Compression of Video and Audio Signals

Video compressionThe header has been reduced, detection and
At present there are used a few compressiontransmission of error correction have been
methods. The oldest of them (JPEG, H-261,abandoned, and new optional modes of operation
MPEG-2) use groups of pixels (covering somehave been integrated.
area, for example 8 bits x 8 bits). These methodsMPEG-4 compression
don't analyze the actual structure of the image.MPEG-4 is the newest video compression
This generation of compressors may be used instandard which quickly becomes popular, also in
the case of lower resolutions when transmissionthe case of video streaming. In comparison with
speed isn't less than 64kbps.previously presented standards, it integrates
More effective compression is offered by secondeffective methods for multimedia data.
generation of algorithms. These algorithms (e.g.New attitude consists in interrelation of data
MPEG-4) carry out a kind of scene analysiscontents with the role for human perception, and
before realization of adequate compressionextraction of so called audio-visual objects (AVO).
procedure. They make out that image can beThere have been integrated solutions allowing
divided into fragments in more effective way.scalability of data representation in many
Image which consists of pixels with the samedimensions, that is space, time, quality and
features can be divided into specific groups, andcomputational complexity.
this operation significantly improves compressionThe standard has been adjusted to many types
process. To achieve such analysis procedure, aof telecommunication networks and can operate
model of human perceiving has been employedat rates from 64 to 1024 kbps.
(HVS - Human Vision System) .The concept of MPEG-4 allows transmission of
Typical second generation encoder consists ofsingle or several audiovisual AV objects from the
following modules: segmentation, motionsender to the receiver, however before the
estimation, outline and texture coding. The systemtransmission begins, the initial exchange of
offers compromise between compression rateinformation between the encoder and decoder
and the quality of the image after decompression.has to be carried out. This way, the convenient
Video compression standards: H-261, H263algorithm classes and the tools essential for
The H-261 system has been created for ISDNeffective use of the link can be settled. Obviously,
networks with 64 kbps bandwidth. The H-261the coder protects audiovisual objects against
standard allows compression rates from 100:1 toerrors.
2000:1, however the higher compression ratio, theAudio compression
lower quality of decompressed image. The codecData stream from audio compression is
works in two modes: intra frame - INTRA andsignificantly smaller than video data stream.
inter frame - INTER.However, the compression is normally performed
During the INTRA compression mode, the frameto minimize the total bandwidth.
is being divided into blocks sized by 8x8 pixels.Simple reduction of link's bandwidth can be
Then they are compressed using Discrete Cosinesachieved by reduction of bit rate (sampling
Transform (similar to JPEG).frequency) and reduction of amplitude resolution.
In INTER mode, only changes which have beenDepending on transmission type, various samplings
occurred between following frames are beingare used:
saved. However, it has to be marked that atSampling frequency - Application
least once every 123 frames the compression in8.00 kHz - Telephony
INTRA mode has to be carried out.16 kHz - Multimedia communication
Development of H.263 standard has been meant22.05 kHz - Personal computers
to secure video signal compression for low32.00 kHz - Digital radio and television
bandwidth networks, about 28.8 to 33.6 kbps.44.10 kHz - CD - Audio
H-263 is a modernized version of the H-26148.00 kHz - DAT cassette recorders, HDTV
standard. In comparison with the original version,96.00 kHz - DVD
half-pixel motion compensation has beenThis is not full version of article.
employed instead of pixel motion compensation.