0195136047.pdf

698 COMMUNICATION SYSTEMS

whereWrmsis the rms bandwidth off(t). GivenW 1 / 2 π= 2 .12 kHz,Wf/ 2 π=15 kHz, and

Wrms/ 2 π= 4 .25 kHz, determineRFMwith and without bandwidth constraints.

Solution

RFM

(with no bandwidth constraint)

=

( 15 / 2. 12 )^3

3

[

( 15 / 2. 12 )−tan−^1 ( 15 / 2. 12 )

]∼= 20. 92 ,or 13.2 dB

RFM

(with bandwidth limitation)

=

20. 92

1 +( 4. 25 / 15 )^2 ( 15 / 2. 12 )^2

∼= 4. 17 ,or 6.2 dB

Thus, the bandwidth constraint has resulted in a loss in emphasis improvement of 13. 2 − 6. 2 =

7 dB.

FM Stereo

Figure 15.2.19 shows the block diagram of an FM stereo transmitter and an FM stereo receiver.

The following notation is used:

• fL(t),fR(t): Left and right messages that undergo preemphasis and are then added to yield
  f 1 (t) and differenced to givefd(t).


• f 2 (t): Given by signalfd(t) when itDSB-modulates (with carrier suppressed) a 38-kHz
  subcarrier.


• f 3 (t): A low-level pilot carrier at 19 kHz that is included to aid in the receiver’s demodulation
  process.


• fs(t): Final composite message whenf 1 (t),f 2 (t),f 3 (t), and SCA (subsidiary communications
  authorization) are all added up.


• SCA: A narrow-band FM waveform on a 67-kHz subcarrier with a total bandwidth of
  16 kHz. It is a special signal available to fee-paying customers who may desire to have
  background music free of commercials or nonaudio purposes such as paging.


• NBPF: Narrow-band band-pass filter.


• LPF, BPF, and NBPF: Appropriate filters that select the spectrum portions corresponding
  tof 1 (t),f 2 (t), andf 3 (t), respectively.
  The output signal-to-noise power ratio is smaller in FM stereo than in a monaural system
  with the same transmitted power, messages, and other parameters. With a loss as high as 22 dB,
  many FM stations can tolerate the loss because of the high power being transmitted.

Comparison of Analog-Modulation Systems

The comparison of analog modulation systems (conventional AM, DSB SC, SSB SC, VSB, FM,

and PM) can be based on three practical criteria:

1. Bandwidth efficiency of the system.


2. Power efficiency of the system, as reflected in its performance in the presence of noise.
   The output SNR at a given received signal power is used as a criterion for comparing the
   power efficiencies of various systems.


3. Ease of implementation of the system (transmitter and receiver).