تکنولوژی ها برای همگرایی نظارت ویدئویی و چند رسانه ای Technologies for multimedia and video surveillance convergence

For this the video on demand server, WCAM is using a video on demand server coming from the multimedia field, that will allow to distribute MPEG-4 AVC streams included in MPEG-2 Transport Streams, as well as MotionJPEG 2000 using RTP protocol. An important function that will be added to this server is the video transcoding. Video transcoding is used to convert a coded video signal to another. It is mostly used to reduce a video’s bit-rate to send it through a lower capacity channel. It is also used to change the video’s spatial and temporal resolution, and to increase error-resilience when the signal is sent on wireless networks. Finally, transcoding is used to connect systems using different technologies, like different video compression standards. A good overview of the transcoding techniques has been published in [5]The easiest way produce a transcoded signal is to decompress the initial signal, and re-compress it with the new constraints like on the figure below. The cost of this method is of course very high, and more efficient methods have been developed. A compromise has to be found between the output video’s quality and the low complexity of the algorithm. Much more efficient algorithms take advantage of the reuse of information in the compressed domain, like motion vectors. This allows avoiding the re-computation of the most costly part of the algorithm. Transcoding functions play several roles in WCAM: Bit-rate reduction. It consists in increasing the compression rate usually done by truncation or modification of scale factors. Spatial resolution reduction. It corresponds to the creation of a reduced resolution of the video, e.g. from HDTV to QVGA size Temporal resolution reduction. It consists in modifying the amount of frames per second, by skipping some of them. Error-resilience Transcoding. Addition or removal of particular error resilience options by recreating a codestream Transcoding form one standard to another. Can be done using some compressed domain information or will need a complete decoding/encoding process. WCAM is considering transcoding MPEG-4 AVC from/to MPEG-2 and MPEG-4 AVC from/to MotionJPEG 2000. Transcoding of encrypted bit streams. This is something implied by the used of secured content fro switching from one Right Management system to another, but also to change access rights. Scalable video codestream management. When using MotionJPEG 2000, temporal and spatial reductions, as well as bit-rate are managed by removing some part of information in the compressed data. This is ensured by a parser function that will identify the role of each subset of information, and decide to keep them or not depending on the transcoding requirements.