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A scanner is a radio receiver that can automatically tune, or ''scan'', two or more discrete frequencies, stopping when it finds a signal on one of them and then continuing to scan other frequencies when the initial transmission ceases. The terms ''radio scanner'' or ''police scanner'' generally refer to a communications receiver that is primarily intended for monitoring VHF and UHF landmobile radio systems, as opposed to, for instance, a receiver used to monitor international shortwave transmissions. More often than not, these scanners can also tune to different types of modulation as well (AM, FM, WFM, etc.). Early scanners were slow, bulky, and expensive. Today, modern microprocessors have enabled scanners to store thousands of channels and monitor hundreds of channels per second. Recent models can follow trunked radio systems and decode APCO-P25 digital transmissions. Both hand held and desktop models are available. Scanners are often used to monitor police, fire and emergency medical services. Radio scanning serves an important role in the fields of journalism and crime investigation, as well as a hobby for many people around the world. == History and use == Scanners developed from earlier tunable and fixed-frequency radios that received one frequency at a time. Non-broadcast radio systems, such as those used by public safety agencies, do not transmit continuously. With a radio fixed on a single frequency, much time could pass between transmissions, while other frequencies might be active. A scanning radio will sequentially monitor multiple programmed channels, or search between user defined frequency limits. The scanner will stop on an active frequency strong enough to break the radio's squelch setting and resume scanning other frequencies when that activity ceases. Scanners first became popular and widely available during the heyday of CB radio in the 1970s. The first scanners often had between four and ten channels and required the purchase of a separate crystal for each frequency received. A US patent was issued to Peter W. Pflasterer on June 1, 1976.〔http://www.google.com/patents/about?id=sAIuAAAAEBAJ〕 An early 1976 US entry was the Tennelec MCP-1, sold at the January 1976 Consumer Electronics Show in Chicago. 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「Scanner (radio)」の詳細全文を読む スポンサード リンク
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