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Switching and timing circuits, or logic circuits, form the heart of any device where signals must be selected or combined in a controlled manner. Applications of these circuits include telephone switching, satellite transmissions, and digital computer operations.

Digital logic is a rational process for making simple †œtrueâà ƒÆ’¢â€šÂ¬Ã‚? or †œfalseâ €Â? decisions based on the rules of Boolean algebra. †œTrueâà ƒÆ’¢â€šÂ¬Ã‚? can be represented by a 1 and †œfalseâ €Â? by a 0, and in logic circuits the numerals appear as signals of two different voltages. Logic circuits are used to make specific true false decisions based on the presence of multiple true false signals at the inputs. The signals may be generated by mechanical switches or by solid state transducers. Once the input signal has been accepted and conditioned (to remove unwanted electrical signals, or †œnoiseâ €Â?), it is processed by the digital logic circuits. The various families of digital logic devices, usually integrated circuits, perform a variety of logic functions through logic gates, including †œOR,âà ƒÂ¢Ã¢â‚¬Å¡Ã‚¬Ã‚?ÃƆ™Ãƒâ€šÃ‚¢ÃƒÂ¢Ã¢â‚¬Å¡Ã‚¬Ãâ₠¬Å¡?AND,ââ‚à ƒâ€šÃ‚¬Ãƒâ€š? and †œNOT,âà ƒÆ’¢â€šÂ¬Ã‚? and combinations of these (such as †œNOR,âà ƒÆ’¢â€šÂ¬Ã‚? which includes both OR and NOT). One widely used logic family is the transistor transistor logic (TTL). Another family is the complementary metal oxide semiconductor logic (CMOS), which performs similar functions at very low power levels but at slightly lower operating speeds. Several other, less popular families of logic circuits exist, including the currently obsolete resistor transistor logic (RTL) and the emitter coupled logic (ELC), the latter used for very high speed systems.

The elemental blocks in a logic device are called digital logic gates. An AND gate has two or more inputs and a single output. The output of an AND gate is true only if all the inputs are true. An OR gate has two or more inputs and a single output. The output of an OR gate is true if any one of the inputs is true and is false if all of the inputs are false. An INVERTER has a single input and a single output terminal and can change a true signal to a false signal, thus performing the NOT function. More complicated logic circuits are built up from elementary gates. They include flip flops (binary switches), counters, comparators, adders, and more complex combinations.

To perform a desired overall function, large numbers of logic elements may be connected in complex circuits. In some cases microprocessors are utilized to perform many of the switching and timing functions of the individual logic elements. The processors are specifically programmed with individual instructions to perform a given task or tasks. An advantage of microprocessors is that they make possible the performance of different logic functions, depending on the program instructions that are stored. A disadvantage of microprocessors is that normally they operate in a sequential mode, which may be too slow for some applications. In these cases specifically designed logic circuits are used.

Recent Developments

The development of integrated circuits has revolutionized the fields of communications, information handling, and computing. Integrated circuits reduce the size of devices and lower manufacturing and system costs, while at the same time providing high speed and increased reliability. Digital watches, hand held computers, and electronic games are systems based on microprocessors. Other developments include the digitalization of audio signals, where the frequency and amplitude of an audio signal are coded digitally by appropriate sampling techniques, that is, techniques for measuring the amplitude of the signal at very short intervals. Digitally recorded music shows a fidelity that is not possible using direct recording methods. Digital playback devices of this nature have already entered the home market. Digital storage could also form the basis of home video systems and may significantly alter library storage systems, because much more information can be stored on a disk for replay on a television screen than can be contained in a book.

Medical electronics has progressed from computerized axial tomography, or the use of CAT or CT scanners, to systems that can discriminate more and more of the organs of the human body. Devices that can view blood vessels and the respiratory system have been developed as well. Ultrahigh definition television also promises to substitute for many photographic processes, because it eliminates the need for silver.

Today's research to increase the speed and capacity of computers concentrates mainly on the improvement of integrated circuit technology and the development of even faster switching components. Very large scale integrated (VLSI) circuits that contain several hundred thousand components on a single chip have been developed. Very high speed computers are being developed in which semiconductors may be replaced by superconducting circuits using Josephson junctions and operating at temperatures near absolute zero.