Guidelines for Selecting Practical MPEG Group of Pictures

The repeated pattern of I, P and B frames in an MPEG stream is known as the Group of Pictures (GOP). Current GOP choices are made using intuition and informal guidelines without the support of theoretical or practical evidence. This paper studies the impact of the choice of GOP by evaluating the effects of GOP on both static MPEG videos and MPEG videos streaming over a lossy network. The static analysis involves encoding raw video images into MPEG files with various GOP patterns to compare and contrast static properties such as the frame size, file size and quality. The streaming analysis varies the GOP length and pattern to study the impact of GOP on a model of the streaming bitrate and playable frame rate. The results consistently suggest two guidelines: 1) the number of B frames between two reference frames should be close to 2, except when limited to less than 2 by time constraints; 2) the number of P frames should be 5 or fewer as there is little performance gain in setting the number of P frames in the GOP larger than 5.