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A ringtone or ring tone is the sound made by a telephone to indicate an incoming call or text message. Not literally a tone nor an actual (bell-like) ring any more, the term is most often used today to refer to customizable sounds used on mobile phones. ==Background== A phone “rings” when its network indicates an incoming call and the phone thus alerts the recipient. For landline telephones, the call signal can be an electric current generated by the switch or exchange to which the telephone is connected, which originally drove an electric bell. For mobile phones, the network sends the phone a message indicating an incoming call. The sound the caller hears is called the ringback tone, which is not necessarily directly related. The electromagnetic bell system is still in widespread use. The ringing signal sent to a customer's telephone is 90 volts AC at a frequency of 20 hertz in North America. In Europe it is around 60-90 volts AC at a frequency of 25 hertz. Some non-Bell Company system party lines in the US used multiple frequencies (20/30/40 Hz, 22/33/44 Hz, etc.) to allow "selective" ringing. While the sound produced is still called a “ring”, more-recently manufactured telephones electronically produce a warbling, chirping, or other sound. Variation of the ring signal can be used to indicate characteristics of incoming calls (for example, rings with a shorter interval between them might be used to signal a call from a given number). A ''ringing signal'' is an electric telephony signal that causes a telephone to alert the user to an incoming call. On a POTS interface, this signal is created by superimposing ringing voltage (volts AC at 20 Hz in the USA ) atop the -48VDC already on the line. This is done at the Central Office, or a neighborhood multiplexer called a "SLC" for Subscriber Line Carrier. (SLC is a trademark of Alcatel-Lucent, but is often used generically.) This ring voltage came from various sources. In large Central Offices, there were 48VDC motor-driven generator sets for both ringing & other signals such as dial tone and busy signals. In smaller offices, a special Sub-Cycle〔http://www.telephonecollectors.info/index.php?option=com_docman&task=doc_view&gid=319&Itemid=2〕 magnetic oscillator was used. More recently, solid-state oscillators have replaced them. Originally this voltage was used to trigger a high-impedance electromagnet to ring a bell on the phone. Fixed phones of the late 20th century and later detect this ringing current voltage and trigger a warbling tone electronically. Mobile phones are fully digital, hence are signaled to ring as part of the protocol they use to communicate with the cell base stations. In POTS switching systems, ringing is said to be "tripped" when the impedance of the line reduces to about 600 ohms when the telephone handset is lifted off the switch-hook. This signals that the telephone call has been answered, and the telephone exchange immediately removes the ringing signal from the line and connects the call. This is the source of the name of the problem called "ring-trip" or "pre-trip", which occurs when the ringing signal on the line encounters excessively low resistance between the conductors, which trips the ring before the subscriber's actual telephone has a chance to ring (for more than a very short time); this is common with wet connections and improperly installed lines. The ringing pattern is known as ring cadence. This only applies to POTS fixed phones, where the high voltage ring signal is switched on and off to create the ringing pattern. In North America, the standard ring cadence is "2-4", or two seconds of ringing followed by four seconds of silence. In Australia and the UK, the standard ring cadence is 400 ms on, 200 ms off, 400 ms on, 2000 ms off. These patterns may vary from region to region, and other patterns are used in different countries around the world. Some central offices offer distinctive ring to identify which of multiple numbers on the same line is being called, a pattern once widely used on party line (telephony). In many systems, including North America Bellcore standards, Caller ID signals are sent during the silent interval between the first and second bursts of the ringing signals. 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「ringtone」の詳細全文を読む スポンサード リンク
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