to be represented as alphanumerics. Early teleprinter systems used a code called the Murray code. Often ASCII (American Standard Code for Information Interchange) is used for the basis of some systems now. Also as FSK systems are widely used on the HF portion of the spectrum where levels of interference are high, the systems use high levels of error checking and acknowledgements when a packet of data has been received. These systems enable the data to be received with very few errors even when some interference is present. Phase modulation Another form of modulation that is widely used is called phase modulation. As phase and frequency are inextricably linked (frequency being the rate of change of phase) both forms of modulation are often referred to by the common name of angle modulation. To explain how phase modulation works it is first necessary to give an explanation of phase. A radio signal consists of an oscillating carrier in the form of a sine wave as shown in Figure 3.10. The amplitude follows this curve moving positive and then negative, returning to the start point after one complete cycle. This can also be represented by the movement of a point around a circle, the phase at any given point being the angle between the start point and the point on the waveform as shown. Speed of of rotation modulated to give phase modulation 90 Direction of rotation 270 o Figure 3.10 Phase modulation 64 Newnes Guide to Radio and Communications Technology Modulating the phase of the signal changes the phase of the signal from what it would have been if no modulation was applied. In other words the speed of rotation around the circle is modulated about the mean value as shown in Figure 3.10. To achieve this it is necessary to change the frequency of the signal for a short time. In other words when phase modulation is applied to a signal there are frequency changes and vice versa. Phase and frequency are inseparably linked as phase is the integral of frequency. Frequency modulation can be changed to phase modulation by sim