uency modulation Frequency Frequency Figure 3.14 Amplitude modulation process The modulated signal s(t) is simply the product of the information signal and the carrier signal and is written as: (t)=Ax(t)cos(2r.ft) The process of amplitude modulation can be implemented in hardware as illustrated in Figure 3.14. The modulation process transfers a baseband signal (e.g. a 0-4 kHz voice signal) to higher frequency band of a carrier signal. Why is Modulation Needed? There are a number of reasons why modulation is necessary. The frequency of the original information signal, called the baseband frequency, is generally very low. Natural phenomena frequencies and bandwidth are not very high. For example, the human voice's maximum fre- quency is some 4 kHz, and video signal variations are in the order of several megahertz. These signals can be transmittable over a number of media, for example air, space and optical fibres. However different media have different capacity to transfer information, and it may be ineffi- cient to dedicate them to only one signal stream. For example, coaxial cables have a band- width of several megahertz and are capable of transferring hundreds of voice calls simultaneously. Should transmissions be carried out at baseband, only one voice signal can be Voice Communications accommodated over the channel. This is because two voice signals transmitted simultaneously at baseband over the same medium would mix and interfere with each other. Modulation to different carrier frequencies allows for these two calls to be carried simultaneously over the same medium. This process is called call multiplexing, and is further explained below. Another reason for modulation is that transmission of signals may not be possible at its baseband frequency over a medium. For example, transmission of voice signals over optical fibres is not possible in the baseband. The signals need to be modulated to higher frequencies - such as those of the visible and invisible light range - for them to be transmittable over optical f