U.S. patent number 3,601,550 [Application Number 04/809,356] was granted by the patent office on 1971-08-24 for loop communication system.
This patent grant is currently assigned to Zenith Radio Corporation. Invention is credited to John G. Spracklen.
United States Patent |
3,601,550 |
Spracklen |
August 24, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
LOOP COMMUNICATION SYSTEM
Abstract
A limited-range, induction-coupled loop communication system for
the transmission of audio information to a receiver located within
the loop uses a frequency-modulated carrier-wave signal to transmit
audio information to one or more receivers located within the loop.
Each receiver has a pair of pickup coils preferably mounted at
90.degree. relative to each other and a phase-shifting network
connected to each of their outputs to thereby create a quadrature
relationship in the received signals to substantially eliminate the
occurrence of null conditions with variations in receiver
orientation.
Inventors: |
Spracklen; John G. (N/A,
IL) |
Assignee: |
Corporation; Zenith Radio
(IL)
|
Family
ID: |
25201130 |
Appl.
No.: |
04/809,356 |
Filed: |
March 21, 1969 |
Current U.S.
Class: |
455/41.1;
455/276.1 |
Current CPC
Class: |
H04B
5/00 (20130101) |
Current International
Class: |
H04B
5/00 (20060101); H04B 001/06 (); H04B 005/00 () |
Field of
Search: |
;179/82
;325/320,349,369 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Helvestine; William A.
Claims
While a particular embodiment of the invention has been shown and
described, it will be obvious to those skilled in the art that
changes and modifications may be made without departing from the
invention in its broader aspects, and, therefore, the aim in the
appended claims is to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
1. S A limited-range, induction-coupled communication system for
the wireless transmission of information to a receiver located
within a predetermined limited space, comprising:
means for providing a carrier-wave signal having a predetermined
center frequency and modulated with a signal representative of said
information;
transmitter means including a closed-loop antenna for transmitting
said carrier-wave signal to said space and establishing therein an
electromagnetic radiation field having a predetermined
direction;
input circuit means for receiving said carrier-wave signal,
including a pair of input transducers each having a
maximum-signal-coupling axis orthogonally oriented relative to each
other, each said input transducer providing a separate output
signal representative of said modulated carrier-wave signal;
phase-shifting networks respectively coupled to said input
transducers and providing phase shifts of substantially 45.degree.
and 135.degree. respectively for establishing a quadrature phase
relation between said output signals;
means for combining said separate phase-shifted output signals;
detector means responsive to said combined output signal for
recovering said modulation signal;
and output transducer means for converting said recovered
modulation signal into output information corresponding to that
transmitted
2. A receiver adaptable for use with a limited-range,
induction-coupled communication system of the type including a
transmitter employing a carrier-wave signal modulated with a signal
representative of information for transmission to a predetermined
limited space by establishing an electromagnetic field having a
predetermined direction in said space, said receiver
comprising:
input circuit means for receiving said modulated carrier-wave
signal, including a pair of input transducers each having a
maximum-signal-coupling axis orthogonally oriented relative to each
other, each said input transducer providing a separate output
signal representative of said audio-modulated carrier-wave
signal;
phase-shifting networks respectively coupled to said input
transducers and providing phase shifts of substantially 45.degree.
and 135.degree. respectively for establishing a quadrature phase
relation between said output signals;
means for combining said separate phase-shifted output signals;
detector means responsive to said combined output signal for
recovering said modulation signal;
and output transducer means for converting said recovered
modulation signal into output information corresponding to that
transmitted.
Description
BACKGROUND OF THE INVENTION
There are various situations in which a limited-range,
induction-coupled communication system is desirable.
Hard-of-hearing members of a theatre audience, for example, can
take advantage of such a system to enable them to adequately hear
the audio portion of the program. By using headphones adapted to
receive the transmitted signal, the audio portion of the program
may be transmitted to the user's ears at a loudness level higher
than that transmitted to the rest of the audience by loudspeakers.
Hospitals provide another situation for a desirable application of
such a system because of the need to communicate with various
hospital personnel in remote locations without disturbing patients,
physicians, and others. A third application involves the classroom
training of hard-of-hearing students. Since such a student normally
wears a hearing aid anyway, it is especially convenient to
incorporate such a transmission system with the hearing aid itself
and thereby eliminate the necessity on the student's part for
additional equipment.
Conventional limited-range, induction-coupled communication systems
typically include a transmitting antenna consisting of a closed
loop of wire disposed in a horizontal plane about the periphery of
the space in which communication is desired. In the classroom
application, such a closed-loop antenna is placed about the walls
of the classroom at a height of approximately 7 feet. The
instructor's voice is picked up by a microphone and then amplified
by an audio amplifier to provide sufficient signal strength for
effective transmission by the antenna. The direction of the
magnetic field established by the closed-loop antenna is
essentially vertical within the loop, and the magnitude of the
field varies proportionally with the audio signal representative of
the instructor's voice. Each student has his own receiver which is
preferably integrated with his hearing aid as mentioned above. A
conventional hearing aid often has a pickup coil for inductively
coupling a telephone output signal to the hearing aid. Hence, by
means of a switch, this pickup coil may also be employed to
inductively couple the closed-loop transmission signal to the
receiver and thereby enable each student to adjust the volume of
his receiver (hearing aid) to provide the loudness level he
requires.
Such systems, however, are plagued with several limitations. The
pickup coil has an axis which, when oriented parallel with the
magnetic field direction, provides maximum inductive signal
coupling. When this maximum-signal-coupling axis deviates from the
parallel orientation, the amount of signal coupling correspondingly
decreases. Quite obviously, since the loudness level of the signal
delivered to the user's ear is dependent upon the amount of signal
coupling, it correspondingly decreases for such a deviation. As
mentioned above, the closed-loop antenna establishes an essentially
vertical magnetic field. Hence, the maximum-signal-coupling axis of
the pickup coil is typically oriented vertically for maximum signal
coupling. When a person wearing such a receiver moves about, for
example when he leans forward to read or write, this axis is no
longer parallel with the direction of the magnetic field. The
amount of signal coupling therefore decreases resulting in a
reduction of the amplitude of the output signal delivered to the
person's ear. In addition to the variations in volume level, such
systems are also subject to the reception of extraneous signals
(crosstalk) from adjacent classrooms utilizing a similar
closed-loop transmission system.
In order to overcome the limitations of the direct audio
transmission system discussed above, conventional systems have been
contrived which use elaborate schemes including a plurality of
closed-loop antennas connected in series or parallel
configurations, and various types of neutralizing loops or
circuits. Some have even replaced the direct audio type of
transmission signal with an amplitude-modulated (AM) carrier-wave
signal. However, these systems are inherently expensive,
complicated, and not always reliable.
While the AM approach partially overcomes some of the
above-mentioned limitations, it still is subject to serious
crosstalk interference from adjacent rooms employing a similar
system unless each room uses a carrier-signal frequency which is
substantially different from and not harmonically related to that
of an adjacent room. Furthermore, in order to enable a user of this
type of multifrequency system to enter different rooms and receive
the transmission, each receiver must be provided with tuning means
for permitting the user to selectively receive the carrier-wave
signal of the particular frequency assigned to each different room.
This approach substantially prevents crosstalk between rooms, but
it requires elaborate tuning circuitry and a good automatic volume
control system, and the user must select a different communication
channel for each room he enters.
Where such a closed-loop transmission system is used to aid a
hard-of-hearing person, it is preferable to combine the receiver
with the person's hearing aid, as mentioned above. By means of a
simple switch, for example, the user may operate his hearing aid
conventionally, or he may selectively operate it as a closed-loop
communication system receiver. Such an incorporation should require
a minimum number of additional parts in order to minimized cost and
prevent an increase in the size of the person's present hearing
aid.
It is therefore an object of the invention to provide a new and
improved limited-range, induction-coupled communication system
which is capable of uniform operation throughout a limited space
and has a high degree of immunity from extraneous signals.
It is a further object of the invention to provide such a
communication system which is conveniently and economically
adaptable to conventional hearing aids.
SUMMARY OF THE INVENTION
A limited-range, induction-coupled communication system for the
wireless transmission of information to a receiver located within a
predetermined limited space, in accordance with one aspect of the
invention, comprises means for providing a carrier-wave signal
frequency modulated by a signal representative of the information.
Transmitter means including a closed-loop antenna are provided for
transmitting said carrier-wave signal to the limited space and
establishing therein an electromagnetic radiation field of a
predetermined magnitude, which magnitude is larger than the
magnitude of any other electromagnetic radiation field present in
the predetermined space. Receiver means responsive to the
electromagnetic radiation field having the largest magnitude in the
space, to the substantial exclusion of other electromagnetic
radiation fields having a smaller magnitude in the space, are
provided for developing an electrical signal representative of the
transmitted carrier-wave signal. Detector means responsive to the
electrical signal are provided for recovering the modulation
signal, and output transducer means are included for converting the
modulation signal into output information corresponding to that
transmitted.
In accordance with another aspect of the invention, a
limited-range, induction-coupled communication system for the
wireless transmission of information to a receiver located within a
predetermined limited space comprises means for providing a
carrier-wave signal modulated with a signal representative of the
information. Transmitter means including a closed-loop antenna are
provided for transmitting the carrier-wave signal to the space and
establishing an electromagnetic radiation field having a
predetermined direction therein. Input circuit means are provided
for receiving the carrier-wave signal, including a plurality of
input transducers each having a maximum signal-coupling axis
angularly oriented relative to the predetermined field direction at
a different predetermined angle, each input transducer providing a
separate output signal representative of the modulated carrier-wave
signal. Also provided are means for combining the separate output
signals and detector means responsive to the combined output signal
for recovering the modulation signal. OUtput transducer means are
further provided for converting the recovered modulation signal
into output information corresponding to that transmitted.
BRIEF DESCRIPTION OF THE DRAWING
The features of the present invention which are believed to be
novel are set forth with particularity in the appended claims. The
invention, together with further objects and advantages thereof,
may best be understood, however, by reference to the following
description taken in connection with the accompanying drawing, in
which the single FIGURE is a block diagram of a preferred
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to the drawing, a preferred embodiment of a
limited-range, induction-coupled communication system is shown in
block diagram form. In accordance with one aspect of the invention,
a microphone 10 converts audio information, such as an instructor's
voice, into an electrical signal which is coupled to an audio
amplifier 20 wherein the signal is amplified sufficiently to
provide a modulation signal for the FM carrier-wave signal
generator 30. Alternatively, various other means may be used for
providing a suitable FM carrier signal, including a playback device
(not shown) utilizing prerecorded records, magnetic tapes, etc., on
which is recorded a carrier-wave signal frequency modulated with a
signal representative of the information desired to be
transmitted.
The output of signal generator 30 is transmitted by means of a
closed-loop antenna 40 to an FM receiver situated in the space
encompassed by antenna 40. The receiver may be a conventional FM
receiver tuned to the frequency of the transmitted carrier-wave
signal or it may preferably be of the type shown in block diagram
form within antenna 40 and constructed in accordance with another
aspect of the invention hereinafter described in greater
detail.
Antenna 40 may take various shapes and forms including the
rectangular form shown in the drawing. It may comprise a single
loop of wire disposed about the desired space as shown or it may be
a multiturn loop or even several different loops respectively
disposed about different portions of the room, depending upon the
field strength/uniformity desired. It is typically placed at a
height of approximately 7 feet about the periphery of the space in
which transmission is desired in order to establish therein an
electromagnetic radiation field of a predetermined magnitude which
is greater than that of any other such field present in the space.
Other such fields may be present, for example, when several
adjacent spaces (rooms) employ this type of system and each room
has a separate closed-loop antenna transmitting a similar signal.
Although the closed-loop antenna essentially localizes its
associated electromagnetic radiation field, there may be some
signal overlapping (crosstalk) between adjacent rooms depending
upon the amount of signal isolation provided by the intervening
wall, the respective signal levels in adjacent rooms, etc.
In order to overcome this crosstalk limitation, some conventional
systems discard a direct-audio transmission signal in favor of an
amplitude-modulated (AM) carrier-wave transmission signal and, as
discussed previously, employ widely separated carrier frequencies
which are not harmonically related. Consequently, in order to
enable a user to go from one room to another and still use the same
receiver, provision must be made for selectively turning the
receiver to the carrier-wave frequency of the particular room
involved. On the other hand, an FM system constructed in accordance
with the invention is not subject to any appreciable crosstalk and
may therefore employ the same carrier-wave frequency in adjacent
rooms.
By employing a frequency-modulated carrier-wave signal, in
accordance with one aspect of the invention, advantage is taken of
a phenomenon peculiar to FM signal reception which is sometimes
referred to as the "capture effect." The FM "capture effect" is the
characteristic of an FM receiver which enables it to effectively
discriminate against weaker, undesired extraneous signals. A
stronger signal "takes over" when its magnitude is approximately 3
decibels greater than that of a weaker signal present at the input
circuit of the receiver. At this signal difference level some
crosstalk is evident, although it is manifested as merely a
relatively low-level, unintelligible noise at the user's ear. As
the difference between signal levels is further increased, however,
the amount of crosstalk rapidly decreases.
This effect has been found to be especially useful for a
closed-loop transmission system. For a typical adjacent room
application, a signal difference level of at least 15 decibels
exists provided there is no appreciable difference between the
signal level of the transmitted signals for each individual room
(i.e., the power delivered to loops in adjacent rooms must be
equal). For such an adjacent room application, and with a
sufficient signal level to provide good amplitude limiting in the
receiver, a rejection of unwanted signals in excess of 30 decibels
has been achieved. Moreover, in extreme situations where additional
signal isolation is desired, the FM receiver may be designed to
have a broader bandwidth than that of the transmitter so that
adjacent rooms may employ carrier frequencies which are slightly
staggered (e.g., one at 95 kHz. and the next at 105 kHz.), yet a
receiver need not be retuned as a user transfers from one room to
another.
The receiver for a limited-range, induction-coupled communication
system constructed in accordance with one aspect of the invention
may, as mentioned above, be a conventional FM receiver.
Alternatively, and also mentioned above, it may be constructed in
accordance with another aspect of the invention and take the form
of the preferred embodiment shown in block diagram form within
antenna 40 in the drawing. Such a receiver includes input circuit
means 50 and 60 for receiving the carrier-wave signal transmitted
by antenna 40. Circuits 50 and 60 are inductively responsive to the
electromagnetic radiation field established by closed-loop antenna
40 to develop corresponding output signals respectively therefrom.
These output signals are coupled through phase-shifting networks 55
and 65, respectively, and are then combined for application to
amplifier 70 wherein the combined signal is amplified sufficiently
to permit detection of the modulation signal. The modulation signal
is recovered by the detector means including switching circuit 80,
pulse clipper 90, and integrator 100. The output of the detector
means is an audio signal which is representative of the transmitted
audio information. It is amplified by audio amplifier 110 to
provide a signal level sufficient to drive an earphone 120. The
entire circuit requires relatively few additional components so
that it may be conveniently incorporated with a conventional
hearing aid without any appreciable increase in physical size. A
simple switch may be provided for changing the operation from that
of a standard hearing aid to that of a limited-range,
induction-coupled communication system receiver. Of course, the FM
receiver unit may also be made as a separate unit and then coupled
to a regular hearing aid.
In accordance with this aspect of the invention, the input circuit
means of the receiver comprises a plurality of input transducers
for inductively coupling the transmitted signal to the receiver. In
the preferred embodiment shown in the drawing, an identical pair of
input circuits, 50 and 60, are provided, although various circuit
modifications can of course be made to suit particular individual
applications. The circuits include input transducers in the form of
pickup coils 51 and 61 which may be respectively associated with
capacitors 52 and 62 and resistors 53 and 63 as shown to form
resonant circuits tuned to the frequency of the transmitted
carrier-wave signal. Of course, the input transducers may be
employed independently of resonant circuit means, depending upon
design considerations. In some applications it may even be
convenient and economical to utilize the telephone pickup coil of a
conventional hearing aid as the input transducer.
Further in accordance with this aspect of the invention, the input
transducers are preferably oriented at different angles relative to
the direction of the electromagnetic radiation field established by
closed-loop antenna 40. More specifically, pickup coils 51 and 61
may be mounted in the receiver such that their
maximum-signal-coupling axes are oriented at a predetermined angle
(e.g., 90.degree.) relative to each other. As the receiver is moved
in the plane of these axes (for example, when the receiver is
embodied in a head-worn hearing aid and the wearer leans forward to
read or write), the amount of signal coupling for each pickup coil
varies. By mounting the two coils at right angles and combining
their respective output signals, the amount of signal-coupling loss
in one coil tends to be compensated by a corresponding amount of
signal-coupling gain in the other coil. Thus, the total amount of
signal is essentially constant.
There are, however, two situations for which the signal induced in
coil 51 is equal in magnitude yet opposite in phase (polarity) from
that induced in coil 61. Hence, a null condition results from the
consequent signal cancellation. To overcome this, in accordance
with this aspect of the invention, two phase-shifting networks 55
and 65 are respectively connected to the outputs of the input
circuits 50 and 60. By making network 55 shift the phase of the
output signal of circuit 50 by 45.degree., and by having network 65
shift the phase of the output signal of circuit 60 by 135.degree.,
the two output signals are maintained in a quadrature or near
quadrature (see below) relationship; that is, they are 90.degree.
out of phase relative to each other. Thus, when combined with the
physical angular orientation of the coils discussed above, there
can never be a situation in which the summation of the two signals
is zero; consequently, null conditions resulting from the user
tilting his head forward or backward are eliminated and a
relatively uniform amount of signal coupling is provided.
Of course, various modifications of the above concepts may be made
to achieve substantially the same results, depending upon the
particular design objective desired. One may even use a single
pickup coil mounted at a given angle (e.g., 30.degree. relative to
the direction of the electromagnetic field) in order to effect an
economical compromise. Alternatively, one may utilize a
two-transducer embodiment yet, because a perfect quadrature
relationship is not necessary for achieving satisfactory reception,
a single phase-shifting network coupled to one of the input
circuits may be employed, although it is much more desirable to use
two phase-shifting networks (one for each input circuit) as shown
in the drawing. For example, a 100kHz. carrier may be employed
using a deviation band of 75 to 125 kHz. Using two input circuits
as shown in the drawing but only one phase-shifting network coupled
to one input circuit (not shown) to provide a 90.degree. phase
difference at the center frequency results in an uneven amount of
phase shift as the signal frequency varies from 75 kHz. to 125 kHz.
Since only one output signal is phase shifted 90.degree. and the
other output signal is not shifted at all, and because the amount
of phase shift varies with frequency, there is considerable
distortion in the resulting combined signal because the amount of
phase shift at 75 kHz. is quite different from that at 125 kHz.
Using two phase-shifting networks, on the other hand, substantially
eliminates this distortion because the amount of phase-shift
variation due to frequency variation is approximately the same for
each network. Thus, although the total phase shift with two such
networks may vary from 90.degree. at the center frequency to
80.degree. at each end of the deviation band, the phase variation
of the one network essentially matches that of the other network so
that the combined signal has no appreciable phase distortion.
The detector circuitry in this receiver may be of the type employed
in conventional FM receivers. It has been found preferable,
however, to employ a switching circuit (such as a Schmitt trigger
or a monostable multivibrator) in the detector means in order to
enhance the detection of the FM signal by further rendering it
immune to amplitude variations. With such a switching circuit, its
output signal is a series of pulses having a repetition rate that
varies in accordance with the modulation signal portion of the
received carrier signal. This output signal is therefore primarily
a function of the number of times the leading and/or trailing
portions of each cycle of the received FM signal go above or below
a preset switching-voltage level, depending on the type of
switching circuit employed. Thus, the detected modulation signal is
essentially unaffected by the amplitude of the received FM signal
so long as the amplitude is greater than the predetermined,
relatively small threshold value required to exceed the
switching-voltage level. The pulse clipper 90 provides additional
amplitude limiting to further render the output signal of switching
circuit 80 immune to amplitude variations and thereby fashion the
signal into a series of pulses of uniform amplitude and duration.
Detection is achieved by integrating these relatively uniform
pulses with integrator 100 to recover the original audio signal for
application to the audio amplifier 110 and earphone 120.
Another variation of the invention comprises a closed-loop
communication system which includes a wireless microphone (not
shown) for receiving an instructor's voice. The microphone may be
coupled to a portable FM transmitter (not shown) worn by the
instructor which transmits an FM carrier-wave signal of a given
frequency (e.g., 50 kHz.) to the loop. A transponder (also not
shown) is coupled to the loop and changes the frequency of the
received carrier signal to a different frequency (e.g. 100 kHz.)
which is transmitted to a receiver in the room.
Thus there has been shown a new and improved limited-range,
induction-coupled communication system which is substantially
immune to extraneous signals and provides uniform reception
throughout the desired limited space. The circuit of the receiver
is easily and efficiently adapted to a conventional transistorized
hearing aid, thereby rendering it simple and economical to
construct and easy to operate. The entire circuit may be
transistorized to provide a relatively small package which may be
incorporated with a conventional hearing aid without appreciably
increasing the physical size thereof. Moreover, the disclosed
invention lends itself to portable, battery-operated application
inasmuch as it requires less power than a direct-audio loop
communication system and it is not affected by signal overload. It
has good noise immunity which renders it substantially free from
the interfering signals created by the close proximity of
fluorescent lights, radios, television, or electric motors.
* * * * *