An amplifier, electronic amplifier or amp is an electronic device that can increase the power of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output; the amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit. An amplifier can either be a separate piece of equipment or an electrical circuit contained within another device. Amplification is fundamental to modern electronics, amplifiers are used in all electronic equipment. Amplifiers can be categorized in different ways. One is by the frequency of the electronic signal being amplified. For example, audio amplifiers amplify signals in the audio range of less than 20 kHz, RF amplifiers amplify frequencies in the radio frequency range between 20 kHz and 300 GHz, servo amplifiers and instrumentation amplifiers may work with low frequencies down to direct current.
Amplifiers can be categorized by their physical placement in the signal chain. The first practical electrical device which could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. Today most amplifiers use transistors; the first practical device that could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. Vacuum tubes were used in all amplifiers until the 1960s–1970s when the transistor, invented in 1947, replaced them. Today, most amplifiers use transistors; the development of audio communication technology in form of the telephone, first patented in 1876, created the need to increase the amplitude of electrical signals to extend the transmission of signals over long distances. In telegraphy, this problem had been solved with intermediate devices at stations that replenished the dissipated energy by operating a signal recorder and transmitter back-to-back, forming a relay, so that a local energy source at each intermediate station powered the next leg of transmission.
For duplex transmission, i.e. sending and receiving in both directions, bi-directional relay repeaters were developed starting with the work of C. F. Varley for telegraphic transmission. Duplex transmission was essential for telephony and the problem was not satisfactorily solved until 1904, when H. E. Shreeve of the American Telephone and Telegraph Company improved existing attempts at constructing a telephone repeater consisting of back-to-back carbon-granule transmitter and electrodynamic receiver pairs; the Shreeve repeater was first tested on a line between Boston and Amesbury, MA, more refined devices remained in service for some time. After the turn of the century it was found that negative resistance mercury lamps could amplify, were tried in repeaters, with little success; the development of thermionic valves starting around 1902, provided an electronic method of amplifying signals. The first practical version of such devices was the Audion triode, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912.
Since the only previous device, used to strengthen a signal was the relay used in telegraph systems, the amplifying vacuum tube was first called an electron relay. The terms amplifier and amplification, derived from the Latin amplificare, were first used for this new capability around 1915 when triodes became widespread; the amplifying vacuum tube revolutionized electrical technology, creating the new field of electronics, the technology of active electrical devices. It made possible long distance telephone lines, public address systems, radio broadcasting, talking motion pictures, practical audio recording, radar and the first computers. For 50 years all consumer electronic devices used vacuum tubes. Early tube amplifiers had positive feedback, which could increase gain but make the amplifier unstable and prone to oscillation. Much of the mathematical theory of amplifiers was developed at Bell Telephone Laboratories during the 1920s to 1940s. Distortion levels in early amplifiers were high around 5%, until 1934, when Harold Black developed negative feedback.
Other advances in the theory of amplification were made by Hendrik Wade Bode. The vacuum tube was the only amplifying device, other than specialized power devices such as the magnetic amplifier and amplidyne, for 40 years. Power control circuitry used magnetic amplifiers until the latter half of the twentieth century when power semiconductor devices became more economical, with higher operating speeds; the old Shreeve electroacoustic carbon repeaters were used in adjustable amplifiers in telephone subscriber sets for the hearing impaired until the transistor provided smaller and higher quality amplifiers in the 1950s. The replacement of bulky electron tubes with transistors during the 1960s and 1970s created another revolution in electronics, making possible a large class of portable electronic devices, such as the transistor radio developed in 1954. Today, use of vacuum tubes is limited for some high power applications, such as radio transmitters. Beginning in the 1970s, more and more transistors were connected on a single chip thereby creating higher scales of integration (small-scale, medium-scale, large-s
A registered jack is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces were first defined in the Universal Service Ordering Code system of the Bell System in the United States for complying with the registration program for customer-supplied telephone equipment mandated by the Federal Communications Commission in the 1970s, they were subsequently codified in title 47 of the Code of Federal Regulations Part 68. The specification includes physical construction and signal semantics. Accordingly, registered jacks are named by the letters RJ, followed by two digits that express the type. Additionally, letter suffixes indicate minor variations. For example, RJ11, RJ14, RJ25 are the most used interfaces for telephone connections for one-, two-, three-line service, respectively. Although these standards are legal definitions in the United States, some interfaces are used worldwide.
The connectors used for registered jack installations are the modular connector and the 50-pin miniature ribbon connector. For example, RJ11 uses a six-position two-conductor connector, RJ14 uses a six-position four-conductor modular jack, while RJ21 uses a 25-pair miniature ribbon connector; the registered jack designations originated in the standardization processes in the Bell System in the United States, describe application circuits and not just the physical geometry of the connectors. The same modular connector type may be used for different registered jack applications. Registered Jack refers to both the female physical connector and its wiring, but the term is used loosely to refer to modular connectors regardless of wiring or gender, such as in Ethernet over twisted pair. There is much confusion over these connection standards; the same six-position plug and jack used for telephone line connections may be used for RJ11, RJ14 or RJ25, all of which are names of interface standards that use this physical connector.
The RJ11 standard dictates a single wire pair connection, while RJ14 is a configuration for two lines, RJ25 uses all six wires for three telephones lines. The RJ designations, only pertain to the wiring of the jack, hence the name Registered Jack. Modular connectors were developed to replace older telephone installation methods that used either hardwired cords, or bulkier varieties of telephone plugs; the common nomenclature for modular connectors includes the number of contact positions and the number of wires connected, for example 6P indicates a six-position modular plug or jack. A six-position modular plug with conductors in the middle two positions and the other four positions unused has the designation 6P2C. RJ11 uses a 6P2C connector; the connectors could be supplied with more pins, but if more pins are wired, the interface is not an RJ11. Registration interfaces were created by the Bell System under a 1976 Federal Communications Commission order for the standard interconnection between telephone company equipment and customer premises equipment.
These interfaces used newly standardized jacks and plugs based on miniature modular connectors. The wired communications provider is responsible for delivery of services to a minimum point of entry; the MPOE is a utility box containing surge protective circuitry, which connects the wiring on the customer's property to the communication provider's network. Customers are responsible for all jacks and equipment on their side of the MPOE; the intent was to establish a universal standard for wiring and interfaces, to separate ownership of in-home telephone wiring from the wiring owned by the service provider. In the Bell System, following the Communications Act of 1934, the telephone companies owned all telecommunications equipment and they did not allow interconnection of third-party equipment. Telephones were hardwired, but may have been installed with Bell System connectors to permit portability; the legal case Hush-A-Phone v. United States and the Federal Communications Commission's Carterfone decision brought changes to this policy, required the Bell System to allow some interconnection, culminating in the development of registered interfaces using new types of miniature connectors.
Registered jacks replaced the use of protective couplers provided by the telephone company. The new modular connectors were much smaller and cheaper to produce than the earlier, bulkier connectors that were used in the Bell System since the 1930s; the Bell System issued specifications for the modular connectors and their wiring as Universal Service Order Codes, which were the only standards at the time. Large customers of telephone services use the USOC to specify the interconnection type and, when necessary, pin-assignments, when placing service orders with a network provider; when the U. S. telephone industry was reformed to foster competition in the 1980s, the connection specifications became federal law, ordered by the FCC and codified in the Code of Federal Regulations, Title 47 CFR Part 68, Subpart F, superseded by T1. TR5-1999. In January 2001, the FCC delegated responsibility for standardizing connections to the telephone network to a new private industry organization, the Administrative Council for Terminal Attachments.
For this delegation, the FCC removed Subpart F from the CFR and added Subpart G. The ACTA derives its recommend
Cable management refers to management of electrical or optical cable in a cabinet or an installation. The term is used for products, planning. Cables can become tangled, making them difficult to work with, sometimes resulting in devices accidentally becoming unplugged as one attempts to move a cable; such cases are known as "cable spaghetti", any kind of problem diagnosis and future updates to such enclosures could be difficult. Cable management both supports and contains cables during installation, makes subsequent maintenance or changes to the cable system easier. Products such as cable trays, cable ladders, cable baskets are used to support a cable through cabling routes; the choice of cables is important. Color-coding of cables is sometimes used to keep track of, which. For instance, the wires coming out of ATX power supplies are color-coded by voltage. Documenting and labeling cable runs, tying related cables together by cable ties, cable lacing, rubber bands or other means, running them through cable guides, clipping or stapling them to walls are other common methods of keeping them organized.
Above drop ceilings, hooks or trays are used to organize cables and protect them from electrical interference Planning is crucial for cables such as Thicknet that do not bend around corners and fiber optic, difficult to splice once cut. Cable strain relief is a mechanical protection for flexible electrical cables, wires and pneumatic hoses, it is regulated by the European standard EN 62444. With a strain relief component, the connection between a flexible electrical line and its connection port is protected against mechanical stress; the lines are fixed by clamping them into single cable clamps made of plastic or metal. Another possibility is to use so called cord grips which consist of weaved wire strands that put a grip around the cables. A more cable-friendly alternative is attaching the lines to special strain relief plates using common cable ties. In case of industrial applications these strain relief plates are as well cost-effective because the packing density is much higher than with common cable clamps which are designed for holding one single line.
Furthermore, most of the available cable clamps are not flexible when it comes to routing lines with varying diameters. That causes higher storage costs; the installation of the single cable clamps can take a lot of mounting time, depending on the laying length of the lines. Strain relief plates are therefore a more flexible solution which allows a parallel routing of several lines with varying diameters. Strain relief is required for terminated electrical lines that are plugged into sockets or ports to prevent unplugging or accidentally ripping out of the connector. At which point the lines have to be strain relieved depends on the application. For PROFINET, for example, used in automation it is recommended to set the strain relief component approx. 1 m / 3.5 ft from the connection point. Strain relief components are used in applications where cables and hoses are exposed to constant dynamic stress. One end of a cable is terminated in the data cabinet; the other end of a cable ends at the desk.
The cable management needs at either end. Buildings and office furniture are designed with cable management in mind; some cables have requirements for minimum bend radius or proximity to other cables power cables, to avoid crosstalk or interference. Power cables need to be grouped separately and suitably apart from data cables, only cross at right angles which minimizes electromagnetic interference; the organized routing of cables inside the computer case allows for optimal cooling. Good cable management makes working inside a computer much easier by providing safer hardware installation, repair, or removal; some PC mod enthusiasts showcase the internal components of their systems with a window mod, which displays the aesthetics of internal cabling as well as the skills and wealth of the modder. The IT industry needs data cables to be added, moved, or removed many times during the life of the installation, it is usual practice to install "fixed cables" between cabling cabinets. These cables are contained in cable trays etc. and are terminated at each end onto patch panels in the communications cabinet or outlets at the desktop.
The circuits are interconnected to the final destination using patch cords. Because large IT infrastructures encompass vast networks of cables -- all of which need to be serviced, added, so on throughout an installation lifecycle -- cable planning, in some fashion, is a necessity. Techs may employ different methods of cable planning, depending upon the level of detail required for proper management. Excel can be employed for organizing information, however techs need a way to visually organize information, which goes beyond Excel’s capabilities. For proper visualization of cabling, companies may opt to use a cable management software package. In hospital situations, cable management can be critical to preventing medical mistakes. Emergency room nurse manager Pat Gabriel said, "My wish is that we could somehow not have spaghetti on the bed; when you look
In electronics, a crossbar switch is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of interconnecting lines between which a connection may be established by closing a switch located at each intersection, the elements of the matrix. A crossbar switch consisted of crossing metal bars that provided the input and output paths. Implementations achieved the same switching topology in solid state semiconductor chips; the cross-point switch is one of the principal switch architectures, together with a rotary switch, memory switch, a crossover switch. A crossbar switch is an assembly of individual switches between a set of inputs and a set of outputs; the switches are arranged in a matrix. If the crossbar switch has M inputs and N outputs a crossbar has a matrix with M × N cross-points or places where the connections cross. At each crosspoint is a switch. A given crossbar is non-blocking switch. Non-blocking switch means that other concurrent connections do not prevent connecting other inputs to other outputs.
Collections of crossbars can be blocking switches. A crossbar switching system is called a coordinate switching system. Crossbar switches are used in information processing applications such as telephony and circuit switching, but they are used in applications such as mechanical sorting machines; the matrix layout of a crossbar switch is used in some semiconductor memory devices. Here the bars are thin metal wires, the switches are fusible links; the fuses are read using low voltage. Such devices are called programmable read-only memory. At the 2008 NSTI Nanotechnology Conference a paper was presented that discussed a nanoscale crossbar implementation of an adding circuit used as an alternative to logic gates for computation. Matrix arrays are fundamental to modern flat-panel displays. Thin-film-transistor LCDs have a transistor at each crosspoint, so they could be considered to include a crossbar switch as part of their structure. For video switching in home and professional theater applications, a crossbar switch is used to distribute the output of multiple video appliances to every monitor or every room throughout a building.
In a typical installation, all the video sources are located on an equipment rack, are connected as inputs to the matrix switch. Where central control of the matrix is practical, a typical rack-mount matrix switch offers front-panel buttons to allow manual connection of inputs to outputs. An example of such a usage might be a sports bar, where numerous programs are displayed simultaneously. Ordinarily, a sports bar would install a separate desk top box for each display for which independent control is desired; the matrix switch enables the operator to route signals at will, so that only enough set top boxes are needed to cover the total number of unique programs to be viewed, while making it easier to control sound from any program in the overall sound system. Such switches are used in high-end home theater applications. Video sources shared include set-top receivers or DVD changers; the outputs are wired to televisions in individual rooms. The matrix switch is controlled via an Ethernet or RS-232 connection by a whole-house automation controller, such as those made by AMX, Crestron, or Control4, which provides the user interface that enables the user in each room to select which appliance to watch.
The actual user interface varies by system brand, might include a combination of on-screen menus, touch-screens, handheld remote controls. The system is necessary to enable the user to select the program they wish to watch from the same room they will watch it from, otherwise it would be necessary for them to walk to the equipment rack; the special crossbar switches used in distributing satellite TV signals are called multiswitches. A crossbar switch consisted of metal bars associated with each input and output, together with some means of controlling movable contacts at each cross-point. In the part of the 20th century, these literal crossbar switches declined and the term came to be used figuratively for rectangular array switches in general. Modern crossbar switches are implemented with semiconductor technology. An important emerging class of optical crossbars is being implemented with MEMS technology. A type of middle 19th-century telegraph exchange consisted of a grid of vertical and horizontal brass bars with a hole at each intersection.
The operator inserted a brass pin to connect one telegraph line to another. A telephony crossbar switch is an electromechanical device for switching telephone calls; the first design of what is now called a crossbar switch was the Bell company Western Electric's coordinate selector of 1915. To save money on control systems, this system was organized on the stepping switch or selector principle rather than the link principle, it was little used in America, but the Televerket Swedish governmental agency manufactured its own design, used it in Sweden from 1926 until the digitalization in the 1980s in small and medium-sized A204 model switches. The system design used in AT&T Corporation's 1XB crossbar exchanges, which entered revenue service from 1938, developed by Bell Telephone Labs, was inspired by the Swedish design but was based on the rediscovered link principle. In 1945, a similar design by Swedish Televerket was i
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
A telephone switchboard is a telecommunications system used in the public switched telephone network or in enterprises to interconnect circuits of telephones to establish telephone calls between the subscribers or users, or between other exchanges. The switchboard was an essential component of a manual telephone exchange, was operated by switchboard operators who used electrical cords or switches to establish the connections; the electromechanical automatic telephone exchange, invented by Almon Strowger in 1888 replaced manual switchboards in central telephone exchanges around the world. In 1919, the Bell System in Canada adopted automatic switching as its future technology, after years of reliance on manual systems. Many manual branch exchanges remained operational into the second half of the 20th century in many enterprises. Electronic devices and computer technology gave the operator access to an abundance of features. A private branch exchange in a business has an attendant console for the operator, or an auto-attendant, which bypasses the operator entirely.
Following the invention of the telephone in 1876, the first telephones were rented in pairs which were limited to conversation between the parties operating those two instruments. The use of a central exchange was soon found to be more advantageous than in telegraphy. In January 1878 the Boston Telephone Dispatch company had started hiring boys as telephone operators. Boys had been successful as telegraphy operators, but their attitude, lack of patience, behavior was unacceptable for live telephone contact, so the company began hiring women operators instead. Thus, on September 1, 1878, Boston Telephone Dispatch hired Emma Nutt as the first woman operator. Small towns had the switchboard installed in the operator's home so that he or she could answer calls on a 24-hour basis. In 1894, New England Telephone and Telegraph Company installed the first battery-operated switchboard on January 9 in Lexington, Massachusetts. Early switchboards in large cities were mounted floor to ceiling in order to allow the operators to reach all the lines in the exchange.
The operators were boys. Late in the 1890s this measure failed to keep up with the increasing number of lines, Milo G. Kellogg devised the Divided Multiple Switchboard for operators to work together, with a team on the "A board" and another on the "B"; these operators were always women until the early 1970s, when men were once again hired. Cord switchboards were referred to as "cordboards" by telephone company personnel. Conversion to Panel switch and other automated switching systems first eliminated the "B" operator and usually years the "A". Rural and suburban switchboards for the most part remained simple. In many cases, customers came to know their operator by name; as telephone exchanges converted to automatic service, switchboards continued to serve specialized purposes. Before the advent of direct-dialed long distance calls, a subscriber would need to contact the long-distance operator in order to place a toll call. In large cities, there was a special number, such as 112, which would ring the long-distance operator directly.
Elsewhere, the subscriber would ask the local operator to ring the long-distance operator. The long distance operator would record the name and city of the person to be called, the operator would advise the calling party to hang up and wait for the call to be completed; each toll center had only a limited number of trunks to distant cities, if those circuits were busy, the operator would try alternate routings through intermediate cities. The operator would plug into a trunk for the destination city, the inward operator would answer; the inward operator would obtain the number from the local information operator, ring the call. Once the called party answered, the originating operator would advise him or her to stand by for the calling party, whom she'd ring back, record the starting time, once the conversation began. In the 1940s, with the advent of dial pulse and multi-frequency operator dialing, the operator would plug into a tandem trunk and dial the NPA and operator code for the information operator in the distant city.
For instance, the New York City information operator was 212-131. If the customer knew the number, the point was direct-dialable, the operator would dial the call. If the distant city did not have dialable numbers, the operator would dial the code for the inward operator serving the called party, ask her to ring the number. In the 1960s, once most phone subscribers had direct long-distance dialing, a single type of operator began to serve both the local and long distance functions. A customer might call to request a collect call, a call billed to a third number, or a person-to-person call. All toll calls from coin phones required operator assistance; the operator was available to help complete a local or long-distance number which did not complete. For example, if a customer encountered a reorder tone, it could indicate "all circuits busy," or a problem in the destination exchange; the operator might be able to use a different routing to complete the call. If the operator could not get through by dialing the number, she could call the inward operator in the destination city, ask her to try the number, or to test a line to see if it was busy or out of order.
Cord switchboards used for these purposes were replaced in the 1970s and 1980s by TSPS and similar systems, which reduced operator involvement in calls. The customer would, instead of dialing "0" for the operator, dial 0+NPA+7digits, after which an operator would answer and provide th
A vision mixer is a device used to select between several different video sources and, in some cases, compositing video sources together to create special effects. This is similar to. A vision mixer would be found in a video production environment such as a television studio, production truck, OB Van or linear video editing bay of a post-production facility. In most of the world, both the equipment and its operator are called a vision video mixer. Besides hard cuts, mixers can generate a variety of transitions, from simple dissolves to pattern wipes. Additionally, most vision mixers can generate color signals. Most vision mixers are targeted at the professional market, with newer analog models having component video connections and digital ones using Serial Digital Interface, they are used in live television, such as outside broadcasts, with video tape recording and video servers for linear video editing though the use of vision mixers in video editing has been supplanted by computer based Non-linear editing systems.
Older professional mixers worked with analog signal inputs. There are still a number of consumer video switchers with composite video, S-Video or FireWire available; these are used for VJing and small multi-camera productions. The most basic part of a vision mixer is a bus, a signal path consisting of multiple video inputs that feeds a single output. On the panel, a bus is represented by a row of buttons. Older video mixers had two equivalent buses, one of these buses could be selected as the main out bus. Most modern mixers, have one bus, always the program bus, the second main bus being the preview bus; these mixers are called flip-flop mixers, since the selected source of the preview and program buses can be exchanged. Some switchers allow the operator to switch between these two modes. Both the preview and program bus have their own video monitors displaying the video selected in their respective buses. Another main feature of a vision mixer is the transition lever called a T-bar or Fader Bar.
This lever, similar to an audio fader, is used to transition between two buses. Note that in a flip-flop mixer, the position of the main transition lever does not indicate which bus is active, since the program bus is always the active or hot bus. Instead of moving the lever by hand, a button can be used, which performs the transition over a user-defined period of time. Another button labeled "cut" or "take", swaps the preview signal to the program signal instantaneously; the type of transition used can be selected in the transition section. Common transitions include dissolves and pattern wipes. A third bus used for compositing is the key bus. A mixer can have more than one key bus, but they share only one set of buttons. Here, one signal can be selected for keying over the program bus; the digital on-screen graphic image that will be seen in the program is called the fill, while the mask used to cut the key's translucence is called the source. This source, e.g. chrominance, pattern or split and can be selected in the keying section of the mixer.
Note that instead of the key bus, other video sources can be selected for the fill signal, but the key bus is the most convenient method for selecting a key fill. A key is turned on and off the same way a transition is. For this, the transition section can be switched from program mode to key mode; the transition section allows background video and one or more keyers to be transitioned separately or in any combination with one push of the "auto" button. These three main buses together form the basic mixer section called Program/Preset or P/P. Bigger production mixers may have a number of additional sections of this type, which are called Mix/Effects sections and numbered. Any M/E section can be selected as a source in the P/P stage, making the mixer operations much more versatile, since effects or keys can be composed "offline" in an M/E and go "live" at the push of one button. After the P/P section, there is another keying stage called the downstream keyer, it is used for keying text or graphics, has its own "Cut" and "Mix" buttons.
The signal before the DSK keyer is called clean feed. After the DSK is one last stage that overrides any signal with black called Fade To Black or FTB. Modern vision mixers may have additional functions, such as serial communications with the ability to use proprietary communications protocols, control auxiliary channels for routing video signals to other sources than the program out, macro programming, DVE capabilities. Mixers are equipped with effects memory registers, which can store a snapshot of any part of a complex mixer configuration and recall the setup with one button press. Since vision mixers combine various video signals such as VTRs and professional video cameras, it is important that all these sources are in proper synchronization with one another. In professional analog facilities all the equipment