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Author:Abstract : The rear projection display technology is in a period of vigorous development. This paper introduces the basic optical path principle of rear projection display technology and the application and development prospects of several major rear projection TV technologies under the current technical background. And give a key introduction to the field of LCOS rear projection that the author is engaged in.
Keywords : rear projection technology, liquid crystal display, digital light processing, silicon-based liquid crystal
1, rear projection display technology
The definition of Rear Projector is relative to the traditional Front Projector. The main difference between the two is the way the image is sourced. In the front projection system, the observer and the projector are located on the same side of the reflective screen, the light projected by the projector is irradiated to the screen, and then reflected to reach the observer; in the rear projection system, the observer and the projector are located on the display screen. On both sides, the light emitted from the projector illuminates the translucent display screen, and part of the light is transmitted to form an image, so the observer sees the transmitted light. The principle is shown in Figure 1:
Figure 1: Rear projection schematic
The multimedia projectors that people usually refer to are mainly front projectors. The advantage of back projection compared with them is that the projector and the screen are integrated in the rear projection system, and the user does not need to make optical adjustments when using them, just like using a normal TV. simple. In addition, the optical projector in the rear projection system is enclosed in a box, and the light projected onto the screen is not affected by external light, so that the image can be displayed perfectly in a dark or bright environment. Based on these principles, rear projection TVs have been produced. Due to the different types of projectors used, rear projection technology can be divided into CRT (cathode ray tube), LCD (liquid crystal), LCOS (silicon liquid crystal), DLP (digital light processing). ) and so on. So far, the technology of CRT rear-projection TV is the most mature, with large production scale and high cost performance. It is still the mainstream product of the domestic rear projection TV market. However, the rear projection of the CRT relies on the phosphor to emit light, which makes it difficult to increase the brightness, and it is easy to age the picture tube. After a long time, the picture will be darkened and the definition will be lowered. In view of this, with the gradual maturity of the other three technologies, the market will be re-segmented, who will occupy the mainstream of the future market? Below I will introduce LCD, DLP, LCOS three rear projection TV projection technology.
2, LCD rear projection technology
The LCD (Liquid Crystal Display) rear projection imaging method is a transmissive type, and the imaging device is a liquid crystal panel, which is a passive projection method. It uses an external light source (metal halide or UHP lamp), so you can increase the brightness by increasing the power of the lamp. It uses more mature liquid crystal projection technology, good color reproduction, brightness and contrast are better than CRT rear projection. With the continuous development of technology, the problem of lamp life that is currently plaguing the industry will also be better solved. At present, the reason why LCD rear projection has not become the mainstream of the market is mainly due to its high cost. In addition, the LCD rear projection is limited to the reason of its working principle. It takes time to start warming up and heat off after shutdown, and it is impossible to follow the CRT rear projection.
The LCD projector is divided into three pieces and a single piece according to the number of liquid crystal panels. At present, the three-piece projector is the main type of liquid crystal panel projector, and its principle diagram is as follows:
The principle of the three-chip LCD panel projector is that the optical system emits the strong light emitted by the light source through the beam splitter to form three beams of R, G, and B, which are respectively transmitted through the R, G, and B liquid crystal panels; the control signal source passes through the A/D. After conversion modulation, it is applied to the liquid crystal panel to control the opening and closing of the liquid crystal cell, thereby controlling the on and off of the R, G, and B light paths, and then the three color lights pass through the color combining optical path, and are concentrated in the color combining prism, and finally After the lens is projected, a color image is formed on the screen.
3, DLP rear projection technology
DLP (Digital Light Processing) refers to digital light processing technology. This technology firstly processes the image signal after digital processing, and the projection display quality is very good. Unlike transmissive imaging with LCD rear projection, DLP is a reflective mode. The core of the system is the Digital Micro Mirror Device (DMD) developed by TI (Texas Instruments). The DMD is the final link for displaying digital visual information. It is on the standard semiconductor process of CMOS, plus one A device formed by a rotating mechanism that modulates a reflecting surface. Typically, the DMD chip has approximately 1.3 million hinged mounted micromirrors, one for each pixel. The principle of DLP rear projection is to homogenize the light source with an integrator. The color wheel is divided into R, G, and B colors by a color wheel with three primary colors. When the micro mirror is tilted toward the light source, the light is tilted. Reflected onto the lens, equivalent to the "on" state of the optical switch. When the micromirror is tilted in the opposite direction to the light source, the light is not reflected on the lens, which is equivalent to the "off" state of the optical switch. The gray level is determined by the number of optical switches per second and the number of switching times. Therefore, the method of synchronizing signals is used to process the electrical signals of the digital rotating lens, and the continuous light is converted into gray scale, and the colors are expressed in three colors of R, G, and B. Finally, the projection imaging can produce high quality and high gray. Degree level image.
At present, DLP projectors mainly include a single-chip DMD machine, a two-chip DMD machine, and three DMD machines. According to their different characteristics, there are different applications. Among them, the monolithic type is mainly applied to portable projection products, the two-piece type is applied to large-scale spliced ​​display walls, and the three-piece type is mainly applied to ultra-high brightness projectors. Generally, DLP rear-projection TV has 4-5 times of resolution of ordinary color TV, and has the advantages of high brightness and high contrast, and can achieve a contrast ratio of 1000:1. In addition, due to the adoption of digital technology, the gray level of the image is improved, the image noise is lost, and the picture quality is more stable. However, Texas Instruments is currently the only manufacturer of DMD chips in the world, resulting in a weak supply of projectors, insufficient supply of core components, low yields, and high prices, thus limiting the development of this product to a certain extent. In addition, in the long run, DLP projection technology is limited in ultra-high resolution (more than 2000 lines).
4, LCOS rear projection technology
LCOS (Liquid Crystal On Silicon) technology combines semiconductor and LCD technology, and its optical imaging principle is the same as DLP. Compared with the two types of rear projection techniques described above, the advantages are high resolution, high brightness, simple structure, and high potential for cost reduction. Although in the current rear projection application, compared with the popular LCD technology and the recent popular DLP projection technology, LCOS can not compete with it, and temporarily ranks third among the three technologies in the short term, but LCOS is still quite optimistic. The most promising projection technology, with the continuous improvement of its optical projection system in weight and brightness, will surely occupy a prominent position in the rear projection TV market. In addition, in terms of China's high-end rear-projection color TV entry point, to establish its own technological advantages, LCOS technology is currently the first choice. From the perspective of the display panel, Japan and South Korea occupy considerable advantages in the field of LCD technology. China's Taiwan region only occupies some middle and low-end markets, and DLP technology is controlled by TI exclusively for its core device DMD. The LCOS technology is not yet mature. At this time, the development of LCOS will have the opportunity to get rid of the situation of people who are subject to LCD and DLP projection technology. Therefore, it can be said that LCOS is an opportunity for China to take the lead in high-end color TV technology. At present, Taiwanese manufacturers in China are quite active in the development of LCOS technology, and the LCOS alliance led by UMC has attracted more attention. The promotion and application of HDTV will accelerate the industrialization of LCOS.
One of the LCOS display panels uses a CMOS chip as the substrate, so that the light cannot pass directly through, so the transmissive imaging method is adopted, so that the rear projection optical system and the LCD rear projector are different. Usually, the LCOS optical system needs to use a Polarization Beam Splitter (PBS) to separate the light incident on the LCOS panel from the reflected light. The PBS is a prism made of two 45-degree isosceles right-angle prisms. When nonlinearly polarized light is incident on the PBS, the PBS reflects the S-polarized light of the incident light (vertical into the ray plane) and allows P-polarized light (parallel into the ray plane) passes. About LCOS optical system technology is still in its infancy, so many manufacturers such as IBM, ColorQuad, Philip, and Hologram have developed different LCOS optical engine architectures. But the main can be divided into two types of single-chip and three-piece, as follows:
1), single chip
The schematic diagram of the monolithic LCOS Color Wheel optical engine is as follows. The fast rotation of the R, G, and B color rings divides the white light from the light source into sequential red, green, and blue monochromatic lights. These three primary colors are synchronized with the red, green, and blue images produced by the driver to form a color separation image. When the frequency is fast enough, the observer can see the color projection picture due to the persistence of the human eye. The monolithic optical engine takes up relatively little space, requires only one panel, and the system architecture is relatively simple, so it has a competitive advantage in cost. However, at present, there are some technical difficulties. In the Color Wheel, after the white light is polarized, the brightness is significantly reduced, and only 1/3 of the energy remains. In addition, since the LCOS panel is to be synthesized under the rapid switching of red, blue and green screens, the response speed of the panel is higher. Similar technologies currently exist: Displaytech's Field Sequential Color structure, Philip's Scrolling Color-Rotating Prism structure, and JVC's Spatial Color-Hologram structure.
2), three-piece
The three-piece LCOS optical engine is the main method currently adopted in the market. Here is an example of a three-piece LCOS optical engine that I have debugged. UHP bulbs are used as the light source. The light is first homogenized by two repeated eye passes, then passed through a layer of PBS prism and lens, and then split by the red, blue and green light beams, and then the beams are respectively projected into three. In the LCOS panel, the reflected three-color image is color-coded by the color-matching system and projected onto the screen. In this system, four square prisms, four PBS prisms, and two fly-eye lenses, and several mirrors are used. It can be seen that the three-piece LCOS optical engine needs to combine a plurality of color separation and color combining optical systems in addition to three panels, so the volume is large and the cost is high. However, high optical efficiency can be achieved. In the LCOS projection technology, the lower substrate of the panel adopts a CMOS substrate, the material of which is monocrystalline silicon, has good electron mobility, and the monocrystalline silicon circuit can be made very thin, so it is easy Achieve high resolution. In addition, LCOS is a reflective image that does not significantly reduce light utilization as the LCD optical engine penetrates the panel due to light, so it has a high optical rate and can generate higher brightness with less power consumption. And with high image quality, it is mainly for higher-end professional use. At present, the three-chip optical engine also has the ColoRQuard architecture adopted by ColorLink and the Prism architecture of Philips.
Here is a brief introduction to the features of the LCOS display driver. LCOS display technology requires a built-in DRAM image control chip, which mainly includes pulse clock generator, row driver circuit (shift register and buffer), column driver circuit (shift register, buffer, latch), D/A Several parts of the converter and the active pixel matrix. Using the active matrix structure pig layer to write data, for each pixel, its working state is equivalent to static driving, so the contrast is high, almost no Cross-talk. The gray level is determined by the applied pulse width. Each pixel corresponds to a switch, and generally occupies four layers of metal in the driver chip, wherein the lower two layers of metal are used for routing, and the driving circuit connections in the row and column directions are implemented thereon; the upper two layers of metal are used for light shielding. And reflective surface electrodes. The video works as shown below: Each pixel consists of a MOS transistor and a storage capacitor. The width and length of the MOS tube mainly consider the influence of the feedthrough on the logic performance of the circuit. The capacitance of the storage capacitor (Cs in the figure) is determined by the leakage constant of the liquid crystal and the capacitance value of the liquid crystal itself (Clc in the figure).
In terms of driving voltage, the "field-by-field phase-in" method is adopted to apply an interactive voltage to the liquid crystal cell to prevent the liquid crystal molecules from being polarized and the electric field orientation characteristics are effective under the action of a single-direction electric field. The specific operation process is to flip the pulse waveform of the data line after the first field signal, and change the positive pulse signal into a negative pulse signal while keeping the scan pulse signal unchanged. When charging the liquid crystal and its storage capacitor, in order to save power, we chose a linear ramp wave charging method in the circuit design.
In terms of the structure of the drive circuit system, there are two types of analog and digital. In the analog mode, the column direction is connected to the video through the horizontal shift register control, the row direction is turned on line by line, and the pixel matrix is ​​connected to the column line through the vertical shift register control. A DAC is used for each pixel in the digital mode structure. To solve the problem that the DAC cannot be limited to smaller pixels, we can add a latch circuit to use one DAC per line.
In general, CRT, LCD, DLP, LCOS, and other rear projection TV technologies have their own advantages. Considering the consumption capacity, CRT will still occupy the main body of China's rear-projection market in the next few years; LCD is the most promising technology to replace CRT as the mainstream technology in terms of technology maturity and application scope; DLP is a technological upstart, currently According to the exhibition, the momentum surpassed LCOS (especially in portable projectors, DLP has formed a certain industrial scale, this article mainly describes the application in rear projection TV, so it is not elaborated here); and LCOS is the most cost The technology of superior potential and image quality advantage, LCOS will have the greatest advantage as people demand the display screen size and pursue more comfortable and clear TV pictures.
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