MPEG VIDEO DATA REQUIREMENTS |
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MPEG Process Requirements The work of MPEG was a natural extension to the work of JPEG. Indeed MPEG uses much of the research and development that was done for JPEG. JPEG uses the DCT (Discrete Cosine Transform) to exploit the spatial redundancy that is present in a still image. MPEG uses the DCT to exploit the spatial redundancy, but also uses the similarity of successive pictures to exploit temporal redundancy.
In creating the compression standard MPEG had to ensure that the following list of processes were possible: Random Access - This is an essential feature of any video storage system or error prone transmission system. It requires that the compressed data stream have access points which allow an application to decode starting in the middle of the data stream. This implies that there exist a set of unique data patterns in the stream that act as unambiguous feature markers. FFWD/REV - The user requires that it be possible to visually navigate around the system during searches. Reverse Playback - It should be possible to play full quality video in reverse. A-V Synch - It is essential that video and associated audio be synchronised. With the compression tools that are used in MPEG this process is not as simple as it may at first be thought to be the case. The interleaved multiplex nature of the video and audio data in the data stream can mean significant storage is needed at a decode station. Time stamps are required to allow automatic synchronisation to occur. Robustness - The system should not exhibit catastrophic failure in the presence of errors. This implies-added redundancy in the form of correction data. The robustness of MPEG is assisted by the requirement for random access to the data stream. Delay - The coding delay of an MPEG coder may be very large. However it is important to be able to select a low delay mode for face to face (two way) working. Editability - It is desirable to be able to edit the pictures in the data stream (by implication on a random access medium) without the need to decompress - edit - recompress. Flexibility - The standard should work with multiple television standards (both existing and future potential standards), and with both progressive and interlaced scanning structures.
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