Srta

Chapter 2

MPEG Video Compression Basics
B.G. Haskell and A. Puri

2.1

Video Coding Basics

Video signals differ from image signals in several important characteristics. Of course the most important difference is that video signals have a camera frame rate of anywhere from 15 to 60 frames/s, which provides the illusion of smooth motion in the displayed signal.1 Another differencebetween images and video is the ability to exploit temporal redundancy as well as spatial redundancy in designing compression methods for video. For example, we can take advantage of the fact that objects in video sequences tend to move in predictable patterns, and can therefore be motion-compensated from frame-to-frame if we can detect the object and its motion trajectory over time. Historically, therehave been five major initiatives in video coding [1–5] that have led to a range of video standards. Video coding for ISDN video teleconferencing, which has led to the ITU video coding standard called H.261 [6]. H.261 is also the baseline video mode for most multimedia conferencing systems. Video coding for low bitrate video telephony over POTS2 networks with as little as 10 kbits/s allocated tovideo and as little as 5.3 kbits/s allocated to voice coding, which led to the ITU video coding standard called H.263 [7]. The H.263 low bitrate video codec is used at modem rates of from 14.4 to 56 kbits/s, where the modem rate includes video coding, speech coding, control information, and other logical channels for data.

1 If the camera rate, chosen to portray motion, is below the display rate,chosen to avoid flicker, then some camera frames will have to be repeated. 2 Plain Old Telephone Service.

B.G. Haskell (*) Apple Computer, 1 Infinite Loop, Cupertino, CA 95014, USA e-mail: BGHaskell@comcast.net L. Chiariglione (ed.), The MPEG Representation of Digital Media, DOI 10.1007/978-1-4419-6184-6_2, © Springer Science+Business Media, LLC 2012 7

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B.G. Haskell and A. Puri

Videocoding for storing movies on CD-ROM with on the order of 1.2 Mbits/s allocated to video coding and 256 kbits/s allocated to audio coding, which led to the initial ISO MPEG-1 (Motion Picture Experts Group) standard [8]. Video coding for broadband ISDN, broadcast and for storing video on DVD (Digital Video Disks) with on the order of 2–400 Mbits/s allocated to video and audio coding, which led tothe ISO MPEG-2 video coding standard [9]. The ITU has given this standard the number H.262. Video coding for object-based coding at rates as low as 8 kbits/s, and as high as 1 Mbits/s, or higher, which led to the ISO MPEG-4 video coding standard [10]. Key aspects of this standard include independent coding of objects in a picture; the ability to interactively composite these objects into a scene atthe display; the ability to combine graphics, animated objects, and natural objects in the scene; and finally the ability to transmit scenes in higher dimensionality formats (e.g., 3D). Before delving in to details of standards, a few general remarks are in order. It is important to note that standards specify syntax and semantics of the compressed bit stream produced by the video encoder, and howthis bit stream is to be parsed and decoded (i.e., decoding procedure) to produce a decompressed video signal. However, many algorithms and parameter choices in the encoding are not specified (such as motion estimation, selection of coding modes, allocation of bits to different parts of the picture, etc.) and are left open and depend greatly on encoder implementation. However it is a requirementthat resulting bit stream from encoding be compliant to the specified syntax. The result is that the quality of standards based video codecs, even at a given bitrate, depends greatly on the encoder implementation. This explains why some implementations appear to yield better video quality than others. In the following sections, we provide brief summaries of each of these video standards, with the...