Digital Video for the Next Millennium


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Section Two: Video Encoding Standards
MPEG-2 (ISO/IEC 13818-2 MPEG-2)

MPEG-2, published as a standard in 1994, is a high-bandwidth encoding standard, supporting a bandwidth range of approximately 2Mbit/sec to more than 20 Mbit/sec. It was originally designed for coding of television broadcast video with CCIR Rec. 601 resolution at data rates below 10 Mbit/sec, but was expanded to encompass HDTV requirements at app. 12-20 Mbit/sec.

MPEG-2 was designed to encompass, and be backward compatible with, MPEG-1 encoding techniques but was also enhanced to support interlaced video, as provided by television input sources. The MPEG-2 standard was designed for scalability and flexibility, supporting many levels of service depending on the needs of the application. It was expected that an MPEG-3 standard would be developed for HDTV (high definition television), but the MPEG-2 standard scaled to encompass the bandwidth requirements of HDTV.

The most common MPEG-2 compression is main level ("CCIR 601") at 720 pixels x 480 lines, 30 frames/second. The sweet points for MPEG-2 support the bandwidth bit rates of 2-6 Mbit/sec, scaling up to 40 Mbit/sec for very high-level HDTV applications.

The MPEG-2 encoding standard builds on, and is backward compatible with, the statistical redundancy compression of MPEG-1. The most important difference between MPEG-1 and MPEG-2 is the encoding of interlaced frames for broadcast TV. MPEG-1 supports only progressive frame encoding, while MPEG-2 provides both progressive frame and interlaced frame encoding. Video movies, originally in a film format, are a progressive frame format. Television broadcasts are an interlaced format. A broadcast frame is created with two separate fields, a top and bottom interlaced field, with the first line of the bottom field appearing immediately after the first line of the top field. MPEG-2 splits frames into two fields for interlacing, so that 30 frames/sec becomes 60 fields/sec.

In addition, MPEG-2 includes the ability to multiplex multiple video streams, additional color subsampling, improved compression and error correction and improved audio, including "low sample rate" and multichannel extension for surround sound. The many profiles and levels of service include NTSC (app. 3Mbit/sec), PAL (app. 4 Mbit/sec) and Broadcast HDTV (12-20 Mbit/sec).