CA1122658A - Compatible am stereo broadcast system - Google Patents

Abstract

A COMPATIBLE AM STEREO BROADCAST SYSTEM

ABSTRACT

A compatible AM stereo broadcast system has a pair of carriers in quadrature are separately modulated with stereo information. The resulting signal is multiplied by the cosine of an angle 0, the angle between the vector sum of the two carriers and a line that bisects the angle between the carriers in quadrature. The signal envelope contains the sum of the information on the two channels, thus providing compatible monophonic reception. In a stereo receiver the two carriers in quadrature can be restored by dividing the signal by the cosine of the angle ?. The original modulation can be obtained from each channel by product detection.

Description

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, ' '.
~r ' - ~ ;

.

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.

-2--' 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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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:

.

-3a-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.

-3b-~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 . -3c-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;

~ .

-3d-~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 ~;~
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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. -- 4 - t- ~ .

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 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 , .' ' ~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 ~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.

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A receiver for receiving a broadcast carrier wave which is amplitude modulated with signal information pro-portional to the sum of first (A) and second (B) intelligence signals, and which is phase modulated with the signal informa-tion proportional to an angle .PHI. having a form .PHI. = arc tan {C1 (A - B)/(C2 + A + B)}

wherein Cl and C2 are constants, the receiver comprising:
input means for receiving and amplifying the broad-cast carrier wave;
mixer means for translating the broadcast carrier wave to one of an intermediate frequency;
intermediate frequency amplifier means for amplifying said intermediate frequency carrier signal and having a band-width sufficient to accommodate said amplitude and phase modu-lation information; and correcting and demodulating means coupled to the amplifier means for providing a correction signal proportional to the angle .PHI. and further employing said correction signal to process a signal at the output of said amplifier means to provide signals essentially equal to the first and second intelligence signals.

2. The receiver according to claim 1 wherein the correcting and demodulating means comprises means for dividing said amplifier means output signal by said signal proportional to the angle .PHI..

3. The receiver according to claim 2 wherein said signal proportional to the angle .PHI. is proportional to the cosine of the angle .PHI..

4. The receiver according to claim 2 wherein the receiver further includes oscillator means, limiter means for limiting a signal proportional to said amplifier means output signal, first multiplier means for receiving the outputs of said oscillator means and said limiter means and for providing an output to the correcting and demodulating means.

5. The receiver according to claim 4 wherein said correcting and demodulating means comprising a corrector means and a demodulator means, said demodulator means comprising second and third multiplier means and the receiver means further includes first phase shifting means connected to shift the output of the oscillator means by 45°, said second multi-plier means receiving and multiplying the outputs of the first phase shifting means and the corrector means, second phase shifting means connected to shift the output of the oscillator means by -45°, and said third multiplier means for receiving and multiplying the outputs of the second phase shifting means and corrector means.

6. A receiver for receiving a carrier wave which is amplitude modulated with a signal proportional to the sum of first (A) and second (B) intelligence signals, and which is phase modulated with a signal proportional to an angle .PHI. having a form .PHI. = arc tan{Cl(A - B)/(C2 + A + B)}

where C1 and C2 are constants, the receiver comprising in combination:
means for selectively receiving the modulated carrier wave;
means for translating the received carrier wave to an intermediate frequency signal;
means for demodulating the intermediate frequency carrier wave to provide a first audio frequency signal pro-portional in amplitude to the product of the first intelligence signal and a function of the phase of said carrier wave, and a second audio frequency signal proportional in amplitude to the product of the second intelligence signal and a function of the phase of the said carrier wave; and corrector means adapted to divide each of the first and second audio frequency signals by a signal proportional to said function of the phase of the said carrier wave, for pro-viding the first and second intelligence signals.

7. A receiver for receiving a 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 signal information proportional to an angle .PHI. having the form .PHI. = arc tan{Cl(A - B)/(C2 + A + B)}

where Cl and C2 are constants, the receiver comprising in combination:
input means for receiving and amplifying the carrier wave and having a bandwidth sufficient to accommodate said amplitude and phase modulation information;

first detector means coupled to the input means for detecting a signal proportional to L cos .PHI.;
second detector means coupled to the input means for detecting a signal proportional to R cos .PHI.; and transducer means for separately reproducing the first and second intelligence signals in relatively distortion-free form at low modulation levels.

8. A receiver in accordance with claim 1 wherein the input means includes means for translating the received carrier wave to one of an intermediate frequency.

9. A receiver for receiving a carrier wave which is amplitude modulated with a signal proportional to the sum of first (A) and second (B) intelligence signals, and which is phase modulated with a signal proportional to an angle .PHI. having the form .PHI. = arc tan{Cl(A - B)/(C2 + A + B)}

where C1 and C2 are constants, the receiver comprising in combination:
input means for selectively receiving the modulated carrier wave;
means for translating the received carrier wave to an intermediate frequency carrier wave;
means for demodulating the intermediate frequency carrier wave to provide a first audio frequency signal propor-tional in amplitude to A cos .PHI. and a second audio frequency proportional in amplitude to B cos .PHI.; and transducer means for separately reproducing first and second intelligence signals which are relatively distortion-free at low modulation levels.

10. A method of receiving stereophonic signal information of the form (C1 + L + R)cos(.omega.ct + .PHI.) where L and R are intel-ligence signals and .PHI. is arc tan{C2(L - R)/(Cl + L + R)}
where C1 and C2 are constants, and comprising the steps of:
selectively receiving and amplifying the transmitted signal;
detecting the signal L cos .PHI. on the amplified signal;
detecting the signal R cos .PHI. on the amplified signal;
coupling the L cos .PHI. and R cos .PHI. signals to audio transducer means for separate reproduction of L and R intelli-gence signals which are relatively distortion-free at low modulation levels.

11. me method of receiving stereophonic signal information in accordance with claim 10 and further including the step of translating the received and amplified signal to an intermediate frequency signal.

12. A method of receiving a signal of the form (C1 + L + R)cos(.omega.ct + .PHI.) where L and R are intelligence signals and .PHI. is arc tan{C2(L - R)/(Cl + L + R)} where C1 and C2 are constants, and comprising the steps of:
selectively receiving the transmitted signal;
amplifying the received signal;
providing a reference oscillator having the frequency of the unmodulated broadcast carrier;
separately phase shifting the output signal of the reference oscillator by .pi./4 and by -.pi./4 to provide first and second oscillator signals respectively; and multiplying the amplified signal by the first and second oscillator signals respectively to provide signals which are substantially L and R at low modulation levels.

13. The method of receiving a signal in accordance with claim 12 and further including the steps of providing a second local oscillator having a frequency differing from the carrier frequency by a predetermined amount; and mixing the selectively received signal and the output signal of the second local oscillator to provide an intermediate frequency signal.

14. 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 proportional to an angle .PHI. having a form .PHI. = arc tan[Cl(A - B)/(C2 + A + B)]

where Cl and C2 are constants, the receiver comprising:
input means for receiving and amplifying the broadcast carrier wave;
mixer means for translating the broadcast carrier wave to one of an intermediate frequency;
intermediate frequency amplifier means for amplifying said intermediate frequency carrier signal and having a band-width sufficient to accommodate said amplitude and phase modulation information; and demodulator means coupled to the amplifier means for providing output signals substantially equal to the first and second intelligence signals.