Abstract:
A microphone system including a microphone for inputting voice data, an output for transmitting the voice data to a device connectable thereto; and a circuit which operates to mute the voice data and provide a signal to the device in order to maintain a suspended state of operation in the device. The device may be a computer that is responsive to voice data, for example, by utilizing voice recognition software.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the field of microphones, and in particular to microphones with mute and keep alive functions. 
     Speech recognition software is a tool for increasing office productivity, particularly for entering text in word processing applications with personal computers. Such software performs best when the voice input is provided via a “close talking” headset microphone, that is a microphone with acoustic cancellation of background noise. Desk or monitor supported microphones may also be used successfully in quiet environments. The microphone signal is applied to the corresponding input connector of the computer sound card. It is evident that by wearing a headset, dictation becomes a hands-free operation and is therefore valuable for individuals with limited use of hands or arms. When the person dictating text into the microphone wishes instead to speak with a person in the area, an awkward situation develops in which the operator has to remember and say “go to sleep” or “stop listening” to the speech recognition software. This will prevent the computer from recording the person to person conversation, however, resumption of proper dictation will require another spoken command such as “wake up” or “listen to me.” 
     It is therefore appropriate to interpose a mute switch between the microphone and the computer sound card. An exemplary headset product is the VXI Corporation model Parrott QD-10 with a QD 500 mute switch. Such switches are also used with telephony headsets where the mute function prevents the calling party (perhaps the customer) from hearing the called party (a service agent) while he or she asks another agent a question. An exemplary telephony headset is VXI Corporation model PB-QD-10-6, again with a QD 500 mute switch. Microphone mute switches are configured for “clickless” operation where electrical transients are suppressed and hence objectionable audible clicks are prevented. The resulting silence while muted to the computer creates an undesirable condition for the software: the speech recognition program hears nothing, attempts to increase microphone sensitivity to process inaudible information and becomes saturated (causing a significant delay of several seconds) when the mic is unmuted and voice returns to the computer. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides in an exemplary embodiment a microphone system including a microphone for inputting voice data, an output for transmitting the voice data to a device connectable thereto; and a circuit which operates to mute the voice data and provide a signal to the device in order to maintain a suspended state of operation in the device. In accordance with one aspect of the invention, the device is a computer that is responsive to voice data, for example, by utilizing voice recognition software. 
     The invention provides a microphone with mute switch and circuitry. When the microphone is muted, a “keep alive” circuit will inject a signal into the computer microphone input. This signal has characteristics unlike speech, thus preventing misinterpretation and is of sufficient amplitude to inhibit attempts by the software to increase microphone sensitivity. The circuit is powered from the current limited mic bias source provided by the sound card, thus requiring no external batteries. A mic bias current is required by the electret, which is the microphone type most frequently used in telephony and voice-enabled computer applications. However, the invention applies equally when the less common dynamic microphones are used instead. Additional circuitry is provided for optimum utilization of the limited DC power available from the sound card when either the microphone or the “keep alive” stage is energized. Personal computer “sound card” circuits are now commonly part of the system board (motherboard), but appear no different at the mic input connector. 
     Related techniques for “keep alive” functions are described in co-pending U.S. patent application Ser. No. 09/130,745 filed Aug. 7, 1998 and for optimum. DC power extraction in Ser. No. 09/115,980 filed Jul. 15, 1998, both of common assignee. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a prior art computer microphone with mute function; 
     FIG. 2A is a schematic diagram of an exemplary microphone with mute switch and timer “keep alive” circuitry in accordance with the invention; 
     FIG. 2B is a schematic diagram of an alternative embodiment of FIG. 2A having op-amp “keep alive” circuitry and improved DC bias; 
     FIG. 3A is a schematic diagram of another embodiment in accordance with the invention having a simplified mute switch; and 
     FIG. 3B is a schematic diagram of an alternative embodiment of FIG. 3A with improved DC bias. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic circuit diagram of an exemplary prior art microphone assembly  100  with a microphone (not shown), a mute circuit  102  and associated plug connections  104  for connection to one of a variety of computer sound cards  106 ,  108 . A JFET transistor  110  is provided inside the micro phone housing and acts as a buffer between the very high impedance level at electret diaphragm  112  and output terminals MIC DRAIN and MIC SOURCE. 
     The sound card  106  includes a VCC source  114  such as +3.0V and a load resistor  116  (e.g., 2.0 kΩ) to enable the JFET to operate as a class A amplifier and are shown as part of an exemplary sound card input circuit  106  at jack  118 . The DC path is from VCC to the resistor  116  to transistor  110  to ground. The amplifier output impedance is approximately equal to the value of resistor  116  and the audio signal is coupled through a capacitor  120  to analog stages in the sound card and ultimately to analog-to-digital converters. 
     The mute circuit  102  includes a mute switch  122  that may be closed in order to suppress the mic output without introducing transients. Prior to switch actuation, a capacitor  124  (e.g., 100 μF) is fully charged through a resistor  126  (e.g., 20 kΩ) to the quiescent voltage at MIC DRAIN. When resistor  126  is shorted by switch  122 , there is no DC change in the circuit, but the capacitor  124  acts a short to ground at voice frequencies. Thus, audible clicks from the switching action are avoided and subsequently no voice data are passed on to the sound card. 
     Connector jack  118  is typically a 3.5 mm stereo jack more commonly seen in portable tape recorders and other consumer electronic products. Connector  118  and the corresponding 3.5 mm connector  104  have TIP, RING and SLEEVE ports. SLEEVE is connected to ground, the TIP and the RING ports at the sound card may be configured in one of several ways for providing DC bias and obtaining audio output. The function of the three ports will be described hereinafter with reference to connector jack  128  associated with sound card  108  representative of a Creative Labs sound card. 
     At present, it is sufficient to consider the direct connection of TIP to RING found on low cost microphones supplied with speech recognition software. Such microphones may use a Primo EM-124 electret element with approximately 200 μA drain current and provide tens of millivolts of voice output data. The quiescent voltage at TIP and RING of connector jack  118  is 3.0V−2.0 kΩ×200 μA=2.6V. 
     Referring now to FIG. 2A, a signal generator will be described that will maintain an active but suspended state in the speech recognition program while the microphone is muted. FIG. 2A is a schematic diagram of an exemplary microphone assembly  200  with a mute switch circuit  202 , a timer “keep alive” generator  204  and a connector plug  205 . A switch  206  is shown in the active mic position where DC bias from the sound card TIP and RING ports is applied directly to MIC DRAIN. A resistor  208  (e.g., 20 kΩ) in series with the “keep alive” generator  204  is also connected to the same ports. The generator is based on a timer circuit  210 , a low power timer such as National Semiconductor LMC555C connected for astable operation. 
     With the switch  206  set as shown, the timer circuit  210  is idle, being current starved by a resistor  208  (e.g., 20 kΩ) positioned in parallel to the switch. When the switch is moved to the mute position, the microphone is current starved by the resistor  212 , but the timer circuit  210  is active. With resistors  214  (e.g., 220 kΩ) and  216  (e.g., 4.7 kΩ), and capacitor  218  (e.g., 0.022 μF), a repetitive pulse waveform is generated at the OUT port of the timer circuit  210 . Time low is approximately 70 μsec, time high is 340 μsec, therefore the repetition rate is about 244 Hz. This rectangular pulse waveform is continuous, unvarying and rich in harmonics. It does not resemble a voice signal. If it can be applied to the computer audio input node at the proper amplitude, the software will remain engaged while the microphone is muted but no spurious word recognition will occur. 
     Since the timer circuit  210  is preferably a CMOS integrated circuit, it draws current primarily during output waveform transitions and this current modulates the voltage at VDD (pin  8 ) because of the resistor  116  of the sound card  106  as shown in FIG.  1 . The voltage excursions at the AUDIO SIGNAL node would be excessively high (several tens of millivolts) without some provision to attenuate them. Components such as resistor  220  (e.g., 100 Ω) and capacitor  222  (e.g., 22 μF) provide a load for this signal with a high pass corner frequency of 72 Hz, in other words for all harmonics of this waveform. 
     The AC voltage divider formed by resistor  220  of FIG.  2 A and resistor  116  of FIG. 1 brings the “keep alive” signal amplitude to the more suitable level of a few millivolts. It is understood that the signal amplitude is chosen freely by scaling the resistor  220 . Finally, resistors  212  and  208  provide current continuity (make before break) when the single-pole double-throw switch  206  is actuated, thus minimizing electrical transients and audible clicks. Mic current and “keep alive” current are nearly equal at 200 μA. 
     FIG. 2B is a schematic diagram of an alternative embodiment of FIG. 2A showing a microphone assembly  250  having an op-amp “keep alive” generator  252  and improved DC bias. Similar results are obtained from the different signal generator based on a low power opamp  254  such as Texas Instruments TLC25L2C. Here a low duty cycle rectangular pulse, rich in harmonics, is generated using negative and positive feedback. The “signature” or constant nature of this waveform is unlike voice and will again keep speech recognition software in a stable, suspended state while the microphone is muted. A repetition rate of approximately 120 Hz is determined by a capacitor  256  (e.g., 0.1μF) and a resistor  258  (e.g., 10 kΩ) at the inverting (−) input of the opamp  254 . A duty factor of about 1% is established by resistors  260  (e.g., 1 kΩ) and  262  (e.g., 10 kΩ) at the non-inverting (+) input. A resistor  264  (e.g., 200 kΩ) adds a small fraction of VCC to the same (+) pin to overcome input offset voltage and ensure start-up. 
     The multivibrator waveform will swing almost rail to rail at the output of opamp  254 . As before, a capacitor  266  (e.g., 22 μF) and a resistor  268  (e.g., 100 Ω) cause attenuation of VCC current swings allowing only a few millivolts of “keep alive” signal at the AUDIO INPUT of FIG.  1 . Resistor  268  may be scaled for the signal amplitude desired. A mute switch circuit  270  includes resistors  272  (e.g., 20 kΩ) and  274  (e.g., 20 kΩ), and a mute switch  276  that are wired for make before break operation as detailed previously with reference to switch  206  of FIG.  2 A. 
     An interface circuit  280  is provided between the mute switch circuit  270  and the connector plug  290 . The interface circuit includes a diode  282  coupled to RING, a diode  284  coupled to TIP, and an arrangement of capacitors  286 ,  287  and  288  coupled therebetween. The function of the diodes  282  and  284 , and the capacitors  286 ,  287  and  288  will be described with reference to the sound card input  128  of sound card  108  shown in FIG.  1 . This and many other sound card circuits for providing mic bias and receiving audio at the TIP and/or RING ports may degrade mic performance, when the TIP and RING ports are tied together as shown in FIG.  1 . For example, as resistors  130  (e.g., 2.2 kΩ) and  132  (e.g., 560 Ω) of sound card  108  are shorted into a common node by the connector jack  128 , the voltage division on VCC allows only 20% of 5.0V minus 0.44V (mic current times resistor  130 ) to appear at MIC DRAIN. In other words, the mic will be starved operating at 0.56V into connector jack  128  versus a more normal 2.6V into connector jack  118 . 
     The interface circuit  280  in FIG. 2B will maintain optimum mic bias with all sound card input wiring variations. The diode  282  will admit bias current that may be presented to the RING port and the diode  284  will admit current from the TIP. The diodes  282  and  284  will also conduct mic current from the sound card connector if TIP and RING are shorted as in connector jack  118 , or even if each of the TIP and RING terminals is independently connected with resistors to VCC. The audio signal from the microphone is presented to both output terminals TIP and RING, as shown in FIG. 2B, equally and symmetrically by the “Y” connected capacitors  286 - 288 . The values of the capacitors  286 - 288  can be, for example 4.7 μF, as the reactance of 4.7 μF represents a short at voice frequencies. 
     In the simplest case, each one of the diodes  282  and  284  could be paralleled with a non-polar electrolytic capacitor for coupling AC signals in the absence of DC current (consequently low impedance) through the diode. Such a non-polar capacitor is expensive and is sometimes replaced with two common polarized electrolytics connected fin series with like polarities at the common node. The cost of four polarized capacitors in place of two non-polar units is lower. Since in the present instance the anodes of the diodes are tied together, it is possible to complete the signal coupling function with three polarized electrolytics at even lower cost by connecting like (+) polarities together to form the “Y” connection of the capacitors  286 - 288 . Schottky diodes are preferred in order to keep voltage drops to about 0.1V instead of 0.6V with conventional diodes. 
     It will be appreciated by virtue of its symmetry that the interface circuit  280  will accept DC bias and provide audio output to sound cards with every combination of bias resistors, terminating resistors and coupling capacitors on the TIP and RING terminals in addition to those shown as connector jacks  118  and  128 . PNP transistors or PMOS FETs may also be used in place of diodes to conduct bias current from TIP and/or RING to the microphone and similarly avoid voltage starvation when connecting to the exemplary input of connector jack  128  of FIG.  1 . 
     Additional exemplary embodiments of the invention may be based on different “keep alive” signals, for example, a discrete two-transistor multivibrator, white noise generated by a zener diode, pink noise, pseudo-random bit streams from shift registers, as well as arbitrary repetitive waveforms. The signal should provide a picket fence of harmonics in the frequency domain (narrow pulses in the time domain) so that it appears dissimilar to voice. 
     Another exemplary embodiment will now be described where the mute switch is configured as a single-pole single-throw. FIG. 3A is a schematic diagram of a microphone assembly  300  that includes a timer “keep alive” generator  302  and a simplified mute switch  304 . This simplified switch is more suitable for a headset implementation where the mic boom may be rotated up, thus actuating an internal tilt or reed switch. Most headsets allow the user to swing the mic boom to the vertical position, away from the mouth to enable the user to cough or drink coffee, for example. 
     It is advantageous therefore to disable the microphone with a tilt switch as the boom is moved from nearly horizontal (active mic position) to nearly vertical (muted mic position) without additional manual steps by the user. This attitude sensitive switch may be embedded in the headset ear cup or in the mic boom, such that rotation of the boom end causes the switch to tilt. Similar results may be achieved with a magnet embedded in the mic boom and a glass reed switch in the ear cup. Boom rotation upwards will increase the distance of the magnet from the reed and the switch will open. 
     For the more conventional instances of a visible, manually operable mute on headset or desk microphones, a single-pole double-throw switch is not unduly complex. The mute circuit of the invention may then be placed in the headset with the mute switch actuator movable up and down at the ear cup. The mute circuit and switch may also be placed in a nodule at chest level on the headset cord. Desk mics may have the mute switch and circuit either at the base or at the top. 
     Returning now to FIG. 3A, the simplified switch  304  is grouped with a pair of PNP transistors  306 ,  308  and resistors  310  and  312  (e.g., 200 kΩ). In the active mic position as shown (closed switch), the switch  304  is conducting transistor  306  base current to ground through the resistor  310 . This results in saturation of transistor  306  and enables the bias and audio path from the sound card via connector plug  314  to the mic. At the same time, the transistor  308  is kept off by the saturation voltage of the transistor  306  appearing across the base emitter junction of the transistor  308 . When the mic boom is tilted up and the switch  304  is open, the transistor  306  is cut off. Now the transistor  308  conducts bias current from the sound card to enable the “keep alive” generator  302 . No interruption in the flow of current results as the switch  304  is opened and again this make-before-break action prevents audible clicks. FIG. 3A depicts the generator  302  as including a timer circuit  316  and associated components configured similarly to that shown in FIG. 2A, but it will be appreciated that any of the aforementioned signal generators may be used. 
     FIG. 3B shows yet another exemplary embodiment of the invention combining the benefits of a single-pole single-throw mute switch with optimum DC bias. FIG. 3B is a schematic diagram of a microphone assembly  350  including a mute switch  352 , an opamp “keep alive” generator  354 , an interface circuit  356  and a connector plug  358 . Optimum DC bias is obtained with the diode and capacitor network of interface  356  as described with reference to interface circuit  280  of FIG.  2 B. The mute switch  352  is grouped with a pair of PNP transistors  360 , and  362 , and resistors  364  and  366 , and operate in accordance with the description of transistors  306  and  308 , and resistors  310  and  312  of FIG.  3 A. In addition, the “keep alive” generator function may be performed with an opamp  370  and associated circuitry as described with reference to generator  252  of FIG. 2B, or any of the timer, multivibrator, white noise, pink noise, etc. generators described herein. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.