The Digital Fact Book - Quantel

MPEG-2ISO/IEC 13818. A family of inter- and intra-frame compression systems designed to covera wide range of requirements from ‘VHS quality’ all the way to HDTV through a series ofcompression algorithm ‘profiles’ and image resolution ‘levels’. With data rates from below 4to 100 Mb/s, this family includes the compression system that currently delivers digital TVto homes and that puts SD video onto DVDs as well as putting HD onto 6.35mm tape for HDV.MIn all cases MPEG-2 coding starts with analyzing 8x8-pixel DCT blocks and applying quantizingto achieve intra-frame compression very similar to JPEG. That produces I-frame only MPEG-2video. Producing much higher compression involves analyzing the movement of 16x16-pixelmacroblocks frame-to-frame to produce vectors to show the distance and direction ofmacroblock movement. Their correctness is a big factor in coders’ quality and efficiency.This vector data is carried in the P (predictive) and B (bi-directional predictive) frames thatexist between I frames (see diagram). SDTV transmissions and DVDs typically contain twoI-frames per second typically using about 4 Mb/s – a big difference from the 180 Mb/sof uncompressed SD video. The set of images between I-frames is a Group of Pictures (GOP)– usually about 12 for 576/50I and 15 for 480/60I transmissions. These are called ‘long GOP’.The GOP length can vary during transmission – a new sequence starting with an I-framemay be forced if there is a big change at the input, such as a cut.MPEG-2 12 frame GOPI2B8B7P3B6B5P2B4B3P1I1B1B2*Note for transmission the last ‘I’ frame isplayed out ahead of the last two ‘B’ framesto form the sequence I1, B1, B2, P1, B3,B4, P2, B5, B6, P3, I2, B7, B8126