Grading Compression Algorithms III

Grading Compression Algorithms III

Decompression Speed Naturally, decompression speed matters to someone decompressing a ˇ le. This is especially important today because many companies are looking for ways to distribute movies and sounds over digital delivery systems. Small ˇ lesare great, but the computer should be able to decompress them on the y. If the decompression process is too slow, then the uncompressed ˇ lemust be stored on a hard disk before it can be played. In many cases, there isn' t enough space on the disk to do the job effectively. Many computers come with special processors designed to decompress ˇ les compressed with the MPEG video algorithm. This special hardware is necessary to display the decompressed images in real time.

Online Suitability Some of the compression algorithms were designed to compress data owing over the Internet. While all compression algorithms that work for data on the hard disk will also help keep transmission costs down,some algorithms offer better performance when the Internet fails to deliverparts of the data stream. For instance, the Real Audio format is designed to recover smoothly if a packet of data from the middle of the ˇ ledisappears along the route. Other basic sound ˇ lesˇ lleach packet with all of the information about n seconds of the ˇ le. This guarantees that there will be seconds of dead air if the packet doesn' t arrive. RealAudio, on the other hand, interleaves the information from k different blocks and packs it into successive packets to ensure that no packet contains more than n/k seconds. is set to be large enough to ensure that many glitches can be smoothed over. 

Clearly this solution requires a bit of preprocessing. While the compression process might not be too complicated, it does take time to start up. If information from k different blocks is interleaved, then k packets, must be processed before the sound can emerge. This means that replay is delayed by k packets, and this is often known as the Latency of the algorithm. This isn' t a great loss if a user is replaying an old recording, but it makes the solution unsuitable for use on a telephone. The delays make it impossible to hold a decent conversation.

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