 use in present broadband systems. Voice Communications Modulation Modulation is the process of carrying desired data contained within a signal through varying a carrier signal. The data can be embedded within the signal amplitude as shown in Figure 3.12. Here an information signal x(t) modulates a carrier signal, in this case a cosine waveform, 2n ft+0), where f represents the frequency of the carrier signal. Since the amplitude of the carrier signal is modulated by the information signal, this method is known as amplitude modulation (AM). In AM systems, a frequency waveform's amplitude is changed relative to a data signal, and as a result the envelope of the modulated signal carries the data. Data can also be embedded in the frequency of a carrier signal. This is known as frequency modulation (FM). In FM systems the frequency of the carrier wave is changed relative to the information signal's amplitude: the higher the amplitude of the information signal, the higher is the frequency of the modulated signal. Conversely, the lower the amplitude of the modu- lating signal, the lower is the frequency of the modulated signal. This is illustrated in Figure 3.13. AM and FM technologies are well known in radio broadcasting. Both AM and FM technologies are commonly used for multiplexing in telephony systems. In general FM is more robust against noise and interference and is used in wireless telecom- munications. For example, the first generation of mobile phones used FM. AM radio is less robust and is therefore used in applications where noise and interference levels are relatively low, such as in fixed line communications. A variation of AM technology was used to multi- plex calls on inter-exchange links. Amplitude modulation can be accomplished using a simple signal multiplication. Assume the information signal is x(t) and the carrier signal has a frequency of f and a signal amplitude of A written as: A cos (2nft) Figure 3.12 Amplitude modulation Broadband Telecommunications Technologies and Management Figure 3.13 Freq