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AM, FM, and Digital Modulated Systems Chap. 5
preemphasis characteristic, the second corner frequency f 2 occurs much above the baseband
spectrum of the modulating signal (say, 25 kHz for audio modulation). In FM broadcasting, the
time constant t 1 is usually 75 µs, so that f 1 occurs at 2.12 kHz. The resulting overall system
frequency response obtained using preemphasis at the transmitter and deemphasis at the
receiver is flat over the band of the modulating signal. In FM broadcasting, with 75-µs preemphasis,
the signal that is transmitted is an FM signal for modulating frequencies up to 2.1 kHz,
but a phase-modulated signal for audio frequencies above 2.1 kHz, because the preemphasis
network acts as a differentiator for frequencies between f 1 and f 2 . Hence, preemphasized FM is
actually a combination of FM and PM and combines the advantages of both with respect to
noise performance. In Chapter 7, we demonstrated that preemphasis–deemphasis improves the
signal-to-noise ratio at the receiver output.
5–7 FREQUENCY-DIVISION MULTIPLEXING AND FM STEREO
Frequency-division multiplexing (FDM) is a technique for transmitting multiple messages
simultaneously over a wideband channel by first modulating the message signals onto several
subcarriers and forming a composite baseband signal that consists of the sum of these modulated
subcarriers. This composite signal may then be modulated onto the main carrier, as
shown in Fig. 5–17. Any type of modulation, such as AM, DSB, SSB, PM, FM, and so on, can
be used. The types of modulation used on the subcarriers, as well as the type used on the main
carrier, may be different. However, as shown in Fig. 5–17b, the composite signal spectrum
must consist of modulated signals that do not have overlapping spectra; otherwise, crosstalk
will occur between the message signals at the receiver output. The composite baseband signal
then modulates a main transmitter to produce the FDM signal that is transmitted over the
wideband channel.
The received FDM signal is first demodulated to reproduce the composite baseband
signal that is passed through filters to separate the individual modulated subcarriers. Then the
subcarriers are demodulated to reproduce the message signals m 1 (t), m 2 (t), and so on.
The FM stereo broadcasting system that has been adopted in the United States is an
example of an FDM system. Furthermore, it is compatible with the monaural FM system that
has existed since the 1940s. That is, a listener with a conventional monaural FM receiver will
hear the monaural audio (which consists of the left- plus the right-channel audio), while a
listener with a stereo receiver will receive the left-channel audio on the left speaker and the
right-channel audio on the right speaker (Fig. 5–18). To obtain the compatibility feature, the
left- and right-channel audios are combined (summed) to produce the monaural signal, and
the difference audio is used to modulate a 38-kHz DSB-SC signal. A 19-kHz pilot tone is
added to the composite baseband signal m b (t) to provide a reference signal for coherent
(product) subcarrier demodulation in the receiver. As seen from Fig. 5–18c, this system is
compatible with existing FM monaural receivers. In Prob. 5–51 we will find that a relatively
simple switching (sampling) technique may be used to implement the demodulation of the
subcarrier and the separation of the left and right signals in one operation.
The FM station may also be given subsidiary communications authorization (SCA) by
the FCC. This allows the station to add an FM subcarrier to permit the transmission of a second
analog-audio program or background music to business subscribers for use in their stores