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ECG068/0798

Depending on the amount of motion in the video, P-frames may not come fast enough to give the
perception of smooth motion. To compensate for this, B-frames are therefore inserted in between
the I and P-frames. B-frames are "bi-directional" in that they use information in the previous I or
P-frame as well as in the future I or P-frame. The information they contain is interpolated
between two endpoints rather than representing actual data, making the assumption that the pixel
information will not change drastically between the end-points. As a result these "dishonest" B-
frames contain the most amount of compression and are the smallest frames. In order for a
decoder to decode these B-frames it must have the corresponding I and P-frames they are based
on, hence the frames may be transmitted out of order to reduce decoding delays.

A frame sequence consisting of an I-frame and its subsequent B and P-frames before the next I-
frame—typically around 15 frames—is called a Group of Pictures (GOP). An I-frame of the GOP
acts as the basic entry access point.

The result of this compression technique is that MPEG format is not easy to edit since you cannot
enter the video at any point. Also the quality of the resulting video will depend on the particular
implementation and the amount of motion in the video. The MPEG encoding/decoding process
can also be seen as computationally intensive which requires the use of specialized hardware or a
PC with a powerful processor.

The compression achieved using this technique enables about 72 minutes of video on a single CD-
ROM—not quite enough for a full-length feature movie, which typically needs about 2 hours or
two CD-ROMs. In addition to compression techniques, the MPEG-1 standard also supports
playback functions such as fast forward, fast reverse, and random access into the bitstream.
However, as mentioned above, these access points are only at the I-frame boundaries. The CD-
ROM quality audio is stereo (2 channel) 16-bit sampled audio at 44KHz. The main MPEG-1
resolution of 360 x 242 is called Standard, or Source, Input Format (SIF) and, unlike the
Common Intermediate Format (CIF) defined earlier, differs for NTSC and PAL systems. The
computer industry has defined its own square pixel version of the SIF format at 320 x 240.

Originally, MPEG-1 decoding was done by hardware add-in boards with dedicated MPEG audio
and video decoder chips because of the processing power required. Currently, MPEG-1 can be
decoded in software on a Pentium 133 or better.

MPEG-2

MPEG-2 was adopted in the Spring of 1994 and is designed to be backward compatible with
MPEG-1. It is not designed to replace MPEG-1, but to enhance it as a broadcast studio-quality
standard for HDTV, cable television, and broadcast satellite transmission. Resolution is full-
screen (ranging from NTSC 720x480 to HDTV 1280x720) playback with a scan rate of 60 fields
per second. The latter enables MPEG-2 to support interlaced TV scanning systems as well as
progressive scan computer monitors. Compression is similar to MPEG-1, but is slightly improved
with the video exceeding SVHS quality, versus VHS for MPEG-1. Audio is also improved with
six channel surround-sound versus CD-ROM 2-channel stereo audio for MPEG-1. The use of
Interframe compression similar to MPEG-1 makes fast-motion scenes the most difficult to
compress. Because of this, MPEG-2 supports two encoding schemes depending on the needs of
the application: variable bit rate, to keep the video quality constant, and varying quality to keep
the bit rate constant.

The variable data rates for MPEG-2 range from 2Mbps to 10Mbps. This typically requires a 4x
CD-ROM drive for playback. A 4x CD-ROM can only store only 18 minutes of video versus a 1x
CD with 72 minutes of video for MPEG-1. MPEG-2 was selected as the core compression