1、 Establishing a new system of broadcasting, film and television technology based on digital and network technologies is the goal and task of technological innovation in the broadcasting, film and television industry during the 15th Five Year Plan period. With the development of technology, the main technologies of digital television have become increasingly mature, and major international and domestic technical standards have been successively promulgated. The related core equipment has also been finalized, and there are various system integration solutions. According to the "Tenth Five Year Plan for the Development of Radio, Film and Television Technology" and the 2010 Long Range Plan of the State Administration, we will take the lead in promoting the application of digital television in cable television networks. At the same time, in order to achieve the vision of the Nanjing Municipal Government's "Digital Nanjing" and solve the contradiction between the growing demand for film and television culture among the people (the need to watch both more and better TV programs) and the shortage of spectrum resources and low transmission quality brought about by the existing cable TV transmission methods, the research and promotion of cable digital TV are extremely important and urgent. Digital cable TV is a system engineering, and its key technologies include digital compression, channel coding and modulation, conditional access control (CA), user management system SMS, middleware technology, set-top box technology STB, etc. Their maturity varies, and when developing system integration solutions, it is necessary to consider the interdependence of the key technologies mentioned above and their practical applications and development, and follow the unified planning of the State Administration for digital television platforms, with a focus on phased implementation

2、 Overall design concept

Guiding ideology: Unified platform, interconnectivity, full consideration of technological development and potential operational risks, phased and layered approach, with technology maturity first, hardware followed by software, and broadcast style followed by interactivity.

Business objective: To support digital TV services and other interactive services for over 500000 users on the existing HFC cable TV network in Nanjing. Considering the long-term and smooth transition period between analog and digital, digital platforms must support all programs on existing analog platforms.

The design of the system hardware platform must follow the following principles:

Should comply with existing national standards, industry standards of the General Administration, or international standards

The system and equipment must be configurable and can be configured in multiple ways according to business needs

The equipment must meet high reliability and the system must have redundant backups

The system and equipment must have scalability and can be smoothly upgraded without interrupting business

Must have manageability and be able to achieve networked management

Compatible and interoperable, able to communicate with designated CA and SMS as needed

Based on the above design concept, the digital TV experimental platform recently mainly provides broadcasting style video services, specifically 56 sets of digital TV programs from the central, provincial, and local governments, EPG, video graphic and text information, and real-time stock market information services.

3、 Technical requirements for system equipment

The core equipment of digital cable TV front-end mainly includes compression coding equipment, stream multiplexing equipment, channel coding and QAM modulation equipment, scrambling equipment, digital satellite receiving equipment IRD and network management system, etc. According to the design philosophy, clear definitions and requirements must be made for the functions and performance of each unit.

1. Technical requirements for coding equipment

Video and audio coding must first comply with the national standard GB/T 17975-2000. For parts not explicitly defined in the national standard, ISO/IEC 13818-2 and ISO/IEC 13818-3 standards should be followed. The audio encoding must support MPEG Musicam encoding and can support Dolby AC3 mode. The interface must comply with DVB ASI specifications or support specifications such as DVB SPI, DVB SSI, DVB PDH, etc. It must also have an analog video and audio interface and must provide an SDI interface. The video encoding format must support MPEG-2 MP&ML 4:2:0, with an adjustable compression rate between 1.5Mbps and 15Mbps, and must support MPEG-2 P&ML 4:2:2; Support statistical reuse function and variable image groups (GOP); Capable of processing VBI data (WST, VPS, 21 rows of data, etc.); Can support the generation of test signals; Can support the insertion of non business data (such as logos, etc.); We can provide options suitable for digital audio input, encoder cascading, multiplexer control, and DVB CA.

2. Technical requirements for reuse equipment

Following the DVB standard for PSI/SI processing, all parameter requirements related to the transmission stream must be based on the DVB-C and MPEG-2 standards. If there is a conflict between the parameter requirements of DVB-C and MPEG-2, the DVB-C standard must be used as the standard. The data encoding follows the DVB-DATA standard. The interface of the multiplexing device must comply with the DVB ASI specification and can support DVB SPI and DVB SSI; the multiplexing device must support MPTS and SPTS inputs. Must support statistical multiplexing of transport flows and reuse of transport flows; Support various PSI/SI generation and replacement of PSI/SI; Support remapping of PID; Support the insertion of EPG; Provide support for comprehensive data encapsulation and reuse, with data transmission formats compliant with TCP/IP standards, and provide IP to DVB conversion devices or interfaces.

3. Technical requirements for modulation equipment

The channel coding and modulation part must comply with the industry standards of the State Administration of Radio, Film and Television of China (SARFT) for cable television channel coding and modulation. Parts that are not clearly defined by national standards or SARFT industry standards must comply with the ETS/EN 300 429 standard.

The interface requires that the input interface of the encoding and modulation equipment must comply with the DVB ASI specification and can support DVB SPI and DVB SSI; the output interface can be RF output or intermediate frequency output. The modulation equipment must support 16, 32, 64, 128, and 256QAM, and must support adjustable symbol rates of 0-6.96MS/s; Must support adjustable output gain.

4. Technical requirements for Conditional Access Systems (CAS)

CAS must comply with the Commercial Password Management Regulations (State Council Order No. 273, issued on October 7, 1999) and relevant national information security regulations. CAS must support a multi-level CA mechanism of "unified management and hierarchical authorization", and the design and implementation of CAS must fully consider the possibility of using a unified CA system nationwide or in a wide region; CAS must support DVB common scrambling algorithm and DVB common interface (CI); Support the insertion of ECM and EMM; The requirement to support CA systems is to implement Simulcrypt on the same platform, and it must be able to support two or more CA systems.

5. Requirements for Network Management Systems

The network management should include the following functions

Configuration management: adding, deleting, and changing network nodes, plugins, and ports, as well as displaying topology structures; During initialization installation, parameters should be configured and software and hardware should be checked; Configure various modules, including I/O cards, ports, and redundant configurations; Protocol and access restriction configurations supported on each module; Setting and modifying node management permission passwords.

Performance management: Record, analyze, and report on system resource utilization, and generate performance management log files. It should be possible to set the threshold values for resource monitoring and alarms; The network management system should provide graphical reports for historical performance data.

Security management: The network management system should have the function of configuring user identifiers, passwords, access control levels and ranges, identifying user permissions, preventing illegal access, recording all login and operation processes, providing security logs, timeout management, security logout and other security management functions.

Fault management: The network management system should be able to issue alarms when equipment fails or indicators deteriorate to a certain extent; The network management system should be able to use methods such as loop back testing to locate faults; The network management system should be able to display the fault location of the network in a graphical manner and provide audible and visual alarms; The network management system should save a certain stage of alarm history records for retrieval and detailed analysis of alarm reports, and can manually or automatically delete alarm reports.

4、 Equipment testing selection

The basic composition of digital TV front-end:

For the equipment selected by 7 or 8 companies, we mainly conducted the following tests

1. PQR testing for encoding and reuse: TEK PQA 200 image quality analyzer is used to send different MEPG-2 standard test sequences, and multiple selected samples are compared for testing to check the degree of damage to the sequence. At the same time, different compression and code rates are changed to select the best equipment. The test results indicate significant differences between the samples, which provides favorable evidence for our selection. PQA testing legend can be found in Figure 1

2. QAM modulation testing: TEK DDS200, R&S EFA, HP8594Q are used to test the main indicators that affect transmission, such as modulation error ratio MER, signal-to-noise ratio SNR, carrier to noise ratio CNR, bit error rate BER, noise margin, and equivalent noise degradation. At the same time, compare the differences under different QAM modulations (32, 64, 128, 256), and propose selection criteria and practical application guidance. The QAM modulation test results are shown in Figure 2

3. MPEG-2 stream analysis: This is a test of the stability, reliability, and availability of a DVB system. Using the TEK MTS200 stream analyzer, the main parameters of the first two categories of the three priority parameters specified in the DVB system testing are recorded, such as synchronization errors, PMT, PAT, PID errors, etc. It assesses the stability and availability of the integrated system of each individual equipment. This is a test for the hardware platform of the system integrator.

4. Comprehensive testing in the network environment: Select several points on the actual HFC network to test all the above items, test the usability of the sample system in the actual network transmission environment, and also examine the adaptability of our network to digital television systems, as well as whether there is mutual influence between analog and digital transmission systems. The test results are shown in Appendix 3.

Through testing and comparison, we have a clear understanding of the performance, differences, and system advantages and disadvantages of the products of the participating companies. Considering the comprehensive cost-effectiveness, system integrity, and technological development, we ultimately chose Barco's solution, as well as some products from domestic Huishiyuan Company.

5、 Design and Principle of Digital Cable TV Front End at Nanjing Television Station

1. Principle:

The system mainly consists of several parts, including source coding compression, stream multiplexing, channel coding, QAM modulation, network transmission and reception. Encoding and multiplexing analog images and accompanying audio are sampled, quantized, and encoded into the basic bit stream ES, which is then compressed into a TS stream for transmission.

Compression coding: mainly includes discrete cosine transform (DCT), differential coding, motion compensation, entropy coding, etc. The TS packet length is 188 bytes, with a header of 4 bytes and the remaining payload. Package header includes A) Package synchronization: for TS

The first byte of the packet is used for channel decoding synchronization; B) Error code indication and scrambling control: Indicates whether there are uncorrectable errors and whether the signal has been scrambled during transmission; C) Package identifier (PID): a recognition symbol for the decoder to recognize various program streams; D) Program Reference Clock (PCR): The timing reference for the decoder. E) Continuous/discontinuous counting: indicates the temporal order of the code stream. The content that TS stream should monitor includes: Is TS stream synchronization lost? Is the synchronization byte correct? Is the TS stream continuous? Are PAT, PMT, and PID correct? These are important criteria for the correct reception of programs.

MPEG-2 defines Program Specific Information (PSI), which is automatically set by the decoder. PSI consists of the following four tables: Program Related Table (PAT): Indicates how many programs are in the TS stream and the corresponding PID of the PMT table. Program Mapping Table (PMT): Indicates how many different bitstreams are present in a program and their respective PID's. Conditional Acceptance Table (CAT): Indicates whether the stream is encrypted, and the decoder uses the CAT table to search for encryption control information (ECM) and encryption management information (EMM). Network Information Table (NIT): Explain the network information provided for the program.

Reuse: It can be divided into general reuse and statistical reuse. Generally, multiplexing involves concatenating multiple input TS streams into a higher bit rate TS stream without changing the bit rate in each TS stream, so the output TS stream bit rate is the sum of the input program bit rates. Statistical multiplexing analyzes the input TS streams and dynamically allocates them as needed. The bit rates occupied by each TS stream in the output TS stream are dynamically changing.

Channel coding and QAM modulation: The purpose of channel coding is to ensure the accuracy of signal transmission and improve the anti-interference ability of the entire system. Channel coding has the following characteristics: A) it has a certain error correction ability, B) it has high coding efficiency and uses the smallest checksum; C) Having spectrum characteristics that match the transmission channel, resulting in low transmission energy loss; D) There is synchronization information and timing information. DVB-C adopts orthogonal amplitude balanced modulation (QAM), which has the following advantages: orthogonal modulation; Amplitude balance; Completely suppress the carrier wave; No non-linear distortion. The elements of a QAM modulator include frequency, level, bandwidth (symbol rate), number of QAM constellation points, whether to fill, spectrum inversion, etc.

2. Design Description:

① Digital source: The satellite receiver (TITAN IRD) receives digital TV programs from satellites, uses ASI interfaces to output ASI stream signals of SPTS/MPTS into the switching matrix, and then goes to the transmission stream multiplexer. The design uses 31 IRDs, two of which are used for redundant backup.

② Simulated signal source: The analog signal to be transmitted is compressed and encoded using an encoder (PYXIS), and a single program stream (SPTS) is output using an ASI interface. The design uses 23 encoders, two of which are used for redundant backup. The ASI signal output by the encoder enters the switching matrix and is then multiplexed by the transmission stream multiplexer.

③ Transmission Stream Multiplexer (PEGASUS): Multiplexes or re multiplexes the TS stream signals from the satellite receiver and encoder that have passed through the switching matrix, with an output rate of 38Mbps (using QAM64 modulation method), enters an independent scrambler for encryption, and then goes to the switching matrix. Use 10 multiplexers, one of which is used for redundant backup.

④ EPG broadcasting system, using a professional industrial computer, installing EPG broadcasting software, using specialized data memory to receive and store EPG files edited by the editing system. After encoding and packaging by the broadcasting software, the TS stream signal is broadcasted out of the card through code stream playback, with a bit rate of about 2Mbps and enters the transmission stream multiplexer.

⑤ The data broadcasting and broadcasting system uses a professional industrial computer, installs data broadcasting and broadcasting software, and uses a dedicated data storage device to receive and store data files edited by the editing system. After encoding and packaging by the broadcasting software, the data is streamed out of the card and TS stream signals are broadcasted. The code rate is about 30Mbps and enters the switching matrix before reaching the QAM modulator.

⑥ The set-top box software automatic update system uses a professional industrial computer to install automatic update playback software and packaging software for new versions. The output bit rate is adjustable and designed to be around 10M, which can be reused by a multiplexer.

⑦ The NVOD system uses one professional video server, installs video playback software, and uses a large capacity data storage to receive and store MPEG-2 format film and television files from the editing system. After the playback software completes encoding and packaging, it uses multiple concurrent data stream video playback cards to complete program stream multiplexing within the video server. It broadcasts three TS signals with multiple program streams, each with a bit rate of about 38Mbps, occupying the bandwidth of three standard analog channels. Enter the independent scrambler for encryption. Broadcast management is carried out by the NVOD management system.

⑧ Conditional Access System: Composed of Independent Scrambler (KRYPTON), Control System Server, and User Management System. The front-end system software consists of User Authorization System (SAS), EMM controller, EMM injector, ECM injector, etc. The EMM information and ECM information are injected into the independent scrambler to complete program encryption.

⑨ Automatic switching of system backup: To cope with sudden failures, redundant backup devices need to be automatically switched to working mode, using a two-level matrix switching and controlled by the network management system. To prevent matrix failure, jumpers are installed at the entrance and exit of the matrix.

⑩ ROSA Network Management System: Through various interfaces (RS 485, RS 232, and Ethernet), it realizes communication with all devices in the system and monitors the operation of each link. It can provide effective error management, system configuration management, network modeling, system state management, and security management. 1) Configuration management: ROSA facilitates the installation and configuration of devices, networks, and services. ROSA can facilitate the setting of specific hardware device parameters and the logical structure of the printing network. 2) Error management: Detailed visualization of error situations can facilitate quick diagnosis and maintenance. ROSA network maps can provide a visual representation of the status of networks and services. ROSA can directly determine the severity of an error condition and accurately identify the cause. If a backup device is configured, ROSA's automated tasks can provide immediate response to errors. 3) System State Management: The ROSA system provides powerful state management functions, which can generate various reports on the status of the network and its attachments to meet different needs. 4) Security management: ROSA has the ability to control and restrict user sensitive configuration settings, information reading, analysis, and reporting tools. The ROSA system supervisor can specify different access permissions for software system users. Access groups for "technicians", "operators", or "supervisors" can be generated. For each group, the supervisor can specify specific ROSA operations, and a specific user's access rights can be restricted to a specific location or device.

The system principle diagram is shown in the following figure

6、 Conclusion

The completion of the Nanjing Cable Digital Television Experimental Platform marks the beginning of the popularization of digital television in Nanjing. After several months of operation, it has had a great response in society and produced good social effects. Of course, there is still a long way to go. How to establish a front-end monitoring and management system that is different from simulation methods; How to conduct multiple businesses on digital platforms; Technically, CA, SMS, middleware, set-top boxes, etc. still need further research. The changes in transmission systems and program exchange methods will inevitably affect the changes in management systems. How to establish an operational system for digital television is a larger issue. We still need to continue working hard.

Excerpt:<<Broadcasting and Television Technology>>