Document Type

Other

Publication Date

8-2005

Abstract

MPEG is one of the most popular open standards for video on the Internet. MPEG uses intra-frame and inter-frame compression with three types of frames: I, P and B frames. The repeated pattern of I, P and B frames in an MPEG stream is known as the Group of Pictures (GOP). The choice of GOP affects static MPEG properties such as frame size and file size and also impacts the streaming MPEG in terms of network bitrate and playable quality. Current GOP choices are made using intuition and informal guidelines without the support of theoretical or practical evidence. This pa- per studies the impact of the choice of GOP by evaluating the effects of GOP on both static MPEG videos and on 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 bi- trate and playable frame rate. The MPEG streaming analysis considers 3 distinct cases over a network model with packet loss: normal streaming with no repair and no capacity limit; streaming with Forward Error Correction (FEC) but no capacity limit; and streaming with FEC and a capacity limit. 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 the en- coding and 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.

DOI

WPI-CS-TR-05-18

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