In an effort to create a realistic sound presentation from recorded music, two microphones are used, one to record the sound as it is perceived on the right side and the second on the left side. During playback the right- and left-hand side signals have to be fed to the right- and left-hand side loudspeakers, respectively. The listener sitting in front of the speakers can distinguish the sounds of the different instruments as coming from their proper positions when the music was recorded. When this technique is applied to the cinema and television, a car approaching from one side of the screen can be heard in the proper position presented in the picture and the sound appears to move with the picture as the car moves across the screen. In a normal recording process, two separate channels are required for a stereophonic system. So it is expected that a stereophonic FM system will require twice the bandwidth of a monophonic channel. In fact, it requires more bandwidth because the system had to be designed in such a way that listeners who own a monophonic FM receiver can tune in the stereophonic transmission and receive the same performance as they would if the original program had been monophonic.
The system diagram shown in Figure 4.24 is used to pre-process the modulating signal before it goes to frequency modulate a sub-carrier using the Armstrong technique discussed in Section 4.4.
The left-hand side signal L(t) and the right signal R(t) are fed to an adder and a subtractor (the signal is first inverted and then added) to produce [L(t) + R(t)] and [L(t) — R(t)], respectively. The [L(t) + R(t)] signal is fed into one of the inputs of a three-input adder as shown in Figure 4.24. The [L(t) — R(t)] signal is one of the two inputs to the balanced modulator. The other input is a 38 kHz signal originally generated at 19 kHz (pilot frequency) and then doubled. The output of the balanced modulator is then the product
where 2op is the angular frequency of the doubled pilot oscillator frequency and A its amplitude. The other two inputs of the three-input adder are the pilot signal and the output of the balanced modulator. The output of the adder is then
M (t) = [L(t)+ R(t)] + [L(t)- R(t)]A cos 2op t + B cos op t
where B is the amplitude of the pilot oscillator. The design of the circuits in all the component blocks shown in Figure 4.24 have been discussed in this and earlier chapters. The baseband spectrum of the stereophonic signal is shown in Figure 4.25.
The processing of the stereophonic signal and its separation into right and left signals will be discussed in Chapter 5.
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