Mobile telephone controller switch - Patent Review 4644105

Having described the invention what is claimed is:

1. A communication switch circuit adapted to permit selective coupling of a telephone line to (i) a transmitter for transmission of signals from the telephone line to a mobile unit and (ii) a receiver for reception at the telephone line of signals from a mobile unit, the communication switch circuit comprising:

a telephone port couplable to a telephone line;

a transmitter port couplable to a transmitter;

a receiver port couplable to a receiver;

first means for coupling said telephone port to said transmitter port when signals are absent from said receiver port, to define a transmission state;

keying means for permitting, in a transmitter-on state, transmission of signals by a transmitter couplable to said transmitter port, signals being present at said telephone port and said first means being in said transmission state defining said transmitter-on state, said keying means further for preventing, in a transmitter-off state, transmission of signals by a transmitter couplable to said transmitter port, said first means further for coupling said telephone port to said receiver port when signals are present at said receiver port and said keying means is in said transmitter-off state to define a receiver state; and

interrupt means, operable when said keying means is in said transmitter-on state, for intermittently, temporarily placing said keying means in said transmitter-off state, whereby the presence of signals at said receiver port while said keying means is temporarily in said transmitter-off state places said first means in said receiving state thereby interrupting transmission of a transmitter couplable to said transmitter port and coupling said telephone port to said receiver port.

2. The communication switch circuit of claim 1 wherein said first means is coupled to said telephone port via a node, said communication switch circuit further comprising:

switch means for coupling said telephone port to said node, to define an off-hook state, said switch means having an on-hook state wherein said telephone port and said node are not coupled by said switch means;

line seizing impedance means coupled to said node for seizing a telephone line couplable to said telephone port when said switch means is in said off-hook state; and

first control means for detecting the presence of telephone signals at said telephone port and for placing said switch means in said off-hook state once said detection occurs.

3. The communication switch circuit of claim 2 further comprising second control means for placing said keying means in said transmitter-off state when said switch means is in said on-hook state.

4. The communication switch circuit of claim 3, further comprising timer means for maintaining said switch means in said off-hook state for a predetermined period of time after first being placed in said off-hook state and thereafter placing said switch means in said on-hook state.

5. The communication switch circuit of claim 4 wherein said timer means is resettable such that said predetermined period of time begins anew when said timer means is reset, said communication switch circuit further comprising reset means for resetting said timer during the presence of signals at said receiver port when said switch means is in said off-hook state, whereby said switch means remains in said off-hook state for said predetermined period of time after the presence of signals at said receiver port terminates.

6. In a simplex system having a transmitter for selectively transmitting first signals from a land line to a mobile transceiver when said transmitter is energized and a receiver for receiving second signals from a mobile transceiver, and for selectively coupling the second signals to the land line, when the transmitter is deenergized, a communication switch circuit comprising:

keying means for energizing and deenergizing said transmitter during the presence and absence, respectively, of said first signals on said land line;

first interrupt means for intermittently, temporarily deenergizing said transmitter during energization thereof by said keying means; and

second interrupt means for preventing energization of said transmitter by said keying means during reception of said second signals by said receiver for a period of time defined as the duration of said reception of said second signals after coincidence of reception of said second signals and deenergization of said transmitter by either of said keying means or said first interrupt means.

7. In the system of claim 6, the communication switch circuit further comprising:

means for coupling said land line to said transmitter when said second signals are not being received by said receiver and for coupling said land line to said receiver when said second signals are being received by said receiver and said transmitter is deenergized.

8. In the system of claim 6, the communication switch circuit further comprising:

a port;

first means for coupling said port to said transmitter when said second signals are not being received by said receiver and for coupling said port to said receiver when said second signals are being received by said receiver and said transmitter is deenergized;

switch means for coupling said land line to said port, to define an off-hook state, said second switch means having an on-hook state wherein said land line and said port are uncoupled; and

first control means for detecting the presence of signals on said land line and for placing switch means in said off-hook state once said detection occurs.

9. In the system of claim 8, the communication switch circuit further comprising second control means for preventing energization of said transmitter by said keying means when said switch means is in said on-hook state.

10. In the system of claim 9, the communication switch circuit further comprising:

timer means for maintaining said switch means in said off-hook state for a predetermined period of time after first being placed in said off-hook state and thereafter placing said switch means in said on-hook state.

11. In the system of claim 10, wherein said timer means is resettable such that said predetermined period of time begins anew when said timer means is reset, the communication switch circuit further comprising reset means for resetting said timer means during reception of said second signals by said receiver when said switch means is in said off-hook state, whereby said switch means remains in said off-hook state for said predetermined period of time after said reception of said second signals by said receiver terminates.

12. A communication switch circuit adapted to permit selective coupling of a telephone line to (i) a transmitter for transmission of signals from the telephone line to a mobile unit and (ii) a receiver for reception at the telephone line of signals from a mobile unit, the communication switch circuit comprising:

a transmitter port couplable to a transmitter;

a receiver port couplable to a receiver;

a telephone port couplable to a telephone line;

first means coupled to said telephone port via a first node for coupling said telephone port to said transmitter port when signals are absent from said receiver port, to define a transmission state;

first switch means for coupling said telephone port to said first node, to define an off-hook state, said first switch means having an on-hook state wherein said telephone port and said first node are not coupled by said first switch means;

combiner means coupled to said telephone port via a second node for unidirectionally coupling said telephone port to said transmitter port and for unidirectionally coupling said receiver port to said telephone port;

first decoder means for generating an access signal in response to a first predetermined signal at said first node;

second switch means for coupling said telephone port to said second node when said access signal is present, to define a mobile-call state, said second switch means having a no-call state wherein said telephone port and said second node are not coupled by said second switch means;

first line seizing impedance means coupled to said first node for seizing a telephone line couplable to said telephone port when said first switch means is in said off-hook state;

second line seizing impedance means coupled to said second node for seizing a telephone line couplable to said telephone port when said second switch means is in said mobile-call state;

first control means for detecting the presence of signals at said telephone port and for placing said first switch means in said off-hook state once said detection occurs;

keying means for permitting, in a transmitter-on state, transmission of signals by a transmitter couplable to said transmitter port when signals are present at said first telephone port and said first means is in said transmission state to define a transmitter-on state, and for preventing, in a transmitter-off state, transmission of signals by a transmitter couplable to said transmitter port, said first means further for coupling said telephone port to said receiver port when signals are present at said receiver port and said keying means is in said transmitter-off state to define a receiver state; and

interrupt means, operable when said keying means is in said transmitter-on state, for intermittently, temporarily placing said keying means in said transmitter-off state, whereby the presence of signals at said receiver port while said keying means is temporarily in said transmitter-off state places said first means in said receiver state thereby interrupting transmission by a transmitter couplable to said transmitter port and coupling said telephone port to said receiver port.

13. The communication switch circuit of claim 12 further comprising falsing timer means for preventing said generation of said access signal until said first predetermined signal has been present at said first node for a predetermined period of time.

14. The communication switch circuit of claim 12 further comprising first timer means for delaying coupling of said telephone port to said second node for a first predetermined period of time after generation of said access signal.

15. The communication switch circuit of claim 14 further comprising falsing timer means for preventing said generation of said access signal until said first predetermined signal has been present at said first node for a second predetermined period of time.

16. The communication switch circuit of claim 15 further comprising second control means for placing said keying means in said transmitter-off state when said first switch means is in said on-hook state.

17. The communication switch circuit of claim 16 further comprising second timer means for maintaining said first switch means in said off-hook state for a third predetermined period of time after first being placed in said off-hook state and thereafter placing said first switch means in said on-hook state.

18. The communication switch circuit of claim 17 wherein said second timer means is resettable such that said third predetermined period of time begins anew when said second timer means is reset, the communication switch circuit further comprising reset means for resetting said second timer means during the presence of signals at said receiver port when said first switch means is in said off-hook state, whereby said first switch means remains in said off-hook state for said third predetermined period of time after the presence of signals at said receiver port terminates.

19. The communication switch circuit of claim 12 further comprising:

second decoder means for generating a dump signal in response to a second predetermined signal at said first node; and

second control means for placing, in response to said dump signal being generated, said second switch means in assume said no-call state and said first switch means in said on-hook position.

20. The communication switch circuit of claim 19, further comprising first timer means for delaying coupling of said telephone port to said second node for a first predetermined period of time after generation of said access signal.

21. The communication switch circuit of claim 19 further comprising:

first falsing timer means for preventing said generation of said access signal until said first predetermined signal has been present at said first node for a first predetermined period of time; and

second falsing timer means for preventing said generation of said dump signal until said second predetermined signal has been present at said first node for a second predetermined period of time.

22. The communication switch circuit of claim 21 further comprising first timer means for delaying coupling of said telephone port to said second node for a third predetermined period of time after generation of said access signal.

23. The communication switch circuit of claim 22 further comprising third control means for placing said keying means in said transmitter-off state when said first switch means is in said on-hook state.

24. The communication switch circuit of claim 23 further comprising second timer means for maintaining said first switch means in said off-hook state for a fourth predetermined period of time after being placed in said off-hook state and thereafter placing said first switch means in said on-hook state.

25. The communication switch circuit of claim 24 wherein said second timer means is resettable such that said fourth predetermined period of time begins anew when said second timer means is reset, the communication switch circuit further comprising reset means for resetting said second timer means during the presence of signals at said receiver port when said first switch means is in said off-hook state, whereby said first switch means remains in said off-hook state for said fourth predetermined period of time after the presence of signals at said receiver port terminates.

26. A communication switch circuit adapted to permit selective coupling of a telephone line to (i) a transmitter for transmission of signals from the telephone line to a mobile unit and (ii) a receiver for reception at the telephone line of signals from a mobile unit, the communication switch circuit comprising:

a transmitter port couplable to a transmitter;

a receiver port couplable to a receiver;

a telephone port couplable to a telephone line;

first means coupled to said telephone port via a node for coupling said telephone port to said transmitter port when signals are absent from said receiver port, to define a transmission state;

switch means for coupling said telephone port to said node, to define an off-hook state, said switch means having an on-hook state wherein said telephone port and said node are not coupled by said switch means;

first control means for detecting the presence of signals at said telephone port and for placing said switch means in said off-hook state once said detection occurs;

line seizing impedance means coupled to said node for seizing a telephone line couplable to said telephone port when said switch means is in said off-hook state;

keying means for permitting, in a transmitter-on state, transmission of signals by a transmitter couplable to said transmitter port when signals are present at said telephone port and said first means is in said transmission state to define a transmitter-on state, and for preventing, in a transmitter-off state, transmission of signals by a transmitter couplable to said transmitter port, said first means further for coupling said telephone port to said receiver port when signals are present at said receiver port and said keying means is in said transmitter-off state to define a receiver state;

interrupt means, operable when said keying means is in said transmitter-on state, for intermittently, temporarily placing said keying means in said transmitter-off state, whereby the presence of signals at said receiver port while said keying means is temporarily in said transmitter-off state places said first means in said receiver state thereby interrupting transmission by a transmitter couplable to said transmitter port and coupling said telephone port to said receiver port;

decoder means for generating a dump signal in response to a first predetermined signal at said node; and

second control means for placing said switch means in said on-hook position in response to said dump signal being generated.

27. The communication switch circuit of claim 26 further comprising falsing timer means for preventing said generation of said dump signal until said first predetermined signal has been present at said node for a first predetermined period of time.

28. The communication switch circuit of claim 27 further comprising third control means for placing said keying means in said transmitter-off state when said switch means is in said on-hook state.

29. The communication switch circuit of claim 28 further comprising first timer means for maintaining said switch means in said off-hook state for a second predetermined period of time after being placed in said off-hook state and thereafter placing said switch means in said on-hook state.

30. The communication switch circuit of claim 29 wherein said first timer means is resettable such that said second predetermined period of time begins anew when said first timer means is reset, the communication switch circuit further comprising reset means for resetting said first timer means during the presence of signals at said receiver port when said switch means is in said off-hook state, whereby said switch means remains in said off-hook state for said second predetermined period of time after the presence of signals at said receiver port terminates.

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BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention pertains to over-the-air paging systems, and more particularly, to a "talk-back" or "two-way" paging arrangement by which the user of a portable or mobile pager unit may respond to pages or, alternatively, may initiate telephone calls.

II. Description of the Prior Art

One-way radio or receive-only "paging" systems have become increasingly popular in recent years including various sophisticated schemes for relaying information to the user. The original and best-known paging system generates a signal from a fixed-location transmitter in response to actuation of the transmitter by a telephone call being received. The transmitted signal is received by a mobile receiver or pager unit worn about the person. When the signal is received by the appropriate pager, the pager emits a "beep" thereby alerting the user that there is a message being held for him at a prearranged location or that he is to call a prearranged number. The user must then find an available telephone and call the prearranged location or number whereupon he is given his message.

Some paging systems may employ multiple tones, or multiple-tone pulse signaling to distinguish between several predetermined locations or numbers to be contacted for messages. By way of example, one particular sounding beep may signify that a call to the office is in order, while a different-sounding beep may indicate that the call should be to home, instead. More recent advances in pager technology include liquid-crystal-displays mounted to the exterior of the pager. The liquid-crystal-displays may be remotely loaded by the telephone caller to indicate, for example, the telephone number on which the caller can be reached.

A disadvantage of all such one-way paging systems is that the user of the pager unit cannot immediately speak with the call-initiating party directly. Instead, the user must locate an available telephone and place a call to the initiating party to return the page. A further drawback to a one-way paging system is that the pager cannot be utilized to initiate contact with someone else. Instead, the user must, again, locate an available telephone.

Alternatively, conventional two-way telephone and land-mobile communication services are available. Such services permit two-way communication such as is commonly encountered in telephone conversations. Conventional two-way radio telephone communications, however, require separate transmit and receive frequencies to permit simultaneous communication from the mobile unit to the telephone and from the telephone to the mobile unit. Thus, while such a duplex system permits telephone-like conversation, i.e., the ability to talk simultaneously and to interrupt the other party, it also requires the use of multiple radio channels. In many installations, this is impractical due to frequency congestion and channel assignments. Additionally, duplex systems may cost more than simplex or single channel systems such as are used for conventional one-way paging.

Simplex systems have also been used for two-way radio communication but with some drawbacks. In a typical simplex system, the transmitter and receiver operate at the same nominal frequency. To avoid interference, and other possible communication disruptive problems, the transmitter and receiver should not be energized at the same time. Hence, a keying circuit is typically provided which energizes the transmitter when the telephone party is talking so as to transmit these signals to the mobile unit. While the transmitter is energized, or keyed, the receiver is deenergized. When the telephone party stops talking, the keying circuit deenergizes the transmitter and re-energizes the receiver. While the receiver is energized, any signals transmitted by the mobile unit will be received by the receiver and heard by the telephoning party. One notable drawback is that the simplex two-way radio communications system does not permit both parties to talk at the same time, nor can one party interrupt or interject a comment while the other person is talking (i.e., transmitting).

Systems designed for simplex operation thus require a certain degree of user education from the standpoint that persons accustomed to conventional telephone communications must refrain from speaking until the other party has completed its transmission. Failure to wait out the transmission of the other party will result in the interrupter's message not being heard. This inability to interrupt may also result in channel capturing whereby one party or the other can lock up the channel precluding its use by others.

As discussed, in a typical simplex two-way communication system, the originating caller may be using a telephone to gain access to a base station having a transmitter and a receiver which are adapted for simplex communications with a mobile transceiver. Once the communication link is established, only one or the other party may talk at any given time. When the telephone party talks, for example, the transmitter is energized such that telephone signals (e.g., the caller's voice) can be transmitted to and received by the mobile transceiver. During transmission by the transmitter, the receiver is deenergized. Hence, so long as the telephoning party is talking, no messages can be received at the base station. As a result, for so long as the telephone party is talking, the mobile transceiver user cannot interrupt to terminate the conversation or to interject a comment. In this way, the telephone caller can lock up or capture the channel precluding others from communicating through the base station.

Accordingly, it is an object of the present invention to provide a communication switch circuit for a simplex two-way radio communication system which avoids channel lockup by the telephoning party.

A further objective of the present invention is to provide a communication switch circuit for a simplex two-way radio communication system which will permit a mobile unit such as a pager adapted to transmit to interrupt a telephoning party to interject a comment.

An even further objective of the present invention is to provide a communication switch circuit to permit a transmitting mobile unit to initiate a call to a telephone party.

A yet further objective of the present invention is to provide a communication switch circuit which permits a transmitting mobile unit to terminate the connection between a telephone party and the mobile unit.

It is a further objective of the present invention to provide a communication switch circuit meeting the above objectives and compatible for use in one-way pager systems and/or with duplex radio communication systems.

SUMMARY OF THE INVENTION

In accordance with the present invention and in its broadest aspects, there is provided a communication switch circuit by which quasi-duplex communication may be had with a simplex two-way radio communication system wherein the keying circuit includes an interrupt circuit to intermittently, temporarily deenergize the transmitter and reenergize the receiver. The communication switch circuit is further provided with a control circuit which prevents energizing of the transmitter (and deenergizing of the receiver) if a signal is received by the receiver during the temporary interrupt thereby permitting a mobile unit to interrupt the telephone party and interject a comment.

More specifically, the present invention provides a communication switch circuit having a first circuit which couples a telephone line to a transmitter when signals are not being received by a receiver to define a transmission state; a keying circuit which permits the transmitter to transmit signals, when signals are present on the telephone line and the first circuit is in the transmission state to define a transmitter-on state, and which prevents the transmitter from transmitting signals in a transmitter-off state; the first circuit, in a receiver state, coupling the telephone line to the receiver when signals are being received by the receiver and the keying circuit is in the transmitter-off state; and an interrupt circuit operable when the keying circuit is in the transmitter-on state, which intermittently, temporarily places the keying circuit in the transmitter-off state, whereby signals transmitted by the mobile unit when the keying circuit is temporarily in the transmitter-off state may be received by the receiver to prevent further transmission by the transmitter during reception of the mobile unit signals. Also, during an interrupting reception, signals received from the receiver will place the first circuit in the receiver state whereby signals received from the mobile unit are coupled to the telephone line.

Preferably the communication switch circuit of the present invention is adapted to permit reception of telephone calls from a telephone party as well as to permit a mobile unit to initiate and terminate calls. In preferred embodiments, the first circuit is coupled to a circuit node rather than directly to the telephone line, and the telephone line is selectively couplable to the node under control of a first control circuit which monitors the telephone line. The first control circuit includes a first switch circuit by which the telephone line and node may be coupled. Preferably the telephone line is normally uncoupled from the node, i.e., the first switch circuit is in an on-hook state and the telephone line is idle. Also, the first circuit is normally in the transmission state. When a telephone party calls in or the telephone line is otherwise in use, i.e., has signals thereon, the first control circuit will couple the telephone line to the node, i.e., the first switch circuit will be placed in an off-hook state, thereby permitting operation as above. Hence, a telephone party can initiate communications.

The communication switch circuit further preferably comprises a second control circuit which monitors the node. When the keying circuit is in the transmitter-off state, which occurs when no signals are present at the node or the first switch circuit is in the on-hook state, the receiver is energized. Hence, signals can be received from the mobile unit. Once signals are received, the first circuit will be placed in the receiver state whereby received signals are coupled to the node to be monitored by the second control circuit notwithstanding that the telephone line is on-hook. In response to a first predetermined signal at the node, the second control circuit couples the receiver to the telephone line thereby placing signals on the telephone line causing the first control circuit to place the first switch circuit in the off-hook state and thereby to initiate a call. In response to a second predetermined signal at the node, the second control circuit will cause the first control circuit to place the first switch circuit in the on-hook state thereby uncoupling the telephone line from the node and terminating a call.

By virtue of the foregoing, a telephoning party cannot lock-up a channel and a mobile or pager unit can interrupt the telephoning party. Further, the pager unit can initiate connection to a telephone line to place a call and can also terminate such a connection.

The communication switch circuit may also preferably be provided with a timer circuit which will automatically terminate the connection after a predetermined period of time. With a two-way communication system, the timer will be reset whenever either the telephone party speaks or signals are received from the mobile unit thereby reinitiating the predetermined period of time. When used with a pager in a one-way system, no signals will be received by the receiver and, hence, the connection will be terminated after the telephone party hangs up. In this way, the communication switch circuit of the present invention may be used in a one-way pager system or in combination with both one-way and two-way communication systems.

For proper operation of a duplex communication system, signals from the receiver must be coupled to the transmitter for retransmission along with signals from the telephone line. To this end, the communication switch circuit of the present invention is also preferably provided with a unidirectional combiner which couples signals from the receiver to the transmitter. Although receiver signals will be present at the transmitter even in a simplex communication system due to the combiner, that is of no consequence due to the non-coincidence of operation of the transmitter and receiver. Hence, the communication switch circuit of the present invention is compatible for use in one-way paging systems and duplex radio communication systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objectives and advantages of the invention will become more readily apparent from the following detailed description taken with the accompanying drawings in which:

FIG. 1 is a block diagram of one embodiment of the communication switch circuit according to the principles of the present invention;

FIG. 2 is a schematic diagram of the voice activation/interrupt, circuit of FIG. 1;

FIG. 3 is a block diagram of an alternative embodiment of the communication switch circuit according to the principles of the present invention;

FIG. 4 is a block diagram of the line control of FIG. 3;

FIG. 5 is a schematic diagram of the timer of FIG. 4;

FIG. 6 is a block diagram of a further alternative embodiment of the communication switch circuit according to the principles of the present invention;

FIG. 7 is a block diagram of the tone decode control of FIG. 6;

FIG. 8 is a further alternative embodiment of the communication switch circuit according to the principles of the present invention; and

FIG. 9 is a schematic circuit of the combiner of FIG. 8.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference to FIG. 1, there is shown a block diagram of one embodiment of a communication switch circuit 10 in accordance with the principles of the present invention. Communication switch circuit 10 is adapted to permit selective coupling of signals between a land line 12 such as a telephone line or other link to a communication network (not shown), a transmitter 14, and a receiver 16 as determined by the state of COR control 18. COR control 18 is preferably a double-throw transistor-driven relay or the like. Terminal 22 of COR control 18 is coupled to telephone or land line 12 via telephone port 24; terminal 26 is coupled to receiver 16 via receiver port 28; and terminal 30 is coupled to transmitter 14 via transmitter port 32 through voice activation/interrupt circuit 34 of keying circuit 20 as will be discussed below.

COR control 18 further has a control terminal 36 coupled to a COR output 40 of receiver 16 via COR port 38. Receivers typically utilized in communication systems are configured to provide a signal from a carrier-on-relay (COR) or the like which indicates that signals are being received by the receiver. That signal is referred to herein as a COR signal. Of course, the receiver must be energized to receive signals. Hence, receiver 16 includes a COR output 40 which provides a COR signal whenever receiver 16 is energized and is receiving signals from antenna 42 within the passband of the receiver, e.g., from a mobile unit (not shown) operating on the same channel. The presence of a COR signal at the COR port 38 thus corresponds to the presence of signals at receiver port 28.

When signals are present at receiver port 28, the corresponding COR signal places COR control 18 in a receiver state whereby terminals 22 and 26 thereof are coupled one to the other. Thus, in the receiver state, signals from receiver 16 are received at telephone line 12. When receiver 16 is either deenergized or otherwise not receiving a signal, there is an absence of signals at receiver port 28, with a corresponding absence of a COR signal at COR port 38. The absence of a COR signal places COR control 18 in a transmission state whereby terminals 22 and 30 are coupled one to the other. Thus, in the transmission state, signals from telephone line 12 are coupled to transmitter 14 for transmission thereby (through a suitable antenna 42).

Keying circuit 20, which selectively energizes and deenergizes transmitter 14 and receiver 16, includes voice activation/interrupt circuit 34, one output 44 of which couples signals at terminal 30 of COR control 18 to transmitter port 32. The other output 46 of voice activation/interrupt circuit 34 provides a control signal to power switch 48 which controls energization and de-energization of transmitter 14 and receiver 16.

Transmitter 14 and receiver 16 are preferably configured for simplex operation and thus operate at the same nominal frequency. As is well understood, proper operation of a simplex system requires that the transmitter and receiver not be energized simultaneously. Hence, power switch 48, which may be a conventional Darlington transistor switch, is adapted to provide power only to transmitter 14 or receiver 16 at any one time, but not both.

Power switch 48 has a transmitter-off state shown in solid line in FIG. 1 wherein a source of supply (A+) is coupled to receiver 16 via power port 50 thereby energizing receiver 16. Transmitter 14 is coupled to power port 52 which is coupled to the source of supply (A+) when power switch 48 is in a transmitter-on state as shown in dotted line in FIG. 1. As will be appreciated, power switch 48 will energize only one of transmitter 14 and receiver 16 at a time, the other being de-energized.

In the transmitter-on state, therefore, receiver 16 is deenergized and transmitter 14 is energized thus permitting transmission over antenna 42 of signals present at transmitter port 32. In the transmitter-off state, receiver 16 is energized and transmitter 14 is de-energized thus preventing transmission by transmitter 14 of signals present at transmitter port 32.

As mentioned, voice activation/interrupt circuit 34 provides signals on control output 46 which place power switch 48 in the transmitter-on and transmitter-off states as appropriate. Voice activation/interrupt circuit 34 is operable in res