Patent Publication Number: US-6215981-B1

Title: Wireless signal transmission system, method apparatus

Description:
This application is a continuation of U.S. Ser. No. 08,427/450 filed Apr. 24, 1995, now U.S. Pat. No. 5,666,658, which is a continuation of U.S. Ser No. 08/259,339 filed Jun. 13, 1994, now U.S. Pat. No. 5,410,735, which is a continuation of U.S. Ser. No. 07/822,598 filed Jan. 17, 1992, now abandoned, which was a continuation-in-part of U.S. Ser. No. 07/665,772 filed Mar. 7, 1991, now U.S. Pat. No. 5,272,525. 
    
    
     BACKGROUND OF INVENTION 
     The present invention relates to wireless signal transmission systems, methods and apparatus, for example, radio transmission apparatus for transmitting audio signals within a local transmission area to a portable radio receiver means carried on the person of the user. 
     Personal wireless audio signal transmission apparatus include systems which transmit audio signals, such as television audio signals, by means of infrared light received by a personal infrared light receiving device worn by a listener. It will be appreciated that such transmission systems require a line-of-sight transmission path, so that the system is not workable if walls, furniture or other objects intervene between the transmitter and receiver. Accordingly, while infrared transmission systems may be useful where, for example, a person is seated several feet from a television receiver to which the infrared transmitter is connected for transmitting television sound, the transmission path may be interrupted if, for example, the listener turns his or her head away from the transmitter or a person walks between the transmitter and the listener. Moreover, it is not practical to utilize an infrared transmission system where, for example, the listener is positioned in another room or outside a building in which the transmitter is located. 
     Local wireless television transmission systems are available which transmit television signals from a local source, such as a television or VCR, within the 900 MHz local television transmission band to a receiver which downconverts the television signals to a frequency band which may be tuned by a conventional television receiver. Such systems, therefore, employ receivers which are designed for use with a stationery television set and which optionally utilize a directional antenna carefully positioned for best reception of the 900 Mhz signals radiated by the local transmitter. It is desirable, therefore, that the receiver act as a stable base for supporting the receiving antenna in the best disposition to receive the locally transmitted signal, and therefore, the receiver is typically of a size and weight not practical for carrying on the person of a listener. 
     Many television stations now include stereo audio signals in their transmissions. It is, therefore, desirable that a personal wireless audio signal transmission apparatus provide the capability of transmitting stereo audio signals reproduced by a television receiver. Conventionally, stereo audio signals are formed by adding the right and left audio channels to form a first signal and subtracting the right and left channels to form a second signal which is modulated on a subcarrier of 38 kilohertz.. The subcarrier is suppressed and the combination of the first signal, the subcarrier suppressed modulated second signal and a pilot signal having a frequency of 19 kilohertz (one-half that of the subcarrier), constituting a multiplexed stereo signal, modulates a carrier for transmission. Conventional integrated circuits for producing such multiplexed stereo signals are available commercially. 
     However, audio signals provided by a television receiver typically contain unwanted components at the horizontal frequency of the video signal (approximately 15.734 kilohertz in an NTSC signal) and harmonics thereof. Applicants have found that the use of a 38 kilohertz subcarrier to form the multiplexed stereo signal causes mixing with the second harmonic of the NTSC signal, resulting in audible beat interference. In an attempt to overcome this problem, applicants have instead employed a subcarrier having a frequency equal to two times the horizonal frequency of the video signal, approximately 31.5 kilohertz. However, similar beat interference problems resulted. Applicants further attempted to overcome this problem with the use of a subcarrier equal to four times the horizontal video frequency, but were unsuccessful due to a loss of stereo separation resulting from the use of an excessively high subcarrier frequency. 
     With the introduction of digital audio recording media, such as compact discs, and digital reproduction techniques, the ability to reproduce high quality audio signals having superior frequency response and wide dynamic range requires the provision of a similarly capable personal wireless transmission system. The transmitter of such a system must be capable of modulating a carrier without introducing audible distortion at the receiver, for example, due to overmodulation. Transmitters typically employ an overmodulation detector which provides a visual indication when the level of the modulating signal is excessive, thus to enable a user to avoid overmodulation distortion while maintaining a desirably high signal-to-noise ratio. 
     Conventional overmodulation detectors utilize a threshold detector whose output changes state when a level of a modulation signal exceeds a predetermined threshold level, and resumes a prior state once the level of the modulation signal falls below the predetermined threshold level. The output of the threshold detector is used to drive a visual indicator, such as an LED. However, modulation signals which exceed the threshold level for only brief intervals might not produce a visible indication by the conventional apparatus. This becomes especially troublesome where the modulation signal is supplied by a source such as a compact disc player which can produce an output signal having much sharper peaks than typical analog reproduction devices such as a phonograph or magnetic tape recorder. Accordingly, it is possible that a conventional overmodulation detector will be unable to provide a visible indication of sharp peaks in the modulation signal, such as those provided by a compact disk player, with the result that objectionable overmodulation distortion is audible at the receiver, but not detectable by the overmodulation detection circuit. 
     It will be readily appreciated that a personal wireless audio receiver must be battery operated in order to permit mobility of the person while the receiver is in use. However, the need to replace worn out batteries from time to time is a nuisance, so that it is desirable to employ rechargeable batteries to power a personal wireless audio receiver. It is also inconvenient, however, to remove rechargeable batteries for recharging and subsequently reinstall the same. In addition, the user may find that the batteries need recharging when it is desired to resume use of the receiver, which is also inconvenient. 
     OBJECTS AND SUMMARY OF THE PRESENT INVENTION 
     Accordingly, it is one object of the present invention to provide a personal wireless signal transmission system, apparatus and method which alleviate the problems and shortcomings of the above described systems and techniques. 
     Another object of the invention is to provide a local wireless signal transmission system and method which both is economical and provides the ability to transmit signals with low distortion. 
     A further object of the present invention is to provide a local wireless signal transmission system and method which employ personal receiver means carryable on the person of the user thereof and which provide both convenient and economical operation. 
     Still another object of the present invention is to provide a local wireless audio signal transmission system, as well as a personal wireless receiver unit for use with such a system, wherein frequency modulated signals transmitted within a first relatively high frequency band are downconverted to a second lower frequency band for reception by miniaturized frequency modulation receiver means, thus to provide an economical, lightweight personal audio receiver which may be worn or carried by a user. 
     It is a further object of the present invention to provide a local wireless audio signal transmission system and method for transmitting high quality audio signals from a local audio signal source, which is subject to high frequency audio noise, such as a television receiver. 
     Yet another object of the present invention is to provide an overmodulation detector apparatus and method capable of generating a visual indication of the occurrence of overmodulation, even when caused by modulation signals of very brief duration. 
     Yet still another object of the present invention is to provide a method and apparatus of transmitting stereo audio signals from a received television signal which avoids the problem of subcarrier beat interference with video signal components of the television signal. 
     In accordance with a first aspect of the present invention, a local wireless signal transmission system comprises: radio frequency oscillator means for producing a radio frequency carrier, the radio frequency oscillator means including a ceramic resonator for establishing a first predetermined frequency of the radio frequency carrier; modulation means coupled with the ceramic resonator for shifting the frequency of the radio frequency carrier in response to a modulation signal for producing a frequency modulated radio frequency signal; means for radiating the frequency modulated radio frequency signal within a local transmission area; and receiver means for receiving the radiated signal within the local transmission area, the receiver means being further operative to demodulate the received signal to reproduce the modulation signal. 
     In accordance with another aspect of-the present invention, a local wireless signal transmission method comprises the steps of: producing a radio frequency carrier having a first predetermined frequency established by a ceramic resonator; shifting the frequency of the radio frequency carrier from the first predetermined frequency in response to a modulation signal to produce a frequency modulated radio frequency signal; radiating the frequency modulated radio frequency signal within a local transmission area; receiving the radiated signal within the local transmission area; and demodulating the received signal to reproduce the modulation signal. 
     In accordance with a further aspect of the present invention, a local wireless audio signal transmission system for transmitting audio signals from a local audio signal source to a person within a local signal transmission area comprises: local radio transmitter means for transmitting the audio signals wirelessly within the local signal transmission area in the form of frequency modulated radio signals within a first, relatively high frequency band; and personal wireless receiver means for receiving the frequency modulated radio signals for reproducing the audio signals therefrom, the personal wireless receiver means comprising: antenna means for receiving the locally transmitted frequency modulated radio signals; downconversion means for downconverting the received frequency modulated radio signals to a second frequency band including signal frequencies lower than signal frequencies included in the first, relatively high frequency band; frequency modulation receiver means for receiving the downconverted frequency modulated radio signals and reproducing the audio signals therefrom; and coupling means for supplying the reproduced audio signals from the frequency modulation receiver means to electroacoustic transducer means. 
     In accordance with a still further aspect of the present invention, a personal wireless receiver unit for receiving locally transmitted frequency modulated audio signals produced by a local wireless audio signal transmitter, the locally transmitted signals being produced within a first, relatively high frequency band, comprises: antenna means for receiving the locally transmitted signals; downconversion means for downconverting the received locally transmitted frequency modulated audio signals to a second frequency band including signal frequencies lower than signal frequencies included in the first, relatively high frequency band of the received locally transmitted frequency modulated audio signals; frequency modulation receiver means for receiving the downconverted frequency modulated audio signals and reproducing audio signals therefrom; and coupling means for supplying the reproduced audio signals from the frequency modulation receiver means to electroacoustic transducer means. 
     In accordance with yet another aspect of the present invention, a local wireless audio signal transmission system for transmitting audio signals from a local audio signal source to a person within a local signal transmission area, comprises: local radio transmitter means for transmitting the audio signals wirelessly within the local signal transmission area in the form of modulated radio signals, the local radio transmitter means including input means for receiving the audio signals, low pass filter means for attenuating noise in the received audio signals above a predetermined frequency value, and means for producing radio frequency transmission signals modulated by the filtered audio signals; and personal radio receiver means for receiving the modulated radio frequency transmission signals and carryable on the person of a user, the personal radio receiver means comprising means for demodulating the received signals to reproduce the filtered audio signals and electroacoustic transducer means for converting the filtered audio signals to signals audible by the user. 
     In accordance with a still further aspect of the present invention, a local wireless audio signal transmission method for transmitting audio signals from a local audio signal source to a person within a local signal transmission area comprises the steps of: receiving the audio signals from the local audio signal source at an input of a local radio transmitter means; low pass filtering the received audio signals to attenuate noise therein above a predetermined frequency value; producing radio frequency transmission signals modulated by the filtered audio signals; receiving the modulated radio frequency transmission signals with the use of a personal radio receiver means carried on the person of a user; demodulating the received signals by means of the personal radio receiver means to reproduce the filtered audio signals; and converting the filtered audio signals to signals audible by the user. 
     In accordance with another aspect of the present invention, a local wireless signal transmission system comprises: radio frequency transmitter means for transmitting a signal within a local wireless signal transmission area, the radio frequency transmitter means including means for coupling to a source of electrical energy; and personal receiver means carryable on the person of a user thereof for receiving the transmitted signal within the local wireless signal transmission area, the receiver means including rechargeable battery means for providing operating power to the receiver means, and first coupling means for receiving power for recharging the rechargeable battery means; the transmitter means including second coupling means for coupling with the first coupling means of the receiver means for supplying recharging power to the rechargeable battery means from the source of electrical energy when the receiver means is placed in contact with the transmitter means; the transmitter means further including switching means operative in a first switching mode for disabling transmissions by the transmitter means when the first coupling means is coupled with the second coupling means for supplying recharging power to the receiver means, the switching means being further operative in a second switching mode for enabling transmissions by the transmitter means when the first coupling means is uncoupled from the second coupling means. 
     In accordance with a further aspect of the present invention, a method of operating a local wireless signal transmission system comprises the steps of: providing radio frequency transmitter means for transmitting a signal within a local wireless signal transmission area; coupling the radio frequency transmitter means with a source of electrical energy; providing a personal receiver means carryable on the person of a user thereof for receiving the transmitted signal within the local wireless signal transmission area, the personal receiver means including rechargeable battery means for providing operating power to the personal receiver means; coupling the radio frequency transmitter means with the personal receiver means to supply power from the radio frequency transmitter means to the personal receiver means for recharging the rechargeable battery means; disabling transmissions by the radio frequency transmitter means when the rechargeable battery means of the personal receiver means is supplied with recharging power from the radio frequency transmitter means; uncoupling the radio frequency transmitter means and the receiver means to disable the supply of recharging power; and enabling transmissions by the radio frequency transmitter means upon uncoupling of the radio frequency transmitter means from the personal receiver means. 
     In accordance with still another aspect of the present invention, an overmodulation detector comprises: input means for receiving a modulation signal; detector means for producing overmodulation detection signals in response to absolute values of the modulation signal exceeding a predetermined signal level, the detector means being operative to produce respective ones of the overmodulation detection signals in response to corresponding narrow peaks of the modulation signal having absolute values which exceed said predetermined signal level for respective periods less than a predetermined duration such that the respective ones of the overmodulation detection signals each have a duration exceeding the respective period of its corresponding peak; and indicator means for indicating an overmodulation condition to a user in response to the overmodulation detection signals. 
     In accordance with a still further aspect of the present invention, a method for detecting overmodulation by a modulation signal comprises the steps of: producing overmodulation detection signals in response to absolute values of the modulation signal exceeding a predetermined signal level, respective ones of the overmodulation detection signals produced in response to corresponding narrow peaks of the modulation signal having absolute values which exceed said predetermined signal level for respective periods less than a predetermined duration each having a duration exceeding the respective period of its corresponding peak; and indicating an overmodulation condition to a user in response to the overmodulation detection signals. 
     In accordance with another aspect of the present invention, a method of transmitting stereo audio signals from a received television signal having a horizonal frequency f H  comprises the steps of: receiving right and left channel audio signals obtained by demodulation from the received television signal; adding the right and left channel audio signals to form a first combined audio signal; subtracting the right and left channel audio signals to form a second combined audio signal; modulating a subcarrier with one of the first and second combined audio signals, the subcarrier having a frequency substantially equal to 3f H  to form a subcarrier modulated signal; combining the other of the first and second combined audio signals with the subcarrier suppressed signal to form a stereo multiplexed audio signal; and transmitting the stereo multiplexed audio signal. 
     In accordance with a further aspect of the present invention, a transmitter apparatus for transmitting stereo multiplexed audio signals from a received television signal having a horizonal frequency f H  comprises: input means for receiving right and left channel audio signals obtained by demodulation from the received television signal; means for adding the right and left channel audio signals to form a first combined audio signal; means for subtracting the right and left channel audio signals to form a second combined audio signal; means for modulating a subcarrier with one of the first and second combined audio signals, the subcarrier having a frequency substantially equal to 3f H  to form a subcarrier modulated signal; means for combining the other of the first and second combined audio signals with the subcarrier modulated signal to form a stereo multiplexed audio signal; and transmitter means for transmitting the stereo multiplexed audio signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a transmitter unit and a receiver unit of a local wireless audio signal transmission system in accordance with an embodiment of the present invention; 
     FIG. 2A is a block diagram of the transmitter unit of FIG. 1; 
     FIG. 2B is a diagrammatic view of a stereo audio cable for use with a modified transmitter unit in accordance with another embodiment of the present invention; 
     FIG. 3 is a block diagram of a power switching circuit and battery charger of the transmitter unit of FIG. 2A; 
     FIG. 4 is a block diagram of an overmodulation detector of the transmitter unit of FIG. 2A; 
     FIG. 5 is a waveform diagram for illustrating the operation of the overmodulation detector of FIG. 4; 
     FIGS. 6 and 6A are block diagrams of radio frequency circuits incorporated in the transmitter unit of FIG. 2A; and 
     FIGS. 7A-7D are a block diagrams of a receiver unit of the FIG. 1 embodiment; and 
     FIG. 8 is a diagrammatic view of a further embodiment of a receiver unit in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
     With reference first to FIG. 1, a local wireless audio signal transmission system in accordance with one embodiment of the present invention includes a transmitter unit  20  for transmitting multiplexed stereo audio signals over the 900MHZ local transmission band extending from approximately 902 MHz to 928 MHz, and a receiver unit  22  for receiving the multiplexed stereo audio signals transmitted from the transmitter unit  20 . The receiver unit  22  includes a stereo headphone unit  24  which enables the receiver unit  22  to be worn on the head of a listener. The headphone unit  24  includes a headband support member  26  together with left and right electroacoustic transducer elements covered by respective earpads  28  and  30 . Receiver circuits of the unit  22  are enclosed by a first housing  32  mounted on the headband support member  26  adjacent earpad  30 , while a rechargeable battery is enclosed within a second housing  34  mounted on the headband support member  26  adjacent the earpad  28 . The receiver circuit is coupled with the rechargeable battery by battery leads (not shown for purposes of simplicity and clarity) supported by the member  26 . 
     Transmitter unit  20  includes a stereo audio input cable  37  for coupling the unit  20  with a stereo audio signal source. Transmitter unit  20  is also provided with a one quarter wavelength transmitting antenna  72  for wirelessly transmitting the 900 MHz multiplexed stereo audio modulated signal to the receiver unit  22  which receives the same at a receiving antenna  210  which is also supported by the headband support member  26 . The rechargeable battery enclosed with the second housing  34  of the receiver unit  22  is coupled with a recharging input jack  35  which, as illustrated in FIG. 1, is shown connected with a recharging plug  36  connected by means of a recharging cable  38  to a battery charger circuit of the transmitter unit  20 , describe in greater detail below. In use for receiving wireless audio signal transmissions from the transmitter unit  20 , the receiver unit  22  is uncoupled from the plug  36  and worn on the listener&#39;s head for reproducing the wirelessly transmitted stereo audio signals from the transmitter unit  20 . 
     With reference now to FIG. 2A, the transmitter unit  20  is provided with a pair of input terminals  40  for receiving stereo audio modulation signals. The pair of input terminals  40  include a first input terminal L for receiving a first stereo audio signal from the left channel of a stereo audio signal source and a second input terminal R for receiving a second stereo audio input signal from the right channel of the stereo audio signal source. The stereo audio signal source may be, for example, a television receiver capable of supplying stereo audio output signals, a high fidelity receiver or amplifier, VCR, compact disc player, video disc player, magnetic tape reproducing apparatus, phonograph, etc. The pair of input terminals  40  are connected with a stereo input jack of the transmitter unit  20  (not shown for purposes of simplicity and clarity), to which the cable  37  of FIG. 1 is removably connected for coupling the unit  20  with a local stereo audio signal source. The first and second input terminals, L and R, are coupled with an input of a volume control  42  which permits a user to adjust the signal levels of the input audio signals in order to achieve a desirably high signal modulation level to maximize the signal to noise ratio of the signal radiated by the transmitter unit  20 . Another, and countervailing, purpose is to avoid overmodulation of the radiated signal which results in signal distortion. The volume adjusted first and second input audio signals are supplied at respective output terminals  44  and  46  of the volume control  42 . 
     In order to permit the user to determine whether overmodulation is taking place, an overmodulation detector  50  in accordance with one aspect of the present invention is provided. Respective first and second input terminals  52  and  54  thereof are coupled with output terminals  44  and  46  of the volume control  42  to receive the volume adjusted first and second input audio signals. The overmodulation detector  50  is operative to produce an overmodulation detection signal whenever an absolute value of either the signal received at the terminal  52  or the signal received at the terminal  54  exceeds a predetermined signal level which indicates an overmodulation condition. An LED  56  is coupled with an output of the overmodulation detector  50  which serves to energize the LED  56  by means of a drive signal produced in response to each overmodulation detection signal. 
     As described in greater detail hereinbelow, the overmodulation detector  50  is operative to produce respective ones of the overmodulation detection signals in response to corresponding narrow peaks of the volume adjusted audio signals having absolute values which exceed the predetermined signal level for respective periods less than a predetermined duration, such that the respective ones of the overmodulation detection signals each have a duration exceeding the respective period of its corresponding peak. In this fashion, it is possible to ensure that the oyermodulation detection signal extends for a minimum amount of time necessary to generate a visually perceptible output from the LED  56 . As will be seen from the more detailed discussion of the overmodulation detector  50  contained hereinbelow, by lengthening the duration of the overmodulation detection signal with respect to the duration of narrow signal peaks of the input signals received at the terminals  52  and  54 , the overmodulation detector  50  enables the user to determine when input signal peaks exceeding a maximum modulation level have occurred, even when such peaks extend for only very brief intervals of time. This is especially useful for transmitting audio signals produced by devices such as compact disc players which produce signals having wide dynamic and frequency ranges. 
     The output terminals  44  and  46  of the volume control  42  are coupled with respective moveable terminals of a double pole, double throw switch  48 . Each of a first pair of fixed terminals of the switch  48  is connected with a respective input terminal of a low pass filter circuit  60 . The circuit  60  carries out low pass filtering of the signals received at the respective input terminals thereof by strongly attenuating frequency components thereof above 12 kilocycles. In this fashion, and in accordance with another aspect of the present invention, high frequency audio noise which may be present in the signals received at the pair of input terminals  40  may be effectively removed. This feature is particularly advantageous where audio signals provided from a television receiver are to be transmitted, since these signals are rich in audible beat interference at the horizontal scanning frequency of approximately 15.734 kilocycles. This feature is likewise useful for suppressing high frequency noise in signals reproduced with the use of a tape deck and for suppressing amplitude modulation beat notes in signals supplied by an AM receiver subject to interference by signals from adjacent stations on the AM band. The filtered signals from circuit  60  are provided thereby to respective inputs of a preemphasis circuit  62 . The circuit  62  carries out preemphasis in a conventional manner in order to boost the levels of high signal frequencies which serves to reduce noise in the transmitted signal, thus to improve the signal-to-noise ratio thereof. 
     For certain applications, such as the transmission of audio signals produced by a compact disc player, it is preferable to bypass the filter circuit  60  in order to preserve high frequency components of the audio signals. Accordingly, each of a second pair of fixed terminals of the switch  48  is connected with a respective input of the preemphasis circuit  62  to permit a user to selectively engage or disengage the low pass filter circuit  60  depending on the presence or absence of high frequency noise (for example, video signal noise in the audio output of a television receiver) in the audio signal. In the alternative, in a modified transmitter unit in accordance with the present invention the switch  48  and low pass filter circuit  60  are omitted from the unit so that the output terminals  44  and  46  of the volume control  42  are connected directly with respective inputs of the preemphasis circuit  62 . 
     With reference also to FIG. 2B, in this alternative embodiment of the transmitter unit a modified stereo audio input cable  300  includes a low pass filter  302  affixed thereto and supported thereby for carrying out the function of the low pass filter circuit  60  of FIG.  2 A. Cable  300  includes a first pair of audio cables  304  each connected at a first end with a respective one of a first pair of audio plugs  306  adapted to connect with a pair of audio output jacks of an audio signal source, such as a television receiver, whose output is likely to include high frequency audio noise. Each of the first pair of audio cables  304  is connected at a second end with a respective input of the low pass filter circuit  302 . Cable  300  also includes a second pair of audio cables  310  connected at a first end with respective outputs of the low pass filter circuit  302  and at a second end with a respective one of a second pair of audio plugs  312  for connecting the cable  300  with corresponding left and right channel inputs of the modified transmitter unit. It will be appreciated that, in accordance with the modified transmitter unit, low pass filtering to remove high frequency audio noise is enabled by using the cable  300  of FIG. 2A, while in the alternative, low pass filtering may be disabled by employing a different audio input cable which does not incorporate a low pass filter circuit. 
     After preemphasis, the left and right channel volume adjusted audio signals are supplied to respective inputs of a stereo multiplexer  64  which is operable to produce a multiplexed stereo modulation signal which it supplies at an output terminal  66  thereof. The stereo multiplexer  64  forms a baseband audio component representing the sum of the left and right channel audio signals (L+R) and a difference signal representing the difference between the left channel audio signal and the right channel audio signal (L−R). The stereo multiplexer  64  produces a subcarrier having a frequency which is substantially equal to three times the horizonal frequency f H  of the video signal associated with the input left and right channel audio signals. Accordingly, where such video signal conforms to the NTSC system, the subcarrier frequency is selected to be substantially equal to 3×15.734 kilohertz, approximately 47.202 kilohertz. The stereo multiplexer  64  modulates the approximately 47.202 kilohertz subcarrier with the difference signal and suppresses the subcarrier to form a suppressed carrier signal representing the difference between the left channel audio signal and the right channel audio signal (L−R). The stereo multiplexer  64  also produces a pilot signal having a frequency equal to one half the subcarrier frequency, that is, approximately 23.601 kilohertz, and combines the baseband audio component with the suppressed carrier signal and the pilot signal to form the multiplexed stereo modulation signal at the output terminal  66  thereof. 
     Applicants have found that the use of a subcarrier having a frequency substantially equal to three times the horizontal scanning frequency provides the ability to avoid beat interference with video signal components at the horizontal scanning frequency and harmonies thereof. Accordingly, beat interference which can occur where the subcarrier frequency is either substantially equal to the horizontal scanning frequency or second harmonic thereof can be avoided, without the loss of stereo separation which can occur where the subcarrier frequency is higher than three times the horizontal scanning frequency. 
     The output  66  of the stereo multiplexer  64  is coupled with a modulation signal input terminal of a radio frequency (RF) transmitter circuit  70  operative to frequency modulate a 913 MHz carrier which is supplied at an output terminal thereof to the transmitting antenna  72  to be radiated within a local transmission area, typically within a radius of approximately one hundred feet from the transmitter unit  20 . Transmitting antenna  72  may be, for example, a quarter wave dipole antenna complying with applicable regulatory enactments. It will be appreciated that the reception range of the transmitted signal will depend not only on the choice of the transmitting antenna  72  and the RF power output by the transmitter unit  20 , but also on receiver sensitivity, the presence and nature of objects within the transmission path, as well as other factors. 
     Power for operating the transmitter unit  20 , as well as for recharging a battery of the receiver unit  22 , is provided, for example, by an AC/DC converter unit which plugs into a wall socket and provides a low voltage DC output. The DC output of the AC/DC converter is coupled with a DC input of the transmitter unit  20  to provide a direct current voltage thereto. A voltage regulator  80  serves to remove spikes in the input DC voltage in conventional fashion and supplies a regulated DC voltage at an output  82  thereof. The output  82  is coupled with an input of a power switching circuit  90  which is shown in greater detail in FIG.  3 . 
     With reference also to FIG. 3, the power switching circuit  90  has a first power output  92  coupled with power input terminals of the overmodulation detector  50 , the stereo multiplexer  64  and the RF transmitter circuit  70  to controllably provide operating power thereto. A second power output  94  of the power switching circuit  90  is coupled with a power input of a battery charger  100  to controllably provide power thereto. Battery charger  100  has a pair of output charging terminals including a positive charging terminal  102  and a negative charging terminal  104  coupled through the recharging cable  38  with the recharging plug  36  of FIG.  1 . 
     Referring to FIG. 3, power switching circuit  90  includes a PNP switch transistor  110  having its emitter connected with the output  82  of the voltage regulator  80  and its collector connected with the first power output  92  of the power switching circuit  90 . The base of the transistor  110  is connected with a first terminal of a fixed resistor  113  whose second terminal is connected with the emitter of transistor  110 , the second power output  94  of the power switching circuit  90 , and a first terminal of a current limiting resistor  126 . A second terminal of resistor  126  is connected to the anode of an LED indicator  124  whose cathode is connected to positive battery terminal  102 . Furthermore, a charge state detection means, comprising an NPN transistor  116  together with resistors  117  and  118 , is provided to shut off the transmitter during battery charging. 
     When the recharging plug  36  is connected with the jack  35  of the receiver unit  22 , the rechargeable battery of the receiver unit causes current to be drawn through current limiting resistor  126 , developing a voltage drop across resistors  117  and  118  which act to divide the voltage thereacross and apply the divided voltage to the base of NPN transistor  116 . Accordingly, the divided voltage change is sensed by NPN transistor  116 , which amplifies this voltage change and applies it as an ON/OFF control signal to the base of PNP switch transistor  110  through a resistor  112 . That is, when recharging current passes through resister  126 , the voltage drop at the base of transistor  116  turns off its collector-emitter circuit, thus turning off transistor  110  to disable the transmitter circuits. However, when recharging plug  36  is uncoupled from jack  35  of the receiver unit  22  so that recharging current no longer flows through resistor  126 , transistor  116  is turned on. Transistor  110  is also turned on as a consequence, so that power is applied to the transmitter circuits. Accordingly, so long as the receiver unit  22  is uncoupled from the transmitter unit  20 , transmitter operation is enabled. However, when the recharging input of the receiver unit is coupled with the battery charger  100  so that current is drawn through resistor  126 , transistor  116  is turned off, thus turning off transistor  110  and disabling the transmitter circuits. 
     With reference now to FIG. 4, the overmodulation detector  50  is illustrated therein in greater detail. The first input terminal  52  of the overmodulation detector  50  is coupled with the input of a first buffer amplifier  140  to provide the volume adjusted first input audio signal thereto. The buffer amplifier  140  is operative to supply an output signal at an output  142  thereof which is proportional to the volume adjusted first input audio signal. The output  142  is connected with the anode of a first germanium diode  144  of a peak detector circuit  146 . The second input terminal  54  of the overmodulation detector  50  is coupled with the input of a second buffer amplifier  150  to supply the volume adjusted second input audio signal thereto. Buffer amplifier  150  is operative to supply an output signal at an output terminal  152  thereof which is proportional to the volume adjusted second input audio signal. The output  152  of the buffer amplifier  150  is connected with the anode of a second germanium diode  154  of the peak detector circuit  146 . 
     The cathodes of the first and second germanium diodes  144  and  154  are connected with the first terminal of a fixed capacitor  160  as well as to the first fixed terminal of a potentiometer  162 . A second terminal of the capacitor  160  as well as a second fixed terminal of the potentiometer  162  are connected to ground. A movable contact of the potentiometer  62  is connected with an output  164  of the peak detector circuit  146 . It will be appreciated, therefore, that the buffer amplifiers  140  and  150  will serve to charge the fixed capacitor  160  through the germanium diodes  144  and  154 , respectively, until the voltage across the capacitor  160  is substantially equal to the voltage of the higher of the two output signal values provided at the output terminals  142  and  152  of the buffer amplifiers  140  and  150 . The germanium diodes  144  and  154  serve to prevent the reverse flow of charge from the capacitor  160  back into either of the output terminals  142  and  152  thus to produce a peak value signal in the form of the voltage across the capacitor  160  substantially equal to the higher one of the signals provided at the output terminals  142  and  152 . It will be appreciated that the germanium diodes  144  and  154  each desirably produce a minimal voltage drop of approximately 0.2 volts. It will also be seen that the potentiometer  162  will bleed charge from the capacitor  160 , so that once a peak value has been reached by a respective one of the output signals at one of the output terminals  142  and  152  and the value thereof decreases from such peak value, the peak voltage thereby produced across the capacitor  160  will decay in value as charge is removed therefrom through the potentiometer  162 . As will be seen below, the rate at which charge is bled from the capacitor  160  is controlled in order to ensure that the value of the voltage across the capacitor  160  decreases after the occurrence of a corresponding narrow peak at a rate less than a rate of decrease of the corresponding narrow peak. 
     The output  164  of the peak detector circuit  146  is coupled with the trigger input of a Schmitt trigger circuit  170 . Schmitt trigger circuit  170  has an output terminal  172  where it supplies an output signal which is either at a first, low voltage level or at a second, high voltage level depending both on the present voltage level of the trigger input as well as the prior history thereof, so that the response of the Schmitt trigger circuit  170  to the trigger input is subject to hysteresis. That is, if the output signal of the circuit  170  at a given point in time is at the first, low voltage level, a transition of the trigger input voltage from a relatively low voltage which is less than an ON trigger level of the circuit  170  to a value higher than such ON trigger level will result in a state change of the output voltage at terminal  172  from the first, low voltage level to the second, higher voltage level. However, a subsequent decrease of the trigger input voltage below the ON trigger level will not thereby result in a state change at the output  172  to the first, low voltage level until the trigger input voltage falls below an OFF trigger level which is less than the ON trigger level. 
     Accordingly, when the voltage at the output of the peak detector circuit  146  rises above the ON trigger level when at least one of the signals provided at the outputs of the buffer amplifiers  140  and  150  exceeds a predetermined overmodulation level at a time when the output of the Schmitt trigger circuit  170  is in its first, low voltage level state, the circuit  170  responds by changing the state of its output to the second, high voltage level, thus to provide an overmodulation detection signal. Since the output level of the peak detector circuit  146  may be adjusted with the use of the potentiometer  162 , it is thus possible to adjust the output of the peak detector circuit so that it exceeds the ON trigger level of the circuit  170  at the point where an overmodulation condition first occurs. The output  172  of the Schmitt trigger circuit  170  is coupled with the input of an LED driver circuit  176  having an output coupled with the LED  56  and operative to energize the LED  56  to emit a visible light signal when the voltage level at the output  172  of the circuit  170  is at the second, high voltage level, thus to provide a visible indication to a user that overmodulation is occurring. 
     As explained hereinabove, certain peak signal values in the form of sharp voltage spikes representing, for example, large high frequency components of either the first or second input audio signals may exceed the overmodulation level for a period of time which is too brief to produce a visible output from the LED  56  if it is energized for only the brief interval during which the audio signal exceeds the overmodulation level. For such narrow audio signal peaks, therefore, the rate at which the voltage across the capacitor  160  of the peak detector circuit  146  decays as it discharges through the potentiometer  162  and the trigger input of the circuit  170 , is selected so that it is slower than the rate at which the level of the input audio signal decreases from its narrow peak value. With reference also to FIG. 5, one of the audio output signals from the amplifiers  140  and  150  having a higher amplitude than the other thereof is illustrated as a solid line waveform  179  plotted along a time axis t and having an amplitude measured by an orthogonal axis A. The output of the Schmitt trigger circuit  170  along the same time axis is represented by the waveform  181  of FIG.  5 . In the waveform diagram of FIG. 5, moreover, the voltage across the capacitor  160  of the peak detector circuit  146  substantially coincides with that of the audio output signal except where the audio output signal is decreasing in value, whereupon the voltage across capacitor  160  diverges from the value of the audio signal as indicated by the dash lines  185  in FIG.  5 . 
     If it is assumed that the output of the peak detector circuit is selected by the potentiometer  162  to be the same as the voltage across the capacitor  160 , and the ON trigger level of the circuit  170  is represented by the one-dot chain line  178  of FIG. 5, at a time t 0  at which a relatively narrow peak of the audio signal  180  first exceeds the ON trigger level  178 , the output  181  of the Schmitt trigger circuit  170  switches from a low to a high voltage level. Once the value of the input audio signal has exceeded a peak amplitude level and falls in value thereafter, the output of the peak detector circuit  146  decays at a relatively slower rate so that its amplitude remains higher over time than the decreasing amplitude of the audio signal. Since the OFF trigger level of the Schmitt trigger  170  is lower than its ON trigger level, the ON output state thereof will persist until a point in time after the output of the peak detector  146  decays below the ON trigger level of the circuit  170 . Accordingly, if the point at which the output from the peak detector circuit  164  falls below the OFF trigger level of the circuit  170  occurs at a time t 1  as illustrated in FIG. 5 corresponding with the point  182  along the waveform  185  representing the output of the peak detector circuit  146 , it will be appreciated that the output  181  of the Schmitt trigger circuit  170  will remain at a high level from time t 0  until time t 1  as shown in the illustration of FIG.  5 . 
     It will also be seen by reference to FIG. 5 that the relatively narrow peak  180  of the audio signal will fall below the ON trigger level of the Schmitt trigger  170  at a point t 2  corresponding with the cessation of the overmodulation condition. Accordingly, it will be seen that for narrow peaks of the input audio signals, such as the illustrative peak waveform  180  of FIG. 5, the Schmitt trigger circuit  170  will output an overmodulation detection signal persisting for a period of time substantially longer than that during which the overmodulation condition persists. The rate of decay in the voltage across the capacitor  160  as well as the difference in the ON and OFF trigger levels of the circuit  170  are selected so that the ON output state of the circuit  170  will be maintained for a predetermined minimum amount of time necessary to produce a visible output by the LED  56  even for corresponding narrow peaks of the input audio signals which exceed the overmodulation level for a shorter period of time. 
     It will be appreciated that, if the rate of decay of the voltage across the capacitor  160  is made sufficiently low, in certain embodiments the Schmitt trigger circuit  170  may be replaced, for example, by a threshold detector or similar device for generating the overmodulation detection signal having a duration longer than that during which predetermined narrow peaks of the input audio signal exceed the overmodulation level. In addition, it will be appreciated that in certain further embodiments, the peak detector  146  may be eliminated if the difference in the ON and OFF trigger levels of the Schmitt trigger circuit  170  are set at substantially different values, thus to lengthen the duration of the ON output state of the circuit  170  to provide a visible indication of signal peaks which exceed the overmodulation level for a relatively shorter period of time. 
     With reference now to FIG. 6, the radio frequency (RF) transmitter circuit  70  of FIG. 2A is illustrated therein in greater detail. The circuit  70  has an input terminal  190  coupled with the multiplexer output terminal  66  (FIG. 2A) to receive the multiplexed stereo modulation signal therefrom. The terminal  190  is connected with the cathode of a varactor diode  192  whose anode is connected to ground. The input terminal  190  is also connected with a first terminal of a fixed capacitor  194  having a second terminal connected with a first terminal of a two terminal ceramic resonator  196  whose second terminal is connected to ground. 
     The ceramic resonator  196 , varactor diode  192  and capacitor  194  connected in the foregoing manner constitute a resonant circuit whose resonant frequency is determined principally by that of the ceramic resonator  196 . The ceramic resonator  196  is loaded by the varactor diode  192  through the fixed capacitor  194 , so that as the multiplexed stereo modulation signal received at the terminal  190  varies the capacitance of the varactor diode  192 , the resonant frequency of the circuit consisting of the varactor diode  192 , capacitor  194  and ceramic resonator  196  varies linearly therewith. The first terminal of the ceramic resonator  196  and second terminal of the capacitor  194  are coupled with an oscillator  200  through a fixed capacitor  198  which is operative to produce an oscillation voltage at the resonant frequency of the circuit consisting of the resonator  196 , diode  192  and capacitor  194 . The resonant frequency of the ceramic resonator is selected as 913 MHz, so that the oscillator  200  produces an oscillation voltage of approximately 913 Mhz frequency modulated by the multiplexed stereo audio signal received at the terminal  190 . The ceramic resonator  196  provides high frequency stability, but possesses a relatively low Q, so that the variable loading produced by the varactor diode  192  in response to changes in the modulation signal produces relatively large variations in the frequency of the oscillation voltage, thus to advantageously achieve a high signal-to-noise ratio thereof. 
     The oscillator  200  has an output terminal  202  coupled with an input of an impedance matching circuit  204  which serves to match the output impedance of the oscillator  200  with the impedance of a transmitting antenna  206  coupled with an output of the impedance matching circuit  204  to receive the frequency modulated oscillation voltage therefrom, thus to radiate a frequency modulated radio signal within a local transmission area. 
     The circuitry of the receiver unit  22  is illustrated in greater detail in FIG.  7 . The receiving antenna  210  mounted on the headband support member  26 , as illustrated in FIG. 1, is coupled with the input of an impedance matching network  212  having an output coupled with the input of a radio frequency (RF) amplifier  214 . The impedance matching network  212  serves to match the impedance of the receiving antenna  210  with the input impedance of the RF amplifier  214 . The RF amplifier  214  serves to boost the level of the received 913 Mhz signal from the transmitter unit  20  and provides the amplified signal to a first input of an active mixer  216 . The RF amplifier  214  is advantageously implemented by a dual-gate MOSFET which provides high gain amplification with low noise. In addition, the second gate serves as a means for adjusting the gain of the MOSFET to achieve automatic gain control (AGC) in response to an AGC voltage supplied in conventional fashion by AGC circuits (not shown for purposes of simplicity and clarity). 
     The receiver unit  22  includes a local oscillator  218  coupled with a second input of the mixer  216  to supply a local oscillation voltage thereto for downconverting the received 913 MHz signal provided by the RF amplifier  214 . The local oscillation voltage produced by the local oscillator  218  has a fixed frequency of approximately 978 MHz stabilized by a SAW resonator. Accordingly, the mixer  216  serves to down convert the 913 MHz signal received from the RF amplifier  214  with the use of the 978 MHz local oscillation voltage. More specifically, it will be appreciated that the mixer  216  down converts a band of frequencies from the 900 MHz local transmission band including the band of frequencies in which the frequency modulated radio signals from the transmitter unit  20  are contained. 
     The downconverted signals are supplied at an output  220  of the mixer  216  which is coupled with a first fixed terminal of a single pole double throw switch  222 . A second fixed terminal of the switch  222  is connected with the receiving antenna  210 . A movable terminal of the switch  222  is coupled with the radio frequency input of an FM receiver  226  so that either the down converted signals output by the mixer  216  or signals picked up by the receiving antenna  210  may be supplied to the FM receiver  226 . The FM receiver  226  in accordance with a preferred embodiment of the present invention comprises an FM broadcast band receiver integrated circuit capable of tuning either the band of downconverted signals provided by the mixer  216  at a frequency of approximately 65 MHz or else the FM broadcast band signals provided thereto directly from the receiving antenna  210 , and is capable of demultiplexing the stereo audio signals included either in the downconverted signals or the FM broadcast signals. A suitable FM receiver integrated circuit is the Sony Model CXA 1238M/S integrated circuit. A switchable tuning network  230  is coupled with the FM receiver  226  and is operative to selectively tune the FM receiver  226  either at approximately 65 MHz or within the FM broadcast band (in the United States, extending from approximately 88 MHz to 108 MHz). The tuning network  230  is coupled with the movable contact of a potentiometer  232  having a first fixed terminal connected to receive a positive supply voltage V+and a second fixed terminal connected to ground. The movable terminal of the potentiometer  232  is coupled with a varactor diode of the tuning network  230  (not shown for purposes of simplicity and clarity) to adjustably tune a selected one of the downconverted 65 MHz signal provided from the mixer  216  or a desired station within the FM broadcast band. 
     The FM receiver  226  provides the demultiplexed left and right channel audio signals produced from the received radio frequency signals at respective outputs  240  and  242  coupled with respective inputs of a headphone driver  244 . The headphone driver  244  amplifies the left and right channel audio signals from the FM receiver  226  which it supplies at respective outputs  250  and  252  for provision to respective electroacoustic transducer elements of the stereo headphone unit  24 . The headphone driver  244  also includes a volume control (not shown for purposes of simplicity and clarity) for controlling the loudness of the acoustic signals produced by the transducers of the headphone unit  24 . 
     The supply voltage V+is provided to the circuits of the receiver unit  22  from a rechargeable battery  260  mounted within the housing  34  of FIG. 1. A positive charging input terminal  264  of the receiver unit  22  connected with the recharging input jack  35  thereof, is coupled with the anode of a diode  262  whose cathode is coupled with the positive terminal of the rechargeable battery  260 , thus to prevent discharge of the battery  260  through the positive charging input terminal  264 . 
     With reference now to FIG. 8, an alternative embodiment of a receiver unit in accordance with the present invention is illustrated therein in which the circuity of the receiver unit is enclosed within and supported by an enclosure unit  400 . Accordingly, the unit  400  encloses and supports each of the elements  212  through  244  as well as the diode  262  and rechargeable battery  260 . The outputs  250  and  252  of the headphone driver  244  are connected with a stereo output jack  410  mounted on the enclosure unit  400  for coupling the stereo input plug  412  of a stereo headphone unit  420  with the headphone driver  244  to supply the stereo audio signals output by the receiver unit to the headphone unit  420 . The enclosure unit  400  is shaped generally as a rectangular parallelepiped and dimensioned to fit within the shirt pocket of a user, so that the receiver unit may be carried conveniently on the person of the user. In addition, by mounting the receiver unit&#39;s circuitry as well as its rechargeable battery within the enclosure unit  400  and apart from the headphone unit  420 , the user may select whatever headphone unit he or she may wish to use with the receiver unit. 
     It will be appreciated that the elements of the receiver unit are readily implemented with a relatively small number of components and that, due to the compact size of the FM receiver integrated circuit, all of the circuitry of the receiver unit  22  may be constructed, as indicated in FIG. 1, as a light weight and miniaturized apparatus which is affixed to the headband support member  26 , or, as illustrated by FIG. 8, supported within an enclosure which fits in the user&#39;s pocket. Accordingly, the receiver unit may be worn by or carried on the user in order to receive either the locally transmitted audio signals from a selected local audio source, or else a desired FM broadcast signal. Since the locally transmitted signals are not line-of-site transmissions, it is possible for the user to move about within the local transmission area while receiving the signals from the transmitter unit  20  despite the intervention of walls and other objects. 
     Although specific embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.