|
A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound, typically and most efficiently the human voice, into electronic signals suitable for transmission via cables or other transmission media over long distances, and replays such signals simultaneously in audible form to its user. In 1876, Scottish emigrant Alexander Graham Bell was the first to be granted a United States patent for a device that produced clearly intelligible replication of the human voice. This instrument was further developed by many others. The telephone was the first device in history that enabled people to talk directly with each other across large distances. Telephones rapidly became indispensable to businesses, government, and households, and are today some of the most widely used small appliances. The essential elements of a telephone are a microphone (''transmitter'') to speak into and an earphone (''receiver'') which reproduces the voice in a distant location. In addition, most telephones contain a ''ringer'' which produces a sound to announce an incoming telephone call, and a dial or keypad used to enter a telephone number when initiating a call to another telephone. Until approximately the 1970s most telephones used a rotary dial, which was superseded by the modern DTMF push-button dial, first introduced to the public by AT&T in 1963.〔Dodd, Annabel Z., ''The Essential Guide to Telecommunications''. Prentice Hall PTR, 2002, p. 183.〕 The receiver and transmitter are usually built into a handset which is held up to the ear and mouth during conversation. The dial may be located either on the handset, or on a base unit to which the handset is connected. The transmitter converts the sound waves to electrical signals which are sent through the telephone network to the receiving phone. The receiving telephone converts the signals into audible sound in the receiver, or sometimes a loudspeaker. Telephones permit duplex communication, meaning they allow the people on both ends to talk simultaneously. The first telephones were directly connected to each other from one customer's office or residence to another customer's location. Being impractical beyond just a few customers, these systems were quickly replaced by manually operated centrally located switchboards. This gave rise to landline telephone service in which each telephone is connected by a pair of dedicated wires to a local central office switching system, which developed into fully automated systems starting in the early 1900s. As greater mobility was desired for commerce and convenience, various radio systems were developed for transmission between mobile customer stations on ships and automobiles from the 1930s by the mid-1900s. Radio systems evolved into various cellular topologies until the first hand-held mobile phone was introduced for personal service starting in 1973 by Motorola. By the late 1970s several mobile telephone networks operated around the world. In 1983, the Advanced Mobile Phone System (AMPS) was launched in the U.S. and in other countries soon after, and offered a standardized technology providing portability for user's within a region far beyond the personal residence or office location. These analog cellular system evolved into digital networks with better security, greater capacity, better regional coverage, and lower cost. The public switched telephone network, with its hierarchical system of many switching centers, interconnects telephones around the world for communication with each other. With the standardized international numbering system, E.164, each telephone line has an identifying telephone number, that may be called from any authorized telephone on the network. Although originally designed for simple voice communications, convergence has enabled most modern cell phones to have many additional capabilities. They may be able to record spoken messages, send and receive text messages, take and display photographs or video, play music or games, surf the Internet, do road navigation or immerse the user in virtual reality. Since 1999, the trend for mobile phones is smartphones that integrate all mobile communication and computing needs. == Basic principles == A traditional landline telephone system, also known as ''plain old telephone service'' (POTS), commonly carries both control and audio signals on the same twisted pair (''C'' in diagram) of insulated wires, the telephone line. The control and signaling equipment consists of three components, the ringer, the hookswitch, and a dial. The ringer, or beeper, light or other device (A7), alerts the user to incoming calls. The hookswitch signals to the central office that the user has picked up the handset to either answer a call or initiate a call. A dial, if present, is used by the subscriber to transmit a telephone number to the central office when initiating a call. Until the 1960s dials used almost exclusively the rotary technology, which was replaced by dual-tone multi-frequency signaling (DTMF) with pushbutton telephones (A4). A major expense of wire-line telephone service is the outside wire plant. Telephones transmit both the incoming and outgoing speech signals on a single pair of wires. A twisted pair line rejects electromagnetic interference (EMI) and crosstalk better than a single wire or an untwisted pair. The strong outgoing speech signal from the microphone (transmitter) does not overpower the weaker incoming speaker (receiver) signal with sidetone because a hybrid coil (A3) and other components compensate the imbalance. The junction box (B) arrests lightning (B2) and adjusts the line's resistance (B1) to maximize the signal power for the line length. Telephones have similar adjustments for inside line lengths (A8). The line voltages are negative compared to earth, to reduce galvanic corrosion. Negative voltage attracts positive metal ions toward the wires. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Telephone」の詳細全文を読む スポンサード リンク
|