Notes by John Peterson
Robin spoke on current and future technologies in digital video.
One of the principle issues in digital video is compression, because
of the tremendous bandwidth required for digital video images.
Some typical bandwidth measures that indicate the problem:
525 Component video
A typical 6 megahertz video channel has only 20 MB/s of bandwidth
Video compression has been evolving for the past 60 years:
Interlacing, one of the oldest techniques, sends half the motion
half the time by alternating the scanlines drawn. However, it
makes subsequent modern compression techniques hard to apply.
NTSC shares bandwidth between color and brightness information,
taking advantage of the fact that the human visual system is much
less critical of color information than tonality. Thus, the color
information is highly compressed and transmitted separately.
Many modern techniques take advantage of "lossy"
methods that throw information away that the human visual system
is not likely to notice, allowing much higher compression. Further
savings are achieved by sending descriptions of the images rather
than the images themselves, such as when a still image appears
on the screen for a few seconds.
Most of the methods for compressing a frame of video are based
on block DCT (discrete cosine transform) encoding. The DCT method
converts a small block of the image from a spatial image to a
frequency-based encoding. This data is re-arranged so the higher
frequency details are removed without producing noticeable artifacts.
The remainder is compressed using standard digital techniques.
The most popular digital video compression standard is MPEG
(Motion Picture Experts Group). This standard has gone through
revisions over time. The first, MPEG 1, was not specified in enough
detail to allow for widespread broadcast and recording use. MPEG
2 was better defined, with both a system and a transport layer.
Improvements that were to become MPEG 3 were later subsumed into
the current MPEG 2 standard. Work on MPEG 4 is still underway,
but Wilson described it as more of a "research project",
still waiting for a breakthrough to achieve its goals. The current
MPEG 2 standard supports a number of different formats and techniques
that broadcasters and receivers must support. For example, it
is possible for broadcasters to switch the image resolution on
the fly. The MPEG 2 format is well accepted, and used in a number
of applications such as direct broadcast home satellite dishes.
Because of the wide number of techniques and formats available
in MPEG encoding, content creators can use a number of tricks
to maximize bandwidth. For some applications, such as creating
a DVD title, the compression can be done "by hand" to
select the best method for a particular scene. Some types of content
remain difficult to compress though, such as football and basketball
Goals for future digital video technologies include lowering
the data rate even further with advanced compression techniques,
transmitting video over the Internet, and directly manipulating
and editing compressed video.
Back to Silicon Valley SIGGRAPH home