easily. Morse has a number of technical advantages. Its relatively slow signalling rate means that it occupies a narrow bandwidth. As a result very narrow filters can be used to cut out most of the interference. It is also found that because the human brain only has to detect the presence or absence of a signal, it can be read at a lower level than a signal carrying speech. These two factors together mean that Morse can be copied at much lower signal levels than several other forms of transmission. If Morse is received by an ordinary domestic portable radio receiver it is simply heard as a series of clicks and pops as the signal is turned on and off. To make the characteristic tone, a circuit called a beat frequency oscillator (BFO) is needed. This circuit generates a signal that mixes or beats with the incoming signal to make the characteristic Morse sounds of a Morse signal. These days the use of Morse is relatively restricted. It is no longer used as a primary mode for maritime purposes, as most ships these days use satellite communications systems that use other modes of transmission. Other services including the military have also ceased to use it, and as such its use is confined to a very few professional applications and radio amateurs. Amplitude modulation Morse code has its advantages but we have all become accustomed to hearing music and speech over the radio. There are a number of ways in which a carrier can be modulated to take an audio signal. The most obvious way is to change its amplitude in line with the variations in intensity of the sound wave. In this way the overall amplitude or envelope of the carrier is modulated to carry the audio signal as shown in Figure 3.3. Here the envelope of the carrier can be seen to change in line with the modulating signal. Amplitude modulation or AM is one of the most straightforward methods of modulating a signal. Demodulation, or the process where the radio frequency signal is converted into an audio frequency signal, is also very simple. It only requires a sim