Click-free muting circuit for headset

A click-muting circuit for a headset microphone connected to a telephone, comprising a user operated mute switch, a click eliminator circuit connected across the microphone for gradually creating a short circuit across the microphone in response to closure of the mute switch, and a hard muting circuit connected in series with the microphone for disconnecting the microphone after the short circuit has been established.

FIELD OF THE INVENTION
 The present invention relates in general to telephone headsets, and more
 particularly to a circuit for providing click-free muting of a telephone
 headset.
 BACKGROUND OF THE INVENTION
 Headsets are useful for hands free telephony in circumstances in which a
 person requires the use of his or her hands for operation of a computer,
 switchboard, etc. Headsets are usually connected to the handset port of a
 telephone set through an interface box. The interface box allows the user
 to select whether the headset or the handset is activated. Prior art
 interface boxes vary greatly in complexity, but most boxes include a
 headset volume control. Some such boxes are battery operated while others
 steal current from the microphone bias circuit.
 One common problem associated with the use of telephone headsets is the
 generation of unpleasant clicking noises which occur when a mute button
 for the headset microphone is depressed or released. Another problem is
 insufficient "hard" muting.
 According to one prior art approach, an AC coupled mute switch has been
 used to provide headset microphone muting. Unfortunately, this approach
 generally fails to sufficiently attenuate the microphone signal (i.e. no
 "hard" muting) and is not capable of performing headset switch detection
 which is a desirable feature of telephone headsets which can function also
 using a regular handset. DC coupled mute switches overcome the problem of
 headset switch detection but produce the aforenoted unpleasant click
 noise. Another approach involves the use of expensive battery powered
 microphone amplifier muting circuits. These circuits are normally
 implemented within cumbersome interface boxes attached to the telephone,
 and often do not provide a hard muting function.
 SUMMARY OF THE INVENTION
 According to the present invention, a circuit is provided which is capable
 of headset detection and microphone mute switch closure detection and
 which overcomes the above-discussed disadvantages of the prior art. The
 circuit provides click-free switching, 80 dB muting and is of inexpensive
 construction. No extra power supply is required, and the circuit is
 capable of providing headset detection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 In FIG. 1, a standard telephone set 1 is shown with a handset 3 thereof
 resting in a handset cradle. The handset 3 is normally connected to the
 telephone set via a handset port 5 (e.g. RJ-11). In the illustrated
 embodiment, a headset 7 is connected to the telephone set 1 via headset
 port 5A. The headset may also be connected to the handset port 5 and in
 that case, the bias circuit 21 (see below) must be designed in such a way
 that the bias circuit functions with both microphones; the handset
 microphone and the headset microphone. Headset 7 includes a pair of
 earpieces 11 and a microphone 13 (e.g. low voltage condenser microphone),
 in a usual manner.
 With reference to FIG. 2 in combination with FIG. 1, according to the
 present invention, a switch 15 is connected to a clickfree muting circuit
 17 which, in turn, is connected across the terminals of microphone 13.
 Thus, the circuit 17 is connected between the microphone 13 and the
 microphone audio line connector 19 of the telephone set 1.
 Headset microphone bias circuit 21 provides bias current for the microphone
 13, in a well known manner. A microphone amplifier 23 is connected
 differentially to the bias circuit 21 for amplifying the low voltage
 output signals therefrom. An output of microphone amplifier 23
 (analog_signal) is connectable to a phone chip (not shown) within the
 telephone set 1, in a well known manner. Although not shown, the earpieces
 11 of the headset 7 and the earpiece of the handset 3 are normally
 connected together and to an output of the phone chip.
 A headset detector circuit 25 is connected to the output of microphone bias
 circuit 21 for providing a signal (headset_detect) to the phone chip for
 indicating that headset 7 is connected to the set 1. Since many headsets
 are connected to telephones via a "quick-disconnect" plug, when the
 handset 3 is left in its cradle and the headset 7 is disconnected, it is
 still possible to use the telephone via the handset operation. The
 detector circuit 25 also generates a signal (headset_switch_detect)
 indicative of a momentary depression of the mute switch 15 for
 interpretation by the phone chip as a hookswitch event for invoking a
 telephone special feature (e.g. soft hold, call forward, etc.). The
 structure and operation of the circuit 25 is discussed in greater detail
 in co-assigned U.S. Pat. No. 5,832,075 entitled "Off-Hook Detector for
 Headset", the contents of which are incorporated herein by reference.
 With reference to FIG. 3, the muting circuit 17 is shown comprising a click
 eliminator circuit 17A and a hard muting circuit 17B. An implementation of
 the circuit 17 is shown according to the preferred embodiment with
 reference to the schematic diagram of FIG. 4.
 In operation, circuit 17 steals current from the bias circuit 21, so that
 no extra power supply is required. When the switch 15 is closed, the click
 eliminator circuit 17A slowly creates a short circuit across the
 microphone 13, thereby suppressing any impulse click noise from being
 created. Once the click eliminator circuit 17A has fully short circuited
 the microphone 13, hard muting circuit 17B completely disconnects the
 microphone 13, thereby resulting in a hard mute (80 dB). When the switch
 15 is released, hard muting circuit 17B re-connects the microphone 13 and
 click eliminator circuit 17A slowly removes the short circuit across the
 microphone 13, thereby again suppressing impulse switch noise during
 resumption of microphone operation after muting.
 More particularly, having regard to the preferred embodiment illustrated in
 FIG. 4, transistor Q3 is normally on (i.e. saturated by the bias voltage
 applied across its base-emitter junction), so that the microphone 13 is
 normally connected to the line. The transistors Q1 and Q2 are normally off
 (i.e. the click eliminator circuit 17A does not normally short circuit the
 microphone 13), capacitor C1 is normally charged to the voltage across R1
 and capacitor C2 is normally charged to a low voltage which is
 insufficient to turn on transistor Q2. Upon closing switch 15, capacitor
 C2 begins to charge slowly through resistors R6, R4, R5 and R7 and
 capacitor C1 begins to discharge slowly thereby gradually turning on
 transistor Q2 so as to create a short circuit across the terminals of
 microphone 13. The speed of the click elimination circuit 17A is governed
 by the choice of capacitors C1, C2 and resistors R1, R6, R5 and R7, and
 further regulated by the turning on of transistor Q1. More particularly,
 the equivalent resistance of transistor Q2 together with the remaining
 resistors forms a voltage divider with resistor R1. The current through
 transistor Q1 is small (in both on and off states) because resistor R1 is
 of high resistance (e.g. 510K ohm). Once the short circuit is fully
 established (i.e. the click eliminator circuit 17A is fully on), Q3 is no
 longer forward biased (as a result of the short circuit through transistor
 Q2) and turns off, thereby completely disconnecting the microphone 13 from
 the line and yielding a hard mute (80 dB).
 Although the switch 15 is shown located at the headset 7, it is also
 contemplated that the switch could be located at a headset interface box
 (not shown) between the headset 7 and telephone set 1. In either case,
 provision of the local switch 15 relieves the user from having to reach
 over to the set in order to mute the microphone 13. It is also
 contemplated that the switch 15 can be installed as a retrofit to existing
 headsets in the form of a series device which can dangle from the cable
 connecting the headset 7 to the telephone set 1 or interface box.
 Other embodiments and variations are possible without departing from the
 sphere and scope of the invention as defined by the claims appended
 hereto.