VOD (Video on Demand) is known as an interactive TV on demand system, meaning that the user needs to play the corresponding video programmes, fundamentally changing the user’s past passive TV watching deficiencies. When you turn on your TV, you can watch the content you wish to watch directly on demand at any time without watching commercials or rushing for a particular programme, just like playing a new film that you have just put into your VCR or VCD player at home, but you don’t need to buy a video tape or VCD disk, nor do you need a VCR or VCD player. This is the dream that information technology can bring to you, delivering video programmes to millions of homes at your own pace via multimedia networks.
Looking at the applications of broadband networks, VOD is the closest to people’s lives, but it is also the most technically difficult. Take the words of China Telecom, that is, the highway has to have a car to run, VOD application is the most eye-catching car on the broadband multimedia network.
VOD technology can be applied not only in the broadband network of telecoms but also in the broadband network of district LAN and cable TV. Nowadays, in the process of building intelligent communities, computer network wiring has become an essential part of the process, and community users can realize VOD video-on-demand applications through computers and televisions (with set-top boxes), which enriches people’s cultural life; cable TV has been transformed in both directions, which allows the majority of TV users to broadcast video programmes on demand through the cable TV network.
As a VOD system mainly consists of three parts.
The server-side system is mainly composed of a video server, a file management server, an internal communication sub-system and a network interface. The file management server mainly undertakes the tasks of user information management, billing, collation of film and television materials and security and confidentiality. The internal communication subsystem mainly completes the transfer of information between servers and the exchange of background film and video materials and data. The network interface mainly realises data exchange with the external network and provides the interface for user access. The video server is mainly composed of storage devices, cache and control management units, whose goal is to realise the compression and storage of media data, as well as the retrieval and transmission of media information on request. The video server differs significantly from traditional data servers in a number of ways and requires the addition of many specialised hardware and software functionalities to support the specific needs of this business. Examples include media data retrieval, real-time transmission of information streams, and encryption and decryption of information. For interactive VOD systems, server-side systems also need to implement the processing of real-time user requests, access licence control, and the simulation of VCR (Video Cassette Recorder) functions (e.g., fast forward, pause, rewind, etc.).
The network system consists of both the backbone network and the local network. As it is responsible for the real-time transmission of the video information stream, it is an extremely critical part of the continuous media network service system that affects its performance. At the same time, the network part of the media service system is a huge investment, so the design must take into account not only the high bandwidth requirements of current media applications, but also the needs of future developments and backward compatibility. Currently, the main physical network media that can be used to create such a service system are: CATV (cable television) coaxial cable, optical fibre and twisted pair cable. The main network technologies used are: Fast Ethernet, FDDI and ATM technology.
At present, there are three main types of VOD systems according to different functional requirements and application scenarios: NVOD, TVOD and IVOD.
NVOD (Near-Video-On-Demand), which can be referred to as Near-Video-On-Demand (NVOD). This type of on-demand TV works by sending the same content to multiple video streams starting at certain intervals in sequence. For example, twelve video streams are launched every ten minutes to send the same two-hour TV programme. If a user wants to watch this TV programme they may have to wait up to ten minutes, and they will choose the nearest start point to watch it. In this way, a video stream may be shared by many users.
TVOD (True Video-On-Demand), call it True TV-on-Demand, which truly supports on-demand playback. When a user requests it, the video server will immediately deliver the video content that the user wants. If another user makes the same request, the video server will immediately start another video stream for him with the same content. However, once the video stream is started, it continues to play continuously until it is finished. In this way, each video stream is redirected to a particular user.
VOD (Interactive Video-On-Demand), known as interactive on-demand television. It is a significant improvement over the previous two methods. Not only does it support on-demand playback, but it also allows the user to have interactive control over the video stream. This allows the user to play, pause, rewind, fast forward and automatically search for programmes in the same way as a traditional VCR.
The user can only contact and interoperate with a service or service provider by using the appropriate terminal equipment. In a VOD system, a TV and a Set-top Box are required and, in some special systems, a computer with a large hard disk may be required to store the video files from the video server. Client systems involve not only the appropriate hardware but also the relevant software. For example, the interface of the client system has to be modified to meet the multimedia interaction needs of the user. In addition, the buffering management of media streams, the synchronisation of sound and video data, and the coordination of network interruptions and acting interruptions all need to be fully considered when performing continuous media playback.
It is clear that although VOD services are tempting, the technology to implement them is not an “easy bone to crack”. However, with the rapid development of network technology, computer technology and storage technology, the day is not far off when the majority of users will be able to fully enjoy the pleasures of VOD. The emergence of VOD has turned the TV into a media that can be accessed randomly, more like a book or a newspaper, which can be browsed and adjusted, no longer limited to a certain time, date and fixed programme.
Although VOD was originally created to better meet the demand for independent video viewing, as VOD technology continues to advance, its widespread use will have a strong impact on popular culture and business models. VOD can not only provide diverse media information streams for end users to expand people’s information channels and enrich their spiritual life; but also entertainment in hospitals, hotels, aeroplanes and other places, staff training in companies, remote market research, advertising business of companies and other fields will gradually be filled with brand new applications of VOD technology.