Weak current College】 【broadband IP network video on demand technology】.

<br> Broadband IP network video on demand technology <br> Video on demand is the multimedia technology and network technology development and the rapid emergence of an integrated technology, the key incorporates digital technology, compression technology, computer technology and network technology. <br .> ① digital technology <br> In video on demand system, making nice program are placed in the program library, the program library is in fact the video storage, we know that memory can be used only to save values (more precisely, you can .only save the binary number), so the program saved to memory, the sound and images into digital form, this technique known as audio, video, digital technology. <br> ② compression technology <br> When it comes to compression techniques, you .must highlight the VOD necessity of data compression. Assuming that does not use compression, a standard personal computer 80 MB hard disk space can only be stored for about 8 minutes with CD quality stereo, or about 3.5 seconds of TV broadcast-quality motion video. .And compressed a CD ROM can store 72 minutes of high-fidelity music. In data transmission, stereo CD-quality sound without compression, ask the network to the constant bit rate transmission 1.5Mbps flow. For PAL video quality digital campaigns, when not compressed .when you need the bandwidth of 160Mbps. This high-bandwidth requirements for wide area network construction is not possible. <br> And data compression are also possible. This is because in the original information, often with lots of redundancy, such as television images .adjacent pixels and a frame around a large spatial and temporal dependencies. On the other hand, in the areas of application of multimedia systems, people are the main recipients of information, the human visual system for the brightness of the image on the edge-sensitive ., rapid change and color information is not sensitive. According to this characteristic, can achieve high compression ratio and the ability to make the recovery image signal is still satisfied with the quality of the subjective. Multimedia technology is the core of the media display technology and .multimedia data compression technology. Multimedia data compression technique of the main issues include: data compression ratio and compression / decompression speed and simple algorithm. To Compress / decompress data is compressed before the original data exactly the same as a standard, you can put the data .compression algorithm for lossless compression (reversible compression) and distortion in the compresses (non-reversible compression). Main video compression standards have H.261, JPEG and MPEG etc. MPEG standard is a high compression ratio, will still be able to guarantee high .quality picture compression algorithm that is most appropriate for video storage, VOD VOD and online transmission. MPEG is not just a technology that has become an industry standard, to promote the development of multimedia technology, for VOD applications system of production and development has an important .role. The main application in VOD will adopt MPEG compression standard MPEG-1 (1.5Mbps) and MPEG-2 (3Mbps) standards. MPEG-1 standard formation of image quality equivalent to VCD, video, effects, and the standard form of MPEG .-2 image quality is equivalent to broadcast-quality effects. <br> ③ computer technology <br> Computer technology including video server technology and multimedia PC (MPC). <br> VOD system requires the adoption of a storage and retrieval system for on .-demand television service, the device referred to as video server. Video server is on-demand TV system in the most important component is the most valuable equipment. Because of the moving image and high speed flow transmission requires large amounts of storage space, video .servers and other computer has a significantly different compared to the server. Video server most major characteristic is called the data flow of stationarity, video stream hope to exact frames per second transmission speed 29.97 too large, too small or delays will affect the video effects, .and unlike most computer network server that want the fastest possible data transfer. Video server is able to better meet this requirement, and has a good interactive, can * and scalability. To meet the needs of multimedia, multimedia computer shall meet the following conditions: .Pentium MMX 166 more CPU, 32M's memory (RAM), 500M bytes of disk storage (hard drive), a transmission speed of operation of 1.2M byte CD-ROM drive. A pixel is 640 × 480 and 16bit color graphic display, a 16 .-bit sampling frequency of A / D and D / A conversion of the sound card, speakers or headphones and a MIDI interface. <br> ④ network technology <br> Network technologies including high speed backbone network and access network. <br> 3 .. VOD system <br> VOD system NCBI by VOD video, VOD user terminals and broadband network is composed of three parts. <br> VOD video information from the video server and MPEG Encoder workstation components, video server to 100M fast Ethernet switch access through .IP backbone network, video server for storing video data while providing users with video on demand service. MPEG Encoder is used for video encoding, the encoder may video signal into various standard for compressed digital signal, such as MPEG-1 or MPEG-2 standard .and instantly via Ethernet will compress better video data files passed to the video server. <br> Broadband network include high-speed backbone network and user access network, video on demand user principal will adopts LAN access modes. 4. VOD implementation strategy <br .> The actual programme implementation for video on demand network services mainly TVOD and NVOD. <br> TVOD programme (pure video on demand) <br> Technically, VOD service is a "pull" mode network services business. "Pull" mode is on .the delivery of service content. By user request and need to get business providers. First user decides business providers all program source choose-when to watch; and secondly be retrieved as needed; and finally to selectedOptional. "Pull" mode of transmission of the .main features are: service provider in the downlink direction with fixed, predetermined format for transferring data to the user. In the "pull" mode, in theory, no two viewers at the same time to watch the same movie. Thus, a different stream .of data connection is different users are established, each user corresponds to a separate connections, each connection takes a certain amount of network bandwidth. NVOD (nvod) <br> "Nvod" is a video of the place of business, using this technology, .the service provider may from time to start playing the same movie, such as 15 minutes, but the same content not more join the channel line. Service providers must decide when to start playing the movie and play in the channel. Visitors can select the same .channel line all the way, in any of this actually is in a particular time period film broadcast. If users need to "rewind" feature, you can switch to the late than his current channel 15 minutes playback channels, you need to "fast forward ."function when you switch to the current channel as early as 15 minutes of the channel. Clearly, this approach does not provide the user with accurate" rewind "and" fast forward "function, but this technology's main purpose is to reduce user connections, .thereby reducing network bandwidth and cost. From a technical perspective, in line with "pushing" pattern definition: business provider data stream sent to the user, instead of the user data from the server "pulls". Compared with the VOD, in user- .generated hardware cost is the same, but the physical servers and network prices a lot cheaper. In order to guarantee the quality of services, VOD servers require a distributed design. According to the user on the future of the business needs of different user groups for .reasonable estimation and divided. In user group focus more on the area that connects to a server, any other users to connect to other servers. Server with high bandwidth connectivity, using replication technology, the Center produced programmes sent to a server by front-end .for sub front-end users. <br> Divide the front-end of VOD system structure, the increase in the number of users does not bring the system into paralysis, VOD may be part of the user on demand needs to share to the front .-end system, thus ensuring system and network can host a number of user application needs. <br>.