The principle of cable digital TV set top box
4, the upper application software: the implementation of service providers to provide a variety of services, such as: electronic program guide, quasi video on demand, video on demand, data broadcasting, IP phones and video phones. Upper layer applications are independent of STB hardware, which can be used on a variety of STB hardware platforms to eliminate application software dependence on hardware. (7) display technology: TV and computer display, CRT display is a mature technology, but with a low resolution display text, especially less than 24 * 24 words, the problem becomes more complicated. The television picture tube is low resolution of big pitch pipe, only suitable for image display 720 x 576 or 640 x 480, its deflection system is fixed, is 525 60Hz or 625 50Hz design, and digital TV has more than 18 kinds of display format. The Internet is in line with the VESA format. Obviously, the TV's display system can't adapt to so many formats. In addition, the TV adopts a low frame rate interlaced scanning method. When displaying graphics and text, the brightness signal has the background flicker, and the horizontal line has the flicker between the lines. If the computer graphic conversion scan to the TV set, will only appear in the horizontal edge odd field or even field, display time close to human visual persistence, will produce the edge flicker phenomenon is severe, so to use Internet TV, the TV show to remedy the defects. According to the technical difficulty and cost, currently with two kinds of methods were improved, an anti flicker filter, the image is in proportion to the sum of three adjacent line up in a row, which only appears in the image reproduction in every single game, called the three line filtering method. The three line filtering method is simple and easy to implement. But it reduces the sharpness of the image and is suitable for interlaced tv.
The main technologies of digital set top box are channel decoding, source decoding, modulation and coding of uplink data, embedded CPU, MPEG 2 decompression, set-top box software, display control and scrambling and descrambling technology. (1) the channel decoding: channel decoding circuit in digital TV set top box is equivalent to the analog TV tuner and if amplifier. In the digital TV set-top box, the tuner is required, but the tuning range of satellite channels, including ground TV, cable TV supplementary channels. According to the existing modulation methods of DTV, channel decoding should include QPSK, QAM, OFDM and VSB demodulation functions. (2) source decoding: after the analog signal is digitized, the amount of information is increasing rapidly, so the corresponding data compression standard must be adopted. Digital TV broadcasting using MPEG-2 video compression standard, image quality for a variety of clarity. At present there are AC3 and MPEG-2 audio two standard. The source decoder must adapt to different encoding strategies and correctly restore the original audio and video data.
In fact, the digital set top box is a transform device that converts digital signals into analog signals. Most of the time we have this kind of doubt. Since the analog signals from TV stations are received, the TV receiver must also be analog signals, so why use digital set-top boxes? Now life, we always hope that their TV can broadcast high-definition TV, from analog TV to high definition digital TV transition, is a leap of transition, can be said that can not be directly compatible, that is to say at present all of the analog TV is not used, so in one step is not realistic. Usually it is: the TV station will first analog signals into digital signals, digital set-top box transfer home after the digital signal into analog signal input to the analog TV now display information, which can effectively avoid interference and lead to loss of TV signal in the transmission process, the TV signal quality has been greatly improved.
(3) modulation and coding of uplink data: to carry out interactive applications, the modulation coding problem of uplink data needs to be considered. At present, there are 3 ways to use the telephone line to transmit uplink data. The uplink data is transmitted by Ethernet card and the uplink data is transmitted through the wired network. (4): embedded CPU embedded CPU is a digital TV set-top box in the heart, when the data channel decoding, first demultiplexing, the transmission stream into the video and audio, video, audio and data from the special digital TV set-top box CPU integrates more than 32 programmable PID filter. Two of them for the video and audio filter, for PSI, SI and the rest of the filtering of Private data. CPU is the platform of embedded operating system, it should complete the network management and operating system, display management, conditional access management (IC card and Smart card), teletext decoding, data decoding, OSD, video signal up and down conversion function. To achieve these capabilities, many new features must be extended on the average 32~64 bit CPU, and the speed is constantly increased to suit the requirements of high-speed networks and 3D games. (5): MPEG-2 decoding of MPEG-2 is one of the key technologies in digital television, the digital video processing technology and practical basically is based on MPEG-2 technology, including MPEG-2 is transmitted from the network to a high-definition TV all specifications. MP@LL, H.263 and H.261 for VCD, videophone conference and videophone are its subsets. is used for DVD, SDTV, and for HDTV. The video image signal processing method of motion prediction, DCT, quantization, variable length encoding 4 step, circuit is a ASIC circuit by the RISC processor as the core. MPEG-2 decompression circuit including video, audio decompression and other functions.
