In 2009, the research firm SNL Kagan estimated that sales of ringtones in the United States peaked at $714 million in 2007. In 2003, the global ringtone industry was worth somewhere between US$2.5 and US$3.5 billion. In 2003 for example, the Japanese ringtone market, which alone was worth US$900 million, experienced US$66.4 million worth of sound file ringtone sales. The rise of sound files also contributed to the popularization of ringtones. According to Fortune magazine, ring tones generated more than $2 billion in worldwide sales during 2005. The Manhattan-based marketing and consulting firm Consect estimated ringtones generated $4 billion in worldwide sales in 2004. The fact that consumers are willing to pay up to $3 for ringtones has made mobile music a profitable part of the music industry. Mobile phones have been fully digital since the early 1990s second-generation ('2G') devices, hence are signaled to ring as part of the protocol they use to communicate with the cell base stations. Fixed phones of the late 20th century and later detect this ringing current voltage and trigger a warbling tone electronically. Originally this voltage was used to trigger an electromagnet to ring a bell installed inside the telephone, or in a nearby mounted ringer box. Typically, solid-state oscillators have replaced them. In smaller offices, special sub-cycle magnetic oscillators were used. Large central offices used motor-driven generator sets for both ringing and other signals such as dial tone and busy signals. Ringing voltage is produced by various sources. Some non-Bell Company system party lines in the US used multiple frequencies for selective ringing. In Europe it is around 60–90 VAC with a frequency of 25 Hz. 90 volts AC with a frequency of 20 hertz. The ringing signal in North America is normally specified at ca. (SLC is a trademark of Alcatel-Lucent, but is often used generically.) Telephones with electromagnetic ringers are still in widespread use. This is done at the Central Office, or a neighborhood multiplexer called a 'SLC' for Subscriber Line Carrier. On a POTS interface, this signal is created by superimposing ringing voltage atop the −48 VDC already on the line.
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