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Active matrix liquid crystal displays (AMLCDs) are gradually gaining popularity among consumers. In the average family, many large-screen TVs in the living room of the home have been replaced with a liquid crystal display. TV screen quality requirements are extremely strict, and the price must be popular, so it is not easy to meet consumer requirements. Manufacturers of flat-panel display TVs using active-matrix liquid crystal displays must seek to increase the clarity of the picture and the brightness of the color, and lower the price, in order to further expand market share.If manufacturers want to transform the production line of liquid crystal displays and produce TV sets, they will have to overcome several problems in display technology. First of all, the difference between the larger screen and the format is a problem that must be solved. Most laptops use the 14-inch XGA format (1024x768 pixels), while most desktop LCD monitors use the 17-inch SXGA format (1280x1024). The entry-level products for large-screen LCD TVs are no less than 30 inches, and both use the widescreen XGA Plus format (1366x768). Large-screen LCD TVs of 40 inches or more are in true high-definition television (HDTV) format (1920x1080) and are high-end products on the market. The high-definition TV format requires more than 2.5 times more data than the widescreen XGA Plus format, based on the data required for each frame.
As more and more liquid crystal displays adopt XGA and SXGA formats, manufacturers must further reduce power consumption and reduce electromagnetic interference. Differential signal transmission technology and data transmission line design have become mainstream solutions in this respect. According to the theory of transmission lines, the signal path should be considered as an electrical waveguide, not just a line connection. This ensures that the signal retains its waveform while it is being transferred. After the new technology is available, digital pixel data can be transferred directly to each column of drivers, and the transfer speed is extremely fast, so that all pixel data can be written to the column driver within a typical frame time of 1/60 second.
The signal input to the TV must have a high degree of integrity, which is no different from a notebook system display and a general monitor. However, large-screen TVs have more new requirements for signals, which cannot be met by the signal transmission technology of notebook system displays and ordinary monitors. In addition to being able to support large screens, the new signal transmission technology has to meet other requirements. Since the screen of the television is large, the transmission distance of the video signal is also necessarily long, so there are more artifacts and beats caused by the impedance mismatch. In addition, the length of the TV column board is generally the same as the width of the screen, but when the screen of the TV reaches about 30 inches, the column board must be divided into two, because the printed circuit board is limited by the production process, the size There is a certain limit, the board is divided into two, which will cause more connection points in the signal path, greatly increasing the chance of signal error, and making the signal path design more complicated. I hope to shorten the circuit to save space. It also fell through. However, the problem is not limited to this. According to the current development trend, the refresh rate of the picture will gradually increase to 90-120 Hz (Hertz), so that the active matrix liquid crystal display will be blurred due to the necessity of performing scanning and holding functions. The phenomenon.
In addition to the above-mentioned stringent requirements for signal integrity, high-definition televisions have more gray levels per computer than computer monitors. LCD TVs must use 30-bit pixels (10-bit grayscale for each of red, green, and blue), rather than the 24-bit pixels commonly used in computer monitors. LCD TVs must use 30-bit pixels to ensure that the edges of the image do not appear when the brightness gradient is shallow, such as when displaying the evening sky or the sea. If we quantify the brightness according to the spatial gradient, we will produce a distinct and abrupt line. Since televisions generally use a design with a strong contrast between light and dark, this kind of flaw is more obvious on TV.
It is self-evident that enhancements such as grayscale, color, and light-dark contrast are necessary, but in addition, image enhancement processing can also ensure image accuracy of up to 30 bits through brightness adjustment. The LCD does not adequately display the light-dark contrast of the dark environment because the display cannot turn off the light valves in each pixel. In other words, the light valve leaks. At present, this generation of high-end LCD TVs adopt advanced light-dark contrast technology, so the light-dark contrast is close to 1000:1, but the light-dark contrast of traditional LCD TVs is only about half of that of high-end products. After the emergence of image processing technology, this situation has been greatly improved. This image processing technology can view the image frame one by one, and expand the main brightness range, so that the image with a smaller brightness range (ie, the light-dark contrast is smaller) can be adjusted to increase its gray level to enhance the grayscale effect.
For the TV set, the most important thing for our generation of consumers is the color effect of the TV set. If different brands of TV sets are put together for comparison, most consumers use their color as their purchase indicator. For this reason, the industry has been working hard to study how to accurately fit different colors of an image into the color space of individual LCD screens. The nesting of colors is done by mapping the image. The entire mapping process turns the image into the appropriate lumens and chrominance space, and some colors of the image are highlighted during processing, and other colors are faded. After such processing, the accuracy of the red, green and blue colors is more accurate than the original image, so the TV must use 10-bit grayscale (30-bit pixels) to properly process and improve the image to avoid a segment. The illusion of segmentation.
The average consumer may feel a bit surprised, the speed of the LCD monitor can not keep up with the video system. They don't know that applications such as computer monitors can tolerate slower pixel response times. But the requirements for television broadcasting are completely different. Each frame -- in the case of interlaced scanning, for every half frame -- must be captured in 1/60 or 1/50 seconds. In terms of motion pictures, the TV picture is more detailed than each picture. In addition, the frame rate at which the television image is played is faster than the rate at which the movie is played per movie. The speed of the TV is so high that it is sufficient to display the TV with a fast response time. For high-definition TVs, response time is especially important. But for other video systems, the speed of response time is not that important.
For the above reasons, liquid crystal display televisions must be equipped with a response time compensation (RTC) overdrive circuit module to compensate for the slower optical response of the liquid crystal display. The response time compensation circuit module is disposed in the timing controller (TCON) and is responsible for intercepting the digital video stream, and then comparing the previous gray scale instruction of each pixel with the latest gray scale instruction, and then from the lookup table (LUT). Pick another preset gray level. The alternate gray value of this pre-written lookup table is experimentally selected to raise the brightness to the target value at the end of the frame. If the new gray level is lighter than the previous level, the system will issue an instruction to provide a lighter gray level. If the new gray level is darker, the system will first issue a command that is much darker than this gray level.
The structure and technology used in LCD monitors and notebook system monitors simply cannot meet the stringent requirements of LCD TVs (see Figure 1). The multi-station differential bus structure uses different transmission lines to transmit digital video data to the column driver, but such a structure is difficult to cope with the heavy workload of the liquid crystal display television. For example, when transmitting a signal, the signal waveform must remain perfect so that the TV can transmit data at a higher speed. However, due to the long transmission line of the signal and the larger screen of the TV set, more column driver stations are used, so the data transmission rate is difficult to meet the requirements. To solve these problems, some companies are developing technologies for cascading structures. The advantage of this configuration is that the traditional bus is used to connect one end of the column driver, and the data is buffered before being transferred to the next column driver. As long as the traditional bus adopts this structure, it can support the point-to-point transmission mode. The advantage is that the signal quality has higher guarantee, but the disadvantage is to add more column driver input/output terminals and related circuits. Re-engineering the bus structure with cascading can improve signal integrity, but still does not meet other requirements, such as providing more accurate grayscale, enhanced light-dark contrast, improved color management, and the addition of other more advanced features.
Figure 1: The main function block of a typical LCD module is equipped with a multi-drop differential bus that can transfer digital video data to a column driver through different transmission lines. Such a structure is only suitable for general monitors and notebook system displays, but it is difficult to cope with the heavy workload of liquid crystal display televisions.
The first point to note is that for traditional architectures using resistor string digital/analog converters (RDACs) and column drivers, it is a daunting task to provide a 10-bit grayscale per color on a cost-effective basis. challenge. All the time, only the notebook system display and the general monitor have adopted the RDAC structure design. The advantage of this architecture is that digital grayscale data is transferred to the column driver over the differential transmission line bus. The column driver transfers the relevant value to one of the voltage nodes on the serial resistor string by mapping. According to the original design of the column driver, the voltage of each node is set at a certain level, and ensure that the set voltage can adjust the brightness of the liquid crystal display to a level that can match a certain gray level. . In other words, the R DAC is not only responsible for performing the digital/analog conversion function, but also for performing the inverse gamma conversion function, and the latter can ensure that the power supply voltage obtained by the liquid crystal display can match the brightness of the display required for each gray level.
The R DAC can perform these two functions quickly in a 64-gray (6-bit) environment. However, for a system with 256 gray levels (8 bits), since the gray level is increased, the column driver must transfer two electrodes of 256 voltages from one end of the chip to the other end, plus each output. Circuitry is required to decode and select a suitable voltage from it, so the die area of ​​the column driver will account for most of the overall die area. If the gray level is increased to 1024 (10 bits), and other conditions are not changed, the occupied die area is too large, so that the entire design can not play its due role.
National Semiconductor's design orientation is very different from the traditional design using R DAC. We have successfully developed a column driver with a linear loop digital/analog converter. Due to the small die area of ​​this digital/analog converter, two identical digital/analog converters can be installed at each output, one for each electrode. While one of the digital/analog converters is constantly converting the input data for use in the next hatch, another digital/analog converter has driven the current hatch with the voltage that has just been converted at the previous hatch. The main feature of this digital/analog converter is the ability to increase bit accuracy with flexible adjustments. To increase resolution, you only need to increase the operating cycle of the same digital/analog converter circuit without increasing the die area. Because of this structure, we are able to provide 10-bit grayscale with a small die on a cost-effective basis. A typical 10-bit point-to-point differential signaling (PPDS) column driver has a smaller die size than a typical 8-bit RDAC column driver and is less than half the size.
Another advantage of this design orientation is that the conversion circuitry of the digital/analog converter is no longer responsible for performing the inverse gamma function. In other words, each column driver output can directly convert the digital voltage value to an analog voltage value. The upstream timing controller (TCON) is responsible for converting the digital gray level to a digital voltage. In other words, the look-up table in the timing controller is responsible for performing the inverse gamma function. This design has greater flexibility so that each gray level can be mapped to match the brightness of the flat liquid crystal display. In fact, the timing controller can provide different look-up tables for each color, and even update the relevant charts in real time, so that adjustments can be made for different image sources, contrast enhancement, color management, and even temperature transitions.
The column driver structure is part of the entire point-to-point differential signaling (PPDS) structure (see Figure 2). As the name suggests, point-to-point differential signaling is not a multi-drop bus, but a line system consisting of multiple independent point-to-point links that provide one channel for each column of drivers. This channel can transfer the control data and digital voltage data of the column driver, and the column driver converts these digital voltage data into analog voltage data. The traditional bus architecture uses burst mode to transfer data to the column driver, and only one column driver can receive data at the same time because the bus is shared. With a PPDS structure, all column drivers can receive data simultaneously. So even if only one differential channel provides data for each column driver, this channel can be used for the entire time period. Therefore, the clock frequencies of these two systems are very different.
Figure 2: The new point-to-point differential signaling (PPDS) architecture is a line system consisting of multiple independent point-to-point links, which is completely different from traditional multi-drop buses. Each channel can transmit the control data and digital voltage data of the column driver, and the column driver is responsible for converting these digital voltage data into analog voltage data.
An important feature of the PPDS system is that each column driver can be individually controlled by the packing headers sent to each column driver row by row. This structure must use this embedded control method, because only in this way, the column driver and the timing controller can eliminate the need to transfer individual dedicated signals to each other, which helps to reduce the system size and cost. Because the system has the flexibility to control the column drivers, special waveform control functions can be performed. To drive a large-screen TV, you must control the waveforms that drive the flat panel display to achieve optimal signal propagation and increase pixel charging rates.
Since the timing controller can provide separate red, green, and blue gamma lookup tables, the color temperature of each gray level can be accurately corrected. Since the red, green and blue gamma look-up tables are independent, each color can be independently gamma corrected, providing a fixed color temperature for all gray levels, and extending the application of this technology to high performance. system. Since the timing controller can directly access the gamma look-up table for access, it is possible to flexibly adjust the gamma correction function in real time according to different contents. The magnitude of the adjustment is determined by the image processing unit in the timing controller. The new generation of configurations is believed to provide different gamma transfer functions in different windows of the display (see Figure 3). The computer operating system can provide window boundary coordinates for the timing controller and then command the timing controller to select different gamma lookup tables depending on the different display areas to be written.
Figure 3: Since the gamma lookup table of the PPDS structure is set inside the timing controller, the picture-in-picture window can perform different gamma mappings, so that each image can exert a unique color effect.
Manufacturers of large-screen TVs are hoping to produce products that rival the effects of cinemas. To ensure that the picture is comparable to the theater, whether the gray level is accurate and whether the color management is perfect is the key to success. The PPDS structure supports full-color accuracy of 30 bits throughout the entire display process from input to output. Thanks to this precision, coupled with the many functions provided by the independent gamma look-up table, the effect of image processing can be greatly improved, so that a variety of images can be presented in front of the audience. In addition, since the color and dark contrast of the image are enhanced, the gray level of the image is more accurate. Since each of the independent gamma look-up tables has its own settings, it is possible to directly control the image effects without using color enhancement or other methods to achieve the brightness required for each pixel.
For monitor and notebook system displays, the LCD module only plays an extremely low-key role, such as receiving grayscale commands, and then providing control signals to the flat panel display to produce the desired gray level. The requirements of the TV are different. The flat display technology should take into account the picture effects, such as enhancing the color depth, improving the color balance, enhancing the dynamic contrast, providing response time compensation and color temperature control. Since the PPDS structure can support these innovative technologies, LCD TVs will undoubtedly become the consumer's favorite, and the prospects for LCD technology can be described as flawless.
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