The Digital Fact Book - Quantel

ITU-R/BT.799See Dual linkiTVSee: Interactive TelevisionJavaA general purpose programming language developed by Sun Microsystems and bestknown for its widespread use in animations on the World Wide Web. Unlike other software,programs written in Java can run on any platform type, so long as they have a Java VirtualMachine available.Website: java.sun.comJPEG (.JPG)Joint Photographic Experts Group (ISO/ITU-T). They have defined a compression knownas JPEG which is a DCT-based data compression standard for still pictures (intra-frame).It offers compression of between two and 100 times and three levels of processing aredefined: the baseline, extended and lossless encoding.JPEG baseline compression coding, which is overwhelmingly the most common in boththe broadcast and computer environments, starts with applying DCT to 8 x 8 pixel blocksof the picture, transforming them into frequency and amplitude data. This itself may notreduce data but then the generally less visible high frequencies can be divided by a high‘quantizing’ factor (reducing many to zero), and the more visible low frequencies by a muchlower factor. The ‘quantizing’ factor can be set according to data size (for constant bit rate)or picture quality (constant quality) requirements – effectively adjusting the compressionratio. The final stage is Huffman coding which is lossless but can further reduce data by2:1 or more.Baseline JPEG coding creates .jpg files and it is very similar to the I-frames of MPEG,the main difference being they use slightly dissimilar Huffman tables.See also: Compression, Compression ratio, DCT, Huffman coding, MPEGWebsite: www.jpeg.orgIJ115