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Modulation can also be used to transmit the information of analogue signals at higher frequencies. This is helpful because low-frequency analogue signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analogue signal must be superimposed on a higher-frequency signal (known as a [[carrier wave]]) before transmission. There are several different modulation schemes available to achieve this (two of the most basic being [[amplitude modulation]] and [[frequency modulation]]). An example of this process is a [[disc jockey|DJ's]] voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel “96 FM”).<ref>Haykin, pp 88-126.</ref>

 

Telecommunication is an important part of modern society. In 2006, estimates placed the telecommunication industry's revenue at $1.2 trillion or just under 3% of the [[gross world product]] (official exchange rate).<ref>[http://www.voip-magazine.com/content/view/1197/ Telecom Industry Revenue to Reach $1.2 Trillion in 2006], VoIP Magazine, 2005.</ref>