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introduction
Terrestrial transmission is the most basic system of digital TV, and terrestrial television broadcasting is also the most popular media. The DMB-T (terrestrial digital multimedia/television broadcasting) transmission scheme proposed by Tsinghua University has been able to meet the requirements of China's terrestrial digital TV standard in terms of technical feasibility, system stability, industrialization, and operational feasibility, and has begun to be in China. Promote popularization.
This article describes a solution for a DMB-TH (terrestrial digital TV receiver set-top box). In this scheme, the source demodulation uses the TDS-OFDM (Time Domain Synchronous Orthogonal Frequency Division Multiplexing) demodulation chip LGS8813 jointly developed by Lingxun Technology and Tsinghua University. The source decoding uses the limited design of Beijing Haier IC. The MPEG-2 decoder chip Hi2011e developed by the company is the first DMB-TH set-top box design that is fully implemented using domestic digital TV chips.
System hardware design
The hardware circuit of the digital TV set-top box solution mainly includes: digital tuner, DMB-TH demodulation, and MPEG-2 source decoding. The system block diagram is shown in Figure 1.
Figure 1 system hardware block diagram
The digital tuner uses the VA1T1ED6060 developed by Sharp to support DMB-TH. The receiving frequency range is 426MHz-862MHz, and the tuner transmits the ground transmission signal received by the antenna through the internal gain and other circuits to output the IF intermediate frequency analog signal. The output of the tuner converts the intermediate frequency analog signal into an intermediate frequency digital signal through an A/D conversion circuit (AD9203).
The source demodulation is implemented by the LGS8813. This is a TDS-OFDM demodulation chip that fully supports the DMB-TH terrestrial transmission standard. It demodulates the IF digital signal and outputs an 8-bit parallel digital transport stream, that is, a TS (transport stream) stream. The LGS8813 requires a 64Mb SDRAM as its workspace.
The source decoding is realized by Haier Hi2011e, and its working principle is as follows:
The decoding chip reads the digital transport stream and demultiplexes it by hardware to solve the audio and video elementary stream and stores it in the off-chip SDRAM. The video decoding module extracts the compressed data from the video compressed data area, and performs variable length code decoding, inverse quantization, inverse cosine transform, motion compensation, and data combining to generate video image data, which is stored in the video image data buffer. The playback module extracts the video image data from the video image data buffer, and mixes with the menu display OSD (On Screen Display) data to output the video image. Hi2011e's video output is digital and analog. Digital video output is YCbCr, 4:2:2 mode, YUV output, conforms to ITU-656 standard, can be connected to devices or devices with ITU-656 interface, such as LCD driver circuit; analog video output supports CVBS, S-VIDEO , YPbPr. The audio decoding module of Hi2011e extracts the compressed data from the audio compressed data buffer, decodes and subband filters, generates audio PCM data, stores it in the audio PCM data buffer, and the playback module extracts the audio PCM data from the audio PCM data buffer. And then press the audio interface to send the audio PCM data. An external HT82V731 digital-to-analog converter converts audio PCM data into an analog audio signal.
Hi2011e has an 8-bit CPU for system control, including peripheral circuit control, TS stream demultiplexing control, OSD control, and application software for running digital TV.
The system provides an IC card interface that can support conditional acceptance of the CA system. At the same time, there is also an RS-232C interface, which can upgrade the software of this system through the serial port. The main components in the entire system communicate via the I2C bus developed by Philips.
system software
Figure 2 shows a schematic diagram of the hierarchy of the set-top box software.
Figure 2 System software diagram of the set-top box
The software's integrated development tool is KEIL's mVision2, which is currently a widely used C51 development tool. The integrated development environment includes: compiler, assembler, real-time operating system, project manager, cross debugger. All the code of KEIL's standard C compiler can be written in ANSI C language, so it has good portability. uVision2 can generate the necessary connection options for off-chip memory: determine the starting address and size, the generated code is downloaded to the Flash of the set-top box (Figure 1), and runs directly from Flash.
The operating system of the set-top box is the embedded real-time operating system mC/OS, which is a portable, curable, tailorable and deprivable multi-tasking real-time kernel. The mC/OS-based set-top box software platform adopts a multi-tasking mechanism, and the functions implemented include: basic protocols of digital television, such as 13818 system protocol, demultiplexing protocol, etc.; control, decoding, and digital television broadcasting of peripheral circuits; The functions and mechanisms of the system; providing a convenient customized and personalized interface; and extending the functions and mechanisms of its personalized application. The software platform is implemented by a modular program structure, and each module uses a message queue to communicate.
The main modules of the platform are:
Initialize the boot module: Initialize all hardware, establish all semaphores, queues, events, messages, tasks, handle I2C drivers, handle tuner status, and operate.
Port processing module: For hardware systems based on the interrupt mechanism, organize and manage various types of interrupt processing, and analyze the state of various interrupt devices, which may be referred to as an interrupt processing module. For a hardware system based on the port roaming mechanism, the wheel-seeking hardware state can be referred to as a port polling module.
Demultiplexing module: processing TSD channel, filter, buffer management, including setting PID, solving PAT table, PMT table, SDT table, etc.; setting filtering condition; managing channel buffer; performing CRC verification.
The descrambling module: processes the management descrambler, sets the control word, and sets the PID of the descrambling stream.
Program storage module: Pre-programmed programs are retained, and the searched programs are added. Realize the management of program information.
Flash module: Read and write Flash driver to handle Flash read and write operations.
Audio and video operation module: Control audio decoder, including audio program judgment, adjust audio play/stop, volume setting, mute setting, channel setting. The control of the video decoder includes video signal judgment, PAL/NTSC judgment and conversion, program play and stop, and the like.
User interface control module: implements control related to user operation, including power supply, panel button and LED driver module; infrared remote controller processing module; graphic display module. The graphics control module is responsible for processing the OSD operation interface display and user operations. The module also includes functions such as remote message distribution and processing, control system implementation and management.
Channel switching control: related to remote control, descrambling, demultiplexing, complete channel switching.
System upgrade module: handles the needs of users to upgrade programs and upgrade applications.
EPG module: Implements processing and display of EPG information.
Chinese font module: realizes the processing of Chinese display, and can cut the size of the font according to the needs of the user.
With the development of digital TV, new applications will be able to expand on this basic platform. Such as multimedia magazines, digital audio broadcasting, games, and so on. The platform can also extend applications that support video-on-demand programming and interactive TV.
Conclusion
This article introduces the DMB-TH terrestrial transmission digital TV set-top box solution, the main chip is a domestic chip, in terms of standard definition set-top box, its cost is lower than the system scheme using foreign chips, which is conducive to terrestrial transmission of digital TV in China. Popularization. At present, the program has been tested in the laboratory and the vehicle environment, and the receiving performance under fixed and mobile conditions is stable and can be used for ordinary users, vehicle systems, and the like.
references:
1. Yu Zhaoming, Yu Zhibian, 'Digital TV Principles', People's Posts and Telecommunications Press, 2004.4
2. [US] Jean J. Labrosse, Shao Beibei, etc., 'Embedded Real-Time Operating System mC/OSII (2nd Edition)', Beijing University of Aeronautics and Astronautics Press, 2003.5
3. Beijing Haier Integrated Circuit Design Co., Ltd., 'Haier Hi2011e User Manual', 2005.7
4. DMB-TH Demodulator ASIC LGS-8813-A1 "Halo" Product Specifications, March 2005 Revision 5.0
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