Independent channel modulation system for AM stereo

An apparatus for modulating signals for use in AM stereo transmission having left and right signal inputs feeding respective inverted and non-inverted inphase modulators and respective inverted and non-inverted quadrature modulators. The inverted and non-inverted inphase outputs are summed, the inverted and non-inverted quadrature outputs are summed, and the resulting inphase and quadrature signals are summed. The resulting signal is fed to a limiter and then to a broadcast transmitter, such as a difference transmitter, as a radio frequency input.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a modulation system and method 
which is to be incorporated as part of an AM stero radio station, and in 
particular, to an AM quadrature modulator having independent channel 
modulation. 
2. Description of the Prior Art 
Many types of AM stereo modulation systems are known in the art. These 
include quadrature modulation systems in which a first signal modulates a 
carrier signal, and a second signal modulates a carrier signal having a 
90.degree. phase difference from the first carrier signal. The second, or 
quadrature, modulator is a suppressed carrier type, such that only the 
sidebands of the modulated signal remain. The quadrature modulated 
sidebands are added to the output signal of the first modulator to produce 
quadrature modulation. 
For AM stereo transmission, the input of the first transmitter is the main 
(L+R) signal and the input of the second transmitter is the stereo (L-R) 
signal. This "pure quadrature" modulation is not compatible with current 
AM receivers. 
A compatible quadrature modulation system, as described in Motorola 
Incorporated's "Introduction to Motorola C-Quam AM Stereo System" by Chris 
Payne, dated 1982, first generates pure quadrature modulation as described 
above, after which the quadrature signal is fed through a limiter which 
removes the amplitude information, i.e., inphase sidebands, and leaves 
only the quadrature phase information in the carrier signal. This 
quadrature phase shifted carrier is fed into a broadcast transmitter as 
the radio frequency input and the main (L +R) signal is fed to the 
broadcast transmitter as the audio input. 
Other AM stereo quadrature modulators are disclosed in U.S. Pat. Nos. 
4,401,853; 4,373,115; 4,324,952; 4,323,731; 4,236,042; and 4,225,751. 
The above quadrature systems perform the summing and difference functions 
prior to modulation of the sum and difference signals. If the two 
modulators in those systems are not precisely balanced, cross talk occurs 
between the two channels. Thus, to adjust for best left-to-right channel 
separation requires a trade-off in right-to-left channel separation and to 
adjust for best right-to-left separation requires a trade-off in 
left-to-right separation. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide non-interacting set-up 
of left and right channel audio information by providing independent paths 
for the left and right channels to give improved channel separation. 
The above object is inventively achieved in an independent channel 
modulation system and method for AM stereo wherein left channel signals 
and right signals are each modulated prior to matrixing or mixing. The 
present invention provides independent paths for the left and right 
channels during each modulation step to reduce interaction between the 
two. The left channel signal is modulated by both inphase and quadrature 
carrier signals, and similar functions are performed on the right channel 
signals, after which the sum and difference of the modulated signals are 
produced. The sum and difference signals, modulated by carriers in 
quadrature, are summed, producing a phase shift in the resultant signal. A 
limiter is used then to remove the amplitude components, leaving a phase 
shifted carrier for use as the radio frequency input for the broadcast 
transmitter. 
The present invention, in a preferred embodiment, further provides 
independent paths for the left and right channel signals in a differential 
broadcast transmitter. The left and right channel signals are fed 
independently to the transmitter to modulate the phase shifted carrier, 
after which the two modulated carrier signals are added to one another for 
transmission. 
In a second embodiment, a standard broadcast transmitter having a single 
audio input is used and the main L +R signal modulates the phase shifted 
carrier. 
The present invention thus provides independent paths for left and right 
stereo signals during modulation so that interaction between the two 
channels is reduced. In the present system, left-to-right and 
right-to-left stereo separation are improved without trade-offs in 
performance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An independent channel modulation system 10 for use in AM stereo 
broadcasting constructed in accordance with the principles of the present 
invention is shown in FIG. 1. The modulation system 10 includes first and 
second input terminals 12 and 14; first, second, third, and fourth 
modulators 16, 18, 20 and 22; a carrier signal generator 24; a limiter 26; 
and a differential broadcast transmitter 28. 
The first input terminal 12, which in the present example receives left 
channel signals, although it could also be used for right channel signals, 
is connected to a signal input 30 of the first modulator 16, which is 
designated the quadrature modulator as will be explained later. The signal 
from the input 12 modulates a carrier signal A being fed into a carrier 
input 32 of the modulator 16. The carrier signal A, which, in a preferred 
embodiment, is a signal of the form A.sub.c sin .omega..sub.c t, is 
generated by the carrier generator 24. The modulated output signal appears 
at a modulated output 34 of the quadrature modulator 16. 
The second input terminal 14, which in the present example receives right 
channel signals although it could also be used for left channel signals, 
is connected to a signal input 36 of the second modulator 18, or inverted 
quadrature modulator. The signal from the second input terminal 14 
modulates a carrier signal -A being fed into a carrier input 38, which is 
the inverted carrier A. The inverted carrier signal A is produced by 
feeding the carrier signal A from the carrier signal generator 24 through 
an inverter 39 producing -A.sub.c sin .omega..sub.c t. The resulting 
modulated signal is produced at a modulated output 40 of the inverted 
quadrature modulator 18. The modulated outputs of the modulators 16 and 18 
are added by a summing means 42 having a first input 44, a second input 
46, and an output 48. It is, thus, subsequent to modulation of the 
individual channel signals that the left and right signals are first mixed 
to form the L-R stereo signal. 
The signal from the first input terminal 12 is also fed to the third 
modulator 20, which is designated the inphase modulator. Before the 
modulator 20, however, a DC signal, such as a 0.5 volt DC signal, from DC 
input 50 is added to the signal from the input 12 in summing means 52. The 
left channel signal with a resulting 0.5 volt offset is then fed into a 
signal input 54 of the inphase modulator 20. The offset left channel 
signal at signal input 54 modulates a carrier A' which is fed to the 
inphase modulator 20 at a carrier input 56. The modulated inphase carrier 
occurs at modulated output 58. 
In similar fashion, the right channel signal from the input 14 is offset by 
a 0.5 volt DC signal in summing means 60. The offset right channel signal 
is then passed through an inverter 62 and fed into a signal input 64 of 
the fourth modulator 22, or inverted inphase modulator. A carrier signal 
-A' is produced by inverting the carrier signal A' in inverter 65 after 
which it is fed into carrier input 66 of the inverted inphase modulator 22 
and a resulting modulated signal is produced at modulated output 68. 
The carrier signal A', which in a preferred embodiment is represented 
mathematically as A.sub.c cos .omega..sub.c t, leads the carrier A.sub.c 
sin .omega..sub.c t by 90.degree. and therefore A.sub.c sin .omega..sub.c 
t can be said to be in quadrature to A.sub.c cos .omega..sub.c t. This 
designation, however, is somewhat arbitrary as the A.sub.c cos 
.omega..sub.c t signal may have instead been designated as the quadrature 
signal, in which case the A.sub.c sin .omega..sub.c t signal would be the 
inphase carrier, the term quadrature referring only to a phase difference 
between the two signals. For purposes of the present invention, it is also 
foreseen to use phase differences other than 90.degree.. Many types of 
carrier generators 24 are known for producing the quadrature signals 
including the use of an oscillator and a Johnson counter. 
The modulated inphase signals from the outputs 58 and 68 are fed into a 
summing means 70 at first and second inputs 72 and 74 thereof. The signal 
from the inphase summing means 70 at output 76 is added to the signal from 
the quadrature summing means 42 in summing means 78. This addition 
operation may be expressed mathematically as: 
##EQU1## 
As can be seen, the addition of these two signals produces a phase shift 
##EQU2## 
It may be understood from the foregoing that the left and right channel 
signals vary as a function of time and therefore that the phase angle 
.theta. also varies with time. The phase shifted signal identified above 
is then fed into an input 80 of the limiter 26 which strips the signal of 
its amplitude modulated components leaving only the phase shifted carrier 
portion A.sub.c cos (.omega..sub.c t+.theta.). 
The limiter 26 of the preferred embodiment shown in FIG. 1 includes both 
non-inverting and inverting outputs 82 and 84, which feed the two signal 
paths of the differential transmitter 28. In keeping with the concept of 
independent paths for right and left channel signals, the phase shifted 
carrier from the non-inverting output 82 is modulated by the offset right 
channel signal from the inverter 62 in a fifth modulator 86. The phase 
shifted carrier from the inverting output 84 is modulated in a sixth 
modulator 88 by the offset left channel signal. An output 90 of modulator 
86 and an output 92 of the modulator 88 are then joined at a summing means 
94 to produce the signal for broadcast. The operation of the differential 
transmitter is represented mathematically as: 
EQU (-0.5-R)[-A.sub.c cos (.omega..sub.c t+.theta.)]+(0.5+L)[A.sub.c cos 
(.omega..sub.c t+.theta.)]=(1+L+R)A.sub.c cos (.omega..sub.c t+.theta.). 
Thus it can be seen that the channel signal information is modulated 
independently by quadrature modulators 16 and 18 and by inphase modulators 
20 and 22 prior to matrixing the channel signals. Furthermore, the 
broadcast transmitter 28 modulates the left and right channel signals 
independently prior to matrixing the signals for broadcast. This preserves 
the independent nature of the right and left channel signals and provides 
improved channel separation without sacrificing system performance. 
In a second embodiment, the left and right channel signals are modulated 
independently by inphase and quadrature carriers, just as in the first 
embodiment. However, as shown in FIG. 2, the second embodiment utilizes a 
single output limiter 96 which receives the output signal from the summing 
means 78 at input 98 and produces the phase modulated carrier at an output 
100. The phase shifted carrier from the limiter output 100 is fed into a 
radio frequency input 102 of a standard AM broadcast transmitter 104 
having a single audio input 106. The left and right channel signals are 
added in summing means 107 to produce the main, or monaural, signal for 
the audio input 106 of the transmitter 104. The modulated output of the 
broadcast transmitter 102 on output 108 is then transmitted in the normal 
fashion. 
The modified independent channel modulator of the second embodiment has 
many of the channel separation advantages of the fully independent 
modulator of the preferred embodiment, while enabling a standard broadcast 
transmitter to be used, thus avoiding cost of purchasing new equipment. 
The 0.5 volt DC offset which is added to each channel prior to inphase 
modulation provides, after summing of the modulated signals, a 1 volt 
reference so that a carrier signal is still broadcast even if no signal is 
present in either the left or right channels. Many other means for 
providing for broadcast of a carrier in the event of no left and right 
channel signals are contemplated, including, for example, providing a DC 
offset to one of the channel signals prior to modulation or directly 
injecting a carrier after summing of the modulated signals. 
FIG. 3 shows a circuit for injecting pilot and subsidary communication 
authorization (SCA) signals into the left and right channels independently 
prior to the first and second input terminals 12 and 14 of the 
above-described independent channel modulation system 10. A 25 Hz pilot 
signal, shown as 0.025 sin 50.pi.t, is added in summing means 110 to 
one-half of the required SCA signal and the result is added to the left 
channel signal in summing means 112, the output of which is fed to the 
left channel input 12. The pilot tone and SCA are fed through an inverter 
114 and added to the right channel signal in summing means 116 prior to 
being fed into the right channel input 14 of the above-described device 
10. A pilot tone of 5 % injection and an SCA signal of desired strength 
are thus present after summing the modulated signals. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.