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This figure consists of four line graphs labelled A to D. Each graph has the X or horizontal axis labelled as time, measured in seconds. The Y or vertical axes are labelled as voltage, measured in volts.

Part A shows how a series of zeros and ones can be represented by a digital signal. Time periods are represented on the graph by a series of equidistant dotted vertical lines. The sequence 0 1 1 1 0 0 0 1 0 1 is shown from left to right. Each 0 and 1 in the series corresponds to one of the time periods created by the dotted vertical lines. The voltage is initially at value zero for one time period, then rises to a value of one for three time periods, and falls to zero for three time periods. It then rises to one for a single time period, drops to zero for a single time period and rises to one for a single time period.

Part B shows a sinusoidal waveform starting at the origin. It rises smoothly to a rounded peak of 1 volt. The line then drops smoothly down to a rounded trough the same distance below the X axis as the peak is above. Then it rises again to the next peak, which is the same height as the previous peak, and then drops again to a trough of the same depth. Many cycles are shown. The frequency of the sinusoidal waveform corresponds to three cycles per time period created by the dotted vertical lines.

Part C is a combination of parts A and B. Where the voltage in part A is zero, no waveform is shown. Where the voltage in part A is one, a sinusoidal segment of 1 volt amplitude is shown. Time periods are represented by a series of equidistant dotted vertical lines, and correspond to those illustrated in part A. The signal is initially at value zero for one time period, then the sinusoid is shown for three time periods, before reverting to zero for three time periods. The sinusoid then appears for a single time period, drops to zero for a single time period and appears again for a single time period.

Part D is similar to part C, but where the voltage in part A is zero, here a waveform is shown with reduced amplitude. Where the voltage in part A is one, a sinusoidal signal of full amplitude of 1 volt is shown. Time periods are represented by a series of equidistant dotted vertical lines, as in parts A and C. The sinusoidal signal has a smaller amplitude of about 0.5 volts for one time period, then the sinusoid is shown at full amplitude for three time periods, before reverting back to the smaller amplitude for three time periods. The sinusoid then appears at full amplitude for a single time period, drops to small amplitude for a single time period and appears again at full amplitude for a single time period.

 1.4 Amplitude-shift keying (ASK)