B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

5.7 FM BROADCASTING SYSTEM
5.7 FM Broadcasting System 241
The FCC has assigned a frequency range of 88 to 108 MHz for FM broadcasting,
with a separation of 200 kHz between adjacent stations and a peak frequency deviation
t,.f = 75 kHz.
A monophonic FM receiver is identical to the superheterodyne AM receiver in Fig. 5.17,
except that the intermediate frequency is 10.7 MHz and the envelope detector is replaced by
a PLL or a frequency discriminator followed by a deemphasizer.
Earlier FM broadcasts were monophonic. Stereophonic FM broadcasting, in which two
audio signals, L (left microphone) and R (right microphone), are used for a more natural effect,
was proposed later. The FCC ruled that the stereophonic system had to be compatible with
the original monophonic system. This meant that the older monophonic receivers should be
able to receive the signal L + R, and the total transmission bandwidth for the two signals (L
and R) should still be 200 kHz, with !::,.f = 75 kHz for the two combined signals. This would
ensure that the older receivers could continue to receive monophonic as well as stereophonic
broadcasts, although the stereo effect would be absent.
A transmitter and a receiver for a stereo broadcast are shown in Fig. 5.18a and c. At the
transmitter, the two signals L and R are added and subtracted to obtain L + R and L - R. These
signals are preemphasized. The preemphasized signal (L - R)' DSB-SC modulates a carrier
of 38 kHz obtained by doubling the frequency of a 19-kHz signal that is used as a pilot. The
signal (L + R)' is used directly. All three signals (the third being the pilot) form a composite
baseband signal m(t) (Fig. 5.18b),
I I
W e t
m(t) = (L + R) + (L - R) cos W e t + a cos 2
(5.33)
The reason for using a pilot of 19 kHz rather than 38 kHz is that it is easier to separate
the pilot at 19 kHz because there are no signal components within 4 kHz of that
frequency.
The receiver operation (Fig. 5.18c) is self-explanatory. A monophonic receiver consists of
only the upper branch of the stereo receiver and, hence, receives only L + R. This is of course
the complete audio signal without the stereo effect. Hence, the system is compatible. The pilot
is extracted, and (after doubling its frequency) it is used to demodulate coherently the signal
(L - R)' COS W e t.
An interesting aspect of stereo transmission is that the peak amplitude of the composite
signal m(t) in Eq. (5.33) is practically the same as that of the monophonic signal (if we ignore
the pilot), and, hence, !::,.f-which is proportional to the peak signal amplitude for stereophonic
transmission-remains practically the same as for the monophonic case. This can be explained
by the so-called interleaving effect as follows.
The I i and R' signals are very similar in general. Hence, we can assume their peak amplitudes
to be equal to A p
. Under the worst possible conditions, L' and R' will reach their peaks
at the same time, yielding [Eq. (5.33)]
lm(t) I max = 2A p
+ a
In the monophonic case, the peak amplitude of the baseband signal (L + R)' is 2A p
. Hence, the
peak amplitudes in the two cases differ only by a, the pilot amplitude. To account for this, the
peak sound amplitude in the stereo case is reduced to 90% of its full value. This amounts to a
reduction in the signal power by a ratio of (0.9) 2 = 0.81, or 1 dB. Thus, the effective SNR is
reduced by 1 dB because of the inclusion of the pilot.