eo circuitry can be switched out of circuit. To generate the stereo signal, a system similar to that shown in Figure 7.3 is used. The left and right signals enter the encoder where they are passed through a circuit to add the required pre-emphasis. After this they are passed into a matrix circuit. This adds and subtracts the two signals 210 Newnes Guide to Radio and Communications Technology (Left) Pre-emphasis Stereo signal to FM Pilot tone 19 kHz Summation transmitter Matrix oscillator circuit 19kHz (Right) Pre-emphasis Frequency doubler Upper and lower sidebands only (no carrier) 8 kHz Balanced mixer Figure 7.3 A simplified diagram of a VHF FM stereo encoder to provide the L + R and L - R signals. The L + R signal is passed straight into the final summation circuit to be transmitted as the ordinary mono audio. The difference L - R signal is passed into a balanced modulator to give the double sideband suppressed carrier signal centred on 38 kHz. This is passed into the final summation circuit as the stereo difference signal. The other signal entering the balanced modulator is a 38 kHz signal that has been obtained by doubling the frequency of the kHz pilot tone. The pilot tone itself is also passed into the final summation circuit. The final modulating signal consisting of the L + R mono signal, 19 kHz pilot tone and the L - R difference signal based around 38 kHz is then used to frequency modulate the radio frequency carrier before being transmitted. Reception of a stereo signal is very much the reverse of the transmission. A mono radio receiving a stereo transmission will only respond to the L + R signal. The other components being above 15 kHz are above the audio range, and in any case they will be suppressed by the de-emphasis circuitry. A variety of methods are available for decoding the stereo signal once it has been demodulated in the radio. The basic method and circuitry for demodulating an FM signal are described in Chapter 5 (Receivers). This produces the baseband signal consisting of the L + R, L - R