Abstract:
A circuit and method for providing an auto-off capability for a wireless transmitter, of a type having an audio plug which mates with the output jack of an audio source. The portable transmitter modulates signals from a baseband signal source onto a carrier and transmits the RF carrier to a receiver. The auto-off capability is provided to prolong battery life and eliminate the transmission of unmodulated RF carriers. The auto-off capability comprises turning power off to the transmitter circuit when baseband input signal is absent for more than a predetermined amount of time, and immediately turning on the transmitter circuit when baseband signal is present.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims the benefit of United States Utility Patent Application Ser. No. 10/540,070, filed Jun. 22, 2005 (Jun. 22, 2005), which is a Section 371 filing of International Patent Application Serial Number PCT/US04/00452, filed Jan. 9, 2004 (Jan. 9, 2004), which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/438,905, filed Jan. 9, 2003 (Jan. 9, 2003). 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     THE NAMES OR PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not applicable. 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT Disc 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of consumer electronics, and, more specifically, to the field of wireless transmitters for use in transmitting a signal from a device such as a CD player, digital audio player, or a car stereo to a remote speaker system, wherein such device has an auto-off capability. 
     2. Discussion of Related Art Including Information Disclosed Under 37 CFR §§1.97, 1.98: 
     With the widespread use of portable AM/FM receivers, cassette, CD, MP3 players, as well as other consumer electronic devices outputting audio and/or data signals, a need has arisen for more convenient methods for delivering those signals to the system user. Currently, users typically wear headphones that are coupled to the signal-generating device by wires. These wires are inconvenient and possibly dangerous. In the case of portable audio devices, for instance, the devices may be employed while their users are doing other things such as jogging, rollerblading, manual labor, driving, etc. During such activities, wires are susceptible to being tangled up or otherwise providing a hindrance to efficient use. The same is true of wires leading from stationary devices such as a personal computer, car dashboard, or rack mounted stereo. 
     Therefore, as signal generating devices have proliferated, so too has the need to make them convenient. One example of a convenient, hands-free environment was disclosed in U.S. Pat. No. 5,771,441 for a Small Battery Operated Rf Transmitter for Portable Audio Devices for Use with Headphones with Rf Receiver, issued Jun. 23, 1998 to John E. Alstatt (hereinafter referred to as “Alstatt”). 
     In Alstatt, there is taught a portable RF transmitter that modulates audio signals from an audio source onto an FM carrier and then transmits such signals to an FM receiver mounted on a headset worn by a user. The RF transmitter uses its own ground circuit and the ground circuit of the audio source as two elements of a short dipole. Products, such as the AUDIOBUG™, available from Aerielle, Inc. of Mountain View, Calif., have successfully embodied such a wireless device. 
     A further example of a solution to the problem of wireless transmission is found where small RF transmitters have been used on electric guitars to transmit audio signals from the guitar transducer to a receiver coupled to a power amplifier. An example of this type of technology is found in U.S. Pat. No. 5,025,704 for a Cordless Guitar Transmitter, issued Jun. 26, 1991 to Richard L. Davis (hereinafter referred to as “Davis”). In Davis, there is taught an electronic device which, when connected to an electric guitar, or other similar stringed instrument, will effect wireless transmission over a selectable frequency of the FM broadcast band. The unit is compact as it uses the metal strings of the guitar as a partial antenna. The unit remains stationary after being plugged into the guitar&#39;s input receptacle, and no transmitting portion of the device has to be attached to the musician&#39;s belt or guitar strap, or to the musician&#39;s person. Furthermore, no antenna extends from the device itself. The device is automatically turned on when plugged in. 
     As devices providing wireless transmission capabilities have improved and become more convenient and accessible at the consumer level, there has also grown a need to eliminate the transmission of unmodulated RF carriers, and to become more efficient in prolonging battery life. Without this efficiency, larger and/or more expensive batteries, or multiple batteries coupled together, are required to drive the transmitters. The alternative has been a drastically reduced battery life. Thus, there has evolved a need for circuits that reduce battery consumption. 
     Several United States patents reflect proposed solutions to this need, including U.S. Pat. No. 5,636,077, to Kim, which discloses a video recording and reproduction device having an automatic power-saving circuit. The circuit determines the existence of an input video signal and controls system functions accordingly. Video recording and reproduction functions continue if an input video signal is present, and, if no video signal exists and no function key is input for a predetermined period of time, the recording/reproducing actions are halted and power is automatically cut-off. 
     U.S. Pat. No. 6,441,804, to Hsien, teaches a wireless cursor control system that includes a pointing device and a receiver. The pointing device has a controller for receiving user input and for providing a control signal, and a transmitter that includes an antenna and a high frequency modulator coupled to the controller for receiving the control signal and for generating an output signal for transmission via the antenna. The high frequency modulator includes a variable frequency modulator circuit for selectively changing the frequency deviation of the control signal, and a high frequency circuit for increasing the frequency deviation of the control signal to produce the output signal. The receiver has an antenna that receives the output signal, and a demodulation circuit for demodulating the received output signal. The transmitter circuit includes a power saving circuit coupled to the high frequency modulator and controller and detects whether controller has received any input from a button circuit. If no input has been received by the controller for a predetermined time period, the power saving circuit automatically switches the transmitter into a power-saving mode by disconnecting the RF amplifier and the buffer circuit. In the power-saving mode, the button circuit, clock generator, and controller are on, and the remaining circuits are deactivated. User activation of any of the buttons of the button circuit causes the transmitter to come out of the power-saving mode. 
     U.S. Pat. No. 6,529,067 to Uen shows a power saving device for a wireless pointer including a first resistor, a second capacitor, a signal generation circuit, a bias control circuit including an n-type channel MOSFET having a drain connected to the signal generation circuit at a second node for driving the signal generation circuit, a switch having one end connected to an n-type channel MOSFET gate at a first node, a semiconductor having an anode connected to the first node gate and a cathode connected to the positive terminal of the power source, and a first capacitor in series connection with the semiconductor means. When the wireless pointer is inoperative, the switch opens automatically to cause the leakage current of the reverse biased semiconductor to charge the first capacitor. When the switch is closed, the first capacitor discharges completely and cuts off the n-type channel MOSFET. The charging and discharging decrease current consumption in a standby mode. 
     U.S. Pat. No. RE37,884 to Chen discloses a transmitter-receiver system including a transmitter unit installed in an audio equipment, and a receiver unit installed in an earphone, wherein the transmitter unit includes an automatic electric level regulator to regulate the electric level of the output signal of audio equipment to a predetermined range, a power control circuit controlled by the output signal of the audio equipment to provide the necessary working voltage, and an inductance antenna to transmit output signal from the audio equipment to the receiver unit. The receiver unit is of low working voltage design, including an automatic 24-time frequency divider circuit to effectively discriminate left and right sound tracks, and an auto-shut off circuit to automatically cut off power supply when the audio equipment does no work. The transmitter unit and the receiver unit further use a respective dual oscillation frequency regulating circuit consisting of an oscillating transistor, a dielectric resonator, and two variable resistors for regulating the range of the frequency. 
     The foregoing patents reflect the current state of the art of which the present inventors are aware. Reference to, and discussion of, these patents is intended to aid in discharging Applicants&#39; acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a new and improved circuit with a power-saving auto-off capability for an audio device. 
     A further object or feature of the present invention is a new and improved circuits and methods for providing an auto-off capability for a wireless transmitter. 
     An even further object of the present invention is to provide a novel circuit having auto-off capabilities for a wireless transmitter that reduces background noise generated by the circuit. 
     Accordingly, an aspect of the present invention is the reduction of power consumption in an audio device by providing an auto-off circuit that will automatically switch off the system when it is not in use. A further aspect of the present invention is reduction of interference, or background noise generated by the system by providing for the use of certain circuit elements. 
     Other novel features which are characteristic of the invention, as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings, in which a preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration and description only and are not intended as a definition of the limits of the invention. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention resides not in any one of these features taken alone, but rather in the particular combination of all of its structures and elements for the functions specified. 
     Briefly stated, the present invention is a circuit and method for providing an auto-off capability for a wireless transmitter, of a type having an audio plug extending therefrom which mates with the earphone or output jack of an audio source such as a portable battery operated CD, tape, or MP3 player. The portable radio frequency (RF) transmitter modulates audio signals from the audio source onto an RF receiver. The auto-off capability is provided to prolong battery life and eliminate the transmissions of unmodulated RF carriers and comprises pinching off a first field effect transistor (FET) when the circuit is in an “off” state. This occurs when a pre-determined threshold in a capacitor is reached by not discharging that capacitor. The capacitor is discharged by dropping the output from an audio-sensing comparator whose input drops below a pre-determined threshold limit when presented with an audio peak, and wherein the drop in value causes open drain comparator output to go low, discharging the capacitor and causing the FET to supply power to the regulator. In a practical application of the circuit, the comparator polarity could be turned around, or reversed, such that it was sensing the positive going peaks to discharge the capacitor. 
     The description herein teaches three basic circuit blocks required to implement the auto-off function: the audio sensing function (comparator), the timing function (a resistor/capacitor time constant), and a switching function (field effect transistor or bipolar transistor). Each of these circuit blocks could be implemented in a variety of ways. 
     For example, the timing block can be implemented in a very inexpensive way using the RC time-constant described. However, this approach would be limited to a maximum timeout value of 2-3 minutes by required component values, and could vary by 10-20% by component tolerances. Replacing the resistor/capacitor with an embedded controller, microprocessor, or other implementation of a digital counter, these timing limitations can be eliminated. 
     As another example, the switching function used to turn power to the circuit off is a FET in the preferred embodiment, but could be implemented using a bipolar transistor or other semiconductor device, or by controlling the “shutdown” function available on many integrated voltage regulators. 
     The audio sensing function can be implemented in a variety of ways as well, depending on audio levels used and power available. Bipolar transistors, FETs, or operational amplifiers could be used to process the audio as input to the timing function. Alternatively, the audio signal itself could be coupled to a digital input (microprocessor input, logic gate, etc.) that is DC biased to very near its switching threshold to detect audio presence. 
     In addition to the foregoing, several novel features characteristic of the invention, as to the preferred circuit design and the methods of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings, in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration and description only and are not intended as a definition of the limits of the invention. The various features of novelty that characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention does not reside in any one of these features taken alone, but rather in the particular combination of all of its structures for the functions specified. 
     There has thus been broadly outlined the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form additional subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based readily may be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein: 
         FIG. 1  is a perspective drawing of a supporting structure or device that can utilize the present invention; 
         FIG. 2  partitioned into  FIGS. 2A-2D  for clarity) is a circuit diagram of an audio transmitter of the type capable of utilizing the disclosed auto-off circuit; 
         FIG. 3  is a circuit diagram of the disclosed auto-off circuit of this invention; 
         FIG. 4  partitioned into  FIGS. 4A-4D ) is a circuit diagram of an alternative embodiment of the invention wherein a comparator detects the presence of audio, and wherein an embedded controller detects the output of the comparator, and provides an additional timing function for extending the duration of transmission in the absence of audio in the auto-off circuit; and 
         FIG. 5  (partitioned into  FIGS. 5A-5D ) is a circuit diagram of an alternative embodiment of the invention, wherein an embedded controller provides a timing function for the auto-off circuit, and detects the presence of audio. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIG. 1 , there is shown a perspective drawing of the supporting structure or device that can utilize the present invention. A battery operated audio source, typically a portable stereo radio, a portable cassette player or a portable compact disk player, generates audio signals from received radio signals or program material recorded on a medium. These audio signals are presented at a headphone or output jack that in turn is transmitted to the RF transmitter  10 . 
     The portable, battery operated RF transmitter  10 , is comprised of a transmitter housing  12  and enclosed integrated circuitry and a male plug  14 , which plugs into the headphone or output jack of the audio source. The RF transmitter  10  could alternatively be hardwired to, or embedded in, the device as well. The audio signals generated by the audio source are amplified at the audio transmitter  10  and modulate an RF carrier. The RF carrier is coupled into an antenna for radiation to a remote receiver. 
     Referring next to  FIG. 2  (partitioned into  FIGS. 2A-2D  for clarity), there is shown a circuit diagram of a transmitter platform capable of utilizing the disclosed auto-off circuit. Audio signals from an external source such as a CD player, cassette tape player, MP3 player, etc., enter the circuit at P 1  via a standard 3.5 mm three conductor audio cable and are attenuated, AC coupled, and routed to right audio input pin  1  and left audio input pin  22  (see  FIG. 2B ). Each channel is routed through a 50 uS pre-emphasis network, a limiter circuit to prevent over-deviation of the transmitter by excessive audio levels, and a 15 kilohertz (KHz) lowpass filter network to remove undesired spectral components outside the audio range. The processed audio signals are then fed to the stereo multiplexer. This circuit does the left-right channel subtraction, modulates a 38 KHz subcarrier provided by the PLL with this signal, and divides the 38 KHz source into two (2) to generate a 19 KHz pilot, then outputs the gain-scaled composite signal on pin  5 . 
     The transmit chain consists of an on-chip phase-locked oscillator with an external AC-coupled tank circuit consisting of L 3 , C 17 , C 19 , C 20 , C 21 , and varactor diodes D 3  and D 4 . The oscillator frequency is sampled on-chip and divided by a programmable divider down to approximately 100 KHz, where it is compared with a 200 KHz reference signal derived from a crystal reference oscillator operating at 7.6 MHz. The result of this phase comparison is output from pin  7  to an external loop filter consisting of Q 1 , C 10 , C 11 , C 24 , R 9 , and R 15 , having a bandwidth of about 14 Hz. The DC output of the loop filter is an error voltage proportional to the difference of the divide down oscillator frequency and the divided down reference signal, and is applied to varactor D 4 , coupled to the oscillator tank circuit by C 21 , thus controlling the oscillator frequency. Capacitor C 20  is selected during manufacture to center the oscillator in the desired range, assuring that frequency lock is maintained over the entire operating voltage and temperature range. 
     Channel selection is performed by changing the divide ratio of the programmable divider in the oscillator chain. Slide switch SW 1 , the diode decoding matrix formed by D 1  and D 2 , and the on-chip decoding of signals D 0 -D 3  (pins  15 - 18 ) allow the selection of four (4) channels in the range of 88.1 to 107.9 MHz. 
     The composite stereo baseband signal from pin  5 , above, is routed through R 16  to varactor diode D 3 , which is coupled to the oscillator tank circuit by C 17 . The change in capacitance of this diode caused by the varying composite baseband signal causes small changes in the oscillator frequency, thus frequency modulating the oscillator with the composite signal. Due to the very narrow loop filter bandwidth, the PLL is unable to track out the modulation. A separate varactor diode may be used for the modulation patch to improve modulation linearity from channel to channel, assuring constant deviation over the operating range. 
     Primary power for the circuit is provided by a CR2 3V lithium battery. As useful power can be obtained from the battery down to about two (2) volts, and circuit operation is degraded below about 2.8 volts, a switching regulator is used to transform the varying battery voltage to 3.75 volts into the regulator filter. This regulator is a PWM type switcher optimized for efficiency, with the switching frequency varying with battery voltage. 
     Referring now to  FIG. 3 , there is shown a circuit diagram of the auto-off feature as used in the preferred embodiment. Two important features of the disclosed circuit are effects of capacitance and “pinchoff.” In the circuit as shown, where Q 2  is a P-channel field effect transistor (MOSFET), as the drain voltage (V D ) within the circuit increases, so does the drain current (I D ), up to a certain level-off value. This is true as long as the gate voltage is constant and not too large. As the gate voltage continues to increase (positively, since this is a P-channel device) a depletion region begins to form in the channel. Charge carriers cannot flow in this region because they must pass through a narrowed channel. Ultimately, if the gate voltage becomes high enough, the depletion region will completely obstruct the flow of charge carriers, a phenomenon known as “pinch-off.” Capacitance, on the other hand, impedes the flow of alternating current (AC) charge carriers by temporarily storing the energy as an electric field. 
     In the off mode, Q 2  and Q 3  are pinched off, with the capacitor labeled C 29  charged to the battery voltage. Momentary contact switch SW 2  discharges C 29  when pressed, turning on Q 2  which supplies power to the regulator. The output of the regulator turns on Q 3  as it rises, enabling the audio sensing comparator output (PFO) to discharge C 29 , keeping Q 2  on. The non-inverting input to the comparator is biased for a threshold of approximately six hundred (600) millivolts (mV). The inverting comparator input (PFI) is biased around six hundred fifty (650) mV and AC-coupled to the audio source. Whenever an audio peak (low) drops U 2 - 2  (PFI) below its six hundred fourteen (614) mV threshold, U 2 - 3  (PFO) goes low, discharging C 29 , which slowly accumulated a charge through R 23 . Should a sufficiently long period of time elapse without any audio pulses discharging C 29  (about seventy seconds, for instance), it will approach the battery voltage, pinching off Q 2 . With the input supply cut off, the drop of the regulator output pinches off Q 3 , disabling the comparator (PFO) output from discharging C 29  during regulator off conditions. In this mode, battery drain is about 3 microamperes. In a practical application of the circuit, the comparator polarity could be turned around, or reversed, such that it was sensing the positive going peaks to discharge the capacitor. 
     Referring next to  FIG. 2C , an alternative embodiment of the auto-off circuit is shown. In the embodiment shown in  FIG. 2C , 3V battery BT 1  is connected to the source terminal of MOSFET Q 2 , and also provides power to pin  2  of comparator U 3 A, along with providing a bias voltage for the non-inverting input (pin  3 ) of comparator U 3 A. This bias is set by the values of resistors R 25  and R 21 . The bias is set to cause comparator U 3 A output to go to the positive supply voltage when the inverting input (pin  4 ) of comparator U 3 A drops below the value of approximately 1 millivolt. Since resistor R 28  pulls the inverting input (pin  4 ) to ground in the absence of an audio signal, until an audio signal is present, comparator U 3 A output (pin  1 ) is kept high, thereby charging capacitor C 29  through resistor R 27 . 
     Battery BT 1  also charges capacitor C 29  through resistor R 23 . This occurs slowly due to the high value of resistor R 3 . When capacitor C 29  is fully charged to the positive 3V supply voltage, the gate of MOSFET Q 2  is pushed positive, and therefore the current through MOSFET Q 2  is pinched off, blocking current to its drain terminal. This removes the supply current available to pin  1  of regulator chip U 2  and removes operating power from the transmitter, thus shutting off the transmission of RF signals. This condition is true until audio is present (via resistor R 13  or resistor R 14  and capacitor C 30 ) at the inverting input (pin  4 ) of comparator U 3 A. 
     When an audio signal arrives at the inverting input (pin  4 ) of comparator U 3 A, and when the audio signal rises above the  1  millivolt value, the output (pin  1 ) of comparator U 3 A goes to ground, thus rapidly discharging capacitor C 29  through resistor R 27 . The value of R 27  is very low, causing rapid discharge of capacitor C 29 . When capacitor C 29  has discharged below a certain value, the current through MOSFET Q 2  is switched all the way on, and the current is delivered to regulator chip U 2 , thus providing power to the transmitter circuitry (turning on the transmitter). As audio continues to arrive at the non-inverting input of comparator U 3 A, some of the audio negative peaks will reduce the voltage on the input to below  1  millivolt, thus triggering comparator U 3 A to push its output (pin  1 ) to near the positive supply voltage. This causes capacitor C 29  to begin to charge through resistor R 27 . However, due to the large value of C 29 , C 29  will not charge sufficiently to turn current through MOSFET Q 2  off before the next positive half of an audio waveform arrives to rapidly discharge capacitor C 29 . 
     In this manner, the auto-off/on circuit shown in  FIG. 2C  provides the power-saving function of the present invention. 
     Referring now to  FIG. 4 , an additional alternative embodiment of the power-saving function of the present invention is shown. Note that the schematic includes, as its primary part, the transmitter circuits of a battery-powered transmitter. These circuits are typical of low-power low-current-drain transmitters current in the art. The power-saving circuitry of the present invention is seen in the power supply circuits, the components for which are found in the bottom third of the schematic drawing. 
     Still referring to  FIG. 4 , it can be seen that a direct-current power source (such as a 12V battery) provides power, via resistor R 20  and then through light-emitting diode (LED) D 3  and resistor R 18 , to the input (pin  1 ) of regulator chip U 2 . A parallel path is also used to provide, via resistor R 22 , LED D 2  and resistor R 18 , current to the input (pin  1 ) of regulator chip U 2 . Capacitor C 31  provides filtering of the input voltage. Capacitor C 34  provides a bypass capacitance. 
     Regulator chip U 2  is set (via the values of resistors R 15  and R 25 ) to regulate its output voltage to approximately +3.6V. Capacitors C 27  and C 28  provide filtering of the output voltage of regulator chip U 2 . 
     The output (pin  5 ) of regulator chip U 2  provides source current to the source terminal of MOSFET Q 2 . The drain terminal of MOSFET Q 2  provides the current source to all of the transmitter circuits. 
     The gate of MOSFET Q 2  is connected to the NTXON (pin  46 ) output of embedded controller U 4  via resistor R 41 . The gate of MOSFET Q 2  is also connected to the positive terminal of capacitor C 33 . When capacitor C 33  is charged to near the positive supply voltage, it causes MOSFET Q 2  to pinch off current to its output drain terminal, thus turning off power to the transmitter circuits. Capacitor C 33  slowly charges from the regulated +3.6V supply via resistor R 14 . Resistor R 14  is very high to create a very long RC time constant with capacitor C 33 . 
     Still referring to  FIG. 4 , comparator U 3  receives +3.6V power directly from the output of regulator chip U 2 . The non-inverting input (pin  3 ) of comparator U 3  is biased to a value of approximately 1 millivolt through the values of resistors R 13  and R 23 . The inverting input (pin  4 ) of comparator U 3  receives an audio input via capacitor C 24  and resistor R 10 . The inverting input (pin  4 ) of comparator U 3  also receives the voltage provided by the AUOTONEXT output (pin  48 ) of embedded controller U 4  via resistor R 10 . 
     In the case where the voltage provided by the AUOTONEXT output (pin  48 ) of embedded controller U 4  via resistor RIO is greater than the bias value of comparator U 3 , the output (pin  1 ) of comparator U 3  is pulled to near ground, thereby rapidly discharging capacitor C 33  through R 17 , thus causing MOSFET Q 2  to allow current flow to its output drain terminal. This causes the transmitter circuits to turn on (or remain on). 
     In the case where the AUOTONEXT output (pin  48 ) of embedded controller U 4  is floated (neither pushed high, nor pulled low, but providing a very high input resistance), the actions of comparator U 3  are dependent on the presence or absence of audio signal at the inverting input (pin  4 ) of comparator U 3 . 
     When audio is present on the inverting input (pin  4 ) of comparator U 3 , the output (pin  1 ) of comparator U 3  is pushed to near ground, thus rapidly discharging capacitor C 33  via resistor R 17 . This causes MOSFET Q 2  to allow current flow to its output drain terminal. This causes the transmitter circuits to turn on (or remain on). 
     When comparator output (pin  1 ) is pulled to near ground, the NDETAUDIO input (pin  47 ) of embedded controller U 4  is pulled to near ground through resistor R 42 . When this occurs, firmware in embedded controller U 4  detects and records the event. Once such an event is recorded, the firmware in embedded controller U 4  watches for the event where the NDETAUDIO input (pin  47 ) of embedded controller U 4  is pushed above a voltage threshold through resistor R 42 . This voltage threshold is less than the positive voltage required to cause MOSFET Q 2  to pass current. When this second event is detected, the NTXON output (pin  46 ) of embedded controller U 4  is pulled low (near to ground), rapidly draining capacitor C 33  through resistor R 17 , thereby keeping the gate of MOSFET Q 2  low and thus keeping power supplied to the transmitter circuits. 
     This condition is maintained while a positive voltage is provided at the AUTOONEXT output (pin  48 ) of embedded controller U 4 . This positive voltage is fed, via resistor R 10  to the inverting (pin  4 ) input of comparator U 3 . This immediately forces the output (pin  1 ) of comparator U 3  to near ground, thus rapidly discharging capacitor C 33 , via resistor R 17 . This condition causes MOSFET Q 2  to continue to provide power to the transmitter circuits. Once the positive voltage is provided at the AUTOONEXT output (pin  48 ) of embedded controller U 4 , embedded controller U 4  floats its NTXON output, and starts an internal timer that measures the amount of time elapsed since the absence of audio was detected. If a predetermined amount of time has elapsed (70 seconds, for example), the AUTOONEXT output (pin  48 ) of embedded controller U 4  is floated, so if no audio is then present, the inverting input (pin  4 ) of comparator U 3  is pulled to ground potential through resistor R 28 . When this occurs, the output (pin  1 ) of comparator U 3  is again pushed to near the supply voltage, charging capacitor C 33  through resistor R 17 , thus causing MOSFET Q 2  to pinch off the current to its output drain terminal (turning off the transmitter circuits). When this happens, the NDETAUDIO input (pin  47 ) of embedded controller U 4  is pulled to near ground through resistor R 42 . When embedded controller U 4  detects this event, it resets and waits to detect the condition where audio is once again present. 
     If audio is present when the internal timer of embedded controller U 4  reaches its predetermined time limit, when the AUTOONEXT output (pin  48 ) of embedded controller U 4  is floated, the inverting input of comparator U 3  detects the audio, thereby continuing to hold its output (pin  1 ) near ground potential, keeping capacitor C 33  discharged, thus causing the gate of MOSFET Q 2  to remain low and allow current to flow to its output drain terminal. This keeps power supplied to the transmitter circuits. In this condition, embedded controller U 4  detects that its NDETAUDIO input (pin  47 ) was never pulled high after the AUTOONEXT output (pin  48 ) of embedded controller U 4  was floated. This condition causes the firmware in embedded controller U 4  to begin watching once again for the absence of audio. 
     By this description it can be seen that the embodiment of the invention shown in  FIG. 4  provides the power-saving function of the present invention by keeping power turned off to the transmitter circuits until audio is present from an audio source. When audio is present, the transmitter circuits are turned on, and when the audio disappears, after a predetermined delay, the transmitter circuits are automatically turned off to save battery power and prevent unnecessary RF transmissions. 
     Now referring to  FIG. 5 , another alternative embodiment of the power-saving function of the present invention is shown in a schematic form. Note that the schematic includes, as its large part, the transmitter circuits of a battery-powered transmitter. These circuits are typical of low-power low-current-drain transmitters current in the art. The power-saving circuitry of the present invention is seen in the power supply circuits whose components can be seen in the bottom third of the schematic drawing. 
     Still referring to  FIG. 5 , it can be seen that a direct-current power source (such as a 12V battery) provides power, via resistor R 20  and then through light-emitting diode (LED) D 3  and resistor R 18 , to the input (pin  1 ) of regulator chip U 2 . A parallel path is also used to provide, via resistor R 22 , LED D 2  and resistor R 18 , current to the input (pin  1 ) of regulator chip U 2 . Capacitor C 31  provides filtering of the input voltage. Capacitor C 34  provides a bypass capacitance. 
     Regulator chip U 2  is set (via the values of resistors R 15  and R 25 ) to regulate its output voltage to approximately +3.6V. Capacitors C 27  and C 28  provide filtering of the output voltage of regulator chip U 2 . 
     The output (pin  5 ) of regulator chip U 2  provides source current to the source terminal of MOSFET Q 2 . The drain terminal of MOSFET Q 2  provides the current source to all of the transmitter circuits. 
     The gate of MOSFET Q 2  is connected to the NTXON (pin  46 ) output of embedded controller U 4  via resistor R 41 . When the output on pin  46  of embedded controller U 4  is pushed high (near to the positive supply voltage), it causes MOSFET Q 2  to pinch off current to its drain terminal, thereby turning off power to the transmitter circuits and halting the transmission of RF signals. When the output on pin  46  of embedded controller U 4  is pulled low (near to ground), it causes MOSFET Q 2  to allow current to flow to its drain terminal, thereby turning on power to the transmitter circuits and starting the transmission of RF signals. 
     The NDETAUDIO input (pin  47 ) of embedded controller U 4  is a comparator input that detects the presence of an audio signal arriving via capacitor C 24  and resistor RIO. Pin  47  of embedded controller U 4  is biased at +1.3V by the values of resistors R 13  and R 23 . 
     When audio is present on pin  47  of embedded controller U 4 , its internal comparator output indicates so, thereby causing internal firmware to pull NTXON (pin  46 ) of embedded controller U 4  to near ground. When this occurs, the gate of MOSFET Q 2  is pulled low, allowing current to flow to its drain terminal, thereby turning on the transmitter circuits. 
     When audio is no longer detected as present on the NDETAUDIO input (pin  47 ) of embedded controller U 4 , embedded controller U 4  starts an internal timer, measuring the duration of the absence of audio on the input pin. If the internal timer reaches a predetermined duration (70 seconds, for example), embedded controller U 4  pushes its NTXON output (pin  46 ) to near the positive supply voltage. This results in the gate of MOSFET Q 2  being pushed high, pinching off the current flow to its drain terminal, thus turning off the transmitter circuits. If audio reappears on the NDETAUDIO input (pin  47 ) of embedded controller U 4  before its internal time reaches a predetermined duration, the timer is deactivated, and the NTXON output (pin  46 ) of embedded controller U 4  remains pulled to near ground, thus leaving MOSFET Q 2  in the mode of providing current to the transmitter circuits. 
     By this description it can be seen that the embodiment of the invention shown in  FIG. 5  provides the power-saving function of the present invention by keeping power turned off to the transmitter circuits until audio is present from an audio source. When audio is present, the transmitter circuits are turned on, and when the audio disappears, after a predetermined delay, the transmitter circuits are automatically turned off to save battery power and prevent unnecessary RF transmissions. 
     Thus, it will be appreciated, that in its most essential form, the power-saving auto-off circuit of the present invention comprises the following elements: (1) a battery to provide power to a voltage regulator; (2) a voltage regulator powered by the battery and providing a regulated voltage output; (3) a current-switching means having as its power input the output of the voltage regulator; (4) a timing circuit having a reset state and a non-reset state and held initially in a non-reset state, and having an output in communication with a controlling input of the current-switching means, such that when the timing circuit is in a non-reset state it switches the current-switching means to an off state at the end of a predetermined amount of time unless the timing circuit is reset, and when the timing circuit is in a reset state it switches the current-switching means to an on state and allows current to flow through the current-switching means to the transmitter; and (5) a baseband signal detection circuit for detecting the presence of the baseband signal and having an input in communication the source of the baseband signal and an output in communication with a controlling input of the timing circuit, and when the baseband signal detection circuit detects the presence of the baseband signal it resets the timing circuit, causing the current-switching means to transition to an on state, thus providing power to the transmitter, and thereafter continues to keep the timing circuit in a reset state while the presence of the baseband signal is detected, and when said baseband signal detection circuit no longer detects the baseband signal, the timing circuit waits up to the predetermined amount of time before switching the current-switching means to an off state, thus cutting off power to the transmitter. 
     While certain embodiments have been described above in terms of the system within which utilization may occur and/or reside, the invention is not limited to such context. The system shown in  FIG. 1  is an example of a host system of the invention, and the system elements are intended merely to exemplify the type of peripherals and components that can be used in support of the invention. 
     The above disclosure is sufficient to enable one of ordinary skill in the art to practice the invention, and provides the best mode of practicing the invention presently contemplated by the inventor. While there is provided herein a full and complete disclosure of the preferred embodiments of this invention, it is not desired to limit the invention to the exact construction, dimensional relationships, and operation shown and described. Various modifications, alternative constructions, changes and equivalents will readily occur to those skilled in the art and may be employed, as suitable, without departing from the true spirit and scope of the invention. Such changes might involve alternative materials, components, structural arrangements, sizes, shapes, forms, functions, operational features or the like. 
     Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, which is defined by the appended claims.