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Vector graphics or geometric modeling is the use of geometrical primitives such as points, lines, curves, and polygons to represent images in computer graphics. It is used by contrast to the term raster graphics, which is the representation of images as a collection of pixels (dots). Overview Virtually all modern current computer video displays translate vector representations of an image to a raster format. The raster image, containing a value for every pixel on the screen, is stored in memory and the entire screen is repainted 30 or more times per second. Starting in the earliest days of computing in the 1950s and into the 1980s, a different type of display, the vector graphics system, was used. In these systems the electron beam of the CRT display monitor was steered directly to trace out the shapes required, line segment by line segment, with the rest of the screen remaining black. This process was repeated many times a second to achieve a flicker-free or near flicker-free picture. These systems allowed very high-resolution line art and moving images to be displayed without the (for that time) unthinkably huge amounts of memory that an equivalent-resolution raster system would have needed. One of the first uses of vector graphic displays was the US SAGE air defense system. Vector graphics systems were only retired from U.S. enroute air traffic control in 1999 and are likely still in use in military and specialized systems. The term vector graphics is mainly used today in the context of two-dimensional computer graphics. It is one of several modes a programmer can use to create an image on a raster display. Other modes include text, multimedia and 3-d rendering. Virtually all modern 3-d rendering is done using extensions of 2-d vector graphics techniques. Plotters used in technical drafting still draw vectors directly to paper.
The signal is the sound waves (pressure fluctuations in air particles) that can be identified as speech. The channel is the air carrying those sound waves, and all the acoustic properties of the surrounding space: echoes, ambient noise, reverberation. Between the speaker and the listener (the receiver), might be other devices that do or do not introduce their own distortions of the original vocal signal (e.g. telephone, HAM radio, IP phone, etc.) The penultimate receiver is the listener's ear and auditory system, the auditory nerve, and the language areas in the listener's brain that will "decode" the signal into meaningful information and filter out background noise. All channels have noise. Another important aspect of the channel is called the bandwidth. A low bandwidth channel, such as a telephone, cannot carry all of the audio information that is transmitted in normal conversation, causing distortion and irregularities in the speaker's voice, as compared to normal, in-person speech. Other Background Bell Labs scientist Claude E. Shannon published A Mathematical Theory of Communication in 1948. This landmark publication was to set the mathematical models used to describe communication systems called information theory. Information theory enables us to evaluate the capacity of a communication channel according to its bandwidth and signal-to-noise ratio. Original theory on communication principles was provided by Harry Nyquist and Émile Baudot after whom the term Baud was conceived to represent a single piece of transmitted information. Early telecommunication systems were predominantly based on analog electronic circuit design and used a single encoding technique. The introduction of mass-produced digital integrated circuits has enabled telecom engineers to take full advantage of information theory and simultaneously use multiple encoding techniques. From the demands of telecom circuitry, a whole specialist area of integrated circuit design has emerged called digital signal processing.
Early phone systems used analog transmission lines between central offices, but in the 1960s digital multiplexed circuits were used to send voice calls over a Time Division Multiplexed (TDM) circuit. This was done at speeds of either 1.544 Mbit/s (a T1), or at 2.04 Mbit/s (an E1). A T1 circuit was capable of carrying 24 voice channels while an E1 was capable of carrying 30 voice channels. Each voice channel uses 64 kbit/s worth of digital bandwidth to convey the analog waveform. The development of the computer modem from 1980 is a clear testimony of increases in information transfer capability through the use of multiple mechanisms. A modem today uses frequency, phase and data compression techniques to squeeze data through what originally seemed an impossibly small bandwidth. Possible imperfections in a communication channel are: shot noise, thermal noise, latency, non-linear channel transfer function, sudden signal drops, bandwidth limitations, signal reflections (echos). More recent telecommunications systems take advantage of some of these imperfections to actually improve the quality of the channel. Modern telecommunication systems often make extensive use of a clock signal which is used to decode a transmitted data stream, synchronization. In order to accumulate and manage such streams a telco always provided the clock signal. With the advent of global communications it became necessary to have a single worldwide standard derived from a master atomic clock, or to secondary clocks synchronised to that clock. Synchronous circuits are often used between routers. Asynchronous Transfer Mode, ATM is a relatively new standard, operating at very high bit rates where synchronization outside of the data stream can result in errors.
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