ibre links. Yet another reason for modulation is the range of transmission. Voice waveforms, in their baseband frequency, can be carried by air and be detected by human ear over a distance mea- sured in tens of metres. The same voice signal may be modulated using amplitude modulation and transmitted to a distance of tens of kilometres using modulating frequency in the AM radio range (500-1300 kHz) over the air. The same voice signal may be modulated in the short wave radio range (35-100 MHz) and transmitted halfway around the earth, since signals transmitted at this range use the earth's stratosphere as a communication channel. Modulation of signals using different carrier frequencies enables different range and quality of transmission. Demodulation At the receiver a signal must be transferred back to the baseband SO that its information is extracted. For example, a sound signal can only be heard by human ear if it is played back at baseband. Principally AM signals are demodulated through multiplication by the carrier signal followed by baseband filtering. This is illustrated in Figure 3.15. Mathematically this process is shown below, where r(t) represents the signal after the received signal s(t) is multiplied by the carrier signal: r(t)=s(t)Acos(2xft) A low pass filter suppresses the signal with 2f frequency and the baseband signal x(t) multiplied by a constant factor is obtained. Filter fc Frequency 0 Frequency Figure 3.15 AM demodulation process Broadband Telecommunications Technologies and Management Call 1 Call 2 Call n Frequency (Hz) Figure 3.16 Frequency division multiplexing process Multiplexing The links that connect exchanges together need to carry multiple calls as discussed above. As shown in Figure 3.6 these links are nowadays mainly made of optical fibres which can carry a large number of calls simultaneously. Before optical fibres these links were made of coaxial cables which were capable of carrying tens to hundreds of calls simultaneously. Each voice call needs its own dedicated channel over the