Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Backg~round of the Invention
This invention relatës to an AM stereo broadcast system
for the transmission of two signals on a single carrier and ~ '
more particularly to an improved system ~or transmitting and r ~
. - - ~rtL~
receiving 'fully compatibIe AM stereo signals on the AM ' ~i~
broadcast ba~d on monaural and stereo receivers without
substantial distortion. , ~. ,
Several systems for transmitting and receiving A~
-stereo signals are known in the art. The simplest system is
,,.~, .
probably an unmodified ~uadrature signal which transmits two
signals, A and B, e.g., left (Lj and right (R), on two
carriers which are identical in frequency but are in phase
quadrature. This system is similar to the system used to
transmit the two color signals on one carrier in the NTSC
standard for U.S. color teIevision transmission. On existing
monaural receivers, using signal current rectifiers to
derive the audio signal, however, there is double frequency,
' '.
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.
distortion which is proportional to the amount of the stereo
difference (L - R) signal. The distortion arises from the
fact that this signal consists basically of the following:
~ (1 + L + R)2 + (L - R) cos(~t + ~)
where the term under the radical is the amplitude and ~hexe
~ = tan l(L - R)/(l + L + R). The monaural receiver, however,
requires that the amplitude of the received signal be sub-
stantially the carrier plus the audio, or (1.+ L ~ R). The
(L - R) term thus represents distortion, and, --- since it iS
a squared term, --- double frequency distortion. The ~ term
.epresents phase modulation and produces no output from a
conventional envelope detector in a.monaural. receiver when
there is no appreciable amplitude or phase distortion present
on the signal in the entire system.
Still another prior system employs the techni~ue of
transmitting a single carrier, which is amplitude modulated
with~ (L.+ R). information and fre~uency modulated with (L - R).
~he. comple~ spectrum.of- the transmitted.signal may give rise
to undesirable distortion in.both monaural and stereo receivers
i~ any frequency or phase distortion is present in the received
signal. When the (L - R~ signal contains low frequency
componentsf the radiated spectrum may contain many sideband
frequencies which are subject to distortion in phase and
amplitude which, in tur~, produces spurious conversion of
- F~ components to amplitude modulation.
Yet another syst2m transmits sum and dif~erence signals
in quadrature, but distorts the (T + R) component to correct
the amplitude of ~le envelope and make it compatible. This s
done by changing the in-phase component frcm (1 + L + R) to
V(l + L + R)2 _ (L - R)~
and ~eeping the magnitude of .ne quadrature ccmponent unchanged.
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' llZZ~58
The phase or stereo information is thus distorted and the
number of significant sidebands is increased, increasing the
potential distortion on both monophonic and stereo receivers.
Summary of the Invention
It is an object of the present invention to provide an
AM stereo broadcast system which is compatible with existing
~M monaural receivers.
It is a~ further object of the invention to provide a
compatible stereo signal requiring minimal change in existing
transmitters and minimal complication in receiver circuitry
designed for stereo decoding.
The above objects are obtained~according to the
invention by a system wherein the transmitted signal includes
both the (L + R) monaural information and the phase or StereQ
information necessary for obtaining the separated stereo signals,
but the envelope does not include the (L - R) or difference
information. Thus, the signal is no different, to monaural
circuitry, from- a normal AM monaural transmission. In the
transmitter, the required changes are minimal and for AM
stereo receivers the circultry is not complex. Basically, the
concept involves multiplying the quadrature signal in the
transmitter by a factor which is related to the phase of the
stereo information, and in a stereo receiver di~iding the
received signal by the same factor, thus restoring the complete,
original ~uadrature signal.
In accordance with the above objects, the present
invention provides a communication system wherein signal
information corresponding to first and second inleiligence
signals is transmitted in ~uadrature and is compatible for bo
monophonic and stereophonic operation.
The system compris2s ln combination:
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,.
2~S~ `
transmitter means for generating a single carrierwave amplitude modulated in accordance with the algebraic
addition of said first and second inte~ligence signals and
phase modulated by an angle whose tangent is the ratio of the
difference between the first and second intelligence signals
to the envelope of the amplitude modulated carrier, and
receiver means for receiving said carrier wave and
demodulating said first and second intelligence signals in
quadrature for stereophonic operation. The carrier wave is
fully compatible for reception and direct monophonic re-
production without substantial distortion.
The transmitter means preferably comprises:
a irst intelligence signal source;
a second signal intelligence source;
a carrier wave source;
first combining. means for combining additively the
first and second intelIigence signals;
second combining-means for combining subtractively
the first and.second intelligence signals;
~0 means for amplitude modulating the carrier wave in
quadrature in response-to the outputs of the first and second
combining means;
means for limiting the amplitude of the modulated
carrier wave; and
means for amplitude modulating the limited carrier
wave in response to the output of the first combining means.
The present invention provides in another aspect a
system for transmitting and receiving îirst (A) and second (D~
intelligence signals on a single carrier wave. The system
includes in combination:
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11;Z2~58
transmitter means for providing the carrier wave
which is amplitude modulated with a signal proportional to
(A + B) and phase modulated with a signal proportional to an
angle ~ having the form
~ = arc tan[Cl(A - B)/(C2 + A + B)]
where Cl and C2 are constants; and
receiver means for receiving the transmitted signal
and including means for separately deriving the first (A) and
second (B) intelligence signals from the received signal.
The present invention provides in still another aspect
a receiver for receiving a broadcast carrier wave which is
amplitude modulated with signal information proportional to
the sum of first (A) and second (B) intelligence signals, and
which is phase modulated with the signal information propor-
tional to an angle ~ having a form
~ =-arc tan[Cl(A - B)/(C2 + A + B)]
where Cl and C~ are constants. The receiver comprises in
input- means for receiving and amplifying the ~roadcast carrier
wave;
mixer means for translating the broadcast carrier
wave to one of an intermediate frequency;
intermediate frequency amplifier means for amplifylng
` said intermediate frequency carrier signal and having a band-
width sufficient to accommodate said am~litude and phase
modulation information; and
corrector means coupled .o the amplifier means -or
providing a signal proportional to the angle ~ for processing
output signals which are substantially equal to the firsl and
second intelligence signals.
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~22~8
The invention provides in a further aspect an AM
broadcast system including transmitter means for generating and
transmitting a single carrier wave signal representative of
first and second intelligence signals in quadrature relation
and which is compatible for both monophonic and stereophonic
operation. The transmitter means comprises in combination:
means for generating an unmodulated carrier wave
signal of predetermined frequency;
means for amplitude modulating said carrier wave
with. he instantaneous vector sum of the first and second
intelligence signals;
phase shifter means coupled to the generating means
for-providing:a second unmodulated carrier wa~e signal of he
predetermined frPquency and of a phase diferent from the first
carrier wave signal;
means for amplitude modulating said second unmodulated
carrier wave signal.with the~ difference of. the first and second
intelligence signals;
adder means. for combining the first and second ca'rrier
waves;
- means for limiting the amplitude variation of said
combined carrier wave to a predetermined value to provide a
signal having only the phase variation due to the com~ined
first and second carrier waves; and
.I means for amplitude modulating the limited carrier
wave signal with the sum of the first and second intelligence
signals.
~, In a still further aspect of this invention there is
provided a transmitter for generating and transmitting rsroad-
5 30 cast carrier wave amplitude modula.ed with the algebraic addi-
i
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llZ~{i S~
tion of first and second intelligence signals and phase
modulated by an instantaneous angle whose tangent is the ratio
of the difference b~tween the first and second intelligence
signals to the envelope of the amplitude modulated carrier.
The transmitter includes in combination:
circuit means for generating an unmodulated carrier
wave of a predetermined frequency;
means for-amplitude modulating said unmodulated carrier-
wave with the algebraic addition of the first and second
intelligence signals;
means for changing the phase of said ~nmodulated
carrier wave and amplitude. modulating the same with the
difference of the first.and second intelligence signals;
adder-and limiter means for combining said amplitude
modulated carrier waves and limitir.g the amplitude variation
thereo to a--single carrier wave having only phase
high leveI moduIation means for ampLitude modulating
said limited:and~phase varying-carrier wave with the algebrais
addition Q~ the first and second. intelligence signals; and
means for transmitting said amplitude. and phase
modulated carrier wave.
In a still further aspect of this invention there is
, provided a method of transmitting signal information represen-
J~ tative of first and second intelligence signals in quadrature
,' relation and which is compatible for ~oth monophonic and
stereophonic operation. The method comprises the steps of:
providing a first unmodulated carrier wave signal of
a predetermined frequency;
amplitude modulating said first carrier wave aisnal
with the sum of the first and second intelligence signals;
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~Z2~S~
providing a second unmodulated carrier wave signal of
the predetermined frequency and of a phase different from the
phase of the first carrier wave signal;
amplitude modulating said second carrier wave with the
difference of the firs~ and second intelligence signals;
combining said first and second modulated carrier wave
signals;
limiting the amplitude variation o said combined
carrier wave signal to a predetermined value to provide a
signal having only the phase modulation due to the two
-amplitude modulated carrier signals;
additively com~ining said first and second intelligence
signals for amplitude modulating the phase modulated and
limited carrier wave signal; and
said phase and amplitude modulated carrier wave being
compatible for reception and direct monophonic reproduction of
the--signal information without substantial distortion.
Brief Description of the Drawing
Fig. 1 is a block diagram illustrative of a prior art
2Q system .or trans.~itting and receiving two signals amplitude
modulated in quadrature on a single carrier.
Fig. 2 is a phasor diagram representative of the
carrier and sidebands of the transmitted signal in the system
of Fig. L.
AP-76819 l~Z2~S8
Fig. 3 is a block diagram of an AM stereo system constructed
in accordance with the present invention. c-
Fig. 4 is a phasor diagram representative of the trans-
mitted signal in the system of Fig. 3.
Fig. 5 is a block diagram of a transmitter compatible
with the operational requirements of the invention. l, -
Fig. 6 is a block diagram of a preferred embodiment o
a receiver compatible with the operational requirements of
the present invention.
) Fig. 7 is a circuit diagram of a portion of the receiver
of Fig. 6.
Fig. 8 is a block diagram of still another receiver
compatible with the system of the present invention.
Fig~ 9 is a block diagram of still another preferred
embodiment of the receiver.
Fig. 10 is a block diagram of a left-right SSB system.
Fig. 11 is a block diagram of a receiver for the system
of-Fig. 10.
Fig. 12 is~-a spectrum- diagram for the transmitted
0 sigr.al of Fig. lQ.
Fig. 13 is a block diagram of another SSB system. ~',;
~'.,q~,
Fig. 14 is a spectrum diagram for the transmitted ~ ~
,J .
signal of Fig. 13. r~
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Detailed Description of the Preferred Embodiments ~;~
. C~
'' ' ~
The ~M quadrature system of the prior art (Flg. 1) and
the compatible system constructed according to the present
invention (Fig. 3) will, for the sake of brevity, be described
ir. terms of a stereo signal having left (L) and right (R)
program channels, nevertheless, it will be understood that
there is nothing inherent in the system to so limit it and
the system is applicable to the transmission and reception
of any two signals on a single carrier. -
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fi~'B
The system according to the invention as shown in
block form in Fig. 3 will be best understood in relation to the
block diagram of Fig. 1 which is an unmodified and thus
incompatible quadrature system. A quadrature transmitter,
represented by a section 10 thereof, includes a program
signal path from an input 11 which provides (1 + L + R) to
a modulator 12 and a second input 13 which provides tL - R)
to a second modulator 14. An RF exciter 15 provides a
carrier s-ignal to the modulator 12 and, through a 90 phase
shifter 16, to the modulator 14. The outputs of ~he two
modulators are summed in signal adder 17 to provide a signal
`whlch is transmitted in ~he conventional fashion. This
signal may be represented mathematically as
~ (1 + L + R)2 + (L - R)2 cos(~t + ~)
where ~ = tan 1 (L - R)/(l + L + R). When this signal is
received by a stereo receiver, as represented by a section
; I8 thereof, and demoduIated in product detectors or-multlpliers
20 and 21,- the-respective signals (1 + L + R) and (L - R)
axe obtained~ However, in the envelope detector 22 of a
monaural receiver, indicated by dashed line 23, the demodulated
output may be represented as
~(1 + L + R)2 + (L - R)2
¦ which it will be appreciated is compatible only for a
signal wherein L = R, i.e. monophonic.
The phasor diagram of Fig. 2 shows the locus 24 of the
modulated transmitted signal for the system of Fig. 1.
! Phasor 25 represents the unmodulated carrier, 1 cos ~ t,
I with the phasors 26 representing the in-phase modulatlng
signal (L + R) and the phasors 27, the quadrature sisnal
1 30 (L - R). ~ indicates the instantaneous phase angle cf a
, _
- AP-76819 - llZZ658
resultant phasor 28 which, as the locus 2.4 shows, cannot
exceed + 45. . ,~__
A compatible AM stereo broadcast system in accordance
with the invention is shown in block diagram form in Fig. 3. .
Again there are the two inputs 11' and 13', for (l + L + R)
and (L - R), which are coupled to the two modulators 12' and r -
14' of a transmitter as partially shown by dashed line 30.
The RF exciter 15' and the phase shifter 16' are as described
in connection wLth Fig. l, The,outputs of the modulators - , .
) 12' and 14' are summed in the a~der 17', amplitude variations
are then removed by a limiter 31, leaving only the phase ~y
information. The resulting phase modulated carrier may then .
be amplitude modulated.by signal component (l.+ L +-R) in a
high level modulator or multiplier 32. The transmitted
signal which ~ay be repres.en~ed as (l +- L + R)cos(~t + ~
This is the eguivalent of the original stereo signal from
adder 17 multiplied by cos 0 or is -
~.''.
(1 +-1.+ R)/~.l + L + R~2 +, ~ _ R)2, ~-,
This latter signal is completely compatible, i.e., when this ~,~.. ;,'
signaL is: receiued by the monophonic.receiver 23 and demodulated
by the envelope detector-22, the output is proportional to
(L + R~. When the transmitted signal is received by a ';:.'.;
stereo receiver as indicated at 33, it is limited in limiter '~
34, The resulting stereo information is then compared in a
multiplier stage 35 with the phase of cos ~ t from a VCO 36
which is locked to the phase of the RF exciter 15 in the
transmitter 30 in a manner to be described hereinafter. ~he
phase difference is cos 0 and the output of the multiplier
35 is proportional to cos 0.
. In a corrector circuit 37, which is further shown in
Fig. 7 and will be described in detail hereinafter, the
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6~;~
signal is divided by the output of the multiplier 35, which
restores the original stereo output of the adder 17 as will be
described. The cos ~ t signal from the VCO 36 is shifted
- 45 in phase shifters 38 and 39 and fed to multipliers 40
and 41 as is the output of the corrector circuit 37. The multi-
pliers 40 and 41 provide outputs of L and R plus DC terms.
Fig. 4, which is the phasor diagram for the transmitted
signal in the system of Fig. 3, has a modified locus 45. Each
point within the locus 45 corresponds to a point or value
within the locus 24 multiplied by cos ~. Multiplication
by cos ~ produces the minimum number of higher order sidebands
- consistent with the transmisslon of a compatible monophonic
signal with minimum distortion.
In Fig. 5 the transmitter is shown in somewhat more
detail. In a monaural transmitter, the carrier ~requency
from the crystal oscillator 15 would be coupled to the
modulator--32. The nece9sary modifying circuits 49 for
converting the oscillator output at this point, according to
the invention are shown within the dashed line. The carrier
frequency from the oscillator 15 is divided and one part is
shifted 90 in the phase shifter 16. The two carriers in
quadrature are then coupled to the modulators 12 and 14 and
the modulator outputs are connected to the adder 17. A
portion of the unshifted and unmodulated carriar is also
connected to the adder 17 through a carrier level control 50
to establish the level of the unmodulated carrier. The
adder 17 output is limited in limiter 31 to remove amplitude
i modulation, there~y leaving the carrier, moduiated with ~he
I phase stereo information only to be coupled to the high level
modulator 32. Each of the program channel inputs 52 (L) and
53 (R) has a program level limiter 54 and 55 and a moni'o~ing
AP-7'~19 ~ 1122~S8
meter 56, 57. The L and R signals are combined (L + R) in
the adder 58 which is connected to the multiplier 12. The R
signal is inverted by the inverter 60 and combined (L ~ R)
in the adder 61 which is connected to multiplier 14. A
second output of the (L + R) adder 58 is connected through a
time delay circuit 62 to the high level modulator 32. The L~_~
time delay 62 provides a delay equal to that of the modifying
circuits 49. The output of the modulator 32 is then a ~
signal which is amplitude modulated with (-L + R) information ~-.
and phase modulated with the stereo information~
Fig. 6 shows the stereo receiver 33 of Fig. 3 in somewhat
more detail. The received signal passed through an RF-
mixer-IF amplifier section 65, the design o~ which is entire-ly
conventional as will be appreciated by those skilled in the
art without further operational description. The amplitude -
modulation on the signal at the output 66 of the section 65 ~
is removed in the limiter 34. The output of the limiter 34 ~---
may be represented as cos(~t ~ ~) is applied to one input of
the-in-phase~ detector or multiplier-35 and also to one input
o~ a quadrature detector or multiplier 70. The multiplier .
70 forms an integral part of a~phase locked loop identified
at 71. A low pass filter 72 pre~ents rapid phase changes
from reaching a VCO 36 while allowing phase drift to pass ~-~O
through. The output of the VCO, then, is controlled very
closely and, since it is in quadrature to the transmitter
oscillator 15, it is coupled to a ~/2 or 90 phase shifter
73. The resultant cos ~ t output of the phase shifter 73 is
connected to a second input of the multiplier 35. The
output 74 of the multiplier 35 which may be represented as
Io cos 0 is coupled to the corrector circuit 37. In the
corrector circuit 37, an embodiment of which is shown in
detail in Fig. 7, the signal appearing at 66 is divided by
- 8 -
the output of the multiplier 35, thus restoring the quadrature
signal. The remainder of the circuit is substantially as
described with regard to Fig. 3.
In Fig. 7, an embodiment of a portion of the receiver
33 is depicted which will satisfactorily provide the above-
described functions of the multiplier 35 and the corrector
circuit 37. The phase detector or multiplier 35 receives an
input 80 from the limiter 34 on terminal 80. The limiter
output switches a differential pair of transistors 81 and
82 in alternately conductive states in synchronism with the
incoming carrier signal from the limiter 34. A reference
input slgnal at terminal 84, derived from the phase locked
loop 71, is supplied to the transistor or current source 83
by the output of the phase shifter 73. The phase shifter 73
also serves as a low pass filter, providing an essentially
sinusoidal. reference current to the transistor 83. A DC
reference.voltage at point 85 is supplied by an emitter
follower: 88 which is coupled to the differential pair 81,
82.. A current mirror 87 balances out any static current
from transistor 83 a~ the diferential pair output 74, so
that the output current is proportional to the cosine of the
angular difference between the input signals 80 and 8a. An
integrating capacitor 86 smooths the current impulses from
the multiplier 35.
- In order that.the multiplier output 74 follow closely
a cosine function, one of the inputs 80 or 81 must be relatively
free of higher order harmonics. By making the phase shifting
networX 73 a low pass fllter, odd order harmonics from the
oscillator's square wave are removed.
The corrector circuit 37 preferably consists of a
differential am?lifier having 2 pair of transistors 100 and
101. Current for the emitters of transistors 100 ard 101 is
AP-76819 ~ ~ ~ z~
supplied by a current source 102. Two transistors 103 and
104 form a current mirror so that the current in the transistor --
104 is equal to the current in transistor 100. When the
currents in transistors 100 and 101 are equal, the current .~
in the transistor 104 equals the current in the transistor
101 and the current Io is zero. L.i-~
The signal voltage derived from the signal input 66 is ~ .
applied between the bases of the transistors 10.0 and 101 ,~
respectively through two resistors 108 and I09, two diodes ~
110 and 111 and a reference voltage.source 112. The reference , ~.
voltage source 112 consists of an emitter foll.ower 113
coupled to a voltage divider means consisting of three .
resistors 114, 115 and 116. The base of the transistor 113
is connected to the junction of the resistors 114 and 115 to
provide a reference voltage. T~e emitter or the emitter
follower 113 provides a low impedance voltage reference for .
the pair of transistors 100 and 101 forming the differential.
ampliier............................................................ ~*
A current Ir from the multiplier 35 flows through the
diodes 110 and 111, the resistors 108 and 109, the voltage
source 112 and the input signal source 66 to provide forward
bias for the diodes 110 and 111.
.The forward impedance of the diodes 110 and 111, together
with resistors 108 and 109, provide a voltage divider so
that the voltage applied between the transistor bases 106
and 107 is reduced by the ratio of the forward resistance of
diodes 110 and 111 to the resistors 108 and 109.
The corrector circuit 37 will now be described in terms
of its currents and the OUtpllt of the multiplier 35, Ir = I~aX
COS ~. The output current may be represented by Io = IlIs/Ir,
where I1 is supplied by a current source 102. IS is the
input signal current at terminal 66 and may be represented
, .' '
--1 0
~12~ai5~3
as eSf2r where 2r equals the sum of the two resistors 91 which
are large value resistors. eS may be taken as equal to
ec(l + L + R~cos~ct + ~), where ec is the amplitude of the
unmodulated carrier. ImaX is the peak signal current in the
transistor 83. Therefore IS = [Iec(l + L + R)cos (~ct + ~)]/2r,
and I = ~I e (1 + L + R)cos(~ct +~)]2rImaxcos ~. Since cos
(1 + L + R)/(l + L + R) + (L - R)2, Io = (Ilec/2rImax)
~(1 + L + R)2 + (L - R)2 cos (~ct + ~) which is the desired
quadrature signal.
Fig. 8 shows a portion of another embodiment of a
receiver compatible with the operational requirements of the
present invention, wherein ~he corrector circuit 37 is in
the audio portion of the receiver, and is, in ract, two
identical corrector circuits 37a and 37b. The output 66 of
the RF-mixer-IF amplifier 65 can now be a single output
connected to multipliers- 40 and 4I. The output of the
multiplier-40 is L cos ~ and goes to corrector circuit 37a
where it is divided by cos ~ providing an L output. The
output o corrector circuit 41 is R cos ~ and is connected
to the corrector circuit 37b where it is divided by cos ~
- providing an R output. The output current at point 74 of
the multiplier 35 is divided and applied to both correctors
37a and 37b.
Fig. g shows still another receiver embodiment similar
to those of Figs. 7 and 8. Here the corrector circuit 37c
has inputs 83 and 74 from the phase shifter 73 and the
multiplier 35 respectively. The output 95 of Lhe corrector
circuit 37c is connected to the inputs cf the phase shifters
38 and 39 and is the reference voltage divided by cos ~. The
outputs of the multipliers 40 and 41 thus become L and R
respectively.
Fig. 10 is a block diagram of a left-right SSB system
ha-Jing a transmitter similar to that or Fig. 5, tha~ is, a
1~2Z~S~
quadrature system with the cos~ change. The L and R inputs are
com~ined additively in adder 58 and subtractively in adder 61.
The output of adder 61 is then phase shifted 90 in phase
shifter 95 and fed to the transmitter as before. The required
stereo receiver would have the decoding angles changed to
derive outputs (L + R) such as indicated at 96 and (L - R)
/ ~/2 such as indicated at 97. The output 97 is phase
~ shifted by ~/2 in a phase shifter 98 and the output connected
to receiver matrix 99 as is the output 96. The output of the
matri~ 99 is, of course, L and R.
Fig. 11 shows a detail of the receiver of Fig. 10
wherein the corrector circ~it 37 is connected to the output
66 of the receiver RE-mixer-IE amplifier 65, the O'ltpUt o
the corrector 37 is- coupled to the multipliers 40 and 41 and
the phase locked loop and phase locked loop and phase sh~ting
networks are the same as described with regard to Fig. 6. As
described above with regard to Fig. 10 the one output 97 is
phasa shifted and both outputs go to a matrix circuit 99 to
provide L and R outputs.
Fig-. 1~ is a spectrum diagram showing that in the
transmitted signal the L signals are contained in one set of
sidebands and the R signals in the other set OL S1aebandS.
The signal, of course, also includes higher order correction
sidebands which are transmitted double sldeband.
Fig. 13 is a block diagram of another single sideband
system similar to that of Fig. 10. In this embodiment one
of the program inpu~ signals, e.g., R, is phase shifted by
90 in phase shifter 95. The phase shited signal then goes
to adder 58 and inverter 60, thence to adder 61. The second
program signal, e.g., L, goes directly to adders 58 and 61.
The outputs of the adders 58 and 61 are ~L + R / ~/2) and
(L - R/ ~/2) respectively. These sisnals are modulated on
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~122~,5~
to the carrier as before in the transmitter having the
cosine correction. When received by a quadrature receiver
with cosine correction, the corrected signals come ou~ as L and
R / ,/2 and the R signal is shifted 90 lagging in phase shifter
98.
Fig. 14 is a spectrum diagram of the transmitted signal
showing that the sum and difference signals are transmit~ed
single sideband. The correction information transmitted
double sideband.
Thus, by multiplying a quadrature signal by the cosine
of an angle ~ before transmission and dividing by the same
cosine in the receiver, the system provides a signal which is
completely compatible in monophonic receivers and easily
- decoded in stereophonic receivers, ~ being defined as the
angle between the vector sum of the initial quadrature
carriers and a line that bisects the anglé between the two
quadrature carriers. The signal as transmitted has all of
the advantages o~ guadratures modulation without causing
distortlon in an envelope detector. It provides a ~inimum of
monophonic coverage lost due to skywave distortion and, at
the same time, optimum stereo performance. The system is
compatible with monophonic receivers using either envelope
detection or synchronous detection. For best performance
with synchronous detectors a corrector circuit is desirable
but reasonable performance can be obtained by an uNmodified
synchronous receiver.
