Advanced Materials Research Online: 2014-05-21 ISSN: 1662-8985, Vol. 933, pp 682-686 doi:10.4028/www.scientific.net/amr.933.682 2014 Trans Tech Publications, Switzerland Construction of Cable Digital TV Head-end Yang Zhang Beijing Information Technology College, Beijing, 100070, China Keywords: Digital TV, Head-end, CAS, SMS zhangyang@bitc.edu.cn Abstract. The basic structure of Cable Digital TV Head-end System is presented in this paper. Cable Digital TV Head-end connected between the TV Signal Source and the Transmission Line is a system which can convert a variety of signal sources into a transport stream to meet the exacting standards of the transmission. The transport streams will finally be sent to service terminal by the transmitting device and the existing network. This paper gives the method of constructing Cable Digital TV Head-end System and some useful suggestions about the selection of equipment. Introduction Digital Television is the third generation of TV technology after black-and-white TV and color TV[1]. It means that digital technology is applied completely in the process of television production which is comprised of recording, editing, production, broadcasting, transmission and reception. Digital television technology provides a larger screen, more clear image and better sound quality. Moreover, the combination of digital television technology and computer technology diverfied the information which can be efficiently delivered. The great advantage of Digital TV is interactive, providing information resources and customizing diversified services such as Video On Demand. The optimization of broadcasting resulted in a great change of the traditional management.but the efficiency and security of the transmission are indispensably for a Cable Digital TV System. The Basic Structure of Cable Digital TV Head-end Digital TV Head-end as shown in fig.1 consists mainly of Program Source Processing Module( Encoder, Satellite Receiver, Network Adapter), Program Stream Multiplexing Module( multiplexer), Transport Stream Modulation Module (QAM / QPSK /COFDM Modulator) and Transmission / Emission Module(Frequency Converter / Microwave Transmitter etc.)[2]. Fig.1 The Basic Structure of Cable Digital TV Head-end Efficiency is the key to realize lower cost and higher quality in the construction of Digital TV Head-end. A set of standard process is very important to achieve high efficiency. It is essential to value the whole system correctly and objectively by means of appropriate instruments and method for engineers and technicians to know exactly about each link in the system. Program Source. The Program Source is some equipment providing digital program. Encoder. Signals coding of Analog Audio and Video, Serial Digital Interface and Digital Audio (Audio Engineering Society / European Broadcasting Union) based on the MPEG-2 standard is the main function of MPEG-2 coder. Firstly, the luminance and chrominance signal of analog pixels All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-08/04/16,20:08:49)
Advanced Materials Research Vol. 933 683 would be sampled. Secondly, the discrete signals in time domain should be processed to amplitude discretization on the basis of a quantized interval. Finally, these signals would be coded according to certain rules to be digital signals which will be transmitted after compression. Video compression algorithm by MPEG-2 standard which provides both progressive scanning and interlaced scanning is suitable for compression encoding of high definition and standard definition. Its internal structure is as shown in fig. 2. Fig.2 Encoder Satellite Receiver. The function of the Satellite Receiver especially used in a project is transforming the RF signal from antenna into streamsignal. It provides bit stream output interface as well as traditional analog output interface so that the output bit stream could be input directly to the multiplexer. It is determined by its function that the settings of Satellite Receiver mainly focus on those parameters related to antenna, satellite and transmitter. Some Satellite Receiver equipments support unified management of all the head end equipments by Ethernet management software so that it can be achieved such as Parameter Setting, Monitoring and Remote Management on-line. Adapter. The fuction of Adapter is adapting TS stream signal for the SDH/PDH network to be transmitted. Program Stream Multiplexing. Multiplexing is a transformation which turn different program transport stream into a complex stream with a plurality of programs. The keys of Multiplexing are the readjustment of the system layer in the PCR domain and the collection and reconstruction of the PSI/SI information, as well as modifying the transport stream in the multiplexer according to the new PSI/SI information. Hense a number of program source inputs are multiplexed into a output stream in the complexer according to the demand. The PSI/SI information is rebuilt according to the program complexed, PID mapping and PCR correction and many other operations are done at the same time. Its principle is as shown in fig.3. Fig.3 Program Stream Multiplexing Program Stream scrambling. The program stream should be scrambled for the interests of operators, so the TV programs could only be watched by means of the set-top box in the terminal which is CAS (Conditional Access System). The scrambling is realized by a independent Scrambler which generate
684 Manufacture Engineering, Quality and Production System III a pseudo random Control Word at regular time, and the CW is taken as scrambling factor which scramble the stream synchronous. The principle is as shown in fig.4. Fig.4 Program Stream scrambling Modulation and Radiating of Program Stream. The fuctions of QAM (Quadrature Amplitude Modulation) Digital Modulator includes ASI/SPI/DS3 Stream Adaptation, DVB-C (Digital Video Broadcasting-Cable) Channel Coding, QAM Modulation and Frequency Conversion[3]. Stream Adaptation is a process based on MPEG-II package structure to signals with different electrical characteristics from sundry input interfaces. Channel Coding is designed to detect and correct the errors so that the reliability of the digital TV signal transmission could be ensured to a certain extent. Then the Medium-frequency signal with high anti-interference ability which is adaptive to be transmitted in analog CATVnetwork could be achived by QAM. The Construction of Cable Digital TV Head-end The construction of Cable Digital TV Head-end is the premise steps of building a digital TV system which is huge and complex system.the fast updating of technology leads to fierce competition in head-end equipment manufacturers at home and abroad. Therefore the construction meeting the future expansion should take into account the development of technology and market to avoid the resource waste. It is necessary to consider a large number of details for building a efficiency Head-end System in the construction of Digital TV System which is a gradual process. Choice of the Program Sources. The construction of the promgram souces contains the following contents. Digitization of TV programs from the province. The programs transport stream can be received from project satellite receiver. Payment programs overseas. These encrypted program could only be got by special satellite receiver (kcalmachine) before coding the A/V signal. Digitization of local analog program. These programs from Television and Cable TV are compressed and encoded to Transport Stream by Encoder. Digital program stream directly from the SDH transmission network. The Transport Stream from the Digital Broadcasting Server. Digital program sources from professional provider. Choosing Suitable CAS for the Local Business Development. CAS (Conditional Access System) changed greatly the operation mode of Digital TV business. It provids Digital TV more advantages than traditional analog television mainly because of controlling every terminal user by the scrambling and encryption to the program streams[4]. It should be considered that not only the current user situation but also the future development demand when choosing CAS in the preliminary stage. So the common scrambling algorithm is
Advanced Materials Research Vol. 933 685 widely used for the advantage that more types of user set-top boxs can be chosen for the network operators facing so many CAS manufacturers, accordingly there will not be bottleneck problem of user development caused by CAS binding. There are two conditional accesses to the pay program packet. That means the local service platform tansmit the central encrypted program directly to the user without decryption. The local network user authorization information is managed by both the central and the local sevice platform. Neither of them has no right to authorize alone the subscriber watch the programs. Establishing SMS Suitable for the Local Situation. The SMS is Subscriber Management System which have functions include system management, general information management, equipment management, classification management, billing management, product management, customer management, subscriber management, statistical inquiry, user login etc.. The structure and script of the system are designed by layer. The corresponding adaptation layer for CA system and database are defined in order to guarantee the fast interconnection with various Conditional Access Systems and Database Systems.It is very necessary and feasible to establish a SMS suitable for the local situation for the subscribers vary from place to place and SMS is very adjustable. Great benefits would be brought to network operators by an efficient SMS. The Multi Service. The amazing advantages of digital TV come from not only digital programs but also the competition of data broadcast, NVOD, and interactive business in the current network status. The head-end of Data Broadcasting System is composed of Management Server, Broadcast Control Server, Content Server and Interaction Server. The most important unit of the system is MPE (Multi-Protocol Encapsulation) with which the server comprise LAN. The output TS from Data Encapsulation is sent to QAM for modulation and frequency conversion and then is processed by Data Broadcast Receiving Card and the Cable TV. There should be access WAN routers in the interactive server head-end system and Modems access to local WAN at the subscriber end for interactive systems. NVOD (Near Video On Demand), also called the nearest type video on demand, means that multiple video streams start at a certain interval in turn sending the same content. For example, twelve video streams start successively every ten minutes sending the same two hour TV show. You may wait no more than ten minutes if you want to watch the program for that they will choose the closest point as a start time. In this way, a video stream could be shared by many subcribers. Selection of a Suitable Modulation Mode for Local Network Condition. What kind of transmission network system the exports will access to is very import for the digitalization of Cable TV head-end. The modulation is determined by the transmission network to get more efficiency. The modulated modes can be selected at present include QAM (Quadrature Amplitude Modulation), QPSK (Quadrature Phase Shift Keying modulation) and COFDM (coded orthogonal frequency division multiplexing) based on DVB-T(Digital Video Broadcasting- terrestrial ) system. Generally the QAM is suitable for HFC (Hybrid Fibre Coax) network transmission and digital MMDS (Microwave Multipoint Distribution System) network. QPSK modulation method was used more in the Backbone of Microwave Network. DVB-T system mainly is applyed to the Digital Terrestrial Mobile TV. Selection of the Equipment The following focal points are very important in the selection of the head-end equipment. Performance parameter should meet the requirements. The equipment should be stable and compatible to find alternative easily. Network Management Protocol should be open in order to the realization of the unified management. The equipment should be able to upgraded according to the development of new business. There should be warm backup of hardware equipment for the security and stability of the broadcast system[5].
686 Manufacture Engineering, Quality and Production System III Summary All the paper described is a summary of how to construct a digital TV platform which is stability and excellent cost performance. The engineers should have foresight to keep pace with the times. They also should analyze in the round and examine carefully in every stage of the construction. References [1] Houjian Wang, Jian Xia: The Gehua cable digital TV head end system, Radio & Television Information, 2004(1). [2] Huaijiang Wen: The construction of Tianjin Television digital TV, Cable Television Technology, 2003(23). [3] Yuhang Hao: The comparison and analysis of the system digital TV broadcasting system, HeiLongJiang Science and Technology Information, 2013(07). [4] Yong Xu, Jian Wu: The construction of Cable Digital TV Head-end in Small and medium sized city, China Digital Cable TV, 2004(5). [5] Feng Cui: Equipment selection of building digital TV platform, Cable Television Technology, 2005(12)
Manufacture Engineering, Quality and Production System III 10.4028/www.scientific.net/AMR.933 Construction of Cable Digital TV Head-End 10.4028/www.scientific.net/AMR.933.682