BACKGROUND OF INVENTION

This invention relates to the communication of audio signals over A.C. power and wiring lines and, more particularly, to a so-called wireless remote system by which stereophonic signals may be transmitted from, for example, hi-fidelity audio equipment to loudspeakers.

The technique of using the A.C. wiring system in a home or office to serve as the transmission medium for audio signals has long been used. One application of this tech¬ nology is the widely available wireless intercom system which allows individuals in different rooms of a house to converse with one another. Unfortunately, conventional wireless intercom systems have met with less than enthus¬ iastic acceptance.

Typical wireless intercom systems rely on A.M. transmitting and receiving principles and, consequently, are susceptible to electromagnetic noise which degrades the audio signal. The A.C. power lines in most households are subjected to electromagnetic noise from many sources, particularly impulse noise or noise spikes from SCR or triac circuits of the type normally found in lamp dimmers. It is common, therefore, for the A.M. signals transmitted over such lines to be so degraded that the information contained therein is not comprehensible when received. Noise reducing circuits, if present in such wireless intercoms, usually fail to provide adequate noise immunity.

With these problems in mind, it is readily apparent that existing wireless intercom techniques cannot be turned to account for the transmission of high fidelity audio signals through household A.C. power lines.

OBJECTS OF THE INVENTION

It is one object of the present invention to provide an F.M. communication system for transmitting and/or receiving hi-fidelity audio signals, such as stereophonic signals over A.C. power lines, while avoiding degradation or loss of information in those signals.

A further object of the invention is to provide a wireless communication system which operates in a true stereo mode, using non-harmonically related carrier frequencies for the respective signal channels.

Another object of this invention is to provide a wireless remote speaker system which transmits and receives the full dynamic range of audio sources such as, for example, compact disc players, tape record/playback devices, and phonographs.

A further object of this invention is to warn the user of a wireless communication system about operating the audio transmitter in an overmodulating mode, which would otherwise lead to distortion of the audio signal.

A further object of this invention is to disconnect or to mute the remote output devices in a wireless communication system when no audio signal is received over that system.

Another object of this invention is to provide a compatible transmitter and receiver in a wireless communication system which will operate properly without the need for periodic tuning or adjustment.

Another object of this invention is to provide a wireless transmission system for use over A.C. power lines having selectable transmission frequencies to prevent possible interference with other wireless units operating on the same A.C. lines.

An additional object of this invention is to provide noise immunity to impulse noise and the like generated by SCR, triac and lamp dimmer circuitry.

Further objects and advantages of the invention will be apparent from the ensuing detailed description of a

preferred embodiment thereof, and the novel features will be particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

In accordance with this invention, a transmitter and receiver are provided; the transmitter accepting audio input signals, modulating the audio signals using F.M. principles and transmitting the F.M. signals over A.C. power wiring and the receiver ^' receiving the F.M. signals, amplifying and filtering the F.M. signals, demodulating the F.M. signals into audio signals and outputting the audio signals.

In accordance with one aspect of this invention, the transmitter and receiver transmit and receive stereophonic signals. The transmitter accepts stereophonic audio inputs and modulates the audio signals on different carrier frequencies. The receiver receives the different carrier frequencies, rejects interference of one carrier with the other, demodulates them, and outputs stereophonic audio signals.

In accordance with another aspect of this invention, the transmitter is provided with an audio signal compressor and the receiver is provided with an audio signal expander. The compressor reduces the dynamic range of the transmitted audio signals and the expander increases the dynamic range of the received audio signals. Using these devices enables the transmitter and receiver to transmit and receive audio signals with very wide dynamic ranges, such as might be provided by a compact disc player, without requiring a large bandwidth signal.

In accordance with yet another feature of this invention, the receiver has a muting circuit which, in one embodiment, automatically disconnects any output device currently coupled to the receiver if no audio signals are present and in another embodiment, a solid-state switch shunts the stereophonic outputs.

In accordance with yet another aspect of this invention, the transmitter has an overmodulation protection circuit which provides a visual indication when the voltage level of the audio input signal is too large.

As a feature of the invention disclosed herein, the respective channels of the receiver (e.g. the left and right channels) are provided with band pass/band reject filters for passing the carrier frequency assigned to that channel and rejecting the carrier frequency assigned to .the other channel„ This enhances channel selectivity and minimizes .interference of one channel with the other.

As another feature, each channel is provided with a noise cancellation circuit which removes impulse noise that might be superimposed onto the demodulated audio signals by, for example, SCR, triac or lamp dimmer circuits • which frequently are connected to household AC wiring.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example, and not intended to limit. the present invention solely to the embodiments shown and described herein, will best be understood in conjunction with the accompanying drawings in whic Ϊ

FIG. 1 is a schematic/block representation of the transmitter section of the present invention; and

FIG. 2 is a schematic/block representation of the receiver section of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

To best understand the advantages attained by the present invention, it is described herein in the environment of a remote stereophonic communication system capable of transmitting stereophonic audio signals (such as left-channel and right-channel signals) from a source to a destination. Examples of suitable sources include stereophonic receivers and amplifiers, which in turn may be connected to various audio sources, such as a magnetic tape

player (for example, a reel-to-reel tape machine or a cassette deck) , a compact disc player, a phonograph, or a tuner. Now that stereophonic television broadcasting has been introduced, a television receiver may serve as a suitable source. Examples of suitable destinations include stereophonic loudspeakers or headphones, or a magnetic tape recorder (such as a reel-to-reel or cassette deck) . For convenience, it will be assumed that the present invention is used to transmit audio signals over A.C. power lines to loudspeakers. However, it will be appreciated that this invention may be used in other environments such as, but not limited to, the remote recording of a transmitted conversation, or the transmission of audio signals created during a studio recording session to remote headphones.

Broadly, the present invention includes a transmitting section and a receiving section separated from each other by any desirable distance, but interconnected by way of the usual A.C. power wiring normally provided in a household, office, or the like. The transmitting and receiving sections may be disposed in different rooms of a home.

TRANSMITTER SECTION

As shown in FIG. 1, the transmitter section of the present invention includes a pair of audio channels, such as right and left channels, comprised of input terminals 10 and 11, input buffers 20 and 21, an audio signal compressor 30, pre-emphasis networks 40 and 41, variable frequency oscillators 50 and 51, and tuned output networks 90 and 91, respectively. For convenience and simplification, elements designated by an even reference numeral are included in the right channel and elements designated by an odd reference numeral are included in the left channel. A circuit common to both channels is identified by an even reference numeral.

Input terminals 10 and 11 couple the transmitter section to a source of stereophonic audio signals. Typically, the source may include such devices as a compact

disc player, a cassette deck, a phonograph or a radio tuner. The terminals themselves can be designed to accept plug-in jacks, commonly called "banana" plugs, screw fasteners, or any other conventional mechanical/electrical connector.

Input buffers 20 and 21 are coupled to input -terminals 10 and 11 via low pass filters 12 and 13, respectively. The low pass filters serve to block higher frequency carrier signals which are modulated" with the audio signals applied to terminals 10 and 11, to be described, from feeding back into the audio signal source. The buffers are adapted to amplify the input signal without increasing the load on the audio source. In the illustrated embodiment, each buffer is comprised of an amplifier 24 or 25 and a variable feedback resistor 26 or 27, the variable resistor enabling the user/operator to control the output level from the buffer by varying the gain thereof, thereby accommodating audio signal sources having widely different signal level outputs, such as CD players and tape players. The need for gain adjustment, more particularly, gain reduction, is indicated by an indicator LED 68 in a level control warning circuit 60, whose construction and operation is described below.

Audio signal compressor 30 is coupled to the output terminals of input buffers 20 and 21 and is adapted -to reduce the dynamic range of audio signals supplied thereto by the buffers. Recently developed audio signal sources such as compact disc players have a very extended dynamic range, frequently as much as 90 to lOOdB. In an F.M. transmission system, such as used by the present invention, the audio signal is converted to F.M. by either increasing or decreasing the frequency of a carrier signal. To modulate a carrier frequency using a signal with the large dynamic range of a compact disc player would require very large deviations (i.e. modulations) in carrier frequency and consequently- a very large bandwidth would be needed to accommodate such frequency shifts. Both large bandwidth and large deviations pose numerous problems, such

as the need for a very linear frequency modulator (preferably, a voltage controlled oscillator) to modulate the signal. Although such oscillators are available, a simpler and more price-effective solution entails the use of a compressor to reduce the total dynamic range of -the audio signal.

Compressor 30 reduces the dynamic range of an audio signal to one-half its original range. In one embodiment, compressor 30 may be formed as, for example, a compressor/expander chip such as Signetics Model NE 570N, having separate .channels therein. Other conventional signal compressors may be used.

Pre-emphasis networks 40 and 41 are coupled to the respective left-channel and right-channel outputs of compressor 30. Each pre-emphasis circuit is adapted to increase the signal strength of higher frequency signals included in the compressed audio signals supplied by the compressor. It is well-known that the distorting effects of noise in a transmission system increase with increasing signal frequency. Consequently, if the proportional strength of the high frequency components of the audio signal is to be maintained, the signal strength of the high frequency signal components should be increased relative to the other frequencies. Pre-emphasis networks 40 and 41, which are comprised of capacitors 42 and 45 and resistors 44 and 43 respectively connected in parallel therewith, perform this function. As the applied signal increases in frequency, the impedance of capacitors 42 and 45 decreases. Thus, more of the higher frequency signals pass through the pre-emphasis networks, and the desired proportional signal strength increase of the higher frequencies is obtained.

Although the pre-emphasis function may be effected with other components, a resistor-capacitor network is simpler and more readily constructed.

Variable frequency oscillators 50 and 51 are coupled to pre-emphasis networks 40 and 41 and are adapted to convert an audio signal supplied thereto into an F.M.

signal. In the present embodiment of the invention a voltage controlled oscillator is used to convert the signal, but any conventional frequency modulator, or even a phase modulator, could be used in alternate embodiments.

The voltage controlled oscillators used in the invention operate in a manner known to those of ordinary skill in the art to produce square wave outputs. The individual components 50 and 51 illustrated as "VCOs" may be formed as, for example, a voltage controlled oscillator chip such as National Semiconductor Model LM 566„

Tuned networks 90 and 91 are coupled to variable frequency oscillators 50 and 51 by amplifiers 80 and 81 which are adapted to increase the strength of the F.M. signal. Any type of amplifier circuit with the requisite gain at the operating frequency of the variable oscillators may be used, such as operational amplifiers. In the present embodiment, transistors 82 and 83 in common emitter configuration provide the amplification needed.

The tuned output networks are coupled to the collectors of transistors 82 and 83, respectively, and are adapted to convert the square waves produced by the variable frequency oscillators 50 and 51 to sinusoidal waves. As is known from Fourier analysis, a square wave consists of numerous harmonically related sinusoidal waves added together. By the proper selection of the inductance of transformers 92 and 93 and the capacitance of capacitors 94 and 95, which comprise the tuned output networks, each network will resonate at a respective frequency. Consequently, each network will pass only a single sinusoidal wave having a designated resonant frequency.

The secondary windings of transformers 92 and 93 of tuned networks 90 and 91 are coupled to filter circuits which are adapted to pass a high frequency signal but to block low-frequency signals. By so doing, the filters, which apply the FM audio signals to the AC power lines, prevent the AC power signal, which is usually 6 ^" 0Hz, from flowing into the transmitter. As shown, the right-channel

filter is comprised of capacitors 100 and 104 connected in series with inductors 90 and 102 and the secondary winding of transformer 92. Additionally, an L-C circuit 96 tuned to the left-channel carrier frequency, is connected in parallel with the secondary of transformer 92. Capacitors 100 and 104 serve to couple the filter to the AC power lines via a conventional plug 110, which may be thought of as an output terminal of the transmitter section. The left-channel filter is of substantially the same construction, with an L-C circuit 97 tuned to the right-channel carrier frequency connected in parallel with the secondary of transformer 93. The secondaries of transformers 92 and 93 exhibit low output impedances and these match the AC power lines. L-C circuit 96 prevents the left-channel carrier from being fed back to transformer 92 and the filters formed of capacitor 100, inductor 98 and capacitor 104, inductor 102 present a high blocking impedance to the left-channel carrier frequency. A similar prevention of cross-channel interference is obtained by L-C circuit 97, capacitor 101, inductor 99 and capacitor 105, inductor 103. Although the filters have been constructed from inductors and capacitors, other methods of filter construction could be used with no alteration in function.

A power supply 120 is coupled to the A.C. power lines by plug 110 and is adapted to convert a 120V. A.C. signal to a 12V. D.C. supply voltage to power the previously described circuits. The power supply is of conventional construction and operation, known to those skilled in the art.

Although not shown in the present embodiment, a selector switch may be coupled to variable frequency oscillators 50 and 51 to permit the user to change operating (or carrier) frequencies. As an example, the switch may couple different resistors to the variable frequency circuits, thereby changing their basic carrier frequencies.

A level control warning circuit 60 is coupled to the output from input buffers 20 and 21 and is adapted to

provide a visual warning (e.g. by means of an LED indicator lamp 68) that the signal level output from the buffers is too high. The warning circuit is comprised of resistors 61, 62, 70, 72, and 74, operational amplifiers 64 and 63, an LED 68 and a transistor 66. The operational amplifiers are configured as comparators using the resistors to establish reference voltage levels to which the signals from buffers 20 and 21 are compared. Resistors 70, 72 and 74 are connected as a voltage divider to a source of reference potential, such as produced by power supply 120. The inverting inputs of amplifiers 64 and 63 are connected to receive the reference voltage produced at the junction of resistors 70 and 72. The non-inverting inputs of these amplifiers are coupled to the outputs from buffers 20 and 21. Whenever a voltage is detected from the buffers which exceeds the reference voltage, amplifiers 64 and/or 63 apply a relatively positive voltage to the base of transistor 66. The transistor is connected as a driver and, when rendered conductive by a positive voltage from amplifier 64 or 63, allows current to flow through LED 68, causing it to light and giving the desired warning. If no visual warning is desired, circuit interruption may be substituted for this warning circuit to provide similar overmodulation protection. If a more precise indication of overvoltage is needed, a voltage meter might be connected to the output of the buffers in place of circuit 60.

In operation, a stereophonic audio signal, comprising a right and a left channel, is applied to input terminals 10 and 11. As mentioned above, this signal may be derived from either a stereophonic amplifier or receiver, or any individual component of a stereophonic system -such as a compact disc player, a tape deck, or a phonograph. As the operation of both right and left channels is virtually identical, with the exception of their different operating (or carrier) frequencies, only the operation of the right channel need be described.

Input terminal 10 supplies the right channel audio signal to low pass filter 12 from which the filtered signal is applied to input buffer 20 for amplification. The buffer is designed to present a high input impedance and a low output impedance to the signal, which allows the transmitter to operate without electrically loading the audio input device coupled to input terminal 10.

Warning circuit 60 detects if the buffer output signal is too high and would result in over modulation. If so, resistor 26 may be adjusted by an operator to reduce the audio signal level. Resistors 26 and 27 may be ganged for simultaneous adjustment. It is recognized that a high signal level applied to variable frequency oscillator 50 may cause the oscillator to over-modulate and produce a very distorted audio output. If the audio signal level is higher than the reference voltage established at the inverting input of operational amplifier 64, transistor 66 is turned "on" and current flows through the transistor and LED 68. In the present embodiment the reference voltage is set at about 700mV. Of course, this threshold level can be altered to accommodate any modifications to the variable frequency oscillators which would enable them to accept higher input voltages. If the LED turns on, an operator may adjust resistors 26 and 27, thereby reducing the audio signal strength at the output of amplifiers 24 and 25, until LED 68 is turned "off".

The amplified audio signal from buffer 20 is applied to signal compressor 30. As mentioned above, the audio signal, especially if it is produced by a compact disc player, may have a very wide dynamic range, perhaps as much as 90 to 100 dB. It is desirable to reduce the dynamic range of the audio signal; and compressor 30 reduces the dynamic range by about one-half.

After being reduced in dynamic range, the audio signal is pre-emphasized by pre-emphasis network 40. The impedance of capacitor 42 decreases with an increase In frequency of the audio signal. Thus, lower frequency

signals are reduced in strength relative to higher frequency signal because more of the lower frequency signal is attenuated by the higher apparent impedance of the network. By pre-emphasizing the higher frequencies of the audio signal, the effects of noise, which are greater with respect to higher frequencies than lower, can be compensated.

Once pre-emphasized, the audio signal is applied to variable frequency oscillator 50, where it is frequency modulated. As the embodiment being described is designed to function in a stereophonic fashion, oscillators 50 and 51 are designed to operate at two different frequencies. In one embodiment the chosen carrier frequencies are 135 kHz for the right channel and 330 kHz for the left. In another embodiment the right channel carrier frequency is 485 kHz and the left channel carrier frequency is 345 kHz. It has been found that at these frequencies (345-485 kHz) , and particularly in the band of about 300-525 kHz, noise spikes generated by other circuits and devices connected to the AC power lines are minimized because they generally exhibit an "energy spectrum" below this band. As 330 kHz is between the second (270 kHz) and third (405 kHz) harmonic of 135 kHz, and 485 kHz is between the fundamental and second harmonic (690 kHz) of 345 kHz, interference between the channels is minimized. Other carrier frequencies may be selected, if desired.

The F.M. signal from oscillator 50 is applied to tuned output network 90 by common-emitter transistor 82 of amplifier 80. Network 90 is tuned to resonate at only the desired frequency for the channel, here 485 kHz (or 135 kHz) . Thus only a sinusoidal signal with a frequency of 485 kHz is transferred to the secondary coil of transformer 92, and is applied by L-C circuits 98, 100 and 102, 104 to output terminal 110 and thence to the A.C. wiring. L-C circuit 96 resonates at the left-channel carrier frequency of 345 kHz to provide a low impedance thereto; whereas L-C circuits 98, 100 and 102, 104 provide a high impedance to this 345 kHz carrier.

RECEIVER SECTION

As shown in FIG. 2, the receiving section is comprised of an input plug 200, input filter circuits 220 and 221, amplifier-filter stages 236 and 237, F.M. detectors 320 and 321, de-emphasis circuits 330 and 331, audio signal expander 350, and output terminals 360 and 361. As in the transmitter, elements designated by an even reference numeral are included in the right channel and elements designated by an odd reference numeral are included in the left channel. A circuit common to both channels is identified by an even reference numeral.

Input plug 200 couples the receiver to the A.C. wiring and is adapted to receive the transmitted F.M. signals. The input plug is also adapted to receive 120v. A.C. power and deliver it to a power supply 210. The power supply is adapted to convert 120v. A.C. to a 12v. D.C. supply voltage (or to a voltage of any other suitable magnitude) to power the circuits included in the receiving section. The power supply is of conventional construction and operation, known to those skilled in the art, and may be similar to aforementioned power supply 120.

Input filter circuits 220 and 221 are coupled to input plug ^' 200 and are each adapted to pass only their respective operating (carrier) frequency. Each filter is formed as a two-stage band-pass/band-reject filter. The first stage of filter 220, comprised of a parallel L-C circuit 370, is coupled to input plug 200 by capacitors 222 and 224 and is connected across the AC power lines. The second stage of filter 220 is comprised of a parallel L-C circuit 372 connected in series with the primary of a transformer 232 and coupled to the AC power lines by capacitors 222 and 224. Each L-C circuit operates as a band-pass filter, and L-C circuit 370 is tuned to the right channel carrier (485 kHz) to present a relatively high impedance to the left channel carrier (345 kHz) . L-C circuit 372 is tuned to block the left channel carrier.

Filter 221 is of similar construction with L-C circuit 371 tuned to the left channel carrier (345 kHz) and L-C circuit 373 tuned to block the right channel carrier. Thus, each filter passes one of the FM carriers and rejects the other. The filters also block AC power from the following receiver circuitry. Improved channel selection is obtained. Transformers 232 and 233 include center-tapped secondaries (whose center taps are connected to ground) and capacitors 234 and 235 respectively coupled across these secondaries., thus forming tuned resonant circuits adapted to filter out all signals except a sinusoidal signal with the desired resonant frequency. In this particular embodiment, the right channel carrier is passed through transformer 232 to amplifier filter 236 and the left channel carrier is passed through transformer 233 to amplifier filter 237.

Amplifier-filter stages 236 and 237 are comprised of cascaded stages 240, 260, 280 and 241, 261, 281, respectively. Amplifier filter stages 240 and 241 are coupled to input filter-circuits 220 and 221, respectively, and are adapted to provide signal amplification and filtering. Although three pairs of amplifier-filter stages, 240 and 241, 260 and 261, 280 and 281, are present in this embodiment of the invention, other embodiments of this invention may use only two pairs of amplifier-filter stages, or only one.

Amplifier-filter stage 240 is comprised of a differential amplifier formed of transistors 242 and 244 and resistor 250. Amplifier-filter stage 241 likewise is formed as a differential amplifier. The differential amplifiers are conventional in construction and operation, known to one skilled in the art. Noise filtering is provided by using transformer 232 to apply the filtered source signal from input filter circuit 220 across the bases of both transistors in the differential amplifier. It has been found that noise signals appear to travel on both leads of the A.C. wiring which acts as the transmission medium for the F.M. signals supplied to the receiver? and these noise

signals tend to be in phase with each other and of the same magnitude. Since differential amplifiers only amplify the difference between the signals applied to the bases of the transistors therein, applying the input signal to both bases in the differential amplifier results in the noise signals cancelling themselves out. The F.M. information signal, on the other hand, appears differentially across the leads of the A.C. wiring. Therefore, applying the received signal from transformer 232 to both sides of the differential amplifier results in differential amplification. Thus, the F.M. signal is amplified and noise is reduced. Additional filtering is provided by transformers 248 and 249 and capacitors 246 and 247 which respectively form tuned resonant circuits coupled to the differential amplifiers. These resonant circuits pass only a sinusoidal wave with the correct operating (carrier) frequency.

Amplifier-filter stages 260 and 261 are coupled to amplifier-filter stages 220 and 221, respectively, and are similarly adapted to provide amplification and filtering. The amplification is provided by the respective differential amplifiers, similar to those in stages 220 and 221, and the filtering is provided by tuned resonant circuits comprised of capacitors 268 and 269 and transformers 270 and 271 respectively.

Amplifier stages 280 and 281 are coupled to amplifier-filter stages 260 and 261, respectively, and are adapted to further amplify the F.M. signal. Each amplifier stage is illustrated as a pair of differential amplifiers and its filtering capacity derives from operating the amplifiers in a limited mode. This method of operation is described below.

F.M. detectors 320 and 321 are coupled to amplifier stages 280 and 281, respectively and are adapted to demodulate the F.M. signal, thereby providing an audio signal which can be used by any device requiring an audio input, such as loudspeakers or a magnetic tape recording/playback device. Preferably, detectors 320 and

321 are phase locked detectors well known to one familiar with the art. In one embodiment, the individual components 320 and 321, illustrated as "PLL Detectors," may be formed as, for example, phase locked loop chips such as R.C.A. Model CD 4046BE, although other conventional phase locked loops may be used.

De-emphasis circuits 330 and 331 are coupled to phase locked loop detectors 320 and 321 by way of impulse noise cancellation circuits 390 and 391, respectively. Each de-emphasis circuit is adapted to decrease the signal strength of the higher frequency components of the audio signal provided by the phase locked loops so as to counteract the pre-transmission boost of the high frequency components by pre-emphasis networks 40 and 41 (Fig. 1) . Resistors 332 and 333 and capacitors 334 and 335 respectively comprise the de-emphasis circuits and are connected in series with one terminal of each capacitor coupled to ground potential. As the applied audio signal increases in frequency, the impedance of capacitors 334 and 335 decreases, whereby high frequency signals are increasingly shorted to ground. Thus, the relative strength of the high-frequency signals is decreased and residual high frequency operating (carrier) signals are also removed.

Each of noise cancellation circuits 390 and 391 preferably is formed as an integrated circuit chip to cancel pulse noise that may be superimposed on the FM carriers because of SCR, triac or lamp dimmer circuitry connected to the AC power line. One example of a preferred noise cancellation circuit is Model LA2110, manufactured by the Semiconductor Division of Tokyo Sanyo Electric Co., Ltd. Since this circuit is conventional and is commercially available, no further description thereof is provided herei .

Variable low-pass filter circuits 340 and 341 are coupled to pre-emphasis circuits 330 and 331 and are adapted to filter out any residual operating (carrier) frequency signals from the audio signal. These residual operating

(carrier) frequency signals could cause audio signal expander 350 to operate when no audio signal is present. Furthermore, since the output level produced by each phase locked loop detector 320 and 321 may differ, due to variation in manufacturing, filter circuits 340 and 341 are. adapted to compensate for such difference to provide the correct audio signal level to signal expander 350. The filter circuits use variable resistors 342 and 343 and operational amplifiers 344 and 345 to provide the appropriate variable amplification.

Audio signal expander 350 is coupled to filter circuits 340 and 341 and is adapted to restore the original dynamic range of the audio signal. The compressor 30 (see FIG. 1) in the transmitter section reduced the dynamic range of the audio signal by about one-half, and expander 350 serves to double the dynamic range. In one embodiment, expander 350 may be formed as, for example, a compressor/expander chip such as Signetics Model NE 570N, although other convention signal expanders may be used.

Output terminals 360 and 361 are adapted to be coupled to the audio signal expander by way of relay contacts, described below. The terminals are adapted to connect the illustrated receiving section to a variety of output devices, such as stereophonic loudspeakers, stereophonic headphones, recording devices and the like. The terminals themselves are adapted to accept plug-in jacks, commonly called "banana" plugs, screw fasteners, or any conventional mechanical/electrical connection. In one embodiment a selector switch (not shown) enables a user to choose between output devices, in a manner obvious to one skilled in the art.

In one embodiment, a muting circuit including a carrier detector 300 is coupled to amplifier stages 280 and/or 281 and is adapted to turn on transistors 392 and 393 coupled to the outputs of FM detectors 320 and 321 to shunt the outputs of the left and right demodulating channels in the absence of a carrier in either channel. Hence, in the

absence of an audio signal (i.e. a modulated carrier) in one or the other channel, undesirable hum and hiss in any output device is greatly reduced. In another embodiment, circuit 300 functions to energize a relay coil to open and close relay contacts in the absence and presence of audio signals, respectively. Circuit 300 is comprised of transistors 304, 310 and 312, diode 302 and resistors 306 and 308. By turning on transistors 392 and 393, output devices coupled to terminals 360 and 361 are muted, or effectively disconnected, from the receiver section in the absence of an audio signal.

In the present embodiment both the left channel and right channel operate in a virtually identical fashion. Consequently only the operation of the right channel needs to be described.

Input plug 200 of the receiving section is inserted into an A.C. power outlet. This serves the dual purpose of energizing the illustrated circuits through power supply 210 and coupling any F.M. signals which might be received to the receiver section. When an F.M. signal is received, input filter circuit 220 removes the 60 Hz A.C, passes the right-channel F.M. signal and rejects the left-channel FM signal. Other low frequency noise also is removed by this filter from the received information signal.

The output from filter circuit 220 is amplified by the differential amplifier in amplifier-filter stage 240. This differential amplifier operates in an unlimited fashion, meaning that all signals, both of high and low voltage, are amplified. As the receiver section can be placed anywhere in a house containing the transmitter unit, the received signal can vary in strength from millivolts to volts. To avoid either "losing" very small amplitude signal or reducing large amplitude signals, the amplifier operates in an unlimited fashion- _*

The tuned transformer circuit comprised of capacitor 246 and transformer 248 resonates at 485 kHz. Hence, only F.M. signals of that carrier frequency are

coupled through transformer 248 to amplifier-filter stage 260. Amplification of the signal is achieved by the differential amplifier comprised of transistors 262 and 264. This amplifier also operates in an unlimited fashion to allow for variations in the strength of the received signals. Another resonant, tuned transformer circuit, comprised of transformer 270 and capacitor 268, provides additional filtering in the same manner as tuned transformer circuit 246 and 248.

After being further filtered and amplified by amplifier-filter 260, the signal is sampled by carrier detector 300. If a carrier signal is not present, transistor 392 is turned on and the output de-emphasis circuit 330 is shunted to ground. Thus, in the absence of a carrier signal, no "hiss" or "hum" appears in the output devices. It is appreciated that transistor 392 may be coupled to the output of FM detector 320, noise cancellation circuit 390, de-emphasis circuit 330, variable gain filter 340 or expander 350. Furthermore, the base electrodes of transistors 392 and 393 are connected in common so that both transistors are turned on if a carrier is not sensed in either demodulating channel.

More particularly, if a signal is present, diode 302 conducts to reduce the voltage at the base of transistor 304 to approximately zero. Thus, transistor 304 is turned off, whereby both the collector and base voltage of transistor 310 are equal. Hence, transistor 310 is non-conductive. Consequently, a relatively higher voltage is applied to the base of transistor 312, and it turns on, lowering the voltage of the base electrodes of transistors 392 and 393. Should no signal be present, transistor 304 turns on which, in turn, turns on transistor 310 to reduce the voltage of the base of transistor 312. Transistor 312 thus is rendered non-conductive to turn on transistors 392 and 393. Alternatively, the conductivity of transistor 312 may control the energizing current through a relay coil to

open or close relay contacts coupled to output terminals 360 and 361.

Assuming an F.M. signal has been received, it is applied to amplifier stage 280 whereat it is amplified and filtered by the two differential amplifiers therein. The amplified F.M. signal then is applied to phase locked loop detector 320 which demodulates the F.M. signal and provides an audio signal output.

Impulse noise is cancelled from the audio signal by noise cancellation circuit 390, and the demodulated audio signal then is de-emphasized by de-emphasis circuit 330, filtered by low-pass filter 340, and then expanded to its original dynamic range by audio signal expander 350. Preferably, filter 340 is adjusted during assembly of the receiver section to insure that the signal level at the output of the filter is at the proper level for audio signal expander 350 to function correctly. This adjustment is done by means of variable resistor 342 which increases or diminishes the signal output from operational amplifier 346.

In audio signal expander 350, for every 1 dB range in the signal supplied thereto, a 2 dB range in the output signal is produced. The expanded, recovered audio signal then is applied to output terminal 360.

In a similar operation, the other channel of frequency-modulated audio information is amplified, filtered, demodulated and expanded to recover the original audio signal. This audio signal is supplied to output terminal 361.

While the present invention has been particularly shown and described with reference to a preferred embodiment, it will be readily appreciated by those of ordinary skill in the art that various changes and modification may be made. For example, if several transmitter units are operating on the same A.C. wiring system, a frequency change switch may be coupled to oscillators 50 and 51 to change the two carrier frequencies generated by these oscillators such that each transmitter

operates with a distinctive set of carrier frequencies and allows non-interfering operation of the multiple transmitters. A similar adjustment may be made to F.M. detectors 320 and 321 to recover distinctive carrier frequencies. Also, an input selector switch might be coupled to output terminals 360 and 361 to enable the user to connect several output devices to his receiver section and then choose among them.

It is intended that the appended claims be interpreted as including the foregoing as well as other changes and modifications.