ferent signals can be transmitted together using a system known as time division multiplexing. Using this type of modulation, short periodic samples are transmitted. The modulating waveform is sampled as shown in Figure 3.15(a) and this information is used to change the characteristics of the pulses in one of a variety of ways. Once at the receiver the samples are used to reconstitute the original signal which can be used as required. Surprisingly few samples are needed and it can be shown that if an audio signal is sampled at a frequency just over twice the highest frequency present, then the samples will contain all the necessary information to recreate the signal in its original format. Take as an example the waveform shown in Figure 3.15(b). If the waveform was sampled once a cycle then each sample would have the same value and there would be no indication about the 68 Newnes Guide to Radio and Communications Technology Voltage Successive sample points (a) Samples taken periodically Voltage (b) Minimum number of samples Figure 3.15 Sampling a waveform actual waveform. However, if the waveform is sampled at twice the frequency of the waveform, the samples alternate either side of the zero axis, and it can be deduced that it is varying and has a certain frequency. There are a number of ways in which pulses derived from samples of a waveform can be used to modulate a carrier. Possibly the most obvious is pulse amplitude modulation (PAM) where the amplitude of the pulses represents the amplitude of the waveform at that point as shown in Figure 3.16. It can also be seen that it is possible to place more than one set of modulating pulses on the signal, by spacing them apart in time. This concept is known as time division multiplexing. At the transmitting end of the link the pulses are placed onto the carrier in a given order. By taking the pulses of carrier at the receiver end in the same order, each signal can be properly reconstituted. Modulation 69 Original waveform (a) Varying the pulse height to represent the 