Since the eighties, with the increase in public information, the establishment of excellent performance, low price of large area of information media has been the engineering and technical personnel of the issues of concern. Depending on the application, the required display performance and the requirements for the display system cost are different, one can choose a different display media. Taking into account the large screen display media for indoor and outdoor display equipment on the color, brightness, video effects, reliability, life and other performance requirements are more demanding.
It requires the display system to display a true color screen that is close to the natural color and has a high display brightness, both in the daytime and at night, sunny or cloudy, to allow the viewer to see the content clearly and According to the external natural light intensity changes automatically show it is playing the video program, with high reliability and cost performance, easy to maintain. Based on a number of factors, engineering and technical personnel through the magnetic replica, vacuum pixel tube, TV wall, LED display comparison, found as a display pixel can meet the above requirements, is a very good indoor and outdoor display media.
At the same time, along with the development of science and technology, today's semiconductor light-emitting diode performance reached a new level, the photoelectric conversion efficiency of 10cd / w or more, especially blue super bright LED devices appear, making the LED device spectrum covered with natural light The range.
Therefore, the use of LED devices to manufacture large flat panel display system has become an important application of LED devices, and thus promote the development of large-scale display system and the formation of an industry. The biggest feature of LED display is that its manufacturing is not limited by the area, can reach dozens or even hundreds of square meters or more, used in a variety of indoor and outdoor public display text, graphics, images, animation, video images and other Kind of information, hundreds of thousands of people at the same time stop to watch, with strong advertising rendering power and shock, the city's social activities have a strong atmosphere rendering role, beautify and enrich the people's living environment.
1, image acquisition technology
LED electronic display to show true color images, you must first solve the real-time video signal acquisition, the analog video signal acquisition for digital video images. The early practice is to use video capture cards and some feature-specific (Feature-connect) VGA card to achieve.
The video capture card is used to capture the video image, and then through the VGA feature port to obtain the field frequency, line frequency, pixel frequency and color lookup table index address, in tracking the CRT image can be obtained by copying the color lookup table to get red and green , Blue-separated digital signals. One way is to use software to copy regularly, and the other is to use hardware theft technology, which is more efficient and fast.
As the above-mentioned technology and the VGA card compatibility is poor, the edge is not clear, poor image quality and other shortcomings, electronic display shows the image quality has also been limited, for this, Beijing Galaxy Computer Corporation in 1998 research and development LED electronic display dedicated video card JMC-LED.
The card is based on the PCI bus, using 64-bit graphics accelerator, the VGA and video functions into one, responsible for video data and VGA data overlay, color space conversion, fundamentally solve the compatibility problem. Application of full-screen resolution acquisition, YUV4: 2: 2 uncompressed storage technology to ensure the optimization of video images, video window using EST edge enhancement technology to ensure that the clarity of the image after scaling. Support for PAL and NTSC, video window can be any zoom, move.
The card can be electronic display to play the video when the required field frequency, line frequency, pixel frequency of several synchronization signals extracted, and the red, green and blue three-color signal separation. Digital RGB format for the 8: 8: 8, each can produce 256 gray, can meet the electronic display real color playback requirements.
2, the real image color reproduction
Full color LED electronic display of the visual principle and color TV, like, through the red, green, blue three different colors to achieve the image color reproduction reproduction. The purity of red, green and blue directly affects the visual effect of image color reproduction. However, the three-color ratio of white light is not a simple three-color superposition.
First, in ensuring the pure frequency of the premise, the requirements of red, green, blue light ratio must be close to 3: 6: 1;
Second, because people perceive the sensitivity of the red, requiring red light source in the space to be scattered distribution;
Third, because people visualize the red, green and blue light intensity of the three different nonlinear curve response, require different light intensity of the red, green and blue to be similar to the TV in the gamma correction;
Fourth, the human visual ability to distinguish color is limited. Therefore, it is necessary to find out the objective indicators of the reproduction of the image color. In order to reproduce the true image color, in the LED electronic display light should meet the following requirements:
① red, green and blue wavelengths should be: 660nm, 525nm, 470nm or so;
② with 4 tube unit with white light better (multi-tube unit can also, depending on light intensity);
③ red, green, blue three gray level of 256;
④ must be used for LED pixel tube non-linear correction. The
Red, green, blue three-color light distribution and non-linear correction can be used to display control system hardware, but also by the playback system software.
3, dedicated display drive circuit
From the current pixel tube several display methods can be divided into: ① scan drive; ② DC drive; ③ constant current source drive. For the indoor dot matrix screen, the general use of scanning; and for outdoor pixel tube screen, to ensure that the displayed image consistency is good, stable, bright, must be used to drive DC constant flow mode.
The older LED electronic display driver circuit is composed of low voltage signal serial and parallel conversion CMOS circuit and high current driven bipolar circuit composed of two (such as 74HC595 + MC1413 / UNL2803, CD4094 / MC14094 + MC1413, 74HC164 +74HC273 + MC1413) , This form of the drive circuit there are many solder joints, high cost, low reliability and other issues.
In response to these shortcomings, the United States TI company to develop and produce TPIC6B595 (TPIC6C595) application specific integrated circuit (ASIC), which will be serial and high conversion current driven, this ASIC has the following notable features: parallel output drive capacity, single Drive current up to 200mA, can directly drive the LED; current and voltage range, the operating voltage can be selected in the 5 ~ 15V flexible; serial input, shift and latch, the clock input port is equipped with Schmidt shaping circuit; And the output current is large, the absorption and supply current are greater than 4mA, cascade convenient; data processing speed, serial clock frequency, fmax ≥ 25MHz is particularly suitable for multi-gray color display LED driver.
China's Wuxi Dongda first Microelectronics Co., Ltd. also produced in 1998 and TPIC6B595 fully compatible with the ASIC chip AMT9094 / 9095, but the price is greatly reduced. Since the parallel output of the TPIC6B595 is only 8 bits, the number of TPIC6B595 is required to drive a higher resolution full color display, and 256 levels of gray control are more cumbersome.
To this end, the United States TI company developed to develop LED electronic screen display driver ASIC TLC5901 / 5902/5903, the advantages of this ASIC is: constant current source output 5 ~ 80mA (or 10 ~ 120mA); drive capacity of 80mA × 16Bits (Or 120mA × 8Bits); 256-level grayscale display for PWM control; adjustable brightness level 32; 8-bit parallel data input for clock synchronization. The chip makes 256 levels of gray control more simple, constant current source mode makes the image display consistency better, TQFP100 package makes the drive board area greatly reduced.
On this basis, TI has developed a better performance LED driver chip TLC5921. Beijing Hua Hong integrated circuit design company also developed to develop excellent performance LED ASIC 9701, this ASIC has the following notable features: containing 8 × 16 × 32 data scanning array, from static to 1/32 dynamic scan; data Input scan array and data output gray scale control using two independent clocks; 8-bit parallel data input and 8-bit parallel data output cascade function; 16 data output, each side of the LED current up to 80mA or more , Each end of the data output voltage is greater than 20V; data output 256 gray; output with a mode selection, can be used for odd, even frame selection; with non-linear correction control input.
4, brightness control D / T conversion technology
LED electronic display is composed of many independent pixels (light-emitting element) arranged, due to the separation of pixels, it determines the light control and drive can only be digital way. The luminous states of these pixels are controlled synchronously by the controller and are driven independently. The true color display of the video means that the brightness of each pixel is controlled separately and is done synchronously within the specified scan time. The large screen is composed of tens of thousands of pixels, which makes the complexity of the system than the two large display screen is greatly increased, and the overall data transmission speed put forward higher requirements. Setting up a regular D / A for each pixel is clearly unrealistic and there is a need to find a solution that minimizes system complexity and performance as high as possible.
Knowing from the visual principle that the average brightness of a pixel to a pixel depends on its light / off duty cycle. In other words, as long as the pixel light / off duty cycle to adjust, you can achieve the brightness control. For LED electronic displays, this means that the D / A conversion of luminance is achieved by converting the number representing the pixel brightness to the time (D / T conversion) of the pixel luminescence.
The interval for setting the screen data refresh is that the data for controlling the brightness of any pixel is n-bit binary number D = bi2i (where bi = 0 or 1), Ton is the light-emitting time corresponding to D, the pixel is lit / Ratio: d = Ton / Ts = D = bi2i. The expression can be implemented with a predictable subtraction counter, but each pixel with a counter will make the display circuit extremely complex. The above formula is rewritten as: Ton = Tsbi2i, which means that Ton can be divided into several time periods, since the continuous Ton is the same as the continuous Ton with the same total length for several separate time periods. Then, for the n-bit binary data D = bi2i, the sub-Ts is n and the appropriate time division function f (i) is selected such that the i-th segment Ti = Tsf (i), where 0 is the pixel Point of the light / off duty cycle. Since the function f (i) can be common to all pixels, the above equation shows that as long as f (i) is used to control each pixel, it is possible to realize the D / T conversion. For a single pixel, the circuit of Figure 1 can be used to achieve the above equation. In the figure, the SFR is an 8-bit shift register. The picture shows the waveform of the time division function f (i).
Large-screen display driver circuit is usually used "serial shift + latch + drive" structure, in order to minimize the data transmission line. To the whole screen at the same time to achieve the above formula, as long as all the ST signal unified by f (i) control can be. Of course, the premise is to ask the shift register is stored in the various pixel control data in the same authority, which can be done by pre-data processing.
5, data reconstruction and storage technology
There are two ways to organize the memory: ① PackedPixELMethod: all the bits of each pixel on the screen are stored in a single bank; ② BitPlaneMethod: Each bit of the pixel is stored in Different in the memory. As a result of the use of multiple banks, they can read more pixel information at the same time. From the two storage structure to analyze, the use of bit-plane structure is conducive to improving the LED screen display.
The entire LED display display control circuit block diagram shown in Figure 3. Among them, the data reconstruction circuit to complete the RGB data conversion, the same pixel with the same bit together, and then stored in the adjacent unit, so in the form of bit to complete the re-combination of the entire data.
Data reconstruction circuit consists of four major components: 8-bit data parallel transmission circuit; 8-bit parallel-to-serial conversion circuit; 8-bit data latch circuit; 8 plus 1 counter. R / G / B 8-bit data from the sync after the pixel frequency into the parallel latch, 8 plus 1 counter output carry pulse LD, 8-bit data at the same time latch to 8-bit and serial conversion circuit , By the clock control circuit to complete and - string conversion circuit clock control. After the data is reconstructed, a memory is no longer a pixel value, but a different pixel value of the same authority. Store all the same bits together to form a bit-plane storage structure in bits. The adjacent weights of each pixel must be taken out at the opposite rule.
The read and write address generator must meet the strict timing. For the same memory chip, it can be divided into N (one pixel value in N bits), each piece represents a bit plane, the pixels are converted to write to the same memory, first write 0, write 1 bit , And finally write N bits. For 8-bl × Row dot matrix display, there is 8-bit × Row bit for each bit-plane. The internal organization of the memory depends on the logical connection of the pixel tube on the drive body. According to the memory organization, the read address generator is driven by the column row, and then by the row drive bit; write address generator is driven by the column, column drive line, which can guarantee read and write synchronization, the correct synchronization display the original image information.
6, the logic circuit design ISP technology
In the early LED electronic display display control circuit, a large number of conventional digital circuit system design, digital circuit combination of complex control logic. In the conventional digital circuit system design, when the circuit design is completed, must first make the circuit board, and then install the components, debugging. If the logic function of the circuit board does not meet the requirements, it is necessary to redesign the circuit board and re-commission it until the logic function is implemented. Obviously, this design method of design cycle is long, high cost, and the finished product reliability is poor, maintenance trouble. The use of ordinary programmable logic devices, although the printed circuit board can reduce the design and production, but in the modification of the logic still can not avoid the device repeatedly plug.
In-system programmable technology (In-SystemProgrammable, abbreviated ISP), refers to the user's own design of the target system or circuit board for the reconstruction of logic device programming or repeated rewriting ability. Conventional PLD in the use of programming is usually the first assembly, and the use of ISP technology PLD is the first assembly after programming, as the product can also be programmed later. In the system of programmable technology, from the practice to achieve a number of logic designers