Referring to FIG. 1, an example of a DMD™ (digital micro-mirror device) system 20 is illustrated, wherein the light from a light source 22 is applied through a first lens 24 and through a color wheel 26, which will typically rotate no less than about 60 revolutions or 60 frames per second. Alternately, the color wheel 26 may make up to five or six revolutions per frame or about 300-350 revolutions per second. The light passing through the color wheel 26 passes through a second lens 28 onto a DMD™ array or chip 30. The DMD™ chip includes an array (on the order of one million) of tiny mirror elements, or micro-mirrors, where each mirror element is hinged by a torsion hinge and support post above a memory cell of a CMOS static RAM as shown in FIG. 2 and FIG. 3.
FIGS. 2 and 3 show a portion of a typical DMD™ array 30 having mirror elements 32 suspended over a substrate 34. Electrostatic attraction between the mirror 32 and an address electrode 36 causes the mirror to twist or pivot, in either of two directions, about an axis formed by a pair of torsion beam hinges 38a and 38b. Typically, the mirror rotates about these binges until the rotation is mechanically stopped. The movable micro-mirror tilts into the on or off states by electrostatic forces depending on the data written to the cell. The tilt of the mirror is on the order of plus 10 degrees (on) or minus 10 degrees (off) to modulate the light that is incident on the surface. For additional details, see U.S. Pat No. 5,061,049 entitled “Spatial Light Modulator” and U.S. Pat. No. 5,280,277 entitled “Field Updated Deformable Mirror Device,” both by Larry J. Hornbeck.
Referring again to FIG. 1, the light reflected from all, selected ones, or none of the mirrors may pass through a projection lens 40 and create images on the screen 42. The DMD™s are controlled by electronic circuitry fabricated on the silicon substrate 34 under the DMD™ array. The circuitry includes an array of memory cells, typically one memory cell for each DMD™ element, connected to the address electrodes 36. The output of a memory cell is connected to one of the two address electrodes and the inverted output of a memory cell is connected to the other address electrode. Data is provided by a timing and control circuit 44 determined from signal processing circuitry and an image source indicated at 46. Once data is written to each memory cell in the array, a voltage is applied to the DMD™ mirrors 32 creating a large enough voltage differential between the mirrors 32 and the address electrodes 36 to cause the mirror to rotate or tilt in the direction of the greatest voltage potential. Since the electrostatic attraction grows stronger as the mirror is rotated near an address electrode, the memory cell contents may be changed without altering the position of the mirrors once the mirrors are fully rotated. Thus, the memory cells may be loaded with new data while the array is displaying previous data.
DMD™ arrays are typically operated in a dark-field mode. In one embodiment of dark-field operation shown in FIG. 4, light 22a from light source 22 is focused on DMD™ array 30 and strikes the individual mirrors of the array 30 at an angle. According to the example shown in FIG. 4, when tilted or rotated to an ON position as indicated by mirror 32a, light 22a incident the mirror 32a will be reflected and focused onto an image plane or viewing screen 42 where it will form part of the image. If a mirror 32b is rotated away from the light source to an OFF position, light 22a incident the mirror 32b will reflect away from the viewing screen 42 and will not form part of the image.
Light incident on and reflected from a DMD™ mirror forms an illuminated dot on the viewing screen 42 for every mirror 32 that is rotated to the “ON” position. Each of these dots represents one picture element, or pixel, which is the smallest individually controllable portion of an image. Using a large array of these tiny mirrors, an image is created by selectively turning some mirrors to the “ON” position while turning some to the “OFF” position, thereby creating a pattern of illuminated dots on the viewing screen.
A major production cost of DMD™ modules or mirror arrays for use as display drive engines is the silicon wafer and corresponding processing costs. Of course, if the number of modules that could be manufactured from a single wafer could be substantially increased, this increase would have a direct affect on the cost of the modules. A diamond-shaped array having the same number of rows and columns of pixels is only half the size of an orthogonal array and uses only half the number of pixels. Comparing the 8 column by 6 row orthogonal array of FIG. 5A with the 8 column by 6, row diamond-shaped array of FIG. 5B illustrates that even though the size of the pixels are the same, the diamond-shaped array is only about one half the size of the orthogonal pixel array. The difference in the overall size and total number of pixels of an 8×6 orthogonal array and an 8×6 diamond array is due to the difference in distance between adjacent horizontal and vertical pixels. For example, for an orthogonal array, and as shown in FIG. 5A, the distance between adjacent rows, as indicated by double-headed arrow 50, is the same or equal to the distance between adjacent columns, as indicated by double-headed arrow 52. However, as shown in the diamond array of FIG. 5B, the distance between adjacent rows, as indicated by double-headed arrow 54, is only half that between adjacent columns, as indicated by double-headed arrows 56a and 56b. This is, of course, because there are actually two sets of columns. Namely, a first set for odd rows as indicated by reference number 58 and a second set of even rows as indicated by 60.
Therefore, to maintain a particular or selected aspect ratio, the number of columns in a diamond array will be one half that of its orthogonal counterpart. Thus, it will be appreciated that if the “orthogonal” digital data format that is typically used with digital displays could be used with a diamond-shaped array, a fifty percent reduction in size would be appreciated. The fifty percent reduction in size would translate to substantially double the number of dies per wafer. Consequently, yield per wafer could be significantly improved by using a diamond array.
It should also be appreciated that the present invention is discussed with respect to reducing the size of the mirror array so as to increase yield. Alternately, however, the number of pixels and, consequently a diamond array used to replace an orthogonal array could remain the same size as an orthogonal array. In this event, rather than an increase in yield, the resolution would be increased. However, as will also be understood from the discussion below, although doubling the number of pixels will increase the resolution, it will not double the resolution.
Conversely, loss in resolution will occur with the conversion from an orthogonal to a diamond array. For example, the bandwidth of the Horizontal and Vertical frequency of a diamond array is illustrated in FIG. 6. As shown, FIG. 6 is a plot of a 128-point two-dimensional fast Fourier transform (FFT) used to determine the frequency response where “0” corresponds to the frequency minus π, and 128 corresponds to the frequency plus π. The frequency plot indicated by the raised area 62 illustrates how the diamond array maintains the highest Horizontal and Vertical frequencies as illustrated by solid arrows 64a and 64b respectively, but only half the bandwidth of the highest Diagonal frequency as indicated by dashed arrows 66a and 66b. That is, the Horizontal and Vertical frequencies indicated at coordinates 68 and 70, respectively, include the full range of frequencies as indicated by arrows 64a and 64b, whereas the Diagonal values indicated by dashed arrow 66a is only half that of the Diagonal frequency for an orthogonal array as indicated by arrow 72.
This change in bandwidth is further illustrated in the display of FIGS. 7A through 9A and FIGS. 7B through 9B showing how the diamond array image was created by sub-sampling the orthogonal array. This sub-sampling was accomplished by removing the even pixels from the orthogonal array for odd rows in a diamond array and removing the odd pixels from the orthogonal array for the even rows in the diamond array. For example, FIG. 7A illustrates how vertical lines 74, 76, 78 and 80 of pixels of four different colors shown in an orthogonal array can be reproduced as lines 74a, 76a, 78a and 80a in the diamond array of FIG. 7B. Likewise, FIG. 8A illustrates that horizontal lines 82, 84 and 86 of different colors in the orthogonal array can be reproduced as lines 82a, 84a and 86a of FIG. 8B. However, since converting from an orthogonal array to a diamond array by this type of sub-sampling results in every other pixel on each line being removed, the diagonal lines 88, 90, 92, 94, 96 and 98 of the orthogonal array of FIG. 9A cannot be reproduced by the diamond array of FIG. 9B. The inability to produce the diagonal rows by this simple sub-sampling method is a result of the distortion caused by the interaction between the signal frequency and the sampling frequency hereinafter referred to as aliasing. The spaces in the angled letter “V” in the city names “Vancouver” and “Victoria” of FIGS. 10A and 10B illustrates the results of aliasing. More specifically, or as is better illustrated in the enlarged view of FIG. 10B, the result of removing the pixels in the conversion is obvious. FIG. 10C shows the visual improvement when the aliasing is removed.
Therefore, methods and apparatus for using a diamond-shaped array without unacceptable loss of resolution and increased artifacts would clearly be advantageous.