Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
This invention relates to an AM stereo broadcast system
for the transmission 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.
Several systems-for transmitting and receiving AM
stereo signals are known in the art. The simplest system is
probably an ~mmodified quadrature signal which transmits two
signals, A and B, e.g., left (L~ 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 television transmission. On existing
monaural receivers, using signal current rectifiers to
derive the audio signal, however, there is double frequency
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~ + (L - R) cos(~t + ~)
where the term under the radical is the amplitude and where
~ = tan (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
represents 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 technique of
transmitting a single carrier, which is amplitude modulated
with (L + R) information and frequency modulated with (L - R).
The complex spectrum of the transmitted signal may give rise
to undesirable distortion in both monaural and stereo receivers
if 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
FM 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
~ + L + R) - (L - R)
and keeping the magnitude of the quadrature component unchanged.
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 o~ the present invention to provide an
AM stereo broadcast system which is compatible with existing
AM 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 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 different, to monaural
circuitry, from a normal AM monaural transmission. In the
transmitter, the required changes are minimal and for AM
stereo receivers the circuitry 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 dividing the
received signal by the same factor, thus restoring the complete,
original quadrature signal.
In accordance with the above objects, the present
invention provides a communication system wherein signal
information corresponding to first and second intelligence
signals is transmitted in quadrature and is compatible for both
monophonic and stereophonic operation.
The system comprises in combination:
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 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 first intelligence signal source;
a second signal intelligence source;
a carrier wave source;
first combining means for combining additively the
first and second intelligence signals;
second combining means for combining subtractively
the first 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
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 first (A) and second (B)
intelligence signals on a single carrier wave. The system
includes in comb:ination:
-3a-
`
' ; ' ', .:
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; ancl
receiver means for recei~ing 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 C2 are constants. The receiver comprises in
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
corrector means coupled to the amplifier means for
providing a signal proportional to the angle ~ for processing
output signals which are substantially equal to the first and
second intelligence signals.
-3b-
5~39~
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 the 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 the
predetermined frequency and of a phase different 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 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 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 invention there is
provided a transmitter for generating and transmitting broad-
cast carrier wave amplitude modulated with the algebraic addi-
s~
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 changing the phase of said unmodulated
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 limiting the amplitude variation
thereof to a single carrier wave having only phase
high level modulation means for amplitude modulating
said limited and phase varying carrier wave with the algebrais
addition of 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-
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 wave signal of
a predetermined frequency;
amplitude modulating said first carrier wave signal
with the sum of the first and second intelligence signals;
-3d-
providing a second unmodulated carrier wave signal of
the predetermined frequency and of a phase differen~ from the
phase of the first carrier wave signal;
amplitude modulating saicl second carrier wave with the
difference of the first and seconcl 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 modulated 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.
Brief Description of the Drawing
Fig. 1 is a block diagram illustrative of a prior art
system for transmitting 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. 1.
-3e-
AP-76819
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 is a block diagram of a transmitter compatible
with the operational requirements of the invention.
Fig. 6 is a block diagram of a preferred embodiment of
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
signal of Fig. 10~
Fig. 13 is a block diagram of another SSs system.
Fig. 14 is a spectrum diagram for the transmitted
signal of Fig~ 13~
Detailed Description of the Preferred E~mbodiments
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
the system is applicable to the transmission and reception
of any two signals on a single carrier.
s~
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 lo 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 (~ - R)
to a second modulator 14. An RF exciter 15 provides a
carrier signal to the modulator 12 and, through a 90 phase
shifter 16, to the modulator 14. The outputs of the two
modulators are summed in signal adder 17 to provide a signal
which is transmitted in the conventional fashion. This
signal may be represented mathematically as
~ (1 + L + R) + (L - R) cos(~t + ~)
where ~ = tan 1 (L - R)/(l + L + R). When this signal is
received by a stereo receiver, as represented by a section
18 thereof, and demodulated in product detectors or multipliers
20 and 21, the respective signals (1 + L + R) and (L - R)
are 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,
with the phasors 26 representing the in-phase modulating
signal (L + R) and the phasors 27, the quadrature signal
(L - R). ~ indicates the instantaneous phase angle of a
--5--
AP - 7 6 819 ~ S~
resultant phasor 28 which, as the locus 24 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 (1 + L + R)
and (L - R), which are coupled to the two modulators 12' and
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 with Fig. 1. The outputs of the modulators
1~ 12' and 14' are summed in the adder 17', amplitude variations
are then removed by a limiter 31, leaving only the phase
information. The resulting phase modulated carrier may then
be amplitude modulated by signal component (1 + L + R) in a
high level modulator or multiplier 32. The transmitted
signal which may be represented as (1 + L + R)cos(~t + 0).
This is the equivalent of the original stereo signal from
adder 17 multiplied by cos 0 or is
(1 + L + R)/~l + L + R)2 + (L - R)2
This latter signal is completely compatible, i.e., when this
signal is received 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. The
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
signal is divided by the output of the multiplier 35, which
restores the original stereo output of the adder 17 as will be
deseribed. The eos ~ 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 loeus 45. Eaeh
point within the loeus 45 eorresponds to a point or value
within the loeus 24 multiplied by cos ~. Multiplication
by eos ~ produees the minimum number of higher order sidebands
eonsistent with the transmission of a eompatible monophonie
signal with minimum distortion.
In Fig. 5 the transmitter is shown in somewhat more
detail. In a monaural transmitter, the carrier frequency
from the crystal oscillator 15 would be eoupled to the
modulator 32. The neeessary modifying eireuits 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 carrier is also
conneeted to the adder 17 through a carrier level eontrol 50
to establish the level of the unmodulated carrier. The
adder 17 output is limited in limiter 31 to remove amplitude
modulation, thereby leaving the carrier, modulated with the
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 monitoring
AP-76819 ~gS9~2 r
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
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 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 + 0) 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
70 forms an integral part of a phase locked loop identified
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 ~/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 ~uadrature
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 ~unctions of the multiplier 35 and the corrector
circuit 37. The phase detector or multiplier 35 receives an
input 80 ~rom 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 signal at terminal 84, ~erived 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
re~erence 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 at the differential pair output 74, so
that the output current is proportional to the cosine of the
angular difference between the input signals 80 and 84. 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 84 must be relatively
free of higher order harmonics. By making the phase shifting
network 73 a low pass filter, odd order harmonics from the
oscillator's square wave are removed.
The coxrector circuit 37 preferably consists of a
differential amplifier having a pair of transistors 100 and
101. Current for the emitters of transistors 100 and 101 is
- AP--76819 ~
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 signal voltage derived from the signal input 66 is
applied between the bases o~ the transi~tor~ 100 and 101
respectively through two resistors 108 and 109, two diodes
110 and 111 and a reference voltage source 112. The reference
voltage source 112 consists of an emitter follower 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 11~ and 115 to
provide a reference voltage. The e~itter of the emitter
follower 113 provides a low impedance voltage reference for
the pair of transistors 100 and 101 forming the dif~erential
amplifier.
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 output of the multiplier 35, Ir = ImaX
cos ~. The output current may be represented by Io = I1IS/Irr
where Il is supplied by a current source 102. Is is the
input signal current at terminal 66 and may be represented
--10
~35~
as eS/2r 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 = [Ile (1 + L + R)cos(~ct ~)]2rImaxcos ~- Since cos
(1 + L ~ R)/(l + L + R) + (L - R)2, I = (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 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 amplifier 65 can now be a single output
connected to multipliers 40 and 41. The output of the
multiplier 40 is L cos ~ and goes to corrector circuit 37a
~here it is divided by cos ~ providing an L output. The
output 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
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
multiplier 35 respectively. The output 95 of the corrector
circuit 37c is connected to the inputs of 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
having a transmitter similar to that of Fig. 5, that is, a
quadrature system with the cos~ change. The L and R inputs are
c~mbined additively in adder 58 and subtractively in adder 61.
The 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 chan~ed 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
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 of the receiver RF-mixer-IF amplifier 65, the output of
the corrector 37 is coupled to the multipliers 40 and 41 and
the phase locked loop and phase locked loop and phase shifting
networks are the same as described with regard to Fig. 6. As
described above with regard to Fig. 10 th~ one output 97 is
phase shifted and both outputs go to a matrix circuit 99 to
provide L and R outputs.
Fig. 12 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 of 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 shi:Eter 95. The phase shifted 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 signals are modulated on
z
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 sum 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
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 angle between the two
quadrature carriers. The signal as transmitted has all of
the advantages of quadratures modulation without causing
distortion in an envelope detector. It provides a minimum 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.
-13- :
