Note: Descriptions are shown in the official language in which they were submitted.
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.
--2--
? ~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:
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- 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.
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? ~ 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;
." ~
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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
.
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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
- 8 -
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
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~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
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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.
.
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