 binary code that enters a circuit called a digital to analogue converter. This regenerates the audio signal into a form of staircase waveform with steps as shown. The steps can be removed by adding a capacitor circuit to smooth the waveform, giving a very close approximation to the original signal. Staircase waveform Reconstituted waveform after smoothing Figure 3.18 Regenerating an analogue signal from its digital representation Modulation 71 The main disadvantage of PCM when compared to PAM or PWM is that far more pulses are transmitted, and this means that a much greater bandwidth is required. If eight pulses are used to represent each sample then the bandwidth will be eight times as much. However, the increase in noise immunity it provides usually more than outweighs the problems with the bandwidth. Spread spectrum techniques In many instances it is necessary to keep transmissions as narrow as possible to conserve the frequency spectrum. However, under some circumstances it is advantageous to use what are known as spread spectrum techniques where the transmission is spread over a wide bandwidth. There are two ways of achieving this. One is to use a technique known as frequency hopping, while the other involves spreading the spectrum over a wide band of frequencies SO it appears as background noise. These can be done in different ways and the two most widely used systems are DSSS and OFDM. Frequency hopping In some instances, particularly in military applications, it is necessary to prevent any listeners apart from those intended from picking up a signal or from jamming it. It may also be used to reduce levels of interference. If interference is present on one channel, the hopping signal will only remain there for a short time and the effects of the interference will be short lived. Frequency hopping is a well-established principle. In this system the signal is changed many times a second in a pseudo-random sequence from a predefined block of channels. Hop rates vary dependent upon the requirements. Typically