In multiplexed FM, we can perform further demodulation on the received message signal to obtain the various signals contained within the message signal: u(t) m(t) 0 FM Demodulation Mono Audio Left + Right 30 Hz 19kHz stereo pilot (10%) 23 15 kHz kHz Stereo Audio Left-Right Low-Pass Filter Figure 2 - In multiplexed FM, we can further demodulate the message signal The multiplexed signals are said to be modulated on subcarriers within the message signal. A typical spectrum of the FM radio signal is shown in Figure 3 below. Note that this is the spectrum of m(t), the multiplex signals contained within the message signal m(t), instead of u(t). Use this as a reference for comparison when looking at the spectrum of the demodulated received FM signal. 38 kHz Band-Pass Filter AM DSB-SC Demodulation FSK Demodulation RBDS (10%) (5%) 57 kHz DirectBand 53 kHz 58.65 kHz 67.65 kHz Mono Audio 76.65 kHz Stereo Pilot Tone Stereo Difference Audio Digital Station/Track data (RBDS) Audos subcarrier (10%) 92 99 kHz kHz Figure 3- A typical spectrum of a FM radio signal, note that DirectBand and Audos subcarrier are not used in Singapore Q2a - Given that originally the left and right audio channels are given by the terms L(t) and R(t) respectively, but the received signals are L(t) + R(t) [Mono audio Left + Right] and L(t) - R(t) [Stereo Audio Left - Right], show how the original left and right audio channels can be recovered from the received signals. Q2b - Why do you think such a method was chosen to encode the stereo audio?