U SOO54l0735A United States Patent [19] [11] Patent Number: 5,410,735 Borchardt et al. - [45] Date of Patent: Apr. 25, 1995
[54] WIRELESS SIGNAL TRANSMISSION 4,739,413 4/1983 Meyer - SYSTEMS, METHODS AND APPARATUS 4,771,344 9/1988 Fallacaro e141- - 4,847,903 7/1989 Schotz ...... 381/3 [76] Inventors: Robert L. Borchardt, 120 E. End '. ' . ' ' Ave" New York, NY. 10028; _(L1st contlnued on next page.) _ William T. McGreevy, 43 Thompson FOREIGN PATENT DOCUMENTS Ave., Babylon, NY. 11702; Ashok _ Naw ge’ 3L7‘) 34th St.’ Apt.#3F, 0040481 2/1988 Japan ...... 358/194.l Astoria, NY. 11105; Efrain L. OTHER PUBLICATIONS §°d"lf1“ez’§6; Y" “New 902-928 MHZ Band Now Open!”, Spec-Com mo yn’ ' ' Journal, Sep/Oct. 1985, cover page and p. 9. __ [21] Appl. No.: 259,339 Federal Register, vol. 50, Aug. 22, 1985, Final Rulemak . . ing re addition of 902-928 MHz band to Amateur Radio [22] E169 J‘m' 13,1994 Service Rules, pp. 33937 through 33940. Related Us. Application Data (List continued on next page.) . - _ Primary Examiner—Reinhard J. Eisenzopf [63] ggéietanuation of Ser. No. 822,598, Jan. 17, 1992, aban Assistant Examiner_Andrew Faile [51] In G 6 H04B 1/00 Attorney, Agent, or Firm-Levisohn, Lerner & Berger t. . > ...... [52] US. Cl...... 455/42; 455/110; [571 ABSTRACT 455/205; 455/ 344; 455/ 351 ; 381/3 Systems, methods and apparatus are provided for con [58] Field of Search ...... 455/42-43, ‘ducting local wireless audio signal transmissions from a 455/66, 95, 110-113, 120-125, 205, 208, 214, 'local audio signal source to a person within a local 344, 351, 352, 127, 343; 348/725, 731, 738; signal transmission area. In certain embodiments, the 381/ 2—3; 379/62; 331/64, 96 transmissions are conducted over the 900 MHz local [56] References Cited transmission band to a portable receiver unit supported on the headband of a stereo headphone unit. The re U'S' PATENT DOCUMENTS ceiver unit serves to down convert the 900 MHz signal 2,287,065 6/1942 Roberts . to a local frequency band which is received by an FM £01121 Ct 81- - receiver of the receiver unit which serves to reproduce 9 9 31' er . - - . . _ 3,590,382 6/1971 Kenney ...... 455/110 the who slgn a1? therefrom Whlch are’ 1“. tum’ con 3 697 941 10/1972 Christ verted to acoustlc slgnals by electroacoustw transduc 3Z934Z201 1/1976 Majefslti ...... 381/14 ers of the headphmle- A transmitter unit includes a 3,984,625 10/1976 Camras . ceramic resonator stabilized FM transmitter, as well a 4,021,737 5/1977 Trask ...... 358/189 ?lter for suppressing high frequency noise in an audio 4,097,893 6/1978 caqlras - modulation signal, a stereo audio multiplexing unit uti gveléltragb et a1- ' lizing a 3f]; subcarrier and an overmodulation detection 4’386’436 3/1983 Kills: ett unit. In certain embodiments, the receiver unit is pow 4:392:022 7/1983 Carlson _ ' ‘ ered by a rechargeable battery which is recharged from 4,434,510 '2/1984 Lemelson . the transmitter unit. 4,484,156 11/1984 Khanna ...... 331/96 4,549,179 10/1985 Ste'ndardo . 4,621,374 11/1986 Micic et a1. . 4,685,133 8/1987 Iggulden . 4,694,338 9/ 1987 Tsinberg . 29 Claims, 12 Drawing Sheets my I 212 1214 f 218 1 218 lnrltill'lll;ED E 1111111111 _ ‘"15" 05111111011
_\220
230 mimic 2“
T (24o (244 250 24 H4 mum: maven umvsn (226 242 252 5,410,735 Page 2
U.S.‘PATENT DOCUMENTS __ 1-18 and 35-10 through 35-15. 4,916,532 4/ 1990 Streck et a1. . “TVC-9 GaAs FET 33 cm ATV Downconverter”, 4,939,770 7/1990 Makino ...... 379/61 Spec-Com Journal, Sep./Oct. 1986, cover page and pp. 4,980,665 12/1990 Schotz . 20-21. 4,984,296 1/1991 Schotz . Jerry Iggulden, Rule Making Petition to Federal Com 5,012,350 4/1991 Streck et a1...... 358/86 munications Commission, Sep. 11, 1985. 5,023,933 6/1991 Karkota, Jr...... 455/307 5,151,668 9/1992 Kim ...... 455/110 FCC Public Notice Report No. 1544, Oct. 21, 1985 (two 5,299,264 3/1994 Schotz et a1...... 381/2 pages) and certi?cation that the same is a true and cor rect copy. OTHER PUBLICATIONS Hap Grif?n, “HAM-TV, Amateur Radio’s Visual Fenton, Radio Electronics, Nov. 1987, p. 4. Mode That Blends, Voice, Video, Photography & Ex “Cordless TV Headphones” SYNC catalog, Spring perimentationl”, A5 Amateur Television Magazine, Sep. 1989, p. 5. 1984, pp. 6-9. “Wireless TV Marriage Saver” DAK catalog Summer P. C. Electronics, Advertisement, A5 Amateur Televi 1989, p. 1. sion Magazine, Sep. 1984, cover/table of contents page Electronic Circuits; Collins Raymond; 1980; p. 289 (?g and pp. 20-23. ure 21-50). Henry B. Ruh, “All About Amateur TV”, QST Maga “A Complete Single Chip AM/FM Radio Integrated zine, Jun. 1981, pp. 11-12. Circuit” Okanobu et a1., IEEE vol. CE-28, No. 3, E. Laird Campbell, “Amateur TV-The Easy Way”, Qst Aug./ 1982; pp. 393-407. -Magazine, Nov. 1962, pp. 33-41 and 150. “Reference Data for Engineers: Radio, Electronics, _ 198] Radio Amateur’s Handbook, published by the Computer and Communications”, Howard W. Sams & American Radio Relay League, cover page and pp. Co., Inc., 7th Edition, 1986, cover page, and pp. 1-5, 14-28 to 14-33. US. Patent Apr. 25, 1995 ~ Sheet 1 of 12 5,410,735
210
26 30
L24 28
38 FIG./
37 US. Patent Apr. 25, 1995 Sheet 2 of 12 5,410,735
212 . 5%
Na952:5:2a 05% 55s: as. 255s:5225.fl
8s23$a5mm AI11»;2552255i:32a5 in21a :22;5::‘Nwt g. k\A\\\sa 55:52.55s_ \2:522? ‘i:,2.a_IJ M:2 “EEG
z US. Patent Apr. 25, 1995 - Sheet 3 of 12 5,410,735
\_—'304/ LPF 3l0
F / 6'; 25
"~92 F ————— —'- —- “1/ 90 I"? {no I ‘ l
* ‘ ___1h‘%%%J€L¥8€E A i "3 I 0mm ______1?° I l | I 102 i ‘ ~94 if W(I26 4/ 0 To , I "a | i - m : HEADPHONE ' "6 ' I RECHARGING : 1 4 ma 1 INPUT I l l _I- | 1 F "7 I L ______J l =- l ' r l L ______J US. Patent Apr. 25, 1995 Sheet 4 of 12 5,410,735
3; 4/l2
E: 52%V 2 52.3 58:: 'r-______lullli'llllllull -___.__.. N2
22 _U.S. Patent Apr. 25, 1995 Sheet 5 of 12 5,410,735
F/6‘.5
OFF-
US. Patent Apr. 25, 1995 Sheet 8 of 12 5,410,735
is_
_. . .N: _ a?mE_ ~ "28%§ noz:.22k-52 n3>_g oN \3:E $6_ 5%.J70/?..1I |.\.W\|FilllI aw?a“W_o: l1I1olLilF1I as; 5ca__ ..m\."\MN EN.I 2$§
@222 o2 2%5:: 3a2N £835: =2;
US. Patent Apr. 25, 1995 Sheet 10 of 12 5,410,735
624 %non 220%6'6 vcc l2v 604 flab>620 / I-SV
600/ V00
U.S.'Patent Apr. 25, 1995 Sheet 12 of 12 5,410,735
F/6.8 5,410,735 1 2 horizontal frequency of the video signal (approximately WIRELESS SIGNAL TRANSMISSION SYSTEMS, 15.734 kilohertz in, an NTSC signal) and harmonics METHODS AND APPARATUS thereof. Applicants have found that the use of a 38 kilohertz subcarrier to form the multiplexed stereo sig This application is a continuation of application Ser. nal causes mixing with the second harmonic of the No. 07/822,598, ?led Jan. 17, 1992, now abandoned. NTSC signal, resulting in audible beat interference. In BACKGROUND OF INVENTION an attempt to overcome this problem, applicants have The present invention relates to wireless signal trans instead employed a subcarrier having a frequency equal mission systems, methods and apparatus, for example, 10 to two times the horizonal frequency of the video sig radio transmission apparatus for transmitting audio sig nal, approximately 31.5 kilohertz. However, similar nals within a local transmission area to a portable radio beat interference problems resulted. Applicants further receiver means carried on the person of the user. attempted to overcome this problem with the use of a Personal wireless audio signal transmission apparatus subcarrier equal to four times the horizontal video fre include systems which transmit audio signals, such as quency, but were unsuccessful due to a loss of stereo television audio signals, by means of infrared light re separation resulting from the use of an excessively high ceived by a personal infrared light receiving device subcarrier frequency. worn by a listener. It will be appreciated that such With the introduction of digital audio recording me transmission systems require a line-of-sight transmission dia, such as compact discs, and digital reproduction path, so that the system is not workable if walls, furni 20 techniques, the ability to reproduce high quality audio ture or other objects intervene between the transmitter and receiver. Accordingly, while infrared transmission signals having superior frequency response and wide systems may be useful where, for example, a person is dynamic range requires the provision of a similarly seated several feet from a television receiver to which capable personal wireless transmission system. The the infrared transmitter is connected for transmitting 25 transmitter of such a system must be capable of modu television sound, the transmission path may be inter lating a carrier without introducing audible distortion at rupted if, for example, the listener turns his or her head the receiver, for example, due to overmodulation. away from the transmitter or a person walks between Transmitters typically employ an overmodulation de the transmitter and the listener. Moreover, it is not tector which provides a visual indication when the level practical to utilize an infrared transmission system 30 of the modulating signal is excessive, thus to enable a where, for example, the listener is positioned in another user to avoid overmodulation distortion while maintain room or outside a building in which the transmitter is ing a desirably high signal-to-noise ratio. located. Conventional overmodulation detectors utilize a Local wireless television transmission systems are threshold detector whose output changes state when a available which transmit television signals from a local 35 level of a modulation signal exceeds a predetermined source, such as a television or VCR, within the 900 threshold level, and resumes a prior state once the level MHz local television transmission band to a receiver of the modulation signal falls below the predetermined which downconverts the television signals to a fre threshold level. The output of the threshold detector is quency band which may be tuned by a conventional television receiver. Such systems, therefore, employ used to drive a visual indicator, such as an LED. How receivers which are designed for use with a stationery ever, modulation signals which exceed the threshold television set and which optionally utilize a directional level for only brief intervals might not produce a visible antenna carefully positioned for best reception of the indication by the conventional apparatus. This becomes 900 Mhz signals radiated by the local transmitter. It is especially troublesome where the modulation signal is desirable, therefore, that the receiver act as a stable base 45 supplied by a source such as a compact disc player for supporting the receiving antenna in the best dispo which can produce an output signal having much sition to receive 'the locally transmitted signal, and sharper peaks than typical analog reproduction devices therefore, the receiver is typically of a size and weight such as a phonograph or magnetic tape recorder. Ac not practical for carrying on the person of a listener. cordingly, it is possible that a conventional overmodu Many television stations now include stereo audio lation detector will be unable to provide a visible indica signals in their transmissions. It is, therefore, desirable tion of sharp peaks in the modulation signal, such as that a personal wireless audio signal transmission appa those provided by a compact disk player, with the result ratus provide the capability of transmitting stereo audio that objectionable overmodulation distortion is audible signals reproduced by a television receiver. Conven at the receiver, but not detectable by the overmodula tionally, stereo audio signals are formed by adding the 55 tion detection circuit. ‘ right and left audio channels to form a ?rst signal and It will be readily appreciated that a personal wireless subtracting the right and left channels to form a second signal which is modulated on a subcarrier of 38 kilo audio receiver must be battery operated in order to hertz. The subcarrier is suppressed and the combination permit mobility of the person while the receiver is in of the ?rst signal, the subcarrier suppressed modulated use. However, the need to replace worn out batteries second signal and a pilot signal having a frequency of 19 from time to time is a nuisance, so that it is desirable to kilohertz (one-half that of the subcarrier), constituting a employ rechargeable batteries to power a personal multiplexed stereo signal, modulates a carrier for trans wireless audio receiver. It is also inconvenient, how mission. Conventional integrated circuits for producing ever, to remove rechargeable batteries for recharging such multiplexed stereo signals are available commer 65 and subsequently reinstall the same. In addition, the user cially. may ?nd that the batteries need recharging when it is However, audio signals provided by a television re desired to resume use of the receiver, which is also ceiver typically contain unwanted components at the inconvenient. 5,410,735 3 4 OBJECTS AND SUMMARY OF THE PRESENT a local transmission area; receiving the radiated signal INVENTION within the local transmission area; and demodulating the received signal to reproduce the modulation signal. Accordingly, it is one object of the present invention In accordance with a further aspect of the present to provide a personal wireless signal transmission sys 5 invention, a local wireless audio signal transmission tem, apparatus and method which alleviate the prob system for transmitting audio signals from a local audio lems and shortcomings of the above described systems signal source to a person within a local signal transmis and techniques. ' sion area comprises: local radio transmitter means for Another object of the invention is to provide a local transmitting the audio signals wirelessly within the local wireless signal transmission system and method which signal transmission area in the form of frequency modu both is economical and provides the ability to transmit lated radio signals within a ?rst, relatively high fre signals with low distortion. quency band; and personal wireless receiver means for A further object of the present invention is to provide receiving the frequency modulated radio signals for a local wireless signal transmission system and method reproducing the audio signals therefrom, the personal which employ personal receiver means carryable on the 15 wireless receiver means comprising: antenna means for person of the user thereof and which provide both con receiving the locally transmitted frequency modulated venient and economical operation. radio signals; downconversion means for downconvert Still another object of the present invention is to ing the received frequency modulated radio signals to a provide a local wireless audio signal transmission sys second frequency band including signal frequencies tem, as well as a personal wireless receiver unit for use 20 lower than signal frequencies included in the ?rst, rela with such a system, wherein frequency modulated sig tively high frequency band; frequency modulation re nals transmitted within a ?rst relatively high frequency ceiver means for receiving the downconverted fre band are downconverted to a second lower frequency quency modulated radio signals and reproducing the band for reception by miniaturized frequency modula audio signals therefrom; and coupling means for supply tion receiver means, thus to provide an economical, 25 ing the reproduced audio signals from the frequency lightweight personal audio receiver which may be worn modulation receiver means to electroacoustic trans or carried by a user. ducer means. It is a further object of the present invention to pro In accordance with a still further aspect of the present vide a local wireless audio signal transmission system invention, a personal wireless receiver unit for receiv and method for transmitting high quality audio signals 30 ing locally transmitted frequency modulated audio sig from a local audio signal source, which is subject to nals produced by a local wireless audio signal transmit high frequency audio noise, such as a television re ter, the locally transmitted signals being produced ceiver. within a ?rst, relatively high frequency band, com Yet another object of the present invention is to pro prises: antenna means for receiving the locally transmit vide an overmodulation detector apparatus and method 35 ted signals; downconversion means for downconverting capable of generating a visual indication of the occur the received locally transmitted frequency modulated rence of overmodulation, even when caused by modula audio signals to a second frequency band including tion signals of very brief duration. signal frequencies lower than signal frequencies in Yet still another object of the present invention is to cluded in the ?rst, relatively high frequency band of the provide a method and apparatus of transmitting stereo 40 received locally transmitted frequency modulated audio audio signals from a received television signal which signals; frequency modulation receiver means for re avoids the problem of subcarrier beat interference with ceiving the downconverted frequency modulated audio video signal components of the television signal. signals and reproducing audio signals therefrom; and In accordance with a ?rst aspect of the present inven- coupling means for supplying the reproduced audio tion, a local wireless signal transmission system com 45 signals from the frequency modulation receiver means prises: radio frequency oscillator means for producing a to electroacoustic transducer means. radio frequency carrier, the radio frequency oscillator In accordance with yet another aspect of the present means including a ceramic resonator for establishing a invention, a local wireless audio signal transmission ?rst predetermined frequency of the radio frequency system for transmitting audio signals from a local audio carrier; modulation means coupled with the ceramic signal source to a person within a local signal transmis resonator for shifting the frequency of the radio fre sion area, comprises: local radio transmitter means for quency carrier in response to a modulation signal for transmitting the audio signals wirelessly within the local producing a frequency modulated radio frequency sig signal transmission area in the form of modulated radio nal; means for radiating the frequency modulated radio signals, the local radio transmitter means including frequency signal within a local transmission area; and 55 input means for receiving the audio signals, low pass receiver means for receiving the radiated signal within ?lter means for attenuating noise in the received audio the local transmission area, the receiver means being signals above a predetermined frequency value, and further operative to demodulate'the received signal to means for producing radio frequency transmission sig reproduce the modulation signal. nals modulated by the ?ltered audio signals; and per In accordance with another aspect of the present sonal radio receiver means for receiving the modulated invention, a local wireless signal transmission method radio frequency transmission signals and carryable on comprises the steps of: producing a radio frequency the person of a user, the personal radio receiver means carrier having a ?rst predetermined frequency estab comprising means for demodulating the received sig lished by a ceramic resonator; shifting the frequency of nals to reproduce the ?ltered audio signals and electro the radio frequency carrier from the ?rst predetermined 65 acoustic transducer means for converting the ?ltered frequency in response to a modulation signal to produce audio signals to signals audible by the user. a frequency modulated radio frequency signal; radiating In accordance with a still further aspect of the present the frequency modulated radio frequency signal within invention, a local wireless audio signal transmission 5,410,735 5 6 method for transmitting audio signals from a local audio In accordance with still another aspect of the present signal source to a person within a local signal transmis invention, an overmodulation detector comprises: input sion area comprises the steps of: receiving the audio means for receiving a modulation signal; detector means signals from the local audio signal source at an input of for producing overmodulation detection signals in re a local radio transmitter means; low pass ?ltering the 5 sponse to absolute values of the modulation signal ex received audio signals to attenuate noise therein above a predetermined frequency value; producing radio fre ceeding a predetermined signal level, the detector quency transmission signals modulated by the ?ltered means being operative to produce respective ones of the audio signals; receiving the modulated radio frequency overmodulation detection signals in response to corre transmission signals with the use of a personal radio sponding narrow peaks of the modulation signal having receiver means carried on the person of a user; demodu absolute values which exceed said predetermined signal lating the received signals by means of the personal level for respective periods less than a predetermined radio receiver means to reproduce the ?ltered audio duration such that the respective ones of the overmodu signals; and converting the ?ltered audio signals to lation detection signals each have a duration exceeding signals audible by the user. 15 the respective period of its corresponding peak; and In accordance with another aspect of the present indicator means for indicating an overmodulation con invention, a local wireless signal transmission system dition to a user in response to the overmodulation detec comprises: radio frequency transmitter means for trans tion signals. mitting a signal within a local wireless signal transmis In accordancewith a still further aspect of the present sion area, the radio frequency transmitter means includ 20 invention, a method for detecting overmodulation by a ing means for coupling to a source of electrical energy; modulation signal comprises the steps of: producing and personal receiver means carryable on the person of overmodulation detection signals in response to abso a user thereof for receiving the transmitted signal within lute values of the modulation signal exceeding a prede the local wireless signal transmission area, the receiver termined signal level, respective ones' of the overmodu means including rechargeable battery means for provid 25 lation detection signals produced in response to corre ing operating power to the receiver means, and ?rst sponding narrow peaks of the modulation signal having coupling means for receiving power for recharging the absolute values which exceed said predetermined signal rechargeable battery means; the transmitter means in cluding second coupling means for coupling with the level for respective periods less than a predetermined ?rst coupling means of the receiver means for supplying duration each having a duration exceeding the respec ' recharging power to the rechargeable battery means tive period of its corresponding peak; and indicating an from the source of electrical energy when the receiver overmodulation condition to a user in response to the means is placed in contact with the transmitter means; overmodulation detection signals. the transmitter means further including switching In accordance with another aspect of the present means operative in a ?rst switching mode for disabling invention, a method of transmitting stereo audio signals transmissions by the transmitter means when the ?rst from a received television signal having a horizonal coupling means is coupled with the second coupling frequency f}; comprises the steps of: receiving right and means for supplying recharging power to the receiver left charmel audio signals obtained by demodulation means, the switching means being further operative in a from the received television signal; adding the right and second switching mode for enabling transmissions by 40 left channel audio signals to form a ?rst combined audio the transmitter means when the ?rst coupling means is signal; subtracting the right and left channel audio sig-' uncoupled from the second coupling means. nals to form a second combined audio signal; modulat In accordance with a further aspect of the present ing a subcarrier with one of the first and second com invention, a method of operating a local wireless signal bined audio signals, the subcarrier having a frequency transmission system comprises the steps of: providing 45 substantially equal to 3fH to form a subcarrier modu radio frequency transmitter means for transmitting a lated signal; combining the other of the ?rst and second signal within a local wireless signal transmission area; combined audio signals with the subcarrier suppressed coupling the radio frequency‘transmitter means with a signal to form a stereo multiplexed audio signal; and source of electrical energy; providing a personal re_ transmitting the stereo multiplexed audio signal. ceiver means carryable on the person of a user thereof for receiving the transmitted signal within the local In accordance with a further aspect of the present wireless signal transmission area, the personal receiver invention, a transmitter apparatus for transmitting ste means including rechargeable battery means for provid reo multiplexed audio signals from a received television ing operating power to the personal receiver means; signal having a horizonal frequency fHcomprises: input coupling the radio frequency transmitter means with 55 means for receiving right and left channel audio signals the personal receiver means to supply power from the obtained by demodulation from the received television radio frequency transmitter means to the personal re signal; means for adding the right and left channel audio ceiver means for recharging the rechargeable battery signals to form a ?rst combined audio signal; means for means; disabling transmissions by the radio frequency subtracting the right and left channel audio signals to transmitter means when the rechargeable battery means 60 form a second combined audio signal; means for modu of the personal receiver means is supplied with recharg lating a subcarrier with one of the ?rst and second com ing power from the radio frequency transmitter means; bined audio signals, the subcarrier having a frequency uncoupling the radio frequency transmitter means and substantially equal to 3f}; to form a subcarrier modu the receiver means to disable the supply of recharging lated signal; means for combining the other of the ?rst power; and enabling transmissions by the radio fre 65 and second combined audio signals with the subcarrier quency transmitter means upon uncoupling of the radio modulated signal to form a stereo multiplexed audio frequency transmitter means from the personal receiver signal; and transmitter means for transmitting the stereo means. multiplexed audio signal. 5,410,735 7 8 on the listener’s head for reproducing the wirelessly BRIEF DESCRIPTION OF THE DRAWINGS transmitted stereo audio signals from the transmitter FIG. 1 is a diagrammatic view of a transmitter unit unit 20. and a receiver unit of a local wireless audio signal trans With reference now to FIG. 2A, the transmitter unit mission system in accordance with an embodiment of 5 20 is provided with a pair of input terminals 40 for the present invention; receiving stereo audio modulation signals. The pair of FIG. 2A is a block diagram of the transmitter unit of input terminals 40 include a ?rst input terminal L for FIG. 1; receiving a ?rst stereo audio signal from the left channel FIG. 2B is a diagrammatic view of a stereo audio of a stereo audio signal source and a second input termi cable for use with a modi?ed transmitter unit in accor nal R for receiving a second stereo audio input signal dance with another embodiment of the present inven from the right channel of the stereo audio signal source. tion; The stereo audio signal source may be, for example, a FIG. 3 is a block diagram of a power switching cir television receiver capable of supplying stereo audio cuit and battery charger of the transmitter unit of FIG. output signals, a high ?delity receiver or ampli?er, 2A; 15 VCR, compact disc player, video disc player, magnetic FIG. 4 is a block diagram of an overmodulation de tape reproducing apparatus, phonograph, etc. The pair tector of the transmitter unit of FIG. 2A; of input terminals 40 are connected with a stereo input FIG. 5 is a waveform diagram for illustrating the jack of the transmitter unit 20 (not shown for purposes operation of the overmodulation detector of FIG. 4; of simplicity and clarity), to which the cable 37 of FIG. FIG. 6 is block diagram of radio frequency circuits 20 1 is removably connected for coupling the unit 20 with incorporated in the transmitter unit of FIG. 2A; and a local stereo audio signal source. The ?rst and second FIG. 7 is a block diagram of a receiver unit of the input terminals, L and R, are coupled with an input of a FIG. 1 embodiment; and volume control 42 which permits a user to adjust the FIG. 8 is a diagrammatic view of a further embodi signal levels of the input audio signals in order to ment of a receiver unit in accordance with the present 25 achieve a desirably high signal modulation level to invention. maximize the signal to noise ratio of the signal radiated DETAILED DESCRIPTION OF CERTAIN by the transmitter unit 20. Another, and countervailing, purpose is to avoid overmodulation of the radiated PREFERRED EMBODIMENTS signal which results in signal distortion. The volume With reference ?rst to FIG. 1, a local wireless audio adjusted ?rst and second input audio signals are sup signal transmission system in accordance with one em plied at respective output terminals 44 and 46 of the bodiment of the present invention includes a transmitter volume control 42. unit 20 for transmitting multiplexed stereo audio signals In order to permit the user to determine whether over the 900 MHz local transmission band extending overmodulation is taking place, an overmodulation from approximately 902 MHz to 928 MHz, and a re 35 detector 50 in accordance with one aspect of the present ceiver unit 22 for receiving the multiplexed stereo audio invention is provided. Respective ?rst and second input signals transmitted from the transmitter unit 20. The terminals 52 and 54 thereof are coupled with output receiver unit 22 includes a stereo headphone unit 24 terminals 44 and 46 of the volume control 42 to receive which enables the receiver unit 22 to be worn on the the volume adjusted ?rst and second input audio sig head of a listener. The headphone unit 24 includes a 40 nals. The overmodulation detector 50 is operative to headband support member 26 together with left and produce an overmodulation detection signal whenever right electroacoustic transducer elements covered by an absolute value of either the signal received at the respective earpads 28 and 30. Receiver circuits of the terminal 52 or the signal received at the terminal 54 unit 22 are enclosed by a ?rst housing 32 mounted on exceeds a predetermined signal level which indicates an the headband support member 26 adjacent earpad 30, 45 overmodulation condition. An LED 56 is coupled with while a rechargeable battery is enclosed within a second an output of the overmodulation detector 50 which housing 34 mounted on the headband support member serves to energize the LED 56 by means of a drive 26 adjacent the earpad 28. The receiver circuit is cou signal produced in response to each overmodulation pled with the rechargeable battery by battery leads (not detection signal. shown for purposes of simplicity and clarity) supported 50 As described in greater detail hereinbelow, the over by the member 26. modulation detector 50 is operative to produce respec Transmitter unit 20 includes a stereo audio input tive ones of the overmodulation detection signals in cable 37 for coupling the unit 20 with a stereo audio response to corresponding narrow peaks of the volume signal source. Transmitter unit 20 is also provided with adjusted audio signals having absolute values which a one quarter wavelength transmitting antenna 72 for 55 exceed the predetermined signal level for respective wirelessly transmitting the 900 MHz multiplexed stereo periods less than a predetermined duration, such that audio modulated signal to the receiver unit 22 which the respective ones of the overmodulation detection receives the same at a receiving antenna 210 which is signals each have a duration exceeding the respective also supported by the headband support member 26. period of its corresponding peak. In this fashion, it is The rechargeable battery enclosed with the second possible to ensure that the overmodulation detection housing 34 of the receiver unit 22 is coupled with a signal extends for a minimum amount of time necessary recharging input jack 35 which, as illustrated in FIG. I, to generate a visually perceptible output from the LED is shown connected with a recharging plug 36 con 56. As will be seen from the more detailed discussion of nected by means of a recharging cable 38 to a battery the overmodulation detector 50 contained hereinbelow, charger circuit of the transmitter unit 20, described in 65 by lengthening the duration of the overmodulation ' greater detail below. In use for receiving wireless audio detection signal with respect to the duration of narrow signal transmissions from the transmitter unit 20, the signal peaks of the input signals received at the termi receiver unit 22 is uncoupled from the plug 36 and worn nals 52 and 54, the overmodulation detector 50 enables 5,410,735 10 the user to determine when input signal peaks exceeding tive one of a second pair of audio plugs 312 for connect a maximum modulation level have occurred, even when ing the cable 300 with corresponding left and right such peaks extend for only very brief intervals of time. channel inputs of the modi?ed transmitter unit. It will This is especially useful for transmitting audio signals be appreciated that, in accordance with the modi?ed produced by devices such as compact disc players transmitter unit, low pass ?ltering to remove high fre which produce signals having wide dynamic and fre quency audio noise is enabled by using the cable 300 of quency ranges. FIG. 2A, while in the alternative, low pass ?ltering may The output terminals 44 and 46 of the volume control be disabled by employing a different audio input cable 42 are coupled with respective moveable terminals of a which does not incorporate a low pass ?lter circuit. double pole, double throw switch 48. Each of a ?rst pair H O After preemphasis, the left and right channel volume of ?xed terminals of the switch 48 is connected with a adjusted audio signals are supplied to respective inputs respective input terminal of a low pass ?lter circuit 60. of a stereo multiplexer 64 which is operable to produce The circuit 60 carries out low pass ?ltering of the sig a multiplexed stereo modulation signal which it supplies nals received at the respective input terminals thereof at an output terminal 66 thereof‘. The stereo multiplexer by strongly attenuating frequency components thereof 64 forms a baseband audio component representing the above 12 kilocycles. In this fashion, and in accordance sum of the left and right channel audio signals (L+R) with another aspect of the present invention, high fre and a difference signal representing the difference be quency audio noise which may be present in the signals tween the left channel audio signal and the right chan received at the pair of input terminals 40 may be effec nel audio signal (L-R). The stereo multiplexer 64 pro tively removed. This feature is particularly advanta 20 duces a subcarrier having a frequency which is substan geous where audio signals provided from a television tially equal to three times the horizonal frequency f}; of receiver are to be transmitted, since these signals are the video signal associated with the input left and right rich in audible beat interference at the horizontal scan channel audio signals. Accordingly, where such video ning frequency of approximately 15.734 kilocycles. This signal conforms to the NTSC system, the subcarrier feature is likewise useful for suppressing high frequency 25 frequency is selected to be substantially equal to noise in signals reproduced with the use of a tape deck 3 X 15.734 kilohertz, approximately 47.202 kilohertz. and for suppressing amplitude modulation beat notes in The stereo multiplexer 64 modulates the approximately signals supplied by an AM receiver subject to interfer 47.202 kilohertz subcarrier with the difference signal ence by signals from adjacent stations on the AM band. and suppresses the subcarrier to form a suppressed car The ?ltered signals from circuit 60 are provided rier signal representing the difference between the left thereby to respective inputs of a preemphasis circuit 62. channel audio signal and the right channel audio signal The circuit 62 carries out preemphasis in a conventional (L-R). The stereo multiplexer 64 also produces a pilot manner in order to boost the levels of high signal fre signal having a frequency equal to one half the subcar quencies which serves to reduce noise in the transmitted rier frequency, that is, approximately 23.601 kilohertz, signal, thus to improve the signal-to-noise ratio thereof. 35 and combines the baseband audio component with the For certain applications, such as the transmission of suppressed carrier signal and the pilot signal to form the audio signals produced by a compact disc player, it is multiplexed stereo modulation signal at the output ter preferable to bypass the ?lter circuit 60 in order‘ to minal 66 thereof. _ . preserve high frequency components of the audio sig Applicants have found that the use of a subcarrier nals. Accordingly, each of a second pair of ?xed termi 40 having a frequency substantially equal to three times the ' nals of the switch 48 is connected with a respective horizontal scanning frequency provides the ability to input of the preemphasis circuit 62 to permit a user to avoid beat interference with video signal components at .selectively engage or disengage the low pass ?lter cir the horizontal scanning frequency and harmonies cuit 60 depending on the presence or absence of high thereof. Accordingly, beat interference which can frequency noise (for example, video signal noise in the 45 occur where the subcarrier frequency is either substan audio output of a television receiver) in the audio signal. tially equal to the horizontal scanning frequency or In the alternative, in a modi?ed transmitter unit in ac second harmonic thereof can be avoided, withoutv the cordance with the present invention the switch 48 and loss of stereo separation which can occur where the low pass ?lter circuit 60 are omitted from the unit so subcarrier frequency is higher than three times the hori that the output terminals 44 and 46 of the volume con zontal scanning frequency. trol 42 are connected directly with respective inputs of The output 66 of the stereo multiplexer 64 is coupled the preemphasis circuit 62. with a modulation signal input terminal of a radio fre With reference also to FIG. 2B, in this alternative quency (RF) transmitter circuit 70 operative to fre embodiment of the transmitter unit a modi?ed stereo quency modulate a 913 MHz carrier which is supplied audio input cable 300 includes a low pass ?lter 302 55 at an output terminal thereof to the transmitting antenna af?xed thereto and supported thereby for carrying out 72 to be radiated within a local transmission area, typi the function of the low pass ?lter circuit 60 of FIG. 2A. cally within a radius of approximately one hundred feet Cable 300 includes a ?rst pair of audio cables 304 each from the transmitter ‘unit 20. Transmitting antenna 72 connected at a ?rst end with a respective one of a ?rst may be, for example, a quarter wave dipole antenna pair of audio. plugs 306 adapted to connect with a pair of 60 complying with applicable regulatory enactments. It audio output jacks of an audio signal source, such as a will be appreciated that the reception range of the trans television receiver, whose output is likely to include mitted signal will depend not only on the choice of the high frequency audio noise. Each of the ?rst pair of transmitting antenna 72 and the RF power output by audio cables 304 is connected at a second end with a the transmitter unit 20, but also on receiver sensitivity; respective input of the low pass ?lter circuit 302. Cable 65 the presence and nature of objects within the transmis 300 also includes a second pair of audio cables 310 con sion path, as well as other factors. nected at a ?rst end with respective outputs of the low Power for operating the transmitter unit 20, as well as pass ?lter circuit 302 and at a second end with a respec for recharging a battery of the receiver unit 22, is pro 5,410,735 11 12 vided, for example, by an AC/DC converter unit which ?rst input terminal 52 of the overmodulation detector plugs into a wall socket and provides a low voltage DC 50 is coupled with the input of a ?rst buffer ampli?er output. The DC output of the AC/DC converter is 140 to provide the volume adjusted first input audio coupled with a DC input of the transmitter unit 20 to signal thereto. The buffer ampli?er 140 is operative to provide a direct current voltage thereto. A voltage 5 supply an output signal at an output 142 thereof which regulator 80 serves to remove spikes in the input DC is proportional to the volume adjusted ?rst input audio voltage in conventional fashion and supplies a regulated signal. The output 142 is connected with the anode of a DC voltage at an output 82 thereof. The output 82 is ?rst germanium diode 144 of a peak detector circuit coupled with an input of a power switching circuit 90 146. The second input terminal 54 of the overmodula which is shown in greater detail in FIG. 3. tion detector 50 is coupled with the input of a second With reference also to FIG. 3, the power switching buffer ampli?er 150 to supply the volume adjusted sec circuit 90 has a ?rst power output 92 coupled with ond input audio signal thereto. Buffer ampli?er 150 is power input terminals of the overmodulation detector operative to supply an output signal at an output termi 50, the stereo multiplexer 64 and the RF transmitter nal 152 thereof which is proportional to the volume circuit 70 to controllably provide operating power 15 adjusted second input audio signal. The output 152 of thereto. A second power output 94 of the power the buffer ampli?er 150 is connected with the anode of switching circuit 90 is coupled with a power input of a a second germanium diode 154 of the peak detector battery charger 100 to controllably provide power circuit 146. thereto. Battery charger 100 has a pair of output charg The cathodes of the ?rst and second germanium di ing terminals including a positive charging terminal 102 odes 144 and 154 are connected with the ?rst terminal and a negative charging terminal 104 coupled through of a ?xed capacitor 160 as well as to the ?rst ?xed termi the recharging cable 38 with the recharging plug 36 of nal of a potentiometer 162. A second terminal of the FIG. 1. capacitor 160 as well as a second ?xed terminal of the Referring to FIG. 3, power switching circuit 90 in potentiometer 162 are connected to ground. A movable cludes a PNP switch transistor 110 having its emitter 25 contact of the potentiometer 62 is connected with an connected with the output 82 of the voltage regulator output 164 of the peak detector circuit 146. It will be 80 and its collector connected with the ?rst power appreciated, therefore, that the buffer ampli?ers 140 output 92 of the power switching circuit 90. The base of and 150 will serve to charge the ?xed capacitor 160 the transistor 110 is connected with a ?rst terminal of a through the germanium diodes 144 and 154, respec ?xed resistor 113 whose second terminal is connected tively, until the voltage across the capacitor 160 is sub with the emitter of transistor 110, the second power stantially equal to the voltage of the higher of the two output 94 of the power switching circuit 90, and a ?rst output signal values provided at the output terminals terminal of a current limiting resistor 126. A second 142 and 152 of the buffer ampli?ers 140 and 150. The terminal of resistor 126 is connected to the anode of an germanium diodes 144 and 154 serve to prevent the LED indicator 124 whose cathode is connected to posi 35 reverse ?ow of charge from the capacitor 160 back into tive battery terminal 102. Furthermore, a charge state either of the output terminals 142 and 152 thus to pro detection means, comprising an NPN transistor 116 duce a peak value signal in the form of the voltage together with resistors 117 and 118, is provided to shut across the capacitor 160 substantially equal to the off the transmitter during battery charging. higher one of the signals provided at the output termi When the rechargingplug 36 is connected with the 40 nals 142 and 152. It will be appreciated that the germa jack 35 of the receiver unit 22, the rechargeable battery nium diodes 144 and 154 each desirably produce a mini of the receiver unit causes current to be drawn through mal voltage drop of approximately 0.2 volts. It will also current limiting resistor 126, developing a voltage drop be seen that the potentiometer 162 will bleed charge across resistors 117 and 118 which act to divide the from the capacitor 160, so that once a peak value has voltage thereacross and apply the divided voltage to the 45 been reached by a respective one of the output signals at base of NPN transistor 116. Accordingly, the divided one of the output terminals 142 and 152 and the value voltage change is sensed by NPN transistor 116, which thereof decreases from such peak value, the peak volt ampli?es this voltage change and applies it as an ON/ age thereby produced across the capacitor 160 will OFF control signal to the base of PNP switch transistor decay in value as charge is removed therefrom through 110 through a resistor 112. That is, when recharging 50 the potentiometer 162. As will be seen below, the rate at current passes through resister 126, the voltage drop at which charge is bled from the capacitor 160 is con the base of transistor 116 turns off its collector-emitter trolled in order to ensure that the value of the voltage circuit, thus turning off transistor 110 to disable the across the capacitor 160 decreases after the occurrence transmitter circuits. However, when recharging plug 36 of a corresponding narrow peak at a rate less than a rate is uncoupled from jack 35 of the receiver unit 22 so that 55 of decrease of the corresponding narrow peak. recharging current no longer ?ows through resistor The output 164 of the peak detector circuit 146 is 126, transistor 116 is turned on. Transistor 110 is also coupled with the trigger input of a Schmitt trigger turned on as a consequence, so that power is applied to circuit 170. Schmitt trigger circuit 170 has an output the transmitter circuits. Accordingly, so long as the terminal 172 where it supplies an output signal which is receiver unit 22 is uncoupled from the transmitter unit either at a ?rst, low voltage level or at a second, high 20, transmitter operation is enabled. However, when voltage level depending both on the present voltage the recharging input of the receiver unit is coupled with level of the trigger input as well as the prior history the battery charger 100 so that current is drawn thereof, so that the response of the Schmitt trigger through resistor 126, transistor 116 is turned off, thus circuit 170 to the trigger input is subject to hysteresis. turning off transistor 110 and disabling the transmitter 65 That is, if the output signal of the circuit 170 at a given circuits. point in time is at the ?rst, low voltage level, a transition With reference now to FIG. 4, the overmodulation of the trigger input voltage from a relatively low volt detector 50 is illustrated therein in greater detail. The age which is less than an ON trigger level of the circuit