Note: Descriptions are shown in the official language in which they were submitted.
1 BACKGROUND OF THE INVENTION
FIELD OF T~E INVENTION
This invention relates to compatible AM stereo trans-
mission techniques, including the basic proposition of .
modulating a carrier wave so that the stereo related intel-
ligence appears as respective upper and lower sidebands of
the carrier wave in the manner fundamentally shown in my
prior U.S. Patent 3,218,393, with optional provision for the
presence of infrasonic frequency (e.g. 15 Hz) modulation of
the carrier wave to provide in the receiver indication of the
, , ~,
1 presence of a stereo modulated signal for tuning and for
receiver mode control purposes.
DESCRIPTION OF T~E PRIOR ART
Compatible stereophonic AM transmission and reception,
involving stereo related upper and lower sidebands, with the
difference stereo signal (L-R) phase modulating the carrier
wave and with the summation stereo signal (L+R~ envelope
modulating the carrier wave, are disclosed in my U.S. Patent -~
3,218,393, together with certain forms of receivers for
stereophonic reception of a carrier wave so modulated. A
further discussion of this compatible AM stereophonic mod-
ulation technique appears in my paper entitled "A Stereo-
phonic System for Amplitude Modulated Broadcast Stations",
which appears in IEEE Transactions on Broadcasting, Vol.
BC-17, No. 2, June 1971, at pages 50-55. -
For a fuller understanding of the manner of stereophonic
modulation characteristic of the present invention, as herein- -~
after more fully discussed, reference can be had to my U.S.
Patent No. 3,350,645, relating to a similar technique for
compatible single-sideband modulation, wherein the second order
sideband rendering the single-sideband modulation compatibly
receivable by conventional envelope detection receivers is
developed by signal splitting, phase shifting, signal seg-
ment frequency doubling and signal combining circuitry.
Also known as stereophinic transmission and reception
systems as disclosed in Shoaf U.S. Patent No. 3,009,151,invol-
ving a two-channel FM-AM stereo system wherein stereo related
- - 3 -
signals are respectively frequency modulated and amplitude
modulated on FM band and AM band carrier waves; Colodny U.S.
Patent No. 3,031,529, disclosing a single channel AM stereo
system employing synchronous detectors in the receiver portion
of the system; Avins U.S. Patent No. 3,068,475, disclosing a
stereo transmission and reception sy`stem wherein one stereo
related signal is amplitude modulated on a carrier wave and the
other stereo related signal is frequency modulated on the same
carrier wave; Barton U.S. Patent No. 3,102,167, disclosing a
two-channel, phase-shifted, double sideband stereo transmission; i
Fink U.S. Patent No. 3,206,550, disclosing visual display of
a stereo presence signal; Holt et al U.S. Patent No. 3,167,614,
disclosing use of an infrasonic tone to indicate stereo signal
presence in an AM/PM type transmission system; and Collins U.S. -~
Patent No. 3,231,672, disclosing an AM stereo system involving
linearly added carrier waves at the same frequency but in
different phase, with each of the carrier waves amplitude
modulated with stereo related signals.
Also known is a system for transmission of stereophonic
signals over telephone lines, as in Almering et al U.S. Patent '
No. 3,803,490, granted June 3, 1974, wherein two different
carrier frequencies are employed with a relatively wide band-
width requirement (e.g. 65 kHz to 103 kHz with 8.06 kHz break),
and with no attempt to make the system compatible from the
point of view of detection of signals by envelope detection
Also notable as being of general interest, in the field , ,
of CSSB and stereophonic signal transmission are the following:
E.S. Purington, U.S. Patent 2,020,327
Nov. 12, 1935.
0. G. Villard, Jr., "Composite amplitude and
phase Modulation", Electronics, Vol. 21, Nov.
1948, pp. 86-89.
, ' ~. -'' ' - . ,. ' ~ . ' . - ' : . ~ - -
L. R. Kahn, "Comparison of Linear Single-Sideband
Transmitters With Envelope Elimination and
Restoration Single-Sideband Transmitters", Proc.
IRE, Vol. 44, December 1956, pp. 1706-1712.
J. Avins, et al, "Compatible Stereophonic System
for the A.M. Broadcast Band", RCA Review, September
1960, pp. 299-359. - -
H. E. Sweeney and C. W. Baugh, Jr., U.S. Patent
3,069,679, December 18, 1962.
Philco Corporation, "Petition to the FCC For The
Institution of Rule Making Proceedings Looking
Toward the Adoption of Compatible AM Stereo
Transmission Standards", filed December ~, 1958.
J. M. Hollywood and M. Kronenberg, "A Stereophonic
Transmission System for AM Broadcasting", Journal
of the Audio Engineering Society, Vol. 9, No. 2,
D. Gabor, "Theory of Communication", Proc. Inst.
Elec. Eng., Vol. 93, 1946, pp. 429-457.
E. Bedrosian, "The Analytic Signal Representation
of Modulated Waveforms", Proc. IRE, Vol. 50, Oct.
1962, pp. 2071-2076.
W. L. Rubin and J. V. DiFranco, "Analytic Represen-
tations of Wide-band Ratio Frequency Signals",
J. Franklin Inst., Vol. 275, Mar. 1963, pp. 197-204.
H. B. Voelcker, "Toward a Unified Theory of Modulation-
Part I: Phase-envelope Relationships", Proc. IEEE,
Vol. 54, Mar. 1966, pp. 340-353.
R. E. Bogner, "Frequency Sampling Filters - Hilbert
Transformers and Resonators", Bell Syst. Tech. J.,
Vol. 48, Mar. 1969, pp. 501-510.
E. C. Titchmarsh, Introduction to the Theory of
Fourier Integrals. New York: Oxford, 1937.
M. Schwartz, W. R. Bennett, and S. Stein, Communica-
tion Systems and Techniques. New York: McGraw-
A. Papoulis, The Fourier Integral and Its Applica-
tions. New York, McGraw-Hill, 1962.
H. E. Rowe, Signals and Noise in Communication
Systems. Princeton, New Jersey: Van Nostrand, 1965.
L. R. Kahn, "Compatible Single-Sideband", Proc. IRE
Vol. 49, Oct. 1961, pp. 1503-1527.
- ~ ` lU38933
Summary o~ the Invention
Characteristic advantages and features of the AM stereo-
phonic transmission system of the present invention includes
- modulation of the carrier with an inErasonic frequency utilized
in the receiver to indicate stereo signal presence and provide
automatic shifting of the reception mode to and from stereo-
phonic and monophonic and/or to provide a carrier tuning
indicator. Also an important characteristic and feature of
the present invention is the presentation of an AM stereophonic
transmission system which is fully compatible with existing
equipment in the sense of being receivable by conventional AM ;
envelope detection type receivers, either by a single such
receiver in which case the reception is of the monophonic mode
and without signal distortion, or by two conventional AM type
envelope detector receivers, each slightly orf-tuned respectively
above and below the carrier frequency, in which event the -
reception is of the stereophonic mode or the monophonic mode
depending upon the nature of the transmitted signal.
A further important characteristic and feature of the
present invention is the development of a compatible AM stereo
transmitted signal by means controlling the phase modulation
component in a manner realizing minimal signal distortion in
transmission and reception of the signal, such phase modulation r'''
component being a composite of the fundamental and the second
harmonic of the stereo difference signal with such second
harmonic component being developed through constant gain frequency
doubler means and with the level thereof being controlled in a
level squarer circuit responsive to the syllabic level of the ~ :
fundamental stereo difference signal, the level of the stereo -
difference second harmonic signal being a square law function
of the level of the fundamental stereo difference signal and
being maintained at about 13% of the fundamental level at full
stereo modulation to minimize out-of-band radiation.
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1 More particularly the method and apparatus of this
invention is for developing a compatible AM Stereophonic
Electromagnetic Energy Transmission. The method comprises
the steps of:
amplitude modulating an electromagnetic carrier wave
with the sum of the stereophonically related signals and .
phase modulating the carrier wave with a composite stereo
difference signal. This difference signal is essentially
comprised of the summation of:
(1) the fundamental of the difference signal, and
(2) the difference signal developed from the second
! harmonics of the stereo related signals, signal
(2) being about 13% of the level of signal (1)
at full stereo modulation and being maintained in
substantially a square law relation with respect
to signal (1).
- 6a -
Other features and advantages of the invention will be
apparent from the following description and discussion of
- certain typical embodiments of the invention.
Brief Description of the Drawings
FIG. 1 is a simplified block diagram of a transmitter
exciter developing amplitude modulated and phase modulated
inputs to the modulation stages of a conventional or standard
AM transmitter such as shown in FIG. 1 of my U.S. Patent 3,218,393; i
FIG. 2 is a showing, partly in schematic and partly in
block form, of the level squarer circuit portion of the trans-
mitter exciter presented in FIG. l;
FIGS. 3(A) 5 3(B) and 3(C) compositely and diagrammatically
present the frequency spectra of the PM modulating component
with only the lower sideband (LSB) active and fully modulated;
FIG. 3(A) portraying the Bessel function distribution for 0.5
radian phase modulation (full stereo modulation of the fundamental
stereo difference signal); FIG. 3(B) showing the second harmonic
phase modulation spectrum (at 0.0665 radian); and FIG. 3(C)
showing the combined PM component spectrum distribution;
FIG. 4 is a diagrammatic showing of the final output
spectrum (PM component with 50% AM modulation) corresponding to
the PM component spectra shown in FIG. 3 and illustrating the :-
transmitted output of the system of the present invention under
the same condition as shown in FIGS. 3(A) - 3(C), i.e. with only ;
the lower sideband (LSB) active.
Description of the Preferred Embodiment
FIG. 1 illustrates in block diagram form a typical trans-
mitter exciter layout embodying the present invention. Stereo i~
related audio input signals L and R, derived in a manner known
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, . - . . .
' , . . . ' ~, ~, ,':,, '. :
per se, are fed to a summation circuit 12, the summation output
14 from which is applied to phase shift network 16 (a -45 phase
difference network; compare network 70 in my patent 3,218,393)
and its output 18 is in turn utilized as the audio input to the
AM portion of the associated AM transmitter. To this extent,
the development of the AM modulation of the transmitted signal
is identical to the AM modulation provided by the transmitter
exciter or adapter shown in FIG. 2 of my prior U.S. Patent 3,218,393.
To develop the phase modulation component applied to the
phase modulator portion of the transmitter, the audio input
signals L and R are also fed through respective low pass filters
20 and 22, the respective outputs 24, 26 from which are fed to
difference circuit 28. The output 30 from difference circuit 28,
after undergoing a relative +45 phase shift in phase difference
network 32 (compare network 72 in my patent 3,218,393), serves
as the fundamental phase modulation component input 34 fed to
summation circuit 36.
The outputs 24, 26 from their respective low pass filters
20, 22 are also fed to separate phase difference networks 38, 40
with zero relative phase shift. Respective network outputs 42,
44 are applied to respective constant gain frequency doublers
46, 48. Suitable, each of the constant gain frequency doublers
46, 48 can be the same type of doubler circuit as shown in FIG. 3
of my U.S. Patent No. 3,350,645, with the doubler circuit in this
instance operated at relatively high levels, e.g. on the order
of a volt or more, so that the gain of the doubler is not a
function of the input level over its normal operating range.
In the operation of the circuit as shown in said Patent 3,350,645
the circuit is operated at relatively low level so that the
. . -
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'' `. ' ;'~
- 1 rectifier output curve in nonlinear and the output second
harmonic followed approximately a square law. In the instance
of the present constant gain frequency doublers 46, 48, the
- input level is sufficient (on the order of 2 volts or more)
so that, although the different circuit provides a second ~
harmonic, the amplitude of the second harmonic is a linear -
function of the imput level.
The respective outputs 50, 52 of the doubler circuits ~s
46, 48 are fed to difference circuit 54, the frequency doubled
difference signal output 56 from which is the signal input to
the variable gain amplifier 58 of level squarer circuit 60. The
level squarer 60 circuit, a typical schematic of which is
shown at FIG. 2, also comprises rectifier 62 controlled by
fundamental component input 34 and in turn providing a variable
lS output 64 (the average level of which is set by potentiometer
66) which in turn controls the gain of the variable gain -~ ~
amplifier 58. As shown in FIG. 2, the time constants of the ~ -
circuitry associated with the rectifier 62 are suitably
selected so that the variable gain amplifier 58 gain is
controlled at a syllabic rate, i.e. at a rate comparable
to the syllabic rate of speech (e.g. by use of time constants
on the order of 0.1 second).
i As indicated in FIG. 2, the level squarer circuit- which functions to develop the proper level of second harmonic
difference signal input for the phase modulation suitably
employs an integrated circuit (IC) type MC1594L, connected as
a wideband amplifier with linear AGC as shown at Figure 24 at
page MC1594-Pg.12 of the Motorola Linear Integrated Circuits
Data Book dated December 1971.
As indicated, control of the rectifier 62 is derived
; from the fundamental diffe~ence signal input 34 (th~phase shifted
~ . , .
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.. . . . . .
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output 30 of di~ference circui-t 28). Thus, when -tlle L dnd R
signals are equal and in phase (i.e. the audio signal intelligence
input is monophonic), the L-R signal is zero and the rectifier 62
. reduces the gain of the variable gain amplifier 58 to zero (it
being notable that the input to the variable gain amplifier 58
under this condition is also zero). However, when the L signal
is at full level and the R signal is zero (representing an
: idealized stereophonic signal input condition), the rectifier
62 controls the gain of variable gain amplifier 58 to be a-t
a given, maximal level (i.e. a gain of X). When the L and R
signals are both present and are in phase but the L signal is
at full amplitude and the R signal is at one-half amplitude,
for example, the gain of the variable gain amplifier 58 is
reduced (i.e. to X/2) to provide the right amount of second
' 15 harmonic component. The second harmonic component output 68
from the variable gain amplifier 58 is applied as an input to
summation circuit 36 along with the fundamental difference
signal component 34, and the summated output 70, with appropriate
time delay in variable time delay circuit 72, constitutes the
- 20 altered stereo difference signal input '74 applied to the phase
modulator 76 wherein the audio input 74 phase modulates the
carrier wave input from high frequency crystal oscillator 78,
with the output 80 from the phase modulator 76, after appropriate
frequency multiplication in multiplier 82 as desired, providing
output 84 which is employed as the phase modulated carrier wave
in the associated AM transmitter in like manner as the phase
modulated RF output 42 in the stereo adapter or exciter shown :
in FIGS. 1 and 2 of my U.S. Patent No. 3,218,393.
In general, and as discussed in more detail in connection
with the discussion of FIGS. 3 and 4 hereof as set forth below,
the level squarer 60 circuit functions to provide a second
harmonic component input 68 at a level which is a square law
function of the level of the stereo difference (L-R) component.
As will also be apparent from the more detailed consideration of
S FIGS. 3 and 4 hereinafter, minimal out-of-band distortion is
achieved when the amplitude level of the frequency doubled
difference signal is about 13~ of the amplitude level of the
fundamental difference signal at full stereo modulation (i.e. with
the phase modulation fundamental component modulated at 0.5 radian
and with 50~ AM modulation).
As will be recognized from the comparison of the manner
of development of the phase modulated RF component in the system
disclosed in my prior U.S. Patent 3,218,393, the manner of
development of an AM stereo phase modulated component as
lS illustrated in FIG. 1 hereof is essentially different, involving
in this instance phase shifting and separate frequency doubling
of respective stereo related components at audio frequency. -~
As will also be recognized from a comparison of this mode of '
development of an altered stereo difference signal to phase
modulate a carrier in a stereo transmitter exciter as shown in
FIG. 1, the utilization of phase shift and frequency doubling
at audio frequency to synthesize the phase modylating wave has
- a very general similarity to the manner of development of the
phase modulated component in a compatible single sideband signal
from a single audio source as disclosed in my prior U.S. Patent
3,350,645. However, in this instance the modulating signal is
developed from a stereo related pair of audio input signals,
separate phase shift network means and frequency doubler means
are employed for each of the stereo signals, and the phase
.-' "' "
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,,; - , , , ' ' ' ` . ' ., .: '
modulating audio component is composed not only of the fundamental
of the stereo difference signal but includes also a controlled
amount of frequency doubled difference signal, the level of the
frequency doubled difference signal being substantially a square
law function of the fundamental stereo difference signal and
being approximately 13% of the amplitude level of the stereo
difference signal at full stereo modulation.
Additional circuit differences based on the specific
needs and purposes of the present invention, as illustrated
in the transmitter exciter shown in FIG. 1, involve modulating
the transmitted signal with an infrasonic frequency signal
(e.g. 15 Hz) which serves to indicate to the receiver the
presence of stereophonic intelligence in the transmitted signal.
By the term "infrasonic frequency" signal is meant a signal of
a frequency below the audio range, as the term is defined in the
Modern Dictionary of Electronics, published by Howard W. Sams
Co. Inc., First Edition, 1962, for example.
The infrasonic frequency signal can be present either
as amplitude modulation of the AM component output 18 or as
frequency or phase modulation of the PM component output 84,
- or both. In the first instance, as shown in solid line in FIG. 1,
a 15 Hz oscillator 86 provides through switch 88 an output 90
of variable amplitude, as determined by attenuator 92, whi~h is
combined with the phase shifted summation output 18. If frequency
` 25 or phase modulation of the infrasonic frequency stereo presence
signal is to be used either conjunctively or alternatively with
the infrasonic frequency modulation input 90, this can be
provided by a 15 Hz oscillator 94 the output from which is
applied through switch 96 to phase modulator 76, with the result
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- . . . . , - ..................... . . . .. . ..... . . . .
1 that the infrasonic phase of frequency modulation appears on
the phase modulated component RF wave output 84.
In a typical transmitter exciter, involving frequency
modulation by the infrasonic frequency signal, the low
frequency oscillator 94 is frequency modulated by a simple
narrow band FM modulator, which may take the form of a variable
capacitance circuit across a crystal oscillator, providing an
: output frequency which provides the desired infrasonic freq~
uency stereo presence signal in the receiver. In a typical -~
instance, with the entire AM stereo wave frequency modulated at :~:
15 Hz and to the extent of a frequency deviation of plus or : .
minus 25 cycles, the narrow band frequency modulation of the
signal does not materially effect the bandwidth of the signal
nor is it detectable by listeners to AM receivers. The modu-
~ 15 lation is kept low, typically about 5 to 10~ in the case of
amplitude modulation of the carrier wave, or typically at less .
than a modulation index of one in the case of FM or PM mod-
- ulation, so even if the receiver responds to the infrasonic
- modulation the audio system of a conventional receiver provides
. 20 appreciable attenuation at 15 Hz and renders the infrasonic
frequency signal inaudible or essentially so.
i. As will be understood, the oscillators 86 and 94 are
controlled, in a manner known per se and schematically indicated
: by respective switches 88 and 96, to be in circuit during periods
of stereophonic transmission.
AS indicated, the stereo presence indication to the
various receivers receiving the transmitted signal can be in the
form of either infrasonic amplitude modulation or infrasonic
, .. . . . .. .
'. ' - ', ' ~'' ': ~ '' '
frequency or phase modulation, or both, and can involve use
of either the same infrasonic frequency tone or two infrasonic
frequency tones, as desired.
FIGS. 3(A), (B) and (C) graphically illustrate the
spectrum of the PM component under a typical operating condi-
. tion with a stereophonic signal input. For illustration
purposes the operating condition considered is the situation
with only one sideband (the lower sideband LSB for example)
active and at full modulation for stereo transmission (i.e.
with L fundamental modulation at 0.5 radian and with no R
signal). FIG. 3(A) shows the spectrum of the PM component
(Bessel function distribution) at the carrier frequency (fc)
and at the first upper sideband (+1), second order sideband
(+2) and third order sideband (+3) frequencies and at the
lower first order sideband (-1), second order sideband (-2)
and third order sideband (-3) frequencies at the 0.5 radian
modulation level. This spectrum of frequency distribution in
the output 80 from phase modulator 76 develops from the stereo
; difference fundamental signal input 34. FIG. 3(B) shows the
phase modulation contribution of the second harmonic input
at 68 with the level of the second harmonic at .133 of the
fundamental level (i.e. at .0665 radian). FIG. 3(C) diagram-
- matically portrays the spectral frequency distribution at
modulator output 80 resulting from the combined fundamental
and second harmonic inputs as such appear at the output of
summation circuit 32 and time delay 72, i.e. FIG. 3(C)
- presents a summation of FIGS. 3(A) and 3(B).
.' FIG. 4 diagrammatically shows the final output spectrum,
i.e. the frequency distribution of the transmitted signal resulting
from the phase modulated carrier wave output 84 and the amplitude
modulating L+R audio output 18 (with the latter correspondingly
14 : ~ :
, .. . ..,, ~ .
. ,, .. , , ~ .
at full stereo modulation, i.e. at 50% Al~ modulation), the
numerical values given in FIG. 4 also including parenthetical
presentation of the relative decibel level of each sideband
as compared with the carrier level. As will be noted, this -
output spectrum, with its values for the carrier fc and the
lower first order sideband and second order sideband (-1) and (-2) -
closely approximates the three component transmitted signal
spectrum desired for compatible single sideband transmission
as set forth in my U.S. Patents 2,989,707 and 3,350,645 and
desired for compatible stereophonic transmission as set forth
in my U.S. Patent 3,218,393, i.e. with the stereophonic ~ ;
intelligence (the L signal input in this instance) appearing
spectrally in the form of a somewhat reduced carrier, a
first order sideband and a relatively smaller but substantial
second order sideband. The putput spectrum shown at FIG. 4
is also significant from the point of view that, except for
the carrier frequency and first and second order lower sideband
components, all other spectral components are at least -35db
below the carrier level, indicating that out-of-band inter-
ference and distortion of the other stereo signal sideband(the upper sideband, containing the stereo distinguishable
intelligence of the R stereo signal input) are well within `
commercially acceptable levels.
From the foregoing, various modifications, rearrange-
25 ments and adaptations of the AM stereo transmission technique
and components presented will occur to those skilled in the
art to which the invention is addressed, within the scope of
the following claims.
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