Chapter 7: Coding of Moving Pictures for Digital Storage Media (MPEG-1)

Overview

MPEG-1 is the first generation of video codecs proposed by the Motion Picture Experts Group as a standard to provide video coding for digital storage media (DSM), such as CD, DAT, Winchester discs and optical drives [1]. This development was in response to industry needs for an efficient way of storing visual information on storage media other than the conventional analogue video cassette recorders (VCR). At the time the CD-ROMs had the capability of 648 Mbytes, sufficient to accommodate movie programs at a rate of approximately 1.2 Mbit/s, and the MPEG standard aimed to conform roughly with this target. Although in most applications the MPEG-1 video bit rate is in the range of 1–1.5 Mbit/s, the international standard does not limit the bit rate, and higher bit rates might be used for other applications.

It was also envisaged that the stored data would be within both 625 and 525-line television systems and provide flexibility for use with workstations and personal computers. For this reason, the MPEG-1 standard is based on progressively scanned images and does not recognise interlacing. Interlaced sources have to be converted to a noninterlaced format before coding. After decoding, the decoded image may be converted back to provide an interlaced format for display.

Since coding for digital storage can be regarded as a competitor to VCRs, then MPEG-1 video quality at the rate of 1 to 1.5 Mbit/s is expected to be comparable to VCRs. Also, it should provide the viewing conditions associated with VCRs such as forward play, freeze picture, fast forward, fast reverse, slow forward and random access. The ability of the decoder to provide these modes depends to some extent on the nature of digital storage media. However, it should be borne in mind that efficient coding and flexibility in operation are not compatible. Provision of the added functionality of random access necessitates regular intraframe pictures in the coded sequence. Those frames that do not exploit the temporal redundancy in video have poor compression, and as a result the overall bit rate is increased.

Both H.261 [2] and MPEG-1 [1] are standards defined for relatively low bit rate coding of low spatial resolution pictures. Like H.261, MPEG-1 utilises DCT for lossy coding of its intraframe and interframe prediction errors. The MPEG-1 video coding algorithm is largely an extension of H.261, and many of the features are common. Their bit streams are, however, incompatible, although their encoding units are very similar.

The MPEG-1 standard, like H.261, does not specify the design of the decoder, and even less information is given about the encoder. What is expected from MPEG-1, like H.261, is to produce a bit stream which is decodable. Manufacturers are free to choose any algorithms they wish, and to optimise them for better efficiency and functionality. Therefore in this Chapter we again look at the fundamentals of MPEG-1 coding, rather than details of the implementation.