dio band. For transmissions like SSB a low-pass filter with a response like that shown in Figure 5.8(b) is generally used. This allows through the low frequency signals and removes those above a certain cutoff frequency. This will help reduce interference from signals off channel which produce high frequency audio interference. Simple direct conversion receivers can only receive a limited number of types of transmission satisfactorily. Take the example of a Morse transmission. This consists of a carrier wave being turned on and off. When the oscillator in the receiver moves close to the frequency of the Morse signal, an audio note equal in frequency to the difference between the two signals will be generated giving the characteristic on/off tone for Morse. Similarly for single sideband, the oscillator will beat with the sideband to give the required audio. However, for the pitch of the audio to be correct the local oscillator must be on the same frequency that the carrier would have been if it was present. An amplitude modulated signal is not quite SO easy to resolve. If there is a difference in frequency between the carrier of the incoming signal and the local oscillator then an annoying beat note is generated. To be able to resolve the signal satisfactorily the local oscillator must be tuned to exactly the same frequency as the carrier of the required signal. If this is done the audio is recovered and can be heard satisfactorily. One mode that cannot be resolved easily is frequency modulation. This has meant that this type of receiver is generally only used on the HF bands where Morse and SSB are widely used. When used for communications applications, one of the problems with a direct conversion receiver is called the audio image and it can be demonstrated as the set is tuned through a steady signal. First, a high frequency audio heterodyne or oscillation is heard. As the receiver is tuned closer to the signal this note falls until eventually the local oscillator of the receiver and the signal are on the same f