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BACKGROUND OF THE INVENTION
The present invention relates to a cordless telephone set, and more
particularly to a cordless telephone system in which a master unit is
provided with various control functions which are controllable by at least
one of a plurality of sub-units, any sub-unit can be called from an
external commercial telephone line, transfer between sub-units is
permitted and speech between sub-units is permitted through the master
unit.
It is an object of the present invention to provide a new and improved
cordless telephone set and application apparatus thereof based on the
following problems (1)-(5) encountered in the conventional cordless
telephone set.
(1) In a conventional cordless telephone set, one sub-unit is used for one
master unit. Accordingly, if a person who carries the sub-unit is present
at a pool side, bath room or rest room, it is troublesome to tell him/her
that he/she is having a telephone call, and in some case it is difficult
to tell him/her. In one aspect of the present invention, a plurality of
sub-units are associated with the master unit and means for transferring
the telephone call to another sub-unit is provided. When there is no
speech with an external telephone, communication and speech between
sub-units are permitted to efficiently use the telephone set.
In the case of using one master unit in common for a plurality of sub-units
in a conventional set, it is impossible for the sub-units to have mutual
communication between any two sub-units since they are designed to use a
transmitted signal and a received signal of the same frequency.
The present invention makes possible a mutual communication between any two
sub-units by way of a master unit. When any one sub-unit A transmits a
speech signal, which is received by and transmitted from the master unit,
another sub-unit B can receive a transmitted speech signal from the master
unit. In such a communication, if the sub-unit B transmits a signal wave,
it will interfere with the signal wave transmitted from the sub-unit A to
disturb and prevent the signal reception of the master unit. In order to
avoid this drawback, the sub-unit B is inhibited from transmitting its own
speech signal when receiving from another sub-unit through the master
unit. Thus any two sub-units A and B can conveniently communicate with
each other using a press-talk system.
(2) In non-speech mode, the master unit and dialing signal senders of the
sub-units are idle. Accordingly, by providing various control functions in
the master unit and controlling them by various signals generated by
complex combinations of dial digits of the sub-unit, the telephone set may
be rendered to function as an automatic responding and recording telephone
set, the sub-unit may be permitted to reproduce the speech done during
absence, a light may be turned on and off or a garage door may be opened
and closed so that the cordless telephone set is more effectively used.
Since all of the functions of the master unit can be controlled by the
sub-units, the master unit may be placed at a high position not accessible
by human being so that a range of reach of its electromagnetic wave is
expanded. If it is housed in a locker, a mounting space can be saved and
trouble by a cord is also eliminated.
(3) A conventional cordless telephone system has also been arranged such
that the master-unit is internally provided with an intercom device and
the cordless telephone could be manually switched to operate as an
intercom device. According to one aspect of the present invention, the
master unit can be controlled by a signal from any one sub-unit to relay
the sub-unit and an intercom device away from the master unit. Thus, the
sub-unit and the intercom device can communicate with each other through
the master unit. Also, in the prior art intercom set, when an attendant
hears a calling tone, he/she goes to a place of the master unit and can
communicate by a press-to-talk method. Accordingly, if he/she cannot go to
the place of master unit because, for example, he/she is taking a bath or
in a rest room, he/she cannot respond to the call. If he/she is at a
distant place that he/she cannot hear the calling tone, he/she cannot
therefore respond. Such trouble also occurs in a commercial telephone set
and it is the cordless telephone set that has been invented to solve this
problem. Namely, if he/she always carries a sub-unit, he/she can hear the
calling tone wherever he/she is and he/she can respond immediately. If the
external telephone set can be replaced by the intercom set, the
disadvantage encountered in the conventional intercom set can be
eliminated as is done for the disadvantage of the cordless telephone set.
(4) In a cordless telephone set having one master unit and a plurality of
sub-units, if the sub-units and master unit have press-to-talk functions,
communication between sub-units is permitted through the master unit, as
explained above. However, this system has the following problem. It is
necessary that a transceiver and the master unit are continuously in
operation during the speech between sub-units When the speech terminates,
the initial state must be immediately restored in order to prevent trouble
in the speech with an ordinary external telephone line. When an external
line is to be transferred from a sub-unit A to a sub-unit B, the external
line must be kept on hold while the speech and exchange are done between
the sub-units. Further, speech between the sub-unit and the external line
must be permitted immediately after the termination of communication
between the sub-units. In order to immediately reset the master unit at
the end of speech between the sub-units, the simplest approach is to use
an electromagnetic wave emitted from the sub-unit. However, if the
transmitted wave between the sub-units ceases for a long period, a timer
circuit is reset and the hold of the external line is released and the
speech is interrupted. If a time constant of the timer circuit is set too
long, it operates for a long time after the end of speech and this may
cause various problems. As shown herein later in FIGS. 6 and 7, in a
system which issues a stop signal then, an additional encoder and decoder
are needed and price is higher accordingly. Further, the sub-unit is of
large size.
In other aspect of the present invention, when the sub-unit B is switched
from the press-to-talk mode to a normal speech mode, a predetermined
signal is automatically sent out for a very short period so that the timer
circuit of the master unit is reset by that signal. A circuit for sending
a signal at the start of speech may be used as it is as a circuit for
transmitting and receiving the predetermined signal at the end of speech
in order to prevent increases of the system cost and size.
(5) In the conventional cordless telephone set, there are two operation
modes, stand-by and speech. In the stand-by mode, only a calling tone is
reproduced by a speaker and no speech tone is reproduced. In the speech
mode, since an electromagnetic wave is emitted, the operation of another
unit can be checked and not be used. Accordingly, there is no risk that
the speech tone is intercepted. However, in the system in which a
plurality of sub-units are associated with the master unit, speech between
the sub-units is permitted through the master unit in the press-to-talk
system. Accordingly, if the switch is thrown to the press-to-talk mode,
the transmission tone between other units can be readily intercepted and
such an interception may not be detected by other unit because no
electromagnetic wave is emitted in such a mode.
In other aspect of the present invention, the above disadvantage is
eliminated by rendering receivers of other sub-units nonoperative while
one sub-unit is in speech.
SUMMARY OF THE INVENTION
In order to achieve various objects as described in the foregoing,
according to one aspect of the present invention, there is provided a
cordless telephone system comprising a master unit connected to a
telephone line and having a radio transceiver, at least one sub-unit
having a radio transceiver and capable of transmitting and receiving
speech to and from the telephone line through the master unit, and means
for sending a control signal from the master unit to a unit other than the
telephone line by activating the controller of the master unit by a signal
from the sub-unit.
In order to achieve the objects described in the items (1) and (2) above,
in accordance with another aspect of the present invention, there is
provided a cordless telephone system comprising a master unit connected to
a telephone line and having a radio transceiver, at least one sub-unit
having a radio transceiver and capable of transmitting and receiving
speech to and from the telephone line through the master unit, and control
means responsive to a control signal from any sub-unit, including at least
one of transfer control means for permitting transfer of speech from an
external line between sub-units, speech control means for permitting
speech between sub-units through the master unit, and equipment control
means for controlling an equipment other than the telephone line connected
from any sub-unit through the master unit.
In order to achieve the object described in the item (3) above, in
accordance with another aspect of the present invention, there is provided
a cordless telephone system with an intercom function comprising a master
unit connected to a telephone line and having a radio transceiver, at
least one sub-unit having a radio transceiver and capable of transmitting
and receiving speech to and from the telephone line, at least one intercom
unit connected to the master unit, control means for controlling the
intercom unit, and means for transmitting a signal between the sub-unit
and the intercom unit through the master unit by controlling the control
means from the sub-unit.
In order to achieve the object described in the item (4) above, in
accordance with another aspect of the present invention, there is provided
a cordless telephone system comprising a master unit connected to a
telephone line and having a radio transceiver, a plurality of sub-units
each having a radio transceiver and capable of transmitting and receiving
speech to and from the telephone line through the master unit to permit
speech in a press-to-talk system between any two of the sub-units through
the master unit, and means for resetting a timer function of a timer unit
which holds the master unit in a speech mode, by a predetermined signal
sent from any one of the sub-units.
In order to achieve the object described in the item (5) above, in
accordance with another aspect of the present invention, there is provided
a cordless telephone system comprising a master unit connected to a
telephone line and having a radio transceiver, a plurality of sub-units
each having a radio transceiver and capable of transmitting and receiving
speech to and from the telephone line through the master unit, the
sub-unit having a means for permitting the reception of speech by a
predetermined signal from the master unit or the sub-unit, the sub-unit
having a means for inhibiting the reception of speech by an output of a
received electromagnetic wave, the sub-unit having a means for permitting
the reception of speech by a speech switch or a press-to-talk switch
whereby interception of the speech of one sub-unit by other such unit is
prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a connection diagram of a first embodiment of a master unit of
a cordless telephone system of the present invention,
FIG. 2 shows a connection diagram of an embodiment of a sub-unit for the
master unit,
FIG. 3 shows a specific circuit of major blocks of the embodiment shown in
FIG. 1,
FIG. 4 shows an auxiliary circuit for controlling a signal of a transceiver
of the master unit in press-to-talk speech between the sub-units,
FIG. 5 shows a specific circuit of a variable frequency oscillator used in
FIG. 2,
FIG. 6 shows a connection diagram of a second embodiment of a simplified
master unit which is a modification of FIG. 1,
FIG. 7 shows a connection diagram of an embodiment of the sub-unit for the
master unit of FIG. 6,
FIGS. 8 and 9 show partial connection diagrams of a third embodiment of a
further simplified master unit and a sub-unit therefor, in which
generation and detection circuits for a predetermined signal indicating
the end of speech are omitted from the embodiments of FIGS. 6 and 7.
FIGS. 10a-10b show a partially modified connection diagrams for calling any
one of the sub-units by the master unit of FIG. 1,
FIG. 11 shows a partially modified connection diagram of the master unit
for preventing a master unit self-hold circuit from being reset in the
transfer between the sub-units in the first embodiment,
FIG. 12 shows a connection diagram of a fourth embodiment of the present
invention in which a multi-purpose controller is provided in the master
unit to control by a signal from the sub-units,
FIG. 13 shows a connection diagram of a fifth embodiment of the present
invention in which an intercom unit is connected to the master unit,
FIG. 14 shows a partially modified connection diagram for using
predetermined signals for starting and releasing a power supply,
FIG. 15 shows a partially modified connection diagram for calling the
intercom unit from the sub-unit,
FIG. 16 shows a connection diagram of major portions of a sixth embodiment
of the present invention in which two intercom units connected to the
master unit are switched by the sub-unit,
FIG. 17 shows a block diagram of major portions of a seventh embodiment in
which a 4-wire intercom unit is connected to the master unit,
FIG. 18 shows a connection diagram of an eighth embodiment of the present
invention which is a modification of FIG. 1 to reset a master unit
repeater by using, at the end of speech, a predetermined signal
transmitter which is also used at the start of speech in the transfer
between the sub-units.
FIG 19 shows a connection diagram of a sub-unit for the master unit of FIG.
18,
FIG. 20 shows a circuit diagram which is a partially modification of FIG.
18 in which a power supply which is self-held in the speech between the
sub-units is reset when a predetermined signal indicating the end of
speech is detected,
FIG. 21 shows a connection diagram of a sub-unit receiver in a ninth
embodiment of the present invention in which the speech of a sub-unit is
prevented from being intercepted by other sub-units,
FIG. 22 shows an embodiment of a controller for the sub-unit receiver, and
FIG. 23 shows a block diagram of a summary of the respective embodiments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a connection diagram of a master unit, with a power supply
circuit being omitted because it is similar to a known one. Numeral 32
denotes a transmission/reception antenna, numeral 1 denotes an RF
amplifier of a receiver, numeral 2 denotes a detector, numeral 3 denotes a
low frequency amplifier, numeral 4 denotes a wire/wireless coupler,
numeral 112 denotes a connection terminal to a commercial telephone line,
numeral 31 denotes a contact of a relay 13 for connecting and
disconnecting the telephone line, numerals 5, 6, 7 and 8 denote signal
selectors which may be known narrow band width filters or phase detectors
or pulse detectors and which produce outputs in response to only
predetermined signals, respectively, and numeral 10 denotes a variable
frequency oscillator which oscillates at a frequency f.sub.1 (for example,
600 Hz) when an output circuit thereof is shorted or a collector circuit
of a transistor 14 is shorted to supply the oscillation output to a
modulator 29 of a transmitter, and oscillates at a frequency f.sub.2 (800
Hz) when the signal selector 6 is actuated and oscillates at a frequency
f.sub.3 (1200 Hz) when the signal selector 7 is actuated to supply the
oscillation outputs to the modulator 29, respectively. Numeral 18 denotes
a detector for a call signal from the telephone line 112, numeral 19
denotes a discharger which is activated in response to a high voltage call
signal, numeral 20 denotes a resistor having a resistance thereof changed
in accordance with a discharge light of the discharger, and numeral 15
denotes a transistor whose collector potential is changed in accordance
with the change of resistance of the resistor 20 to open or close the
collector circuit of the transistor 14. Numeral 11 denotes a driver for
the relay 13. When an output of a squelch circuit 9 is positive, the relay
13 is actuated to close a contact circuit 31, and when the signal selector
8 is activated, the relay is reset to open the contact circuit 31. It
permits dialing similar to that of a telephone set. Numeral 22 denotes a
self-hold circuit which starts the hold operation when a short period
output of a specific signal detector 21 and an output of the squelch
circuit 9 are coincidentally applied thereto, holds the signal even after
the output of the detector 21 has been terminated, and releases the signal
when the output of the squelch circuit 9 extinguishes. Numeral 23 denotes
a power supply circuit which supplies a power during the operation of the
self-hold circuit 22, and numeral 24 denotes a power supply circuit which
supplies a power when the squelch circuit 9 alone operates. They serve to
supply the power in the speech between the sub-units by a press-to-talk
system without regard to the telephone line.
FIG. 2 shows a connection diagram of a sub-unit. Numeral 33 denotes a
transmission/reception antenna, numeral 34 denotes an RF amplifier of a
receiver, numeral 35 denotes a detector, numeral 36 denotes a low
frequency amplifier, numeral 37 denotes a speaker of a handset, numeral 38
denotes a frequency selector such as a known narrow band filter or phase
detector or pulse detector, numeral 41 denotes a transistor switch which
opens or closes the amplifier 36 in response to the output of the
frequency selector 38, numeral 45 denotes a switch for selecting one of
three modes, stand-by (SB), press-to-talk (PT) and talk (TK), numerals 43
and 44 denote diodes which control the transistor 41 to activate the low
frequency amplifier 36, numeral 46 denotes a push-button switch for the
press-to-talk speech, numeral 49 denotes a specific signal generator which
is activated by a click generated upon power-on by a switch 45, produces a
specific signal for a short period to activate the control circuit
starting from the specific signal detector, as shown in FIG. 1, numeral 57
denotes a dialing signal generator which generates the same number of
pulse signals as a digit marked on a button as a conventional push-button
telephone set does to control a variable frequency oscillator 54 to start
and stop the oscillation at a frequency corresponding to the frequency
selector 8 of FIG. 1, numerals 55, 55', 56 and 56' denote push-button
switches for calling sub-units C and B, with numeral 61 power supply,
numeral 53 denotes a microphone for transmitting speech, numeral 52
denotes a low frequency amplifier, numeral 51 denotes a modulator and
numeral 50 denotes an RF power amplifier. In FIG. 1, when a call signal
appears on the telephone line 112, the resistance of the resistor 20
changes in accordance with the signal and base voltages of the transistor
15 and 14 change accordingly so that the collector circuits thereof are
opened or closed in accordance with the signal. The variable frequency
oscillator 10 repeatedly starts and stops the oscillation at a
predetermined frequency f.sub.1 (600 Hz) in accordance with the signal and
the oscillation output is supplied to the modulator 29. On the other hand,
a current flowing through the transistor 15 flows through the diode 27 and
is supplied to the power amplifier 28. Thus, an electromagnetic wave
modulated by the call signal is supplied to the antenna. In a sub-unit,
for example, the sub-unit A in FIG. 2, the switch 45 is normally at the
position SB (stand-by) so that no voltage is applied to the base of the
transistor 41 and the low frequency amplifier 36 is opened and no signal
tone is reproduced by the speaker. If a signal electromagnetic wave
described in connection with FIG. 1 is received, it passes through the RF
amplifier 34 and is detected by the detector 35, and a calling signal tone
which is an on-off tone at 600 Hz is applied to the filter 38. The filter
38 has the output terminal thereof shorted only when a predetermined
signal is applied thereto. The amplifier 36 is activated in response to
the call signal and the call signal applied to the input of the detector
is amplified to drive the speaker to inform one of the call from the
telephone line. In the called sub-unit A, the switch 45 is thrown to the
talk (TK) position. Thus, the power is supplied to the base of the
transistor 41 through the diode 44 to short the collector circuit of the
transistor 41 so that the low frequency amplifier 36 is activated. The
power is also supplied to the power amplifier 50 through the diode 48 so
that the transmission electromagnetic wave is emitted. The specific signal
generator 49 is activated by the click generated when the switch 45 is
thrown to the position TK, and the output thereof causes the variable
frequency oscillator to start and stop the oscillation in accordance with
the signal. This signal is also emitted as the electromagnetic wave as
described above. When this electromagnetic wave is received in FIG. 1, the
specific signal emitted from the sub-unit A (usually f7 and for the
purpose of explanation, 5.6 KHz) is demodulated to activate the specific
signal detector 21. Only when the output of the specific signal detector
21 and the output of the squelch circuit (SQ) 9 which is activated when
the received electromagnetic wave is present are coincident, the self-hold
circuit 22 and the power supply circuit 23 are activated and the output
current flows through the diode 26 to the transmitter 28 which emits the
electromagnetic wave. After the self-hold circuit has been activated, it
holds the signal even after the output of the specific signal detector 21
has ceased and it is reset after the received electromagnetic wave has
extinguished and the output of the squelch circuit 9 has ceased, to stop
the supply of power. As described above, since the master unit and the
sub-units are now in operation, the sub-unit can transmit and receive
speech by the microphone 53 and the speaker 37 as a conventional telephone
set does. The transmission frequencies of the master unit and the
sub-units must be selected to be non-interference frequencies. When it is
found that the sub-unit who responded to the call from the telephone line
is not the sub-unit A but the sub-unit B, the gang switch 56, 56' for
calling the sub-unit B is depressed in the sub-unit A of FIG. 2. Thus, the
variable frequency oscillator 54 oscillates at 3.9 KHz (in general at
f.sub.5) by the depression of the switch 56 and it is carried by the
electromagnetic wave to actuate the signal selector 5 of FIG. 1. The
variable frequency oscillator 10 thus oscillates at 800 Hz which is
carried by the electromagnetic wave of the master unit. In the sub-unit B,
only the frequency of the frequency selector 38 in FIG. 2 is different,
that is, it is at 800 Hz but the rest is same as that of FIG. 2. Thus, the
low frequency amplifier 36 is activated to reproduce the 800 Hz calling
tone. When the switch 45 is thrown to the speech position, speech can be
made by the sub-unit B. In the above operation, in the sub-unit A, the
electromagnetic wave emitted by the master unit is received by the
receiver of the sub-unit A and the received tone is picked up by the
microphone of the sub-unit A, resulting in howling. The diodes 115 and the
transistor 42 of FIG. 2 serve to eliminate the above trouble by grounding
the base voltage of the transistor 41 to stop the operation of the
amplifier 36 when the power supply is activated to start the transmission.
When any telephone set is to be called from the sub-unit A through the
commercial telephone line, the switch 45 is thrown to the talk (TK)
position and buttons of the dialing signal generator 57 arranged in the
same manner as those of a conventional push-button telephone set are
depressed in accordance with the telephone number to be called so that
pulse signals are generated in accordance with the telephone number. The
variable frequency oscillator 54 starts and stops the oscillation at 5.6
KHz (f7) in accordance with the pulse signals and the oscillation output
is carried by the electromagnetic wave. When the master unit shown in FIG.
1 detects it, it passes through the signal selector 8 to activate the
driver 11 so that the relay 13 and its switching circuit 1 is opened or
closed and the dialing signal is sent out to the line. In this manner, the
desired unit can be called in the same manner as that in the conventional
telephone. When the sub-unit A is to call the sub-unit B to make speech
while there is no call from the commercial telephone line, the switch 45
is thrown to the press-to-talk (PT) position and the call switches 56 and
56' are depressed to call the sub-unit B. Since the power is supplied to
the transistor 41 through the diode 43, the low frequency amplifier 36 is
in operation and the response from the called unit can be reproduced by
the speaker 37. When speech is to be sent from the sub-unit A, the
press-to-talk switch 45 is depressed so that the power is supplied to the
transmitter through the diode 47 and the speech can be sent through the
microphone 53. If the sub-units A and B are simultaneously in the
transmission mode, the receiver of the master unit would receive two
electromagnetic waves simultaneously and this would cause trouble.
Accordingly, the press-to-talk system is used so that the transmitter is
in operation only when the speech is to be sent. The speech tone of the
sub-unit passes through the receiver of the master unit and is transmitted
from the transmitter. If it is again received by the receiver of the
sub-unit, it would cause a trouble. Accordingly, when the sub-unit sends
the speech, its receiver is rendered inoperative. The transistor circuit
42 is provided for this purpose. By grounding the base of the transistor
41 by the speech sending power supply, the low frequency amplifier 36 is
opened. If the sub-unit B is busy when the sub-unit A calls the sub-unit B
to make speech, the speech would be disturbed if an electromagnetic wave
is emitted The squelch circuit 39 and the light emitting diode 40 are
provided to prevent such a circumstance. If one of the sub-units is busy
through the master unit, the electromagnetic wave is always emitted from
the master unit and the squelch circuit 39 is in operation. When the
squelch circuit 39 is in operation, the light emitting diode 40 is fired
so that the busy state of the line is indicated.
FIG. 3 shows a specific control circuit for the variable frequency
oscillator 10, relay driver 11, self-hold circuit 22 and power supply 22
shown in FIG. 1. A transistor 62 and resistors 64, 65, 66 and 66',
capacitors 67 and 67' and potentiometers 68, 69 and 70 connected across
the collector and the base of the transistor 62 constitute the oscillator
10. When the collector circuit of the transistor 14 is shorted, the
potentiometer 70 is grounded through the diode 73, and the emitter of the
transistor 62 is grounded through the diode 76. Thus, the circuit
oscillates and the oscillation frequency is adjustable by the
potentiometer 70. In the illustrated embodiment, it oscillates at 600 Hz.
When a signal of 4.7 KHz is received, the output terminal of the signal
selector 5 is grounded and the oscillator again oscillates at 600 Hz. When
a signal of 3.9 KHz is received, the oscillator oscillates at 800 Hz, and
when a signal of 3.2 KHz is received, the oscillator oscillates at 1200
Hz. When a signal of 5.6 KHz is received, the base of the transistor 12 is
grounded through the diode 19 so that the relay 13 is reset and the
contact circuit 31 thereof is opened. When the signal of 5.6 KHz is not
received, the relay 13 is actuated by the + input of the squelch circuit 9
and the power supply. The + output of the squelch circuit 9 is applied to
the base of the transistor 82 so that the collector thereof is kept
shorted. Under this condition, if a + signal appears at the output of the
specific signal detector 21, a collector current flows through the
transistor 81 so that a base potential of the transistor 85 becomes
negative relative to an emitter potential. Thus, a collector current flows
through the transistor 85 and it is supplied to the transmitter 28 and
also to the base of the transistor 81 through the resistor 110. Thus, the
power supply 85 is in operation even after the output of the detector 21
has ceased, and it stops the operation when the electromagnetic wave
ceases, the + output of the squelch circuit 9 ceases and the transistor 82
is opened.
As described above, if the switch 45 is thrown to the press-to-talk (PT)
position in the sub-unit of FIG. 2 and the press-to-talk switch 46 is
depressed, the specific signal is not sent out but only the
electromagnetic wave is emitted. Thus, as shown in FIG. 3, the output
appears only in the squelch circuit 9 in the master unit, and only the
power supply 24 is activated to activate the transmitter. Thus, the
transmitter of the master unit is controlled by the open/close state of
the press-to-talk switch 46 of the sub-unit, and the speech between the
sub-units is attained in the press-to-talk system through the master unit.
In this case, a signal or speech received by the receiver of the master
unit must be transmitted to the low frequency amplifier of the transmitter
as it is. Since the relay is not actuated, the telephone line is opened by
the contact 31. Namely, a secondary circuit of the wire/wireless coupler 4
has no load and matching is not attained. As a result, most portions of
the output of the receiver flow into the input of the transmitter. If a
volume to the transmitter is insufficient, an auxiliary circuit shown in
FIG. 4 may be used. FIG. 4 shows only a portion of FIG. 1 and the like
numerals to those of FIG. 1 are used. Since the output terminal of the low
frequency amplifier 3 and the input terminal of the low frequency
amplifier 30 of the transmitter are coupled through capacitor 91 and
resistors 89 and 90, the volume from the receiver to the transmitter can
be freely selected. In the speech with the conventional telephone line,
the specific signal detector 21, self-hold circuit 22 and power supply 23
are activated and the resistors 89 and 90 are shorted by the collector
circuit of the transistor 94. As a result, the received sound does not go
into the transmitter to disturb the speech. In the speech between the
sub-units, the power is supplied from the separate circuit as shown in
FIG. 1 and hence the transistor 94 is not activated.
FIG. 5 shows a specific embodiment of the variable frequency oscillator 54
shown by the block in FIG. 2. A transistor 225, resistors 220-223 and
capacitors 96, 224 and 105 constitute the oscillator. Potentiometers 97,
98 and 99 are provided to adjust the oscillation frequencies f.sub.5 (e.g.
3.9 KHz), f.sub.4 (4.7 KHz) and f (5.6 KHz), respectively. When a switch
56 is closed to call the sub-unit B, the emitter of the transistor 225 and
the potentiometer 98 are grounded by the diode 101 and the variable
frequency oscillator 54 oscillates at 4.7 KHz. The base of the transistor
of the power supply 61 is grounded by the diode 104 and a current flows
into a collector circuit of the transistor 108. The subsequent operation
is same as that of FIG. 2. The operation is same when the dialing signal
generator 57 is activated and the output terminal thereof is grounded. In
this case, since the output terminal of the generator 57 is repeatedly
grounded and opened in accordance with the signal, the 5.6 KHz oscillation
is switched on and off accordingly.
In the embodiment of the present invention shown in FIG. 1, only when the
specific signal is sent from the sub-unit, the power supply of the master
unit is activated and the master unit is connected to the external
telephone line in order to prevent the speech from being intercepted by
others. To this end, after the master unit has received the specific
signal and the power supply 23 has been activated, the supply of the power
is continued until the output of the squelch circuit ceases. In this
system, however, if the electromagnetic wave is interrupted for a short
period by some reason, the power supply is deactivated and will not
activated until the specific signal is again received. FIG. 6 shows other
embodiment of the present invention which overcomes the above problem. In
the present embodiment, a specific signal for stop is sent at the end of
speech.
In FIG. 6, numeral 139 denotes a stop specific signal detector. An output
thereof is normally positive (+) to short the collector circuit of the
transistor so that it does not affect to the operations of the self-hold
circuit and the power supply. When the end signal is received, the
positive output of the detector 139 extinguishes and the transistor 115 is
opened so that the self-hold operation is reset.
In FIG. 1, when the sub-unit A sends the 3.9 KHz signal, the master unit
converts it to 800 Hz to cal | | |
