations more compact than standard SRAM cells, DRAM registers have reduced tolerance to variation of transistor parameters such as leakage, and hence tend to have higher failure rates, especially when implemented not in a specialty DRAM process but in a standard logic process as are most microdisplay backplanes. The display-specific difficulties in using redundancy techniques common in the memory art to map around defective registers has made DRAM registers an unattractive alternative to SRAM registers for pixel-based frame buffers.
This pixel size estimate can be contrasted with pixel pitches found in current commercial microdisplays, which range downwards from around 13 μm to certainly as small as 7 μm. Thus, straightforward implementation of digital sequential-color format conversion results in pixels with areas more than 10 times larger than is commercially competitive. For a given display resolution, a large pixel size results in a large backplane die size, which correspondingly results in few backplane die per silicon wafer and low backplane die yield, compounding to give an undesirably high backplane die cost.
Outside of the limitations imposed by pixel and buffer size are other limitations imposed by power dissipation. Conventional memory architectures, whether SRAM or DRAM, rely on sense amplifiers located peripheral to the array of registers. For a frame buffer located under the pixels of a microdisplay, such an arrangement requires charging a wire of length comparable to the size of the display each time a bit is read from the buffer. This technique was employed in a microdisplay architecture disclosed in U.S. Pat. No. 7,283,105, which describes a microdisplay backplane with integrated frame buffer capable of accepting standard raster-order video signals and displaying in color sequential mode. The architecture in this disclosure comprises an array of SRAM registers largely beneath an array of pixel electrodes. To help overcome the size limitations discussed above, this architecture utilized a lossy compression scheme whereby the frame buffer stored a representation of the image that was compressed by a factor two—e.g. a standard 24-bit/pixel input image representation could be stored as a 12-bit/pixel representation, halving the number of registers required. Digital gray scale was implemented using pulse-width modulation (PWM), which required reading back the 12-bit stored image data for each pixel on each of 2G−1 time steps per color field, where each color had G=8 bits of gray scale. The frame buffer was organized so that it each pixel had three rows and eight columns of registers, the 24 registers/pixel allowing double buffering of the 12-bit image representation. Only half of the pixel's eight columns were read out during a given frame. Thus, the total number of read operations per color field in this architecture was equal to (2G−1)(3Y)(4X), where the display has X columns and Y rows of pixels. The gray-scale value for each of the three colors was displayed four times during one video frame; thus the color field rate was for 60 Hz video input was 720 fields/sec. The capacitance GB of the element of bit-line (column wire) length associated with each register was about 1.2 fF; thus, the total capacitance of each complete bit line was 3YCB. (three rows of register per each of Y rows of pixels). A bit-line voltage swing of VS=0.28 V was sufficient for sense amplifiers at the ends of the columns to complete a readout; thus, the energy CBVS2 associated with charging one register's piece of bit-line was about 0.1 fJ. In this case the power P associated with gray-scale display based on the readout all X columns of the stored image was equal to
P=[(2G−1)·3Y·4X]·720·3YCBVS2·(½)=(0.1 fJ)(12960 Hz)(2G−1)XY2,
the final factor of ½ coming from a statistical assumption that with an equal number of ones and zeroes stored in the frame buffers the bit-line will change state on only half of the reads. For displays of a given aspect ratio, for example X:Y=4:3, the power scales as the cube of the number Y of rows, leading to high power dissipations for high resolution displays. For example, with the above parameters, the readout of a quarter-VGA display (X=320, Y=240) with 8-bit gray scale would consume only 6.1 mW, but the readout of a 1280×960 display would consume 64 times as much, or 390 mW. The power consumption associated with frame buffers implemented as external chips may not scale exactly the same as described above for a frame buffer implemented on a microdisplay backplane, but in general the interconnect capacitances in the case of the external frame buffer will be higher as will the corresponding power dissipation. Power dissipation for high-resolution external-frame-buffer microdisplay systems known in the art is measured in multiple watts.
A consideration of the timing of the readout operations illustrates another very important limitation on frame-buffer architectures for pixel arrays. As detailed in the above example, each column of registers is read out (2G−1)(3Y) times per color field. For the quarter-VGA display with 720 Hz field rate, the amounts to a read time of 7.6 ns. To implement the same gray-scale and color-sequential scheme on a 1080-line display reduces the time allowed for a read to 1.7 ns (a read rate of 600 Mb/s on each column). It would be very difficult to accomplish this with columns having a total capacitance of nearly 4 pF while keeping the detection voltage for the sense amplifier as low as 0.28 V.
In summary, while it is desirable to implement on a single pixel-array-sized substrate low-power microdisplays that accept input video data in the standard pixel-by-pixel order, but perform digital gray scale and color-sequential display techniques by utilizing the input data in an order different from that supplied. However, the factors described above have prevented this until now. Straightforward partition of the substrate into a pixel array surrounded by memory blocks requires a larger-than-necessary substrate and results in a microdisplay with higher-than-desired power consumption. Placing SRAM registers under the pixels (rather than outside the perimeter of the pixel array) can reduce the size of the substrate, but still requires substantial area outside the pixel array unless expensive nanometer-scale CMOS processes are used, and still leaves power consumption unaffected. Substituting DRAM for SRAM can reduce the area overhead associated with the frame buffer, but at the penalty of more complicated sense circuitry and a higher defect rate. The lowest power consumption comes from reducing the distance between frame buffer memory registers and their destination pixels to size of the pixel or a few times that. The resulting association between a register and the pixel that displays its data imposes the need for very effective error correction or fault tolerance techniques if the display is not to be marred by many visually defective pixels. At the same time it precludes the use of error correction and fault tolerance techniques known in the art since the size of the circuit block on which they must work comprises one or only a few pixels and thus at most a few hundred registers—any circuit employed in such a small block must be extremely simple to not dwarf the few pixels and registers it serves.
With the above difficulties in mind, we can now discuss the present invention. One example of an application in which the present invention may be employed is a camera 30, as shown in
The microdisplay 44 is shown in
The above discussion of the operation of the display panel 64 is not intended to limit the present invention, as other types of spatial light modulators could also be utilized in the present invention, such as spatial light modulators depending on miniature mechanical minors, for example. A variety of different kind of light sources could be used with spatial light modulator (SLM) displays. For sequential color SLM displays the light source could preferably be made of red, green, and blue light emitting diodes, either organic or inorganic. Alternately, the light source could be made of red, green, and blue lasers, particularly semiconductor lasers or solid-state lasers. Also, display panels that emit their own light could be used. In addition, while the discussion involves linearly polarized light of two different orthogonal directions, it is also possible to utilize the present invention in a system in which unpolarized light or different types of polarization are used. Further details on the operation of liquid crystal spatial light modulators can be found in U.S. Pat. Nos. 5,748,164, 5,808,800, 5,977,940, 6,100,945, 6,507,330, 6,525,709, and 6,633,301, and in U.S. Patent Publication No. US2004/0263502, the contents of each of which are incorporated herein by reference.
The display panel 64 is shown in greater detail in
The liquid crystal material 76 may include any of several types of liquid crystals including, but not limited to, ferroelectric, nematic, or other types of liquid crystals. In this embodiment, ferroelectric liquid crystals (FLC) are utilized. In the FLC embodiment, it is advantageous to use FLC materials that are multi-component component mixtures. The mixtures may comprise an achiral host mixture plus chiral dopants that provide, for example, a desired magnitude of spontaneous polarization, and provide separate compensation of the nematic-phase and smectic-C*-phase helical pitches. Appropriate design of the mixture formulation provides a wide-temperature smectic C* phase, preferably having a low freezing point and a high melting point. Freezing points below −10° C., or even below −20° C., or even below −30° C. are desirable, while having the temperature at which the smectic C* phase melts to the next less-ordered phase above +60° C. is preferred, with melting temperatures above +70° C. or even +80° C. are more preferred. Selection of low-viscosity host mixtures formulated with appropriate dopants provides suitable FLC materials with switching times at room temperature with drive voltages of ±5 V of less than 300 μs, or even less than 200 μs, desirably with drive voltages less than ±2 V.
Alternatively, other types of display devices such as digital micromirror and other microelectromechanical (MEMS) devices, plasma displays, electroluminescent displays, organic or inorganic light-emitting diodes, and the like could be employed as part of the display panel. As can be appreciated, these alternatives may either be spatial light modulators, either transmissive or reflective, that modulate light from a light source or they may be light emissive devices that do not require a separate light source.
The silicon backplane 70 includes an area on a top surface thereof where an array 80 of reflective pixel electrodes is located. As can be appreciated, the image is formed in this area of the display panel 64, which is known as the “active area” of the display panel. The silicon backplane 70 is shown in
The display panel 64 is illustrated in further detail in
The control unit 84 may also interface with several other devices, not all of which are shown in
Each pixel in the group shares a common decision logic circuit 108 and a select/read circuit 106. Digital image data utilized by the pixel group is stored in a set 104 of image data registers. The image data stored in the registers, which data may represent gray-scale images and/or multi-color or full-color images, may be provided from an external image data source by way of digital control logic 84 and a column control unit 86. If each of the k pixels in the group displays, for example, an m-bit gray-scale image in each of three colors (to make a full-color field-sequential display), and the image data registers provide double-buffered storage, then a total of p=2·3·m·k single-bit registers are required for the group (unless the image data is stored in compressed form or is shared between pixels, in which case fewer registers may be needed). If the display active area is made up of an N×M array of pixels then there will be NM/k pixel groups. The number k of pixels per group could range from 1 (each pixel having its own image data registers, its own select/read circuit, and its own decision logic circuitry) up to M (each column of pixels sharing a set of image data registers and a select/read circuit and a decision logic circuit), or to an even larger number.
The image data registers may be implemented in any of the various ways known in the electronic memory art. For example, they may be implemented as conventional six-transistor (6T) static random-access memory (SRAM) cells, or as other forms of static logic such as any of the many other static latch circuits, shift-register stages, and so on. Alternately, the image data registers may be implemented as one-transistor (1T) dynamic random-access memory (DRAM) cells or by storing the image data as charge on a FET transistor gate, such as at the input of some other logic gate. The image data memory registers are written with data that represents an image. The input image may be supplied from a source external to the display, such as broadcast video or the output of a video player such as a DVD player, or from a computer graphics output, or from an image-sensor or camera system, or similar. Various transformations to the input image data may be applied before it is stored in the image data memory registers. Such transformations include compression, rescaling, clipping or over-scanning, color-space transformations, various coding schemes, and the like. The control unit 84 cooperates with the column control unit 86 to ensure that input image data corresponding to a certain display pixel is written into the appropriate registers, i.e. those registers that are associated, either logically or physically, with that pixel.
After the image data are written into the various registers, they are held there until they are needed, at which time the needed register is selected and read out by the select/read circuit 106. For many of the various types of possible image data register implementations, the read operation will sense some relatively small stored value and convert it to full logic levels. For example, in the case of DRAM registers, the image data are represented as small charges stored on register capacitors. In this case, a sense amplifier in the select/read circuit 106 may be used to convert stored charge values above a threshold value to a logic 1 and stored charge values below the threshold value to a logic 0. Alternately, in the case of SRAM registers, where there is capacitance loading the register outputs, arising for example from shared interconnections used to multiplex multiple registers within a group of pixels onto the to the shared select/read circuitry, a sense amplifier or detection circuit within the select/read unit 106 may act to precharge the capacitance loading the register output, and to then detect relatively small changes in the voltage developed across this load, thereby speeding up the read operation.
The decision logic unit 108 acts on the image data read out by the select/read unit 106 to produce signals that control the drive waveform provided by the pixel driver 116 to the pixel electrode 118, in order to produce the desired or called-for gray-scale response. Sophisticated, many-transistor implementations of select/read unit 106 enable more sensitive detection of the state of the registers in the image data memory 104, and hence enable the use of simpler, more compact register forms. Similarly, more sophisticated functionality implemented at the cost of increased transistor count in decision logic unit 108 enables higher-performing digital gray scale pixel-drive waveforms such as pulse-width modulation where the output gray-scale intensity is determined by the width of a single pulse. To accommodate the increased layout space associated with increased sophistication and corresponding greater transistor count of units 108 and 106 while preserving an overall high display pixel density, the select/read unit 106 and decision logic unit 108 may be made to serve a greater number k of pixel within a group of pixels. While such a design strategy may appear to provide desired pixel density and drive waveform sophistication, it demands increasing clock rate as k is increased, and produces power dissipation that increases faster than k. The novel embodiments of the present invention, however, as illustrated by the following examples, show how the apparently contradictory requirements of compact image data registers, sophisticated pixel-drive waveform generation, and low-power, low-speed small k can simultaneously be met.
Each storage cell pair 112 generates an
If the data bit stored at the gate terminal of the FET switch 132 is a zero (low state), then the FET switch 132 is turned off. If the data stored at the gate terminal of FET switch 132 is a one (high state), then the FET switch 132 is turned on and the
The FET switches 136 and 138 operate in a similar fashion to store B image data therein and control the state of the
In this way the states of multiple selected registers can be read out in parallel and contribute simultaneously to the decision reached by the decision logic unit. In the embodiment described with reference to
The pixel driver 116 illustrated in
The circuitry described above with reference to
That this can produce a PWM drive signal is seen by considering the simplified version of such an algorithm tabulated in
It is known in the liquid crystal art that liquid crystal pixels perform best when driven with drive waveforms that have an average voltage of zero, that is, when driven with waveforms that are “
The above descriptions portray driving the entire array of pixels synchronously with the same global sequence. This is not necessary. Different sequences could be distributed to different rows in the display. It is known in the projection art to illuminate a microdisplay with “scrolling” illumination where bands of red, green, and blue illumination are moved across the panel in sequence, in a way that the panel may at a given instant be illuminated over one portion with a band of light of one color and over a different portion with a band of light of a different color. By providing each row with its own sequence, delayed slightly in time from the same sequence supplied to the previous row, the display pixels can produce a time-sequential gray-scale pattern appropriate for producing color-sequential display with such illumination.
The decision logic unit 114 of the above embodiment offers considerable advantages over prior comparator-based circuits for providing pulse-width modulation. A circuit to compare one digital word (the stored image data) with another (the sequence code), for example a multi-input XOR circuit, requires inputs of both the data value and its complement and the code value and its complement, or four inputs per bit. This results in a decision circuit with an undesirably high transistor count and that produces a pixel that is undesirably large. On the other hand, the PWM scheme employed by the above embodiment of the present invention does not compare the two signals. The fact that the NOR circuit (which is has far fewer transistors than, for example, an XOR-based comparator) would, if considered to be a comparator, produce erroneous matches as described with reference to
According to the above description, it can be seen that the LCOS display panel 64 displays data in the fashion shown in
In order to change the gamma characteristic of the display systems described herein, it is possible to vary the timing of the sequence signal.
Using digital pixel drive waveforms to produce gamma values different from one by the above-described technique relying on varying time intervals between the sequence states has significant advantages over the method previously described in U.S. Pat. No. 7,238,105 that relied on constant intervals between the sequence states while the display illumination intensity was linearly ramped. For illuminators that have a maximum allowable output intensity, linearly ramping the intensity from zero to the allowed maximum produces an average intensity of half the maximum, and thus underutilizes the illuminator. The scheme described here allows the illumination to be continuously at its maximum value for better illuminator utilization. The degree of illuminator utilization can be quantitatively compared by examining the variance or standard deviation illumination vs. time. In the case of an illuminator whose intensity I is linearly ramped from zero to a maximum value over a time interval of length τ (I(t)=IMAXt/τ), the intensity values are uniformly distributed, and thus have a mean value IMAX/2 and a standard deviation of IMAX/√(12). For the constant illumination available under the gamma method described here (I(t)=IMAX), the mean value is IMAX and the standard deviation is zero. The method described here thus advantageously delivers gamma values greater than one with intensity vs. time functions having fractional standard deviations smaller than ½/√(12), or smaller than 28.9%.
According to a second control method, the circuitry depicted in
To provide bit-plane gray scale, the image data registers may be divided into an A bank and a B bank to provide double buffering, with writing of input image data as described above. To read out a selected bank, though, the function of sequence generator 92 in control logic 84 is changed so that it sequences through the select/read lines one at a time instead of driving the select/read lines with a ramp waveform, as described above with respect to PWM gray scale. This can be understood in more detail by means of an example.
Again suppose that it is desired for the display system to accept conventional 24-bit color video signals (one 8-bit gray-scale value for each pixel for each of the red, green, and blue primary colors), and convert this input signal to sequential color with bit-plane digital gray scale drive to each pixel, and again that it is desired to double-buffer the image data. As before, this can be accomplished by providing each pixel with 24 register pairs (24 registers in bank A and 24 in B), resulting in each pixel having 24 select/read lines,
The signals
The sequence of selecting one of the registers by making only its
According to either the first digital gray-scale method (PWM) or the second digital gray-scale method (bit-plane), the pixel circuitry 110 described with reference to
It is a further characteristic of the present invention that this refresh and level-restoration operation is local. That is, the operation of sensing the level stored in the image data register 112 and restoring it can be performed by circuitry located close to the register. The present invention provides that the sensing and restoration circuitry is located closer to the register than one-half of the length of a pixel-array column (or row), and may, in fact, be with the size of a few pixels such as 48 pixels, or even 12 pixels of the register. In fact, the sensing circuitry can be, according to the embodiments of the present invention, within a distance of six pixels or even one pixel of the register. The present invention further provides that the sensing and restoration circuitry may be utilized only by a small group of pixels, the group containing 48 pixels or fewer, or even that the sensing and restoration circuitry be utilized by only a single pixel. This characteristic of local sense and refresh has the advantage that power consumption is minimized, since the energy used in a refresh operation is determined by the energy associated with charging and discharging the wiring that interconnects the register and the sense/restore circuitry.
Applicants have found that, although it is feasible to design dynamic registers with median retention times of many milliseconds, a small fraction, say perhaps somewhat less than 100 parts per million (ppm) might have retention times shorter than 100 μs. An even smaller fraction, perhaps 10 ppm, might have retention times shorter than 10 μs. It is possible to increase register retention times, for example by increasing the area of the gate of FET transistors 132 and 138, but this might undesirably increase the minimum achievable size of the pixel. Thus, it may be advantageous that the pixel registers be refreshed at a rate higher than the 50 Hz or 60 Hz rate at which new video data is supplied, or even higher than sequential-color color field rates, which typically fall in the range of 150-720 Hz. It may even be advantageous to have refresh rates higher than 1 kHz, or even higher than 10 kHz, all of which are feasible with the pixel circuitry 110 described above.
With the LCOS display panel 64 described herein, it is possible to minimize the impact of defective storage registers on the displayed image.
First, in process step 240, a display or microdisplay like the one previously described is provided having an array of pixels and a DRAM frame buffer. As described herein, the DRAM frame buffer is distributed throughout the pixel array, however this process would also work in a situation where the DRAM frame buffer was not distributed throughout the array or even if the frame buffer used a type of memory cell other than DRAM. Next, the defects in the frame buffer are identified in process step 242. These defects can be identified in any number of ways, including visual inspection and automatic testing. After this, information indicative of the location of the defects is stored in one or more memory registers in process step 244. For example, these memory registers may be in the storage unit 98 associated with the control unit 84, which storage unit could include non-volatile memory so that the testing operation need only be performed once. Alternately, these memory registers could be on the backplane of a microdisplay and the location of the defects could be determined by built-in self test every time the microdisplay was powered on. Subsequently, in process step 246, a mapping process is performed so that image data placed in the location of the defective storage cells is based on the significance of the data, both by bit and by color. For example, the first defective cell could be used to contain the least significant bit of the color blue or red, since the eye is less sensitive to these colors than to green. Additional defective cells in the area of that same pixel could be used to contain the next least significant bit of one of the less significant colors, and so forth.
One embodiment of the above mapping process relies on a row-by-row mapping. Suppose that somewhere in one of the display's rows of pixels there were a defective memory cell (say, a cell that would store an ith bit of the q image data bits associated with that pixel) that would, if not otherwise mapped, correspond to an image data bit of high visual significance. This defective cell is written to by activating one of the ith writeA or writeB lines and read or selected by activating the ith read/select line. Hereinafter, this situation will be referred to as the defective cell being in the ith register row. (Thus, the display has N pixel rows and each pixel row has q register rows.) Programmable circuitry in the row control/select block 88 could be used to swap, for this pixel row, all the memory cells in the ith register row with the cells in another register row, say the jth row. This would improve the appearance of the display provided that there were no defective memory cells in that pixel row's jth register row, and provided that what was originally the jth bit of the q image data bits was of less visual significance that what was originally the ith bit. Suppose further that it was determined that of the q gray-scale bits it would be acceptable to re-map as many as r of them in any pixel row. For example, if defects were tolerable in the least significant green bit and in the two least significant blue and red bits, then r would have the value of five.
Then a given display with defective memory cells corresponding to no more than r register rows in any pixel row could be made acceptable by row-based re-mapping. Such row-based re-mapping could be implemented by many different techniques, of which one will be described with reference to
The circuitry can be operated as follows to map defective memory cells so the effect of the defect is inoffensive or imperceptible. The locations of the defective registers in the array of pixels are first found by testing as described above with respect to
After all the latch banks are loaded the display can be operated as described with reference to
Although in the above description of error mapping the mapping is described as operating on rows, it is to be understood that this aspect of the present invention is not to be limited to row-based mapping, but can be used with pixels or registers connected into any desired logical group.
Other techniques may be useful for minimizing the effect of defective memory cells on the displayed image quality. If a memory cell is more likely to fail by sticking one way than the other, the polarity of the data stored in the storage cells can be selected so as to provide a situation where the more probably failure of a storage cell would result in a darker pixel than intended, rather than a brighter pixel than intended. As an alternative to mapping defective cells from one image data value to another, extra memory cells can be provided in each pixel row. For example, to display images with 8-bits of gray-scale information for each of three colors with double-buffering to prevent the tearing artifact, 48 registers are needed per pixel. The display design could provide more than 48, for example 50, registers per pixel. Then when a defective register row was discovered an extra row could be mapped in its place using the same type of map decode circuitry described with reference to
The fault detection and re-mapping feature of the present invention as described above operates to reduce the visual significance of defects in the frame buffer registers and pixel circuits. This means that after the fault detection and re-mapping process is completed a human viewing the display sees a more pleasing displayed image than if the process had not been carried out. The detectability of defects in the buffer memory and pixel circuits by the eye is reduced by carrying out the process compared to what it would be otherwise. At error rates in the range of a few hundred parts per million carrying out the described process can transform a display with glaring pixel defects into a display with no defects detectable under normal viewing conditions.
The invention, including the circuitry described above with reference to
DC balance of the liquid crystal pixel can be ensured by generating pulses as described above by alternately switching the window electrode and the pixel electrode between the same voltage values (0 V and V) for time intervals of the same duration τ, and always alternately applying pulses of opposite sign.
Another embodiment of the present invention can be used to provide analog pixel drive waveforms implemented with digital control signals. Certain ferroelectric liquid crystals are know to exhibit an analog switching characteristic know in the art as “V-shaped” switching, as described by M. J. O'Callaghan et al. in “Charge controlled, fixed optic axis analog (‘v-shaped’) switching of a bent-core ferroelectric liquid crystal,” Applied Physics Letters volume 85, pages 6344-6346 (2004), and in “Switching dynamics and surface forces in thresholdless “V-shaped” switching ferroelectric liquid crystals,” Physical Review E volume 67, pages 011710-011712 (2003), and in “High-tilt, high-Ps, de Vries FLCs for analog electro-optic phase modulation,” Ferroelectrics volume 343, pages 201-207 (2006). It has been found that improved analog switching characteristics can be obtained under drive conditions where the analog value of the pixel drive charge is controlled by the drive circuit (rather than the more usual case where the drive circuit controls the drive voltage).
Constant-charge pixel drive can be provided by a digitally-controlled circuit that relies on the time response of the FLC polarization to a drive step using, for example, the pixel drive circuit shown in
The time during the switching process at which the
For typical FLC materials both the switching charge 2Ps and the switching time vary with temperature. In the case of the “switch & open” driver described with reference to
As an alternative to relying on advance characterization of the FLC material parameters, they could be sensed in situ as described below. For example, a circuit could be integrated into the LCOS backplane to sense the current from a “reference” pixel, located perhaps on the periphery of an active pixel array. If the pixel electrodes of the main pixels in the array were to be driven from 0 V (
Charge-control drive reduces FLC v-shaped switching hysteresis by a factor of 30 compared to voltage-source drive, without the undesirable consequence of increased saturation voltage, and can reduce small-signal optical response rise and fall times by a factor of compared to the response times obtained with voltage-source drive.
While the benefits of charge-control drive for controlling the intermediate FLC device states needed for analog modulation are described above, this type of drive may also provide benefits for devices relying on binary FLC switching. Consider that the electrostatic explanation for V-shape analog switching, as described by N. A. Clark, et al. in “Electrostatics and the electro-optic behaviour of chiral smectics C: ‘block’ polarization screening of applied voltage and ‘V-shaped’ switching,” Liquid Crystals vol. 27, pp. 985-990 (2000) models the FLC material as a slab of uniform polarization, which occurs when FLC spontaneous polarization is high. Ferroelectric charge σF on the surface of the slab is determined in the usual way by the orientation of the polarization vector P, with σF=P·ŝ, where ŝ is unit vector normal to the surface of the slab. Provided that the charge σA applied by the external drive circuit is smaller than the FLC's spontaneous polarization Ps=|P|, then, according to this model, P just takes the orientation that makes σA+σF=0. This implies that the electric field within the liquid crystal is zero. According to this model, the behavior of the ions that cause image sticking—the elimination of which usually necessitates
Image sticking is caused by electrical fields produced by the separation of free ions within the FLC material. The electrical fields modify the applied electric fields, producing a drift of device electrical characteristics which manifests itself as a slightly visible residue of previously applied image pattern. The ion separation is driven by applied electric fields in the regions of non-zero ionic concentration, i.e. non-zero fields within the FLC material. As described above, the use of a high-polarization FLC material can substantially reduce the electric field within the liquid-crystal material itself. Thus, the action on any ions with the FLC is also substantially reduced, so the ions have much less drive to separate and produce unwanted internal electric fields. While FLC materials with polarizations in the range 15-30 nC/cm2 have typically been used for binary-switching applications, the polarization-stiffening effects that tend to exclude applied electric fields become most apparent at polarizations of 100 nC/cm2 or greater. The benefit of using high-Ps materials is that bringing the time-average of the applied voltage to zero is no longer the only way to reduce image sticking. By allowing drive waveforms with an unbalanced ratio of
As described herein, the use of especially high polarization FLC materials combined with new drive techniques provide unexpected advantages for the operation of FLC electro-optic devices. For analog operation, a new “switch & open” drive provides an especially compact driver implementation suitable for LCOS devices. For binary operation three principles, each effective on its own but more effective when combined with the others, provide freedom to vary drive waveforms away from
The display systems and microdisplay panel described above have a number of advantages over previously disclosed systems. For example, as was described above, a shift-register-based system for buffering and re-sequencing image data and providing a PWM drive signal would require 772 transistors per pixel in the case of image data consisting of three colors with 8-bits of gray scale per color. By contrast, in the case of the embodiments of the present invention described with reference to
For a 720 Hz video field rate, having a field duration of 1.39 ms, the teaching above with regard to producing a gamma=2 characteristic by variable time intervals between sequence states indicated that the minimum interval would have a duration of 1.39 ms/65025 in the case of 8-bit gray scale. Thus, this interval would have duration of 21 ns, setting the minimum required read time. This compares very favorably with the 7.6 ns read time required in a prior-art quarter-VGA display described above, and even more favorably with the 1.7 ns read time required in a prior-art 1080-line display.
Applicants have found that utilizing the embodiments of the invention described above they could make a VGA (640×480) display that displays two each of red, green, and blue color fields per 60-Hz video frame time (with another two of each used for DC balance, without being illuminated), while needing only 24 data input lines operating at a 25 MHz bus rate, directly accepting standard digital video input and requiring no other ASIC or external memory. They have similarly found that they could make an SVGA (800×600) display that still needed only 24 data input lines, now operating a bus clock rates as low as 30 MHz, easily accommodating the standard clock rate for this resolution of closer to 40 MHz. This can be compared with an SVGA display sold by Texas Instruments under the DLP (Digital Light Processing) brand. Applicants' examination of such a display used in the Mitsubishi PK20 projector revealed that this display had 150 interconnection pins. The DLP panel was connected, via a 90-line flex circuit, to another board with a 564-pin control ASIC and a 32 Mb external frame buffer memory.
In the case of the embodiments of the present invention described with reference to
While the microdisplay 44 and LCOS display panel 64 have been described thus far in conjunction with the use of a camera 30, it is also possible for the microdisplay 44 and display panel 64 to be used in a rear projection application such as an HDTV as shown in
The foregoing description has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain variations, modifications, permutations, additions, and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such variations, modifications, permutations, additions, and sub-combinations as are within their true spirit and scope.
This application is a divisional of U.S. patent application Ser. No. 11/969,734, filed Jan. 4, 2008, entitled “Digital Display,” which claims priority from U.S. Provisional Patent Applications Nos. 60/883,492 filed Jan. 4, 2007, entitled “Digital Display”; 60/939,307 filed May 21, 2007, and also entitled “Digital Display”; and 60/883,474 filed Jan. 4, 2007, entitled “Charge-Control Drive of Ferroelectric Liquid Crystals,” wherein the entire contents of each of the foregoing priority applications are incorporated herein by reference.
Some aspects of this invention were made with Government support under contract FA8650-04-M-5443 awarded by the United States Air Force Research Laboratory. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60883492 | Jan 2007 | US | |
| 60939307 | May 2007 | US | |
| 60883474 | Jan 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11969734 | Jan 2008 | US |
| Child | 13247005 | US |
