| Compression History |
At its most basic level, compression is performed
when an input video stream is analyzed and information that is indiscernible
to the viewer is discarded. Each event is then assigned a code -
commonly occurring events are assigned few bits and rare events
will have codes more bits. These steps are commonly called signal
analysis, quantization and variable length encoding respectively.
There are four methods for compression, discrete cosine transform
(DCT), vector quantization (VQ), fractal compression, and discrete
wavelet transform (DWT).
Discrete cosine transform is a lossy compression algorithm that samples an image at regular intervals, analyzes the frequency components present in the sample, and discards those frequencies which do not affect the image as the human eye perceives it. DCT is the basis of standards such as JPEG, MPEG, H.261, and H.263. Vector quantization is a lossy compression that looks at an array of data, instead of individualvalues. It can then generalize what it sees, compressing redundant data, while at the same time retaining the desired object or data stream's original intent. Fractal compression is a form of VQ and is also a lossy compression. Compression is performed by locating self-similar sections of an image, then using a fractal algorithm to generate the sections. Like DCT, discrete wavelet transform mathematically
transforms an image into frequency components. The process is performed
on the entire image, which differs from the other methods (DCT),
that work on smaller pieces of the desired data. The result is a
hierarchical representation of an image, where each layer represents
a frequency band. |
| Compression Standards |
MPEG |
Stands for the Moving Picture Experts Group
MPEG is an ISO/IEC working group, established in 1988 to develop
standards for digital audio and video formats. There are five MPEG
standards being used or in development. Each compression standard
was designed with a specific application and bit rate in mind, although
MPEG compression scales well with increased bit rates. They include:
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MPEG-1 |
Designed for up to 1.5 Mbit/sec Standard for the compression of moving pictures and audio. This was based on CD-ROM video applications, and is a popular standard for video on the Internet, transmitted as .mpg files. In addition, level 3 of MPEG-1 is the most popular standard for digital compression of audio--known as MP3. MPEG-1 is the standard of compression for VideoCD, the most popular video distribution format thoughout much of Asia . |
|
MPEG-2 |
Designed for between 1.5 and 15 Mbit/sec Standard on which Digital Television set top boxes and DVD compression is based. It is based on MPEG-1, but designed for the compression and transmission of digital broadcast television. The most significant enhancement from MPEG-1 is its ability to efficiently compress interlaced video. MPEG-2 scales well to HDTV resolution and bit rates, obviating the need for an MPEG-3. |
|
MPEG-4 |
Standard for multimedia and Web compression.
MPEG-4 is based on object-based compression, similar in nature to
the Virtual Reality Modeling Language. Individual objects within
a scene are tracked separately and compressed together to create
an MPEG4 file. This results in very efficient compression that is
very scalable, from low bit rates to very high. It also allows developers
to control objects independently in a scene, and therefore introduce
interactivity. |
|
MPEG-7 |
This standard, currently under development,
is also called the Multimedia Content Description Interface. When
released, the group hopes the standard will provide a framework
for multimedia content that will include information on content
manipulation, filtering and personalization, as well as the integrity
and security of the content. Contrary to the previous MPEG standards,
which described actual content, MPEG-7 will represent information
about the content. |
|
MPEG-21 |
Work on this standard, also called the Multimedia
Framework, has just begun. MPEG-21 will attempt to describe the
elements needed to build an infrastructure for the delivery and
consumption of multimedia content, and how they will relate to each
other. |
|
JPEG |
Stands for Joint Photographic Experts Group.
It is also an ISO/IEC working group, but works to build standards
for continuous tone image coding. JPEG is a lossy compression technique
used for full-color or gray-scale images, by exploiting the fact
that the human eye will not notice small color changes. |
|
JPEG 2000 |
An initiative that will provide an image coding
system using compression techniques based on the use of wavelet
technology. |
|
DV |
A high-resolution digital video format used
with video cameras and camcorders. The standard uses DCT to compress
the pixel data and is a form of lossy compression. The resulting
video stream is transferred from the recording device via FireWire
(IEEE 1394), a high-speed serial bus capable of transferring data
up to 50 MB/sec. |
|
H.261 |
An ITU standard designed for two-way communication
over ISDN lines (video conferencing) and supports data rates which
are multiples of 64Kbit/s. The algorithm is based on DCT and can
be implemented in hardware or software and uses intraframe and interframe
compression. H.261 supports CIF and QCIF resolutions. |
|
H.263 |
Based on H.261 with enhancements that improve
video quality over modems. It supportsCIF, QCIF, SQCIF, 4CIF and
16CIF resolutions. |
|
H.264 |
H.264, also known as MPEG-4 AVC (Advanced Video
Coding), is a video compression standard that offers significantly
greater compression than its predecessors. The standard offers up
to twice the compression of the current MPEG-4 ASP (Advanced Simple
Profile), in addition to improvements in perceptual quality. The
H.264 standard can provide DVD-quality video at under 1 Mbps, and
is optional for full-motion video over wireless, satellite, and
ADSL Internet connections. |
|
DivX Compression |
DivX is a software application that uses the
MPEG-4 standard to compress digital video, so it can be downloaded
over a DSL/cable modem connection in a relatively short time with
no reduced visual quality. The latest version of the codec, DivX
4.0, is being developed jointly by DivXNetworks and the open source
community. DivX works on Windows 98, ME, 2000, CE, Mac and Linux.
|
|
Terms |
Lossy compression - reduces a file by permanently
eliminating certain redundant information, so that even when the
file is uncompressed, only a part of the original information is
still there. |
|
ISO/IEC |
International Organization for Standardization
- a non-governmental organization that works to promote the development
of standardization to facilitate the international exchange of goods
and services and spur worldwide intellectual, scientific, technological
and economic activity.
International Electrotechnical Commission - international standards and assessment body for the fields of electrotechnology Codec - A video codec is software that can compress a video source (encoding) as well as play compressed video (decompress). CIF - Common Intermediate Format - a set of standard video formats used in videoconferencing, defined by their resolution. The original CIF is also known as Full CIF (FCIF). QCIF - Quarter CIF (resolution 176x144) |