The SNA model is concerned with both logical and physical units. Logical
units (LUs) serve as points of access by which users can utilize the network.
LUs can be viewed as terminals that provide users access to application
programs and other services on the network. Physical units (PUs) like
LUs are not defined within SNA architecture, but instead, are representations
of the devices and communication links of the network.
SNR scalability
A type of scalability where the enhancement layer(s) contain only coded
refinement data for the
DCT
coefficients of the base layer.
Spatial scalability
A type of scalability where an enhancement layer also uses predictions from
pel
data derived from a lower layer without using
motion vectors.
The layers can have different frame sizes, frame rates or chroma formats
Standard bodies
Any country have national standard body where experts from
industry and universities develop standards for all kinds of
engineering problems. Among them are, for instance,
ANSI American National Standards Institute USA
DIN Deutsches Institut fuer Normung Germany
BSI British Standards Institution United Kingdom
AFNOR Association francaise de normalisation France
UNI Ente Nazionale Italiano di Unificazione Italy
NNI Nederlands Normalisatie-instituut Netherlands
SAA Standards Australia Australia
SANZ Standards Association of New Zealand New Zealand
NSF Norges Standardiseringsforbund Norway
DS Dansk Standard Denmark
and about 80 others.
The International Organization for Standardization, ISO, in Geneva is the head organization of all these national standardization bodies. Together with the International Electrotechnical Commission, IEC, ISO concentrates its efforts on harmonizing national standards all over the world. The results of these activities are published as ISO standards. Among them are, for instance, the metric system of units, international stationery sizes, all kinds of bolt nuts, rules for technical drawings, electrical connectors, security regulations, computer protocols, file formats, bicycle components, ID cards, programming languages, International Standard Book Numbers (ISBN). Over 10,000 ISO standards have been published so far and you surely get in contact with a lot of things each day that conform to ISO standards you never heard of. By the way, ISO is not an acronym for the organization in any language. It's a wordplay based on the English/French initials and the Greek-derived prefix iso- meaning same.
Within ISO, ISO/IEC Joint Technical Committee 1 (JTC1) deals with information technology.
The International Telecommunication Union, ITU, is the United Nations specialized agency dealing with telecommunications. At present there are 164 member countries. One of its bodies is the International Telegraph and Telephone Consultative Committee, CCITT. A Plenary Assembly of the CCITT, which takes place every few years, draws up a list of 'Questions' about possible improvements in international electronic communication. In Study Groups, experts from different countries develop 'Recommendations' which are published after they have been adopted. Especially relevant to computing are the V series of recommendations on modems (e.g. V.32, V.42), the X series on data networks and OSI (e.g. X.25, X.400), the I and Q series that define ISDN, the Z series that defines specification and programming languages (SDL, CHILL), the T series on text communication (teletext, fax, videotext, ODA) and the H series on digital sound and video encoding.
Since 1961, the European Computer Manufacturers Association, ECMA, has
been a forum for data processing experts where agreements have been
prepared and submitted for standardization to
ISO,
CCITT
and other standards organizations.
Start codes [system and video]
32-bit codes embedded in that coded bitstream that are unique. They are used
for several purposes including identifying some of the structures in the
coding syntax.
Stuffing (bits); stuffing (bytes)
Code-words that may be inserted into the compressed bitstream that are
discarded in the decoding process. Their purpose is to increase the
bitrate
of the stream.
Sub band coding
Sub-band coding for images has roots in work done in the 1950s by
Bedford and on Mixed Highs image compression done by
Kretzmer in 1954. Schreiber and Buckley explored
general two channel coding of still pictures where the low spatial
frequency channel was coarsely sampled and finely quantized and the
high spatial frequency channel was finely sampled and coarsely
quantized. More recently, Karlsson and Vetterli have
extended this to multiple subbands. Adelson et al. have shown
how a recursive subdivision called a pyramid decomposition can be used
both for compression and other useful image processing tasks.
A pure sub-band coder performs a set of filtering operations on an image to divide it into spectral components. Usually, the result of the analysis phase is a set of sub-images, each of which represents some region in spatial or spatio-temporal frequency space. For example, in a still image, there might be a small sub-image that represents the low-frequency components of the input picture that is directly viewable as either a minified or blurred copy of the original. To this are added successively higher spectral bands that contain the edge information necessary to reproduce the original sharpness of the original at successively larger scales. As with DCT coder, to which it is related, much of the image energy is concentrated in the lowest frequency band.
For equal visual quality, each band need not be represented with the same
signal-to-noise ratio; this is the basis for sub-band coder compression. In
many coders, some bands are eliminated entirely, and others are often
compressed with a vector or lattice quantizer. Succeedingly higher frequency
bands are more coarsely quantized, analogous to the truncation of the high
frequency coefficients of the
DCT.
A sub-band decomposition can be the intraframe coder in a predictive loop,
thus minimizing the basic distinctions between
Do not confuse S-Video with S-VHS. S-VHS is a tape/signal standard.
S-Video is a hardware standard that defines the physical cable jacks.
S-Video allows you to bypass the comb filter in a device. Generally,
less processing of the signal results in a better picture. The comb
filter separates the chroma (color) and luma (brightness) components
of a video signal into separate parts.
This is also called Y/C, where Y represents brightness
and C color.
When color and brightness are not separated, when they are
combined in the signal, it is called a composite signal.
S-Video cables have separate wires for the color and
brightness. That is, they carry a Y/C signal.
The best picture comes when the color and brightness is
separate from the source. VCRs record this way, and DSS
broadcasts this way too. Laserdiscs store a composite
picture rather than Y/C separated.
Even when the signals have been combined at some point on
their way to the monitor, different comb filters perform to
different degrees of quality, so one can pick how to connect
one's components to try to use the best comb filter.
Some older sets with S-Video input jacks may actually
combine the Y/C in a crude way, making the S-Video input no
better than a typical composite signal. Newer sets probably
do not do this anymore.
S-Video
S-Video is the standard for the way a signal is carried on the cable
itself. The industry has settled on a 4-pin mini plug connector.
S-Video does not have any relation to the resolution or refresh rate
of the signal.