Digital set-top box technology details

The cable radio and television network is developing in the direction of digitization, networking and industrialization, and eventually built into a comprehensive broadband information network. Relying on the cable broadcast television network to provide the key equipment for integrated information services. This is the user terminal equipment digital set-top box (STB). After introducing the definition, function and classification of digital set-top boxes, this article focuses on the key technologies of digital set-top boxes.

1. Definition of set-top box

For set-top boxes, there is currently no standard definition, the traditional saying is: "a box placed on top of the TV". It uses a cable TV network as a transmission platform and a TV as a user terminal to improve the performance or increase the functions of existing TVs. Due to different functions and uses, this makes the concept of "set-top box" somewhat ambiguous, such as early supplementary channel set-top boxes, teletext set-top boxes, pay-TV set-top boxes, etc.

Digital TV set-top box is one of the information appliances. It is a consumer electronic product that enables users to watch digital TV programs on the existing analog TV and carry out interactive digital entertainment, education and commercialization activities.

2. Function of digital set-top box

The basic function of the digital set-top box is to receive digital TV broadcast programs, and has all the functions of broadcasting and interactive multimedia applications, such as:

(1) Electronic Program Guide (EPG). Provide users with an easy-to-use, friendly interface, and a way to quickly access the programs they want to watch. Users can use this function to see one or more channels or even all TV channels that will be broadcast in the near future;

(2) High-speed data broadcasting. Can provide users with various news such as stock market prices, ticket information, electronic newspapers, popular websites, etc .;

(3) Online software upgrade. Software online upgrade can be regarded as one of the applications of data broadcasting. The data broadcasting server broadcasts the upgraded software according to the DVB data broadcasting standard. The set-top box can recognize the version number of the software, receive the software when the versions are different, and update the software stored in the memory;

(4) Internet access and e-mail. The digital set-top box can easily realize the Internet access function through the built-in cable modem. Users can surf the Internet and send e-mail through the built-in browser of the set-top box. At the same time, the set-top box can also provide various interfaces to connect with the PC, and use the PC to connect to the Internet;

(5) Conditional access. The core of conditional access is scrambling and encryption, and digital set-top boxes should have descrambling and decryption functions. In short, so far, various value-added services have been developed around the three core functions of digital video, digital information and interactive applications of digital set-top boxes. See the table below for details:

3. Classification of digital set-top boxes

At present, the set-top boxes that have appeared on the market can be basically divided into three categories: digital TV set-top boxes, WebTV (WebTV) set-top boxes and multimedia (MulTImedia) set-top boxes.

Among them, the main function of the digital TV set-top box is to convert the received digital TV signal into an analog TV signal, so that users can watch digital TV programs without replacing the TV. The image quality is close to 500 lines, but there is no Internet function. According to different transmission media, digital TV set-top boxes are divided into digital satellite set-top boxes (DVB-S), terrestrial digital TV set-top boxes (DVB-T) and cable TV digital set-top boxes (DVB-C). Digital satellite set-top boxes are currently widely used.

The network TV set-top box was produced under the catalysis of Microsoft's "Venus Plan" and Case's "Nuwa Plan". Its main function is to enable China's existing 320 million analog TV users to pass the PSTN (Public Telephone Switching Network ) And two-way CATV network to achieve Internet, send and receive e-mail, game entertainment, online learning, etc.

Multimedia set-top boxes are produced by synthesizing the functions of the first two set-top boxes, and are also called integrated service set-top boxes or full-function digital set-top boxes. It can support almost all broadcast and interactive multimedia applications, including watching ordinary TV programs, digitally encrypted TV programs, on-demand multimedia programs and information, electronic program guide (EPG), sending and receiving e-mail, Internet browsing, online shopping, distance education, etc. , The required condition is a two-way CATV network.

4. The key technology of cable TV digital set-top box

The technical content of the cable TV set-top box is very high, it reflects the development level of multimedia, computer, digital compression coding, encryption and descrambling algorithm, encryption and decryption algorithm, communication technology and network technology. The key technologies of digital set-top boxes are as follows:

(1) Real-time operating system (RTOS)

The real-time operating system is responsible for the management of local resources and network resources, and provides basic operation functions and device access control.

When the set-top box is started, the boot program is downloaded from the central control system through the network. The RTOS of Shenzhen Dike Netcom digital set-top box is designed to boot from FLASH ROM. Its boot program functions include: a system self-test. Test some hardware of the system, such as DRAM, etc .; b system settings. Users can set the necessary parameters of the system through the remote control, such as basic frequency, auxiliary frequency, symbol rate, DTV program information table, etc .; cDTV function. Users can watch digital TV programs without on-demand service; d system upgrade. By identifying the name and version number of the system or station logo, download and upgrade the system or station logo data in the FLASH ROM.

(2) Middleware (Middle Ware)

Middleware is a software environment that isolates applications from the underlying operating system and hardware details. It is usually composed of various virtual machines. Such as HTML virtual machine, JavaScript virtual machine, Java virtual machine, MHEG-5 virtual machine, etc. A complete digital set-top box consists of a hardware platform and a software system, which can be divided into 4 layers, from bottom to top: hardware, bottom software, middleware, and application software. The hardware provides the hardware platform of the set-top box; the underlying software provides the operating system and various hardware drivers; the application software includes locally stored applications and downloadable applications; the middleware separates the application software from the underlying hardware-dependent software, so that The application does not depend on the specific hardware platform.

(3) Adding descrambling technology

The scrambling and descrambling technology is used to encrypt and decrypt digital programs. The basic principle is to use scrambled control word encryption transmission method, and the user end uses IC card to decrypt. In the MPEG transport stream, there are two data streams related to the control word transmission: authorization control information (ECMs) and authorization management information (EMMs). The control word encrypted by the service key (SK) is transmitted in the ECMs. It also includes program information such as program source, time, content classification, and program price. The service key that encrypts the control word is transmitted in the authorization management, and the service key must be encrypted by the user's personal distribution key (PDK) before transmission. The EMMs also include the address and user authorization information, such as what the user can see The program or time period, the viewing fee paid by the user, etc. The user's personal distribution key (PDK) is stored in the user's smart card (Smart Card). On the user side, the set-top box obtains the PID values ​​of EMM and ECM according to the CA-descriptor in the PMT and CAT tables, and then filters out the ECMs and EMMs from the TS stream and sends them to the Smart Card through the Smard Card interface. The Smart Card first reads the user's personal distribution key (PDK), decrypts the EMM with the PDK, and takes out the SK, then uses the SK to decrypt the ECM, takes out the CW, and sends the CW to the descrambling engine through the Smart Card interface. CW can descramble the disturbed transport stream. The scrambling and descrambling technologies are divided into the same encryption and multiple encryption technologies. The same secret technology is to apply two or more conditional access (CA) systems to the same network platform. From the perspective of a cable TV station, it is an environment for technology selection and competition. Multi-density technology requires the set-top box to use CI technology, so that the same set-top box can receive encrypted programs from different CA systems. From the user's perspective, it will not be restricted by the purchase of a set-top box that is a CA, and users have the possibility to choose their CA services.

(4) Cable modem

So far, various research institutions have conducted extensive research on cable modems, and these research results have finally formed standards defined by several international organizations, IEEE802.14 protocol defined by IEEE, DAVIC protocol defined by DAVIC, and DOCSIS defined by MCNS. And EuroDOCSIS agreement.

Zhou Yi, deputy director of the Science and Technology Department of the State Administration of Radio, Film and Television, pointed out at the 2000 (8th) International Cable TV Technology Symposium Chinese and Foreign Experts Cable TV Forum: For the CA system, China has determined to use the same secret technology and hopes that cable TV can be applied as soon as possible. There is a CA system with independent intellectual property rights in China. For the formulation of the standard of China's cable modem (CM), we tend to EuroDOCSIS, and add our own things on this basis. In the EuroDOCSOS standard, the downstream symbol rate is 6.9652mb / s, and the upstream frequency range is 5-65MHz.

5. Conclusion

The digital set-top box is not only a user terminal, but also a network terminal, it can make analog TV from passive receiving analog TV to interactive digital TV (such as video on demand), and can access the Internet, so that users can enjoy TV, data, language Location-based information services. At present, some people think that the set-top box is used to make the TV online, which is a misunderstanding. In fact, the digital set-top box is a member of the broadband integrated information network before the popularization of digital TV sets. With the development of digital technology, multimedia technology and network technology, the built-in digital set-top box and the overall cost will be reduced, allowing most users to realize multiple services such as entertainment and Internet access on ordinary analog TV sets.