The Research On Core Technology Of Time-shifting Based On Index File For Digital Set-Top Box

With the rapid development of digital television, people start to require more and more for digital Set-Top Boxes. Set-Top Box is now changing from the low-end consumer electronics which only can receive and display digital program to a high-end, powerful and multifunctional product. With hard disk or other storage devices intergraded, Set-Top Boxes with time-shifting function are changing people's traditional watching custom observably. Users will get more and more fire-new experience with the functions such as PAUSE, FAST FORWARD and BACKWARD when watching a real-time broadcasting program.The back-end of a digital Set-Top Box is basicly composed of a Transport Stream demultiplexing module and a MPEG-2 audio & video decoder. While most of the time, the former one is the bottleneck of the whole system.The traditional solution to implement time-shifting function in Set-Top Box is storing Transport Stream directly, but the Transport Stream must get through the demultiplexing module both in recording and playback, the redundant data path will lead to a performance bottleneck. On the other hand, it's unable to offer the frame by frame control abitily. The new time-shifting module based on index file records the Packetized Elementary Stream (PES), not Transport Stream. As each video frame is packed into a PES package, it's feasible to establish a Frame Index structure via the analysis of PES header. The PES data will be injected into the buffers of audio and video decoder in time-shifting playback, which makes the system architecture more clearly, and brings better performance. Besides, with Frame Index file, users will get the powerful ability to jump to and display every frame in the recorded program at their pleasure.According to IDE/ATA standards, a hard-disk storage sub-system is implemented based on STi7710DTR, that is a powerful digital Set-Top Box develop platform provided by STMicroelectronics. In order to meet the requirement for high-speed data access ability, a top-layer special storage mechanism is designed to cover the shortcomings in existed file system which is not suitable for time-shifting application, through the absolute continuous allocation mode, file and frame index table, and the addition of time stamps.A three-lever software architecture and call-back mechanism is established to ensure modularization and portability for core technology. And a dual-buffer technology is used to achieve the balance between read and write load. Regarding the audio and video synchronization, a new method is designed which make the audio stream as the principal media-stream, while the video stream as the subordinate one. Testing and analysis indicate that the functions of a time-shifting system are completely implemented, pictures display smoothly in time-shifting mode with no mosaic, video and audio synchronization works very well.