efficient mode of transmission. There are two main stages in the generation of a single sideband signal. The first is that the carrier is removed. This does not contribute to carrying the sound information, and is only used during the demodula- tion process. As a result it is possible to remove it in the transmitter as shown in Figure 3.8, enabling power to be saved. It is also found that only one sideband is needed. Both the upper and the lower sidebands are exact mirror images of one another, and either can be used equally well for conveying the sound information. If two sidebands are present on a signal twice the bandwidth is used. To save on the use of bandwidth it is possible to remove one sideband without Position of Amplitude the carrier Position of the One remaining removed sideband sideband Frequency Figure 3.8 Spectrum of a single sideband signal 58 Newnes Guide to Radio and Communications Technology degrading the signal. This can be done because both sidebands carry exactly the same information; they are exact mirror images of one another. The other advantage of transmitting only one sideband is that filter bandwidths can be reduced in the receiver to cut out more unwanted interference and give better reception. It is then possible to remove the carrier to conserve power. This form of modulation is known as single sideband suppressed carrier (SSBSC), or single sideband (SSB) for short. To demodulate the signal the carrier has to be reintroduced in the receiver using a beat frequency oscillator (BFO) which may also be called a carrier insertion oscillator (CIO). The BFO must be on the correct frequency relative to the sideband being received. Any deviation from this will cause the pitch of the recovered audio to change. While errors of up to about 100 Hz are acceptable for many communications applications, if music is to be transmitted the carrier must be reintroduced on exactly the correct frequency. This can be accomplished by transmitting a small amount of carrier, and using circuitry in the receiver