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AM and FM Receivers 12-73

with an I detector. The two signals are compared, and the resultant error is used to gain-modulate
the input of the I and Q demodulators.

When the transmitted signal is L + R (monaural, no stereo), it is pure AM (only I sidebands).

In this case the envelope detector and the I demodulator see the same thing. There is no error sig-
nal, the inverse modulator does nothing, and the signal passes without change. However, when a
left- or right-only signal is transmitted, both AM and PM are present, and the input signal is
shifted in phase to the I demodulator, which loses some of its I amplitude. The envelope detector
sees no difference in the AM because of the phase modulation. When the envelope detector and
the I demodulator are compared, there is an error signal. The error signal increases the input level
to the detector. This makes the input signal to the I and Q demodulators look like a pure quadra-
ture signal, and the audio output yields the L – R information. The demodulator output is com-
bined with the envelope-detector output in a matrix to reconstruct the left and right audio
channels.

12.3.5 References

1.

Rohde, Ulrich L, and Jerry C. Whitaker: Communications Receivers, 3rd ed., McGraw-
Hill, New York, N.Y., 2000.

2.

Rohde, Ulrich L.: Digital PLL Frequency Synthesizers, Prentice-Hall, Englewood Cliffs,
N.J., 1983.

3.

Amos, S. W.: “FM Detectors,” Wireless World, vol. 87, no. 1540, pg. 77, January 1981.

12.3.6 Bibliography

Benson, K. Blair, and Jerry C. Whitaker: Television and Audio Handbook for Engineers and

Technicians, McGraw-Hill, New York, N.Y., 1990.

Whitaker, Jerry C. (ed.): NAB Engineering Handbook, 9th ed., National Association of Broad-

casters, Washington, D.C., 1999.

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AM and FM Receivers

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AM and FM Receivers

12-75

Chapter

12.4

Stereo Television

K. Blair Benson

12.4.1 Introduction

The adoption of the Zenith-dbx system for multichannel television sound (MTS) in December
1983 paved the way for the commercial transmission of stereo audio and second-language pro-
gramming in the U.S. The MTS nomenclature is a mixture of borrowed FM and some new MTS-
specific terminology. This chapter discusses the broadcast-equipment specifications that have
the most significant impact on MTS-system performance and gives brief explanations of the
MTS terminology.

A typical origination-to-end-user block diagram is given in Figure 12.4.1. Alternate paths are

shown in the transmitter section, one for discrete encoding and the other for composite encoding
in the studio-transmitter link (STL). The transmission components may be grouped into the fol-
lowing categories:

•

Audio chain

•

Stereo generators

•

Composite STL

•

Aural exciter

12.4.2 Audio Chain

The audio chain includes distribution amplifiers, mixers, switchers, processors, studio-transmit-
ter link (STL), wiring, and connections between the audio source and the TV MTS generator.
The most important performance specifications for equipment in the audio chain are frequency
response, signal-to-noise ratio (S/R), distortion, separation, headroom, and channel-to-channel
tracking.

12.4.2a Stereo Generator

The TV stereo system is a modification of the standard FM stereo system [1–3]. The main differ-
ences are:

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Source: Standard Handbook of Audio and Radio Engineering

12-76 Radio Receivers

Figur

e 12

.4.1

 T

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p

ic

a

l t

e

le

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si

on

 MT

S

 sy

s

te

m

 b

lo

c

k 

di

ag

ra

m.

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Stereo Television

Stereo Television 12-77

•

25-kHz deviation for main channel (L + R).

•

50-kHz deviation for subchannel (L – R). This is 2 times, or 6 dB, greater than for the main
channel.

•

Noise reduction in the subchannel (L – R).

•

Pilot frequency equal to f

H

 (15,734 Hz).

Compatibility with existing monophonic receivers was the reason behind the selection of 25-kHz
deviation for the main channel audio. The increased deviation of the subchannel and the use of
noise reduction was intended to maintain at least 50 dB S/N in outlying areas. With the noise
reduction compandor in the stereo subchannel and the 6-dB increase in level, the L and R S/N
should be dependent only on the main-channel S/N, which was found to be approximately 63 dB
in system tests. The choice f

H

 as the pilot was to minimize buzz-beat interference.

Figure 12.4.2 shows the FM-versus-TV stereo baseband spectrum. Table 12.4.1 shows the

aural carrier-modulation standards for the TV stereo system. Figure 12.4.3 is a block diagram of
a typical TV stereo generator.

Some of the specifications for TV stereo generators are not different from those for other

audio equipment except that they cannot be verified without use of a decoder and that the
decoder contribution to performance must be recognized.

There are at least two operational modes for TV stereo generators: the normal (noise reduc-

tion) operational mode and a test mode called 75-

µs equivalent mode. In the 75-µs equivalent

mode, the noise reduction system in L – R is replaced with a 75-

µs preemphasis network identi-

Figure 12.4.2

 Baseband frequency allocations: (

a) stereophonic FM, (b) BTSC TV stereophonic

system.

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Stereo Television