CA1120106A - 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 0. The original modulation can be obtained from each channel by product detection.

Description

Back~round of the Invention This invention relatës.to an AM stereo broadcast system for the trànsmission of two signals on a single carrier and more particularly to an- improved.system for transmitting and receiving 'fully compatible AM stereo signals on the..AM
broadcast band on monaural and.stereo receivers without '~
substantial distortion.
SeveraI systems for transmitting and receiving A~
signals are known in the art., The simplest system is probably an unmodi~ied quadrature signal which transmits two signals, A and B, e.g~, left tL) and right IR)~ 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 television transmission. On existing monaural receivers, using signal current rectifiers to derive'the audio signal, however, there is double frequency, ~ . ,, .. , - ,;
" ,~,, ,,, , ,,, , , ~F '.:'.
. . .

:..... .. .: ..
.

_ - llZ0106 distortion which is proportional to the amount of the stereo difference (L - R) signal. ~he distortion arises from the fact that this signal consists basically of the following:

_ _ .
~ /(1 + L + R)2 + (L - R)2 cos(~t + ~) where the term under the radical is the amplitude and where ~ - tan l(L - R)/(l + L + R). The monaural receiver, however, requires that the amplitude of the received signal be sub-stantially the carrier pIus the audio, or (~ +- L + R). The ~L - R) term thus represents distortion, and, --- since it is a squared term, --- double frequency distortion. The ~ term represents phase modulation and produces no output from a conventionaL enve-Iope detector in a monaural receiver when there is no appreciable-amplitude or phase distortion present on the signal i~ the entire system.
Still another prior system employs the technique of transmltting a single carrier, which is amplitude modulated wlt~ +~ R) information and frequency modulated with (L - R).
The compIex spectrum o~ 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 components, the radiated spectrum may contain many sideband frequencies which are subject to distortion in phase and amplitude which, in turn, produces spurious conversion of F~ components to amplitude modulation.
~ Yet another system transmits sum and difference signals ; in quadrature, but distorts the (L + R) component to correct the amplitude of the envelope and make it compatible. This is done by changing the in-phase component from (1 + L + R) to ~ (1 + L + R)2 _ (L - R)2 and ~eeping the magnitude of the quadrature component unchanged.

? ~lZ0~06 The phase or stereo information is thus distorted and the number of signiicant sidebands is increased, increasing the potent~al distortion on both monophonic and stereo receivers.
Summary of the Invention ~ It is an object of the present invention to provide an A~ stereo broadcast system which is compatible with existing AN 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 o~tained according to the invention by a system wherein the transmitted signal includes both the ~ + R) monaural information and the phase or stereo 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 dif~erent, to monaural circuitry, from a normaL AM monaural transmission. In the transmitter, the re~uired changes are minimal and for AM
stereo receivers the circuitry is not~complex~ Basically, t~e concept involves multiplying the quadrature ,ignal ln the transmitter by a factor which is related to the phase of Lhe stereo information, and in a stereo receiver aividing the ; received signal by the same factor, thus restoring the complete, original ~uadrature signal.
In accordance with the a~ove objects, the oresent invention provides a communication svstem wherein signal information corresponding to first and second inteiligence signals is transmitted in auadratu~e and is compatible or both monophonic and stereophon-c operation.
The systam comprises in combination:

- l~Z01 06 transmitter means for generating a single carrier wave amplitude modulated in accordance with the algebraic addition of said first and second intelligence signals and phase modulated by an angle whose tàngent 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 demodula~ing said ~irst and second intelligence signals in quadrature for stereophonic operation. The carrier wave is fully compati~le for reception and direct monophonic re-production without substantial distortion.
~ The~ transmitter means prefera~ly comprises:
: a first intelligence signal source;
a second.signal intelligence source:
a carrier wave source;
first combinin~ means for combining additively the ~irst. and second intelligence signals;
secQnd combinin~ means. for combining subtractively the- firs~-and second intelligence signals;
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 f means ~or 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 (~) and seccnd ~) intelligence signals on a single carrier wave. The s~stem :~, 30 includes in combination:
'~
-3a-~lZ0~06 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.
~lO The present invention provides in still another aspect a. receiver for receiving a broadcast carrier uave 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 1 A + B)~
where Cl and C~ are constants. The receiver comprises in inpu~ means for. receivin~ and amplifying the broàdcast carrier wave;
mixer means. for translating the broadcast carrier wave to one o an intermediate frequency;
intermediate frequency amplifier means for a~plifying said intermediate frequency carrier signal and ha~ing a band-width sufficient to accommodate said am~litude and phase . modulation information; and corrector means coupled to the amplifier means ror providing a signal proportional to ~he angle ~ ror proc~sing output signals which are substantially equal to the rirst and second intelligence signals.

-3b-? ~ 0~06 The invention provides in a further asp~ct 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 o~ predetermined fre~uency;
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 wave signal of ~he predetermined frequency and of a phase different from the irst carrier wave signal;
means for amplitude modulating said second unmodulated carrier wave s.igna~ 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 combi~ed carrier wave to a predetermined value to provide a signal having only the phase variation due to the combined first and second carrier waves; and 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 lnvention there ;s provided a transmitter for generating and transmitting broad-cast carrier wave amplitude modulated with the algebraic addi-1~
--3c-, ~:lZ0106 tion of first and second intelligence signals and phase modulated by an instantaneous angle whose tangent is the ratio of the difference between the first and second intelligence signals to the envelope of the amplitude modulated carrier.
The transmitter includes in combination:
circuit means ~or 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 chang~ng the phase of said unmodulated : càrrier.~wave~and amplitude modulating the same with t~e difference o the first and second intelligence signals;
adder and limiter means for combining- said amplitude modulated carrier waves and limitir.g the amplitude ~ariation ,~ o ~ 1,,, thereo to a~singIe;carrier wave ha~ing only phase hig~ leveL mDdulation means for amplitude modulating said.lLmited.an~ phase varying carrier wave with the algebrais addition o~ the-first and second.intelligenc~ signals; and 20- means; ~or transmittinq said amplitude and phase modulated carrier ~ave.
; In a still further aspect of this invention there is provided a method of transmitting signal information represen-tative of first and second intelligence signals in quadrature relation and which is compatible for both monophonic and stereophonic operation. The method comprises the steps of:
providing a first unmodulated carrier wa~e signal of : a predetermined requency;
amplitude modulating said first carrier wave s~`snal with the sum of the first and second intelligence signals;

." ~
-3d-ilZ0'~ 06 providing a second unmodula~ed carrier wave signal of the predetermined frequency and of a phase different from the phase o the first carrier wave signal:
amplitude modulating said second carrier wave with the difference of the first and second intelligence signals;
co~bining 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 10- signal having only the phase modulation due to the t~o ~amplitude modul~ated carrier signals;
additively combining said ~irst and second intalligenc~
signaIs for amplitude modulating the phase modulated and :
limited carrier wave signal; and said phase and amplitude modulated ca_rier wave being compatible for reception and direct monophonic reproduction of ~ the signaL Lnformation without substantial distortion.
f Brief` Description of: the Drawin~
.
Fig. 1 is a block diagram illustrative of a prior art system for transmitting and receiving two signals amplitude modulated in quadrature on a sing}e carrier.
Fig. 2 is a phasor diagram representative of the carrier and sidebands of the transmitted signal in the system of Fig. 1.

., .
.
:i ~0 .

-3e-AP-76819 11~0106 --Fig. 3 is a block diagram of an AM stereo system constructed in accordance with the present invention. ' !, _ Fig. 4 is a phasor diagram representative of the trans-mitted signal in the system of Fig. 3.
Fig. 5 i5 a block diagram of a transmitter compatible with thè operational,requirements of the invention. ~ ~,'c' Fig. 6 is a block diagram of a prefer,red embodiment Oe a receiver compatible with the operational requirements of the present invention. ' I-, Fig. 7 is a circuit diagram of a portion of the receiver of Fig. 6. , Fig. 8 is a block dia~ram of still another receiver compatible with the system of the present invention.
~ ig. 9 is a bloc~ 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 o~ Fi~. 10.
~' Fig. 12 is a spectrum diagram for the transmitted , - signal of Fig,. lQ.
Fig. L3 is a bloc~ diagram-o another,SSB system.
Fig. 14 is a spectrum diagram for the transmitted ~ ,,' , ~, .
' signal of Fig; 13. ,~
~, ,~ ,:
Detailed Description of the Preferred Embodiments The AM quadrature system of the prior art (Fig. 1~ and -`
the compatible system constructed according to the present invention (Fig. 3) will, for the sake of brevity, be described in 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 ~--z- ' the system is applicable to the transmission and reception of any two signals on a single carrier.

ll;~V~06 The system according to the invention as shown in bloc~ form in Fig. 3 will be best understood in relation to the block diagram of Fig. 1 which is an unmodified and thus incompati~le guadrature system. A quadrature transmitter, represented by a section lO thereof, includes a program signal path rom an input ll which provides (1 + ~ ~ R) to - a modulator 12 and a second input 13 which pro~ides (L - R) to. a second modulator 14. An RF exciter 15 pro~ides a car~ier sig~al to the m~dulator 12 and, through a 90 phase shi ter 16, to the modulator 14. The outputs of the two modulators are summed in sLgnal adder 17 to provide a signal which isAtransmitted.in.the co~ventionaL ~ashion.. This signal may be represented mathematically as /
- \/(1 ~ L + R) 2 + (L ~ R) 2 cOS (ll~t + ~) . where ~ = tan l (L ~ R)/(l +- ~. + R). When this signal is rec2ived~by a stereo receiver, as represented by a section I8: thereo~, and demodulated in product detectors or multipliers 20: and 21r the-respectiv~ signals: (1 ~ L I R) and (L - R~
; are o~tained~ ~owever, in the envelope detector 22 of a 20 monauràl receive~, indicated by dashed. line 23~ the demodulated output may be represented as ~tl + L + R)2 + (L - R) ! which it will be appreciated is compatible only for a .. signal wherein L = R, i.e. monophonic.
The phasor diagram or 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, ¦ with the phasors 26 representing the in-phase moduLating I signaL (L + R) and the phasors 27, the quadrature signal ¦ 30 (L - R). ~ indicates the instantareous phase angle of a _5_ AP-768'19 ~ O106 s r~sultant phasor 28 which, as the locus 24 shows, cannot exceed'+ 45.
A compatible'AM stereo broadcast system in accordance wi.th the invention is shown in ~lock diagram form in ~ig. 3.
Again there are the two inputs 11' and 13', for (1 + L + R) and (L - R), which are coupled to the two modulators 12' and ' ~.-14' o a transmitter as partially shown by dashed line 30.
'The RF exciter 15'.and the phase shifter 16' are as described in connection with Fig. 1. The.outputs of. the modulators - ' .
12' and 14'. are summed in the adder 17', amplitude variations are then removed.by a limiter 31, leaving only the phase ,~,y . 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 may be represented as (1 + L + ~)cos(/~t +
This is the equivalent of the origi~al~stereo signal from. .
adder- 17'`multiplied by cos ~ or is ~ ' ' . (L I L.+ R~/~.I + ~..+ R)2 + (~ _R~2.

ThisG latterA signal is.completely compatible, i.e., when this ~ .
s-ignal is.received by the monophonic receiver 23 and.demodulated . ~
- . ~
by the~envelope detector 22, the output is proportional to '.~'-(L +'~). 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 RE exciter 15 in the transmitter 30 in a manner to be described hereinafter. ~he phase difference is cos ~ and the output of the multiplier 35 is proportional to cos ~
o In a corrector circuit 37, which is further shown in Fig. 7 and will be described in detaiL hereinafter, the .

-- 6 -- ..
.. . i^;~

11'~0106 signal is divided by the output of the multiplier 35, which restores the original stereo output of the adder 17 as will be descri~ed~ 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 o~ Fi~.. 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 mini~um.number of higher order sidebands consistent with the t~ansmission.o~ a compatible monophonic signal with minimum distortion.
In Fig. 5 the transmitter is shown in somewhat more detail.. In a monaural transmit~er, the carrier frequency f~om the crystal oscillator 15 would be coupled to the modulator 3~.. The necessary modifving circuits 49 for converting the oscillator output at this point, according to the inventio~ are shown-within the dashed line. The carrie~
frequency from the oscillator 15 is divided and one part is shifted 90 in the phase.shifter 16. The two carriers in ~uadrature 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 carrier 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 llmited in limiter 31 to remove amplitude ~ modulation, thereby leaving the carrier, modulated wi~ ~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 limite~ 54 and 55 and a moni~o ing ) , -7-~P-76819 11'~0106 --meter 56, S7. The L and ~ 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 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 ~ig. 3 in somewhat ~-~
more detail. The received signal passed through an RF-mixer-IF amplifier section 65, the design of which is entirely 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 of a quadrature detector or multiplier 70. The multiplier n~
70 forms an integral part of a phase locked loop identified i.;.,,, `
at 71. A low pass filter 72 prevents rapid phase changes ?-~
from reaching a VCO 36 while allowing phase drift-to pass ~.
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 ~i2 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 l~Z0106 -' the output of the multiplier 35, thus restoring the quadrature signal. The remainder of the circuit is substantially as described with re~ard 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 diferential pair o~ transistors 81 and 82 in alternately conducti~e states in synchronism with the incoming carrier signal from the limiter 34. A reference input signal 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 shiftar 73 also serves as a low pass filter, providing an essentially sinusoidal reference current to the transistor 83. A DC
; reference: vo~tage-at point 85 is supplied by an emitter follower 88 which.is coupled. to the differential pair 81, 8Z_ A current.mirror 87 balances out any static current from transistor 83 at the differential pair output 74, 50 that the output current is proportional to the cosine of the ; angular difference between the input signals 80 and 84. ~n : integrating capacitor 86 smooths the current impulses from the multiplier 35.
In order that.the multiplier-output 74 follow closely - - a cosine ~unction, one of the inputs 80 or 81 muat be relatively free of higher order harmonics. By making the phase shifting network 73 a low pass filter, odd order harmonics from tn~
oscillator's square wave are ramoved.
The corrector circult 37 preferably consists o a differential am~lifier ha~ing a pair of transistors 100 and 101. Current for the emitters of trar.sistors 100 ard 101 is _g_ ' AP-~6819 .. llZ0~06 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 lOl~and the current IO is zero. - . ' The-signat voltage derived from the signal input 66 is applied between the bases of the transistors 10.0 and 101 js~
respectively through two resistors.108 and I09, two diodes ~' :
llO ànd 111 and a reference.voltage .source 112. The reference f ;~`
vo:ltage~source 112 consists of an emitter follower 113 '1.-,, ,~ ooupled~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~ The emitter of the emitter ..
, ~follower 113 provides a low impedance voltage reference for .the pair o~ transistors 100 and lOL forming the di~ferential ~ -.- ',amplif,ier... , -~ ' `A.current Ir from the multiplier 35' flows through the L~s~.
Dl~'~ diodes 11~ and,111, the resistors 108 and.109, the voltage :'~, source 112~and.the,input signaL source 66 to'provide forward blas for the diodes llO and 111. ' ;~
` ~The forward impedance of the diodes 110 and.lll, together ,~ with rosistors 108 and 109, provide a ~oltage divider so that the ~oltage 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 .~nd the output of the multiplier 35, Ir = I~aX
o ' cos 0. The output current may be represented by Io = IlIs/Ir, where Il is supplied by a current source 102. Is is the input signal current at terminal 66 and may be represented , .
' -10 - ' ' ''".'``-`
~ ~ ` "~ "

1120~06 as eS/2r where 2r equals the sum o~ 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. ~herefore IS = ~Iec~l ~ L + R)cos (~ct I ~)3/2r, and I = ~Ile (1 I L + R)cos(~ct ~ 2rImaxcos ~- Since cos R)2 + (L - R)2~ Io = (Ilec/ max ~ (1 ~ L +- R) + (L - R)2 cos (~ct ~ ~) which is the desired quadrature signal.
Fig. 8 shows a portion of another embodiment of a recei~er compatible with the operational requir~ments of the presen~ invention, wherein the corrector circuit 37 is in ; the audio portion of the receiver, and is, in fact, two identical corrector circuits 37a and 37b. The output 66 of the RF-mixer-IF amp~ifier 65 can now be a single output connected to multipliers 40 and 41. The output of the multiplier 40 is h cos ~ and goes to corrector circuit 37a where i~ is divided by cos ~ providing an L output. The outpu~ of 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 i the multiplier 35 is divided and applied to both correctors 37a and 37b.
Fig. 9 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 muItiplier 35 respectively. The output 95 of the corrector circuit 37c is connected to the inputs ~f ~he phase shifters 38 and 39 and is the rererence voltage divided ~y cos ~. The outputs of the multipliers ao and 41 thus become L ar.d R
respectively.
Fig. 10 is a block diagram of a left-right SSB syst~m h~-Jing a transmitter simila. to that oî Fig. 5, tha~ is, a ~lZ0106 quadrature system with the cos~ change. The L and R inputs are com~ined additively in adder 58 and subtractively in adder 61.
~he output of adder 61 is then phase shi~ted 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 shifte~ 98 and the output connected to receiver matrix 99 as is the output 96. The output of the matrix 99 is, of course, L and R.
Fig. 11 shows a detail of the receiver of Fig. 10 wherein the corrector circuit 37 is connected to the output 66 o~ the~ receiver RF-mixer-IF amplifier 6~, the output o~
- the corrector 3f is coupled to the multipliers 40 and 41 and the phase locked loop and phase locked loop and phase sh-ftlng networks are the same as described with regard to Eig. 6. As described above;with regard:to Fig. 10 the one output 97 is phase shi~ted and both outputs go to a matrix circuit 99 to provide L and R outputs.
Fig. ~2 is a spectrum diagram showing that in the transmitted signaL the L signals are contained in one set of sidebands and the R signaIs in the other set OL sidebands.
The signal, of course, also includes higher order correction sidebands which are transmitted double sideband.
Fig. 13 is a block diagram of another single sideband system similar to that of Fig. 10. In this embodiment one of the program input signals, e.g., R, is phase shifted by 90 in phase shifter 95. The phase shi~ted signal then goes to adder 58 and inverter 60, thence to adder 61. The second program signal, e.g., L, goes directly ~o adders 58 and 61.
The outputs of the adders 58 and 61 are ~L + R / ~/2) and (L - R/ ~/2) respectivelv. These signals are modulated on i 11;~0106 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 out 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 s~m and difference signals are transmitted 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 comp-letely compatibl~ i~ monophonic receivers and easily decoded in stereophonic receivers, ~ being deIined as the angle between the vector sum of the initial quadrature carriers and a line that bisects the angle bet~een the two quadrature carriers. The signal as transmitted has all of the advantages o~ quadratures modulation without causins distortion i~ an envelope detector. It provides a minimum of monophonic coverage Iost 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 (7)

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

1. In an AM broadcast system, 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 mono-phonic and stereophonic operation, comprising in combination:
means for generating an unmodulated carrier wave signal of predetermined frequency;
means for amplitude modulating said carrier wave with the vector sum of the first and second intelligence signals;
phase shifter means coupled to the generating means for providing a second unmodulated carrier wave signal of the predetermined frequency and of a phase different from the first carrier wave signal;
means for amplitude modulating said second unmodu-lated carrier wave signal with the difference of the first and second intelligence signal;
adder means for combining the modulated first and second carrier 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 combined first and second carrier wave; and means for amplitude modulating the phase modulated and limited carrier wave signal with the sum of the first and second intelligence signals.

2. A transmitter for generating and transmitting a broad-cast carrier wave amplitude modulated with the algebraic addition of first and second intelligence signals and phase modulated by an instantaneous angle whose tangent is the ratio of the difference between the first and second intelligence signals to the envelope of the amplitude modulated carrier, said transmitter including 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 intel-ligence signals;
means for changing the phase of said unmodulated carrier wave and amplitude modulating the phase-shifted carrier with the difference of the first and second intelligence signals;
adder and limiter means for combining said amplitude modulated carrier waves and limiting the amplitude variation thereof to a carrier wave having only phase variation;
high level modulation means for amplitude modulating said limited and phase varying carrier wave with the algebraic addition of the first and second intelligence signals; and means for transmitting said amplitude and phase modulated carrier wave.

3. A transmitter for generating and transmitting a broadcast carrier wave which is amplitude modulated with signal information proportional to the sum of the first (A) and second (B) intelligence signals, and phase modulated with signal information proportional to an angle 0 having a form 0 = arc tan{C1(A - B)/(C2 + A + B)}

where C1 and C2 are constants, the transmitter comprising in combination:
means for providing a carrier wave of a predetermined frequency which is amplitude modulated by the sum of the first and second intelligence signals;
means for providing another carrier wave of said pre-determined frequency but differing in phase and which is amplitude modulated by the difference of the first and second intelligence signals;
means for combining said amplitude modulated carriers and limiting the combined carriers to provide resultant signal information having only phase variation; and means for amplitude modulating said resultant phase varying carrier signal with the sum of the first and second intelligence signals.

4. A transmitter for generating and transmitting a broadcast carrier wave which is amplitude modulated with signal information proportional to the sum of first (A), shifted in phase by 90°, and second (B) intelligence signals, and phase modulated with signal information proportional to an angle 0 having a form 0 = arc tan{C1(A/.pi./2 - B)/(C2 + A/.pi./2 + B)}

where C1 and C2 are constants, the transmitter comprising in combination:
means for providing a carrier wave of a predetermined frequency which is amplitude modulated by the sum of the first and second intelligence signals;
means for providing another carrier wave of said pre-determined frequency but differing in phase and which is amplitude modulated by the difference of the first and second intelligence signals;
means for combining said amplitude modulated carriers and limiting the same to provide a resultant signal information having only phase variation; and means for amplitude modulating said resultant phase varying carrier signal with the sum of the first and second intelligence signals.

5. A transmitter for generating and transmitting a broad-cast carrier wave which is amplitude modulated with signal information proportional to the sum of first (A) and second (B) intelligence signals, and phase modulated with signal informa-tion proportional to an angle ? having a form ? = arc tan{C1(A - B)/.pi./2/(C2 + A + B)}

where C1 and C2 are constants, the transmitter comprising in combination:
means for providing a carrier wave of a predetermined frequency which is amplitude modulated by the sum of the first and second intelligence signals;
means for providing another carrier wave of said pre-determined frequency but differing in phase and which is amplitude modulated by the difference of the first and second intelligence signals said difference being shifted in phase by 90°;
means for combining said amplitude modulated carriers and limiting the combined carriers to provide resultant signal information having only phase variation; and means for amplitude modulating said resultant phase varying carrier signal with the sum of the first and second intelligence signals.

6. A transmitter for generating and transmitting a single carrier wave signal representative of first (L) and second (R) intelligence signals in quadrature and which is compatible for both monophonic and stereophonic operation, said transmitter including in combination;
a first intelligence signal source;
a second intelligence signal source;
a carrier wave source;
first combining means for combining additively said first and second intelligence signals;
second combining means for combining subtractively said first and second intelligence signals;
means for separately amplitude modulating said carrier wave in quadrature in response to the outputs of said first and second combining means;
means for limiting the amplitude of the modulated carrier wave to provide a signal having phase modulation pro-portional to arc tan{(L - R)/(1 + L + R)}; and means for amplitude modulating said limited carrier wave in response to the output of said first combining means.

7. A method of transmitting signal information repre-sentative of first and second intelligence signals in quadra-ture relation and which is compatible for both monophonic and stereophonic operation, comprising the steps of:
providing a first unmodulated carrier wave signal of a predetermined frequency;
amplitude modulating said first carrier wave signal with the sum of the first and second intelligence signals;
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 first and second intelligence signals;
combining said first and second modulated carrier wave signals;
limiting the amplitude variation of said combined carrier wave signal to a predetermined value to provide a signal having only the phase modulation due to the two amplitude modu-lated carrier signals;
additively combining 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.