Introduction to STB:
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The set-top box is an interface device that provides analog audio and video interfaces to enable the current analog TV to receive programs. It also provides digital TV, digital broadcast interfaces, and interactive functions. It connects to the coaxial cable of the cable network and connects to the TV set of the user's home. Designed for users who do not plan to buy a computer or use a computer. SA (Scientific-Atlanta) designed the first Digital Set-Top Box for Time Warner Cable's Full Service Network (FSN) in Orlando in 1994, using SGI's MIPS chip and software technology for digital set-top boxes. Technology is brought into the field of home appliances. In addition to providing digital TV and digital broadcast interfaces, set-top boxes provide interactive functions, including telephone interfaces, computer data interfaces, and even camera and game console interfaces. It can receive and store certain commands from the user, and has a central processing unit that controls the operation of the various parts of the on-board converter. It can perform heavy numerical calculations and decompress and output digital video/audio signals. A set-top box is actually a versatile computer. Here, the hardware structure of the digital set-top box is introduced, which allows everyone to have an overall understanding and understanding of the hardware structure of the digital set-top box.
First, the digital set-top box contains several important modules:
a. Modulation and demodulation module
The digital set-top box works in the cable TV network state, and the cable TV network uses analog transmission. Therefore, the digital signal must be modulated and demodulated to transmit on the analog channel. The modem is a key component of the system and is technically similar to the current telephone modem. The principle, but adopts a higher modulation method, 64QAM or 256QAM is used in the downlink, 64QAM is adopted as the standard modulation method in DVB-C (Digital Video Broadcast by Cable) and DAVIC, and the MC92305QAM demodulation chip of Motorola is taken as an example. The digital signal rate using 64QAM modulation on the 7M analog bandwidth can reach 42Mbit/S. The uplink currently uses two methods. One is to use the telephone line as the uplink channel, and the other is to use the uplink channel of the bidirectional HFC network, using HFC. The network uses QPSK as a modulation scheme. The QAM or QPSK modulator modulates the MPEG format data stream into a standard PAL channel and combines it with other video modulated signals.
b. Codec module
Due to the use of analog channels, forward error correction coding is essential to ensure data transmission reliability and low bit error rate. DVB uses Reed Solomon coding, and RS code is a type of multi-ary BCH with strong error correction capability. code.
c. MPEG II decoding module
The core of the digital set-top box is digital video technology. The decoding module of MPEGII can be called the core module other than CPU. The interleaving coding is used in MPEG II digital transmission. Firstly, the code stream is deinterleaved, and the video, audio and data streams are separated. , and the decoding of video code. Through the above steps, the MPEGII stream becomes a video (CCIR656 format) and an audio digital signal. The MPEG multiplexer combines the program streams and data streams together, and transmits them to the RF modulator in an MPEG2 data format of 188 bytes for one frame and provides an Electronic Program Guide (EPG).
d. Digital Video Encoder and Audio DAC
The "peripherals" of digital set-top boxes are televisions and audio systems. Digital audio and video signals must be converted to analog audio and video signals. Take MC55024 from Motorola as an example. MC44724 can convert ITU 601 and 656 standard 4:2:2 parallel video data. Converted to PAL or NTSC format video, S-Video, Y/Cb/Cr or R/G/B, the extended VBI (Vertical Blanking Interval) information input port is used to display graphic information. Audio DACs are very common and are not covered here. Modern audio systems support Dolby Pro Logic and Lucasfilm Home THX home theater systems. Digital audio signal processing chips are required to process digital audio signals before audio DACs. Currently, there are dedicated processing chips and DSPs processed by digital signal processing chips. the way.
e. Graphic Control Module
And the video graphics mixer digital set-top box on the other hand need to provide computer-like complex graphics or even three-dimensional graphics, the system needs to have powerful 2D and 3D graphics acceleration chips, and the graphics and video signals need to be mixed and superimposed display, such The function is similar to the existing captioning machine.
f. Media Access Control Module MAC
The digital set-top box can be regarded as a Cable Modem in network communication. The IP router receives the on-demand signal transmitted by the UE through the PSTN (Public Telephone Network) and transmits it to the Internet server/router. Currently mainly referring to the DAVIC and IEEE 802.14 standards, IEEE 802.14 advocates the use of ATM and defines the ATM layer on the 802.14 MAC access layer. DAVIC also defines the Ethernet and ATM standards.
g. Rewritable DVD
The existing digital set-top box works completely in the network environment, and can only use smart cards to store information such as personal accounts and decoding numbers. It does not have the local big data storage function. All information needs to be obtained from the Internet, regardless of the current network status or the future. CD-ROM such media is still a major way of transferring large amounts of information. The disadvantage of CD-ROM is that information cannot be updated. The use of rewritable DVD can not only use the huge information on the CD, but also effectively update the data through the Internet to keep the data on the DVD disk up to date. The DVD should become an important backup module for the digital set-top box.
h. Add descrambling module and copyright protection module
In cable TV operation, pay TV is a major service, requiring digital set-top boxes to have the descrambling function of TV signals. Due to the use of digital signals, the addition of descrambling is easier than the analog signal plus de-interference and high density. DVB pair The addition of the descrambling function has a definition. On the other hand, the use of digital signals has increased the difficulty in copyright protection. Currently, the Macrovision generator is used to protect moving images.
Second, the working principle of the set-top box:
Each module of the set-top box works in cooperation in the high-speed access of the Internet. The user first requests a URL by sending a command. During a transaction using the PPP peer-to-peer protocol, the command eventually reaches the modem shared rack of the Internet service provider. Then through dynamic allocation, the user gets the IP address used in the transaction and sends the request to the Internet. When the content of the Internet is found, it is then sent to the Internet Service Provider (ISP), which is then sent by the ISP's router to the cable TV network and finally back to the user's set-top box. In the set-top box of cable TV, the information content was intercepted. The set-top box completes the task of a gateway between the television and the cable network. Its task is to manage the communication traffic of IP, and has the ability to control the user's access to the network. Once the IP packet arrives at the set-top box, the video signal is separated from the packet, the data is decoded, and then sent to the browser for preparation. Displayed on the TV.
Because Cable Modem requires users to configure a computer to access the Internet, which affects the expansion of the user layer, the use of set-top boxes does not require a computer, and a TV set is sufficient, so the market prospect of set-top boxes may be promising. Information users transition from enterprise to home, network bandwidth transitions from narrowband to broadband, and user network access equipment transitions from PC to TV with set-top box (STB). The interface is used to transition from Windows to TV remote control interface, and information content is from enterprise service. The transition to service for people's lives is the general direction of the development of network services. The set-top box (STB) is obviously a stage in this general direction.
Digital TV set-top boxes receive digital TV programs, process data services and perform analysis of multiple applications. The various types of sources are encoded in two stages before entering the cable television network. The first stage is the source code of the video and audio signals, and all the sources are encapsulated into a transport stream, and the second stage is the channel code for transmission. Corresponding to the front end, the digital TV set-top box first extracts the channel coded signal from the transmission layer, completes the channel demodulation, and then restores the compressed source coded signal, restores the original video and audio stream, and simultaneously completes the reception and analysis of the data service and various applications.
The working process of the digital TV set-top box: the digital TV set-top box selects the channel through the network interface module, performs demodulation and channel decoding processing, outputs MPEG-2 multi-program transport stream data, and sends it to the demultiplexer, the demultiplexer from MPEG -2 transported streamed video and audio elementary stream (PES) data of a program, including video PES, audio PES and auxiliary data PES, the demultiplexer includes a descrambling engine, which can be in the transport stream layer and The PES layer descrambles the scrambled data, and the demultiplexer outputs the descrambled video and audio PES. The video PES is sent to the video decoder, and the MPEG-2 video data is taken out and decoded, and then output to an analog encoder, encoded into an analog video signal, and then output through a video output circuit. The audio PES is sent to the audio decoder, the MPEG-2 audio data is taken out and decoded, and the PCM audio data is output to the audio D/A converter, and the audio D/A converter outputs an analog stereo audio signal, which is output through the audio output circuit. Its structure is shown in Figure 1.
Digital TV set-top boxes include both hardware and software. The hardware provides a hardware platform for the digital TV set-top box to realize the decoding of audio and video. In digital TV technology, software technology occupies a more important position than hardware, because the reproduction of TV program content, the realization of the operation interface, the realization of data broadcasting service, and the interconnection of set-top box and Internet all need software to realize.
1, digital TV set-top box hardware components
(1) Network Interface Module (NIM): The network interface module performs channel demodulation and channel decoding functions to deliver a transport stream (TS) containing video and audio and other data information.
(2) Source data transmission stream demultiplexer: The transmission stream generally includes a plurality of audio and video streams and some data information, and the transport stream demultiplexer is used to distinguish different programs and extract corresponding audio and video streams and data streams. Send to the AV decoder and the corresponding parsing software.
(3) Conditional receiving module: For pay TV, the conditional receiving module also performs descrambling on the audio and video stream, and uses a smart card containing the identification user and the memory function to ensure that the legitimate user can watch normally.
(4) Video and audio decoder and post-processing: MPEG-2 decoder completes decompression of audio and video signals, restores analog audio and video signals through video encoder and audio D/A conversion, and displays high quality on analog TV Images and multi-channel stereo programs are available.
(5) Embedded CPU and memory module and interface circuit: The embedded CPU is the heart of the digital TV set-top box. It is used with the memory module to store and run the software system and control each hardware module. The interface circuit provides a wealth of external interfaces, including universal serial interface USB, Ethernet interface and RS232, analog, digital AV interface, data interface and so on.
2, digital TV set-top box software system
In a set-top box, the software system is an important part. The work of the main controller is done by the execution of the software.
The basic structure of the set-top box software is shown in Figure 2. The operating system generally uses a real-time operating system. In this operating system, it mainly performs process scheduling, interrupt management, memory allocation, inter-process communication, exception handling, and clock extraction. The hardware driver part provides drivers for peripheral hardware devices, including I2C bus, asynchronous serial communication port, parallel communication port, nonvolatile memory, keyboard, remote controller, tuner, channel decoding module, and the like. The graphical interface is mainly used to complete the graphic display function, in order to provide users with a friendly graphical user interface. Audio decoding and video decoding drivers are used to control the operation of audio decoding and video decoding hardware. The demultiplexing and data table extraction module is mainly for controlling the code stream demultiplexing and data table extraction operations. The application programming interface maps all hardware-related low-level functions to a unified interface and provides some hardware-independent public processing functions such as network protocols, graphics format analysis, and business information table analysis. The conditional reception driver is used to complete the work and software interface of the conditional reception process. The application programming interface provides a common programming interface for the application, shielding the application from the hardware, making the application hardware-independent. This makes it easy to implement application portability.
(1) Middleware
The middleware is the software platform of the digital TV receiving system, providing an operating environment and software interface for digital TV applications. As a separate software layer in the digital set-top box, the middleware isolates the application software from the underlying hardware and operating system, defines a unified interface for the operating system and drivers, and defines a uniform interface for the application, as well as regular numbers. Broadcast TV services and value-added services also provide a unified interface. Middleware defines a relatively complete and standard set of application interfaces that make the application independent of the operating system and hardware platform, making application development easier and more open and portable. It usually consists of a Java virtual machine, a web browser, an image and multimedia module, etc. The middleware separates the application software from the hardware-dependent driver layer software, so that the application software does not depend on a specific hardware platform.
(2) SI/EPG
Service Information (SI) includes: Program Service Group Association Table (BAT), Program Service Description Table (SDT), Program Segment Information Table (EIT), Operation Status Table (RST), Time and Date Table (TDT), Time Offset Table (TOT) and so on. Through these data tables, the set top box can provide the user with information such as the content description of the program, the type of the program, the playing time of the program, the rating of the program, and the like. The set-top box extracts these data tables in the demultiplexing process, analyzes and processes them, and then provides them to the user through the user interface to form an electronic program guide (EPG). The user browses the information and makes a selection, and informs the set-top box through the input device such as a remote controller, a keyboard or a button to extract a corresponding program stream or data stream according to the user's selection.
(3) Program business search
In the analog TV system, the search of the TV channel only needs to be performed one by one according to the frequency. In the digital television system, the channel and the program service are no longer in a one-to-one correspondence, and the parameters of the channel transmission system are also configured in multiple ways. Therefore, the search for the program service by the set top box is designed corresponding to the front-end broadcast system. Generally, a fixed channel is designated by the operator as a publishing channel for channel configuration information. The channel parameters are pre-built in the set top box.
(4) Overprinting subtitles
Since the digital television system itself has multi-service functions, the overprint display is also a basic function of the set-top box, so the overprinting subtitle function can be better implemented in the digital television set-top box. Overlays can be turned on and off depending on the user's selection.
Because the software in the set-top box is very complicated, there are some errors. In addition, as the business continues to change, the functionality of the software needs to be continuously improved. Therefore, software updates become an important requirement. Software update implementations include online broadcast download updates and stand-alone local updates. The stand-alone local update programs the program into the FLASH memory through the dedicated data port on the set-top box, or through the replacement program RON. The online broadcast download update is to insert the update software into the code stream through a specific protocol at the front end of the system and transmit it to the set-top box.
Third, the function of digital TV set-top box
The digital television set-top box is capable of receiving MPEG-2 digital television transmission streams and various data information, and viewing digital television programs and various data information on an analog color television set by demodulation, demultiplexing, decoding, and video and audio encoding. At present, the basic function of the digital TV set-top box is to receive digital TV broadcast programs, while having all the functions of broadcast and interactive multimedia applications, including:
(1) Electronic Program Guide: It provides users with an easy to use, very user-friendly interface, and quick access to the way they want to watch the program. Users can see the TV programs that will be played on each channel in the near future.
(2) Support interactive applications such as quasi-video on demand, video on demand, interactive games, etc.
(3) High-speed data broadcasting: It can provide users with various information such as stock market quotes, ticket information, electronic newspapers, and popular networks.
(4) Internet access and e-mail: The digital TV set-top box can realize Internet access through a built-in cable modem. Users can access the Internet via the built-in browser of the set-top box, send e-mails, or provide various interfaces to connect to the PC and use the PC to access the Internet.
(5) Software online upgrade: can be regarded as one of the applications of data broadcasting. The data broadcast server transmits the upgrade software to the set top box, and the set top box can recognize the version number of the software, receive the software when the version is different, and update the software stored in the memory.
(6) Conditional reception: The core of conditional reception is scrambling and encryption, and the digital TV set-top box should have descrambling and decryption functions.
With the development of digital TV and network technology, the function of digital TV set-top box will be more perfect, especially the development of single-chip PC technology, which will make digital TV set-top box highly integrate various parts of hardware in physical structure to form STB core chip. Reduce volume, reduce costs and improve performance. The external interface will be more abundant, and can be connected to the digital camera through the USB interface, and the hard disk can be connected to the program storage through the IDE interface. The interactive set-top box will become the mainstream of digital TV set-top boxes, and users can not only watch digital TV on the analog color TV, but also realize entertainment and Internet access.
Note: Standards and specifications related to set-top boxes
Some technical standards and specifications related to digital TV set-top boxes include:
GB/T 17975.1-2000 Information technology - General coding for motion pictures and associated audio information - Part 1
â— GB/T 17975.2-2000 Information technology - General coding for motion pictures and associated audio information - Part 1: Video
â— GB/T 17975.3-2000 Information technology - General coding for motion pictures and associated audio information - Part 1: Audio
â— GB/T 17971.3-1997 information technology - Digital storage media moving image with 1.5 Mbit / s data rate and its associated audio encoding Part 3: Audio
â— GB 2312-80 basic set of Chinese character coded character set for information exchange
â— GB 13000.1-1993 Information technology common multi-bit coded character set (UCS) Part 1: Architecture and basic multi-language plane
â— GB/T 170-2001 Cable Digital Television Broadcast Channel Coding and Modulation Specification
â— GB/Z 174-2001 Digital TV broadcasting service information specification
â— GB/Z 175-2001 Digital TV Broadcast Conditional Access System Specification
◠GB/T 155-2000 video parameter values ​​for HDTV production and exchange
â— Cable Digital TV EPG Application Guide
â— Cable digital TV broadcasting service information application guide
â— Cable Digital TV CA System Application Guide
â— Universal encoding of moving images and their accompanying sound signals
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