Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
This invention is directed to loudspeaker
systems and in particular to loudspeaker systems for use
as a loudspeaker telephone terminal.
Telephone apparatus is normally designed to
acoustically isolate the microphone from the loudspeaker
to assure the prevention of sing-around pat~s. However,
it has become desirable to operate the telephone in a
hands-off manner so that other tasks may be performed while
talking over the telephone, and more essentially so that
one or more groups of people may talk and listen at various
locations in the form of a conference.
Loudspeaker telephone apparatus which has
been developea to allow hands-free operation, operates on
the basis that at each location, the apparatus is voice-
switched such that when a person speaks into the microphone,
at one location, a switch operates to disconnect the speaker
at that location. This type of system operates quite
satisfactorily for a conversation between two people at
two locations, however, as the number of telephone locations
in a conversation or telephone conference increases, the
chances of two people speaking at the same time increases.
To avoid this situation in which two or more loudspeakers
are off at the same time, the conference must become very
formal with the interplay of participants being controlled
by a chairman.
Summary of the Invention
It is therefore an object of the present
invention to provide a loudspeaker telephone apparatus
wherein loudspeakers are used as bidirectional receive and
transmit transducers.
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It is a further object of the present
invention to provide loudspeaker telephone apparatus wherein
- the receive and transmit transducers are not acoustically
isolated from one another.
These and other objects are achieved in loud-
speaker telephone apparatus including a hybrid circuit
having line terminals for connecting the telephone apparatus
to a two-wire telephone line, balance terminals and first
and second amplifier terminals. A line balance impedance
is connected to the hybrid balance terminal for balancing
the impedance of the telephone line. The apparatus further
includes a receive amplifier having an output, and an input
connected to the hybrid first amplifier terminals, and a
transmit amplifier having an input, and an output connected
to the hybrid second amplifier terminals. A pair of similar
electro-acoustic loudspeakers, each having two similar coil
groups formed from coils having substantially identical
resistance, inductance and mutual inductance, are connected
to form a bridge circuit. The coil groups in the first
loudspeaker are connected in series-aiding and the coil
groups in the second loudspeaker are connected in series-
bucking. The bridge circuit is connected to the output
of the receive amplifier and the input of the transmit
amplifier such that the loudspeakers convert received electrical
signals to acoustic signals and acoustic signals to trans-
mitted electrical signals.
In one embodiment, the output of the receive
amplifier is connected across the two coil groups of each of
the first and the second loudspeakers and, the input of the
transmit amplifier is connected between the coil groups of
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; each of the first and second loudspeakers.
In a second embodiment, the apparatus includes
a feedback amplifier having its output connected across the -
two coil groups of each of the first and second loudspeakers
and its input connected between the coil groups of each of
the first and second loudspeakers. This apparatus may
further include a second hybrid circuit having line terminals,
balance terminals and first and second amplifier terminals
connected respectively to the output of the receive amplifier
and the input of the transmit amplifier. The second hybrid
line terminals are connected across one coil group in the
bridge circuit, and balance terminals are connected to a
balance circuit to balance the effective impedance of the one
coil group.
Other objeets and aspects of the invention
will be clear from the detailed description of the drawings.
Brief Description of the Drawings
In the drawings
Figure 1 illustrates the prior art voice-
switched loudspeaker telephone apparatus;
Figure 2 illustrates one embodiment of loud-
speaker telephone apparatus in accordance with the present
invention; and
Figure 3 illustrates a second embodiment of
loudspeaker telephone apparatus in accordance with the present
invention.
Descri tion of the Preferred Embodiments
p
Figure 1 illustrates a conventional voice-
switched loudspeaker telephone. A hybrid coil circuit 1 is
used to connect the telephone to a two wire line 2. Hybrid 1
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has line terminals to which line 2 is connected, and
balance terminals to which a line balance impedance 3 is
connected. Hybrid 1 also has two further sets of terminals
for connection to a receive amplifier and a transmit ampli-
fier. The hybrid 1 is used to isolate the incoming signals
from the outgoing signals. Typical hybrid circuits are
described in the Electronics Engineers Handbook - McGraw-
Hill Book Company, 1975, Section 22-32. A loudspeaker 4 is
connected to the hybrid 1 through a variable gain receive
amplifier 5. A microphone 6 is also connected to the
hybrid 1 through a variable gain transmit amplifier 7.
Finally, a bias circuit 8 for amplifiers 5 and 7 is con-
nected to microphone 6 and is controlled by the voice
signal from the microphone 6.
The transmi~t amplifier 7 and the receive
amplifier 5 require a total gain that is in excess of the
transhybrid loss provided by the line balance 3. Under
normal conditions, the gain of the receive amplifier 5 Ls
at a maximum value and that of the transmit amplifier 7 at
a minimum value. Under quiescent conditions speech enter-
ing the microphone circuit will switch the receive ampli-
fier 5 to its "off" condition and the transmit amplifier 7
to its "on" condition. The set is thus voice-switched from
the receive condition to the transmit condition, and under
these circumstances, it is assumed that all other sets
connected into the system will remain at the receive
condition. When the voice signal stops, the receive
amplifier 5 will return to maximum gain or "on" and the
transmit amplifier 7 will return to minimum gain or "off".
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One embodiment of loudspeaker telephone
apparatus in accordance with the present invention is shown
in figure 2. The apparatus includes a hybrid circuit 11 of
the type described with regard to figure 1, to connect the
apparatus to a two wire line 12. A balance impedance 13 is
connected to the hybrid 11 to balance the impedance of the
two wire line. The hybrid is further connected to a
receive amplifier 15 and a transmit amplifier 17. The
receive amplifier is connected across two multi-filar
moving coil loudspeakers 14 and 16 at terminals 18 and 19.
The multi-filar moving coil loudspeakers are
of the type described in United States Patent Number
4,300,022, which is entitled, "Multi-filar Moving Coil
Loudspeaker" which issued on November 10, 1981, to Richard
Hastings-James and George W. Holbrook. Loudspeakers 14 and
16 each have similar coil groups 20,21 and 22,23 respect-
ively, each group being made up of coils having near iden-
tical resistance, inductance and mutual inductance as they
all cut the same magnetic flux lines. Coil groups 20 and
21 are connected together at junction 24 in series-aiding
between terminals 18-19, whereas coil groups 22 and 23 are
connected together at junction 25 in series-bucking between
terminals 18-19. Since all of the coil groups 20, 21, 22
and 23 are similar, speaker 16 will act as a resistance,
but speaker 14 will provide the acoustic output. In
addition, since the four coil groups are similar,
essentially none of the signal output from receive
amplifier 15 appears across junctions 24-25. In this
manner, junctions 24 and 25 are isolated from the input and
a sing-around path is prevented. When an acoustic wave
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impinges on the loudspeakers 14 and 16, as when a person
speaks~ an electrical signal is generated in each coil
- group, however since coil groups 20 and 22 are connected in
series-aiding across junctions 24-25, they generate a signal
between these junctions. Coil groups 21 and 23 are connected
in series-bucking and therefore do not contribute to the
output signal. The transmit amplifier 17 is connected across
unctions 24-25, amplifies the output signal and applies it
to the two-wire line 12 via hybrid 11.
As stated above, signals applied to terminals
18-19 are severely attenuated across junctions 24-25. This
attenuation has been found to be greater than 70 dB over the
telephone speech band of 200-3200 hz. As connected in figure
2, the groups of windings of the two multi-filar speakers 14
and 16 act as a highly balanced hybrid circuit~ The two-wire
line 12 is connected via a hybrid ci_cuit 11 to the receive
~mplifier 15 and transmit amplifier 17. The nominal balance
of the hybrid 11 is obtained by a line balance 13 and a trans-
hybrid loss of 10 dB can be achieved. The transhybrid loss of
the two speakers 14 and 16 is, however, greater than 70 dB
and, consequently, the gains of amplifiers 15 and 17 can be
increased to a total of slightly less than 80 dB. By adjustiny
the amplifier gains to the correct level, good loudspeaker
reception is obtained with a signal of -10 dB incoming on the
line 12 and an output on the line 12 of 1 mW which is obtained
with normal speech spoken into the loudspeaker.
Figure 3 illustrates another embodiment of
loudspeaker telephone apparatus in accordance with the
present invention. As in figure 2, the terminal includes
a hybrid circuit 31 to which the two wire line 32 is connected
and is balanced by a line balance impedance 33. In addi~ion,
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a receive amplifier 35 and a transmit amplifier 37 are con-
nected to the hybrid 31. However, in this embodiment, the
amplifiers 35 and 37 are connected to a second hybrid circuit
38. A highly efficient mutli-filar mo~ing coil loudspeaker
circuit 40 is connected into the hybrid 38 and is balanced by
a balance 39. The efficient loudspeaker circuit 40 may be of
the type described in U.S. Patent 4,300,022, referred to
above, however, the feedback circuit 40 described with regard
to figure 3 is preferred.
The circuit 40 includes two similar multi-
filar loudspeakers 34 and 36 which each have two similar coil
groups 41, 42 and 43, 44, respectively, as described with
reference to figure 2. The coil groups are connected in a
bridge arrangement such that coil groups 41 and 42 in speaker
34 are connected at junct~ion 47 in series-aiding across term-
inals 45-46, and coil groups 43 and 44 in speaker 36 are con-
nected at junction 48 in series-bucking across terminals
45-46. The four coil groups 41, 42, 43 and 44 thus form a
highly balanced bridge circuit and signals impressed across
terminals 45-46 will produce only a small signal across junc-
tions 47-48. It has been found that the ratio of these sig-
nals is greater than 70 d~. To improve the efficiency of the
loudspeakers 34 and 36, a feedback amplifier 49 is used with
its input connected across junctions 47-48 and its output
connected across terminals 45-46. Since the ratio of the
signals between terminals 45-46 and junctions 47-48 is
greater than 100:1, the gain of the amplifier 49 may be
adjusted to this level which increases the effective
impedance of each winding group by that amount without
affecting the stability of the circuit. If the nominal
impedance of each winding group is in the order of 2 ohms,
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the effective impedance of each group will be in the order
of 200 ohms. This increase in impedance directly increases
the apparent efficiency of the loudspeakers 34 and 36 as seen
by hybrid 38 when acting as a loudspeaker or as a microphone.
To connect the loudspeakers into the telephone
terminal, hybrid 38 is connected across coil group 44 and the
impedance o~ the coil-group 44 is balanced by a balance
impedance 39 which may consist of a simple resistance 50 and
capacitance 51. A transhybrid loss of over 30 dB over the
speech band can be obtained in hybrid 38 and therefore the
gains of receive amplifier 35 and transmit amplifier 37 can
be set at 40 dB. The increase of impedance between terminal
46 and junction 48 by a factor of 100 represents a ~ain of
20 dB in each direction. Therefore, there is an effective
loop gain of 80 dB and adequate speech in both directions
is obtained.
Modifications to the above described
embodiments of the invention can be carried out without
departing from the scope thereof and therefore the scope of
the present invention is intended to be limited only by the
appended claims.
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