Abstract:
A local wireless system for transmitting a modulated RF carrier audio signal from a base unit to a receiver unit is provided. The base unit has a pair of audio input connections of which are coupled to an audio source amplification device for receiving left and right audio signals. The receiver unit has a pair of electroacoustic transducers (speakers) for reproducing demodulated left and right audio signals modulated upon the RF carrier audio signal. The base unit encloses a transmitting circuit having a first antenna and first, second and third circuits which modulate the left and right audio signals onto an RF carrier signal in the 900 MHz range. A receiver unit encloses a receiver circuit and is coupled to the pair of speakers. The receiver circuit performs a single downconversion of the modulated RF carrier signal from the 900 MHz range to a useable 10.7 MHz left and right signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the wireless transmission and reception of audio signals utilizing a modulated carrier RF signal. More particularly, it relates to the wireless transmission and reception of audio signals for a set of audio headphones wherein a modulated RF carrier signal in the 900 MHz range is employed to transmit the audio signals from a first stationary location (base unit) to a set of wireless audio headphones. 
     2. Background of the Prior Art 
     The transmission and reception of audio signals utilizing modulated RF carrier frequencies is well known in the prior art. The use of such technology to transmit audio signals from a base unit to a pair of wireless headphones is also known. In such use, the base unit is coupled to an audio processing device such as an audio receiver or amplifier which in turn is coupled to a CD player, phonograph player, radio receiver or other like audio producing device. The audio signal produced by one of these devices and processed by the audio receiver/amplifier is wirelessly transmitted to the audio headphones by way of the base unit coupled to the audio receiver/amplifier. High frequency carrier waves are employed wherein the audio information is modulated upon the high frequency carrier wave, transmitted by an antenna coupled to the base unit, received by a receiver unit (wireless headphones) also having an antenna, subsequently demodulated and thereafter converted to an audio signal which is reproducible by the wireless headphones. 
     The use of high frequency carrier waves in the 900 MHz range is known and became a standard for such wireless technology after the US government made the 900 MHz frequency range available for use by consumer electronic manufacturers. However, the manner in which these carrier frequencies are modulated and subsequently downconverted has remained complicated. One example can be seen in U.S. Pat. No. 6,215,981 to Borchardt et al. In such patent, a 900 MHz modulated RF carrier signal is used to transmit an audio signal from a base unit to a local receiving unit, such as, for example, a pair of wireless headphones. When the 900 MHz carrier frequency is received, it is downconverted a first time to an IF (intermediate frequency) of about 65 MHz. Thereafter, a second downconversion is affected to produce a lower frequency that can be reproduced by an electro acoustical transducer (the speakers within the pair of headphones). A standard FM radio receiver is coupled to the first downconverter and contains the second downconverter therewithin. The first downconversion converts the 900 MHz carrier signal to an intermediate frequency (IF) of 65 MHz signal. The second downconversion converts the signal to 10.7 MHz which is then demodulated into right and left audio signals which are reproducible by the electroacoustic transducers (speakers) of the wireless headphones. The second downconversion occurs in the standard FM radio receiver through the use of a VCO (voltage controlled oscillator) and a mixer. This prior art invention requires two downconversions, since the 65 MHz IF signal can not be demodulated into reproducible right and left audio signals. The 65 MHz IF signal is downconverted to a useable 10.7 MHz signal by the mixer after tuning the VCO to a suitable frequency level. 
     Since it would be desirous to simplify the transmission and reception process for this technology, improvements would be welcome. One such improvement could be to eliminate the need for two downconversions and hence the need for an IF carrier signal. This would certainly be an improvement in the art and represent a simplification of the transmission and reception process for wireless audio headphones. 
     SUMMARY OF THE INVENTION 
     We have invented an improved audio wireless headphone system utilizing modulated 900 MHz carrier signals to transmit audio signals emanating from a base unit coupled to an audio processing device to a receiver unit located within a pair of wireless audio producing headphones. Our improved wireless headphone system does not require two frequency downconversions. The IF carrier input signal is eliminated such that a modulated RF carrier frequency in the 900 MHz range is transmitted from the base unit to the wireless headphones and downconverted once within the headphones from the transmitted carrier frequency directly to a useable 10.7 MHz signal which is demodulated into reproducible right and left audio signals. Any VCO and mixer within the FM receiver of the headphones is not used as a second downconverter as practiced in the prior art. Instead, within an UHF module of the receiver, having a built in local oscillator and phase lock loop (PLL) circuit, the frequency can be changed by adjusting an outside crystal tuning circuit. In particular, the RF signal received by the antennae is mixed with the local oscillator frequency whereby the mixer directly converts the mixed signals to a 10.7 MHz signal which is subsequently demodulated into reproducible right and left audio signals. Accordingly, wherein we have invented a wireless transmission system for use with audio headphones whereby a variable frequency tuning system is employed with a built in local oscillator employing a single downconversion, the prior art utilizes a local oscillation frequency, two downconversions with the tuning system in the receiver block portion of the circuit and not in UHF module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be best understood by those having ordinary skill in the art by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: 
         FIG. 1  is a block diagram of a transmitter used in the wireless headphone system of the present invention; 
         FIG. 2  is a block diagram of a receiver used in the wireless headphone system of the present invention; and 
         FIG. 3  is a block diagram of a UHF module of the receiver used in the wireless headphone system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the following detailed description, the same reference numerals refer to the same elements in all figures. 
     With reference to  FIG. 1 , a transmitter circuit  10  used in the present invention is shown. Transmitter  10  is enclosed within a base unit (not shown) having a pair of audio input jacks  12  which couple to an audio amplifier which in turn is coupled to one of any type of audio producing devices, such as, for example, a CD player, a phonograph player, a cassette player or an FM/AM radio receiver. The input jacks couple directly to a first part of transmitter  10 . Transmitter  10  is divided into three parts. A first part is the audio signal processing circuit. The second part is the micro control unit and control circuit. And finally, the third part is the power supply circuit and the charge circuit. 
     With continuing reference to  FIG. 1 , it is shown in the first part of transmitter  10  that input jacks  12  couple directly to an auto level control amplifier (ALC) circuit  14  thereby feeding an audio signal from the audio amplifier emanating from one of the many audio producing devices. After being amplified by auto level control amplifier circuit  14 , both right and left signal are sent through an audio frequency (AF) low pass filter and pre-emphasis circuit  16 . Thereafter, the audio signal is sent to a stereo multiplexer IC  18  which outputs a stereo multiplexed audio modulated signal. The multiplexed signal includes a left channel audio signal, a right channel audio signal and a pilot tone signal. The stereo multiplexed audio modulated signal is then sent to a UHF module  22  which modulates the signal up to a 912.5 MHz RF carrier signal (although other signals in the 900 MHz range could be employed). A transmitting antenna  24  coupled to UHF module  22  then sends the modulated RF carrier signal out to a receiver unit within the local area. 
     With continuing reference to  FIG. 1 , in the second part of transmitter  10 , a CPU  26  is employed which is coupled to UHF module  22 . CPU  26  contains a micro control unit and a control circuit which is in turn coupled to an auto power circuit made up of a first and second component  28  and  30 . CPU  26  is controlled by the auto power circuitry and its ON/OFF signal and a band choose/change signal from a slide switch (not shown) and a change signal from a change signal circuit (also not shown). The micro control unit of CPU  26  sends a control signal to a phase lock loop (PLL) circuit within UHF module  22  to choose or change the RF frequency. 
     Again, with continuing reference to  FIG. 1 , the third part of transmitter  10  is shown wherein a 12V AC/DC power supply adapter  32  is employed which is coupled to an 8V regulator circuit  34 . This third part of transmitter  10  supplies all necessary supply power to all components of transmitter  10  with the voltage that is required of each component. 
     Auto level control (ALC) amplifier  14  is a monolithic integrated circuit consisting of a dual equalizer amplifier. The stereo audio signal input jacks  12  connect directly to auto level control amplifier  14 . If the input level is larger than the standard level, the output level would be limited and the output/input ratio would change. 
     Right and left audio frequency pre-emphasis circuits, a portion of AF filters  16 , receive the audio signal output from ALC amplifier  14  and send it to the audio frequency low pass filters of AF filters  16  after passing through a resistor net. The two channel low pass audio frequency filters effectively remove high frequency audio noise above 15.625 kilocycles so that noise is reduced in the transmitted signal. This filtered signal from audio frequency filters  16  is then sent to a pre-emphasis circuit. Frequencies higher than 2 kilocycles are pre-emphasized, which is later de-emphasized by a de-emphasis circuit in the receiver headphone. This serves to improve the signal-to-noise ratio thereof. The resulting audio signal is then sent to stereo multiplexer IC  18 . 
     Stereo multiplexer IC  18  is an integrated circuit used to generate a stereo composite signal. Stereo multiplexer IC  18  forms a baseband component representing the sum of left and right audio signals and a difference signal representing the difference between the left and right channel audio signal. This is sent to a built-in time-division-MPX which produces a multiplexed signal output. A left and right channel volume adjustor unit (not shown) can adjust the balance between the two audio channels. After combining the signals, the multiplexed signal and the 19 KHz pilot signal are sent to UHF module  22 . 
     UHF module  22  includes a VCQ (Voltage Controlled Oscillator), a PLL (Phase Lock Loop) circuit and a radio frequency amplifier. The VCO circuit produces a radio frequency of about 912 MHz. The PLL circuit, controlled by a micro control unit (MCU) produces a voltage signal for the VCO circuit for choosing an appropriate radio frequency. This radio frequency is modulated by the combined multiplexed signal and the 19 KHz pilot signal. After been amplified, this modulated RF signal is sent to the one-quarter wavelength transmitting antenna  24 . And at last, the transmitted signal is radiated within a local transmission area which typically is within a distance of about one-hundred feet from the transmitter unit. 
     Transmitter  10  is controlled by micro control unit IC (MCU)  26 . In the preferred embodiment, MCU  26  is an 8 bit micro controller with 1*13K of EPROM. In transmitter  10 , MCU  26  deals with the power ON/OFF signal and the charge signal of first and second auto power components,  28  and  30  respectively, to control two LED lights (not shown) and the power supply of UHF module  22 . MCU  26  further is designed to judge the state of a slide switch and an output control signal to UHF module  22  for producing different radio frequencies. All of this control work is completed by firmware loaded onto MCU  26 . When MCU  26  is operating, at first, it will judge the state of a charge signal. If there are batteries connected to transmitter  10 , MCU  26  will shut off all other outputs so that transmitter  10  only works as a battery charger. If there are no batteries connected to transmitter  10 , then MCU  26  will judge the power ON/OFF signal from another location. If there are no audio signal outputs from ALC amplifier  14 , then there are no voltage signals to be sent, which makes a measurable voltage high whereby MCU  26  will then output low voltage signals so that a LED (not shown) is OFF. However, if there are audio signals outputted from ALC amplifier  14 , the voltage of an output of MCU  26  is turned low causing MCU  26  to output a high voltage at another output to light the LED. MCU  26  then checks the state of the slide switch, wherein each of three states of the switch means different frequencies to be radiated out. MCU  26  checks the state and sends a control signal to the PLL unit within UHF module  22 . MCU  26  can also receive a frequency signal from the PLL unit of UHF module  22  for comparing with the frequency created before. If these two frequencies are not the same, MCU  26  will send out a voltage control signal which will continue to operate until these two frequencies are the same. 
     With reference now to  FIG. 2 , a receiver circuit  36  is shown. Receiver  36  includes a UHF module  38 . A receiver antenna  40  is coupled to an input network  50  (see  FIG. 3 ) which is located inside UHF module  38 . The input network  50  is a high pass filter with its output connecting to an RF amplifier. UHF module  38  also includes a voltage controlled oscillator (VCO) with a phase locked loop (PLL) circuit and a mixer circuit. The RF amplifier is employed to boost the level of the received 900 MHz range RF signal from antenna  40  (in the preferred embodiment, a 912.5 MHz RF signal is employed). This amplified signal is then passed to the mixer circuit  52  in UHF module  38 , as shown in  FIG. 3 . 
     The local oscillation is created by the VCO and controlled by the PLL circuit. The VCO frequency is detected by the PLL circuit and divided by a 64 prescaler. Thereafter, the divided signal is compared with a reference frequency, produced by a control circuit, for obtaining an error voltage. This error voltage is used to lock the VCO frequency. 
     With continuing reference to  FIG. 3 , the mixer circuit  52  of UHF module  38  serves to downconvert the received signal from the RF amplifier with the local oscillation frequency to create a useable 10.7 MHz signal. This 10.7 MHz signal is amplified and filtered and then outputted to an IF amplifier  54  built within UHF module  38 . Thereafter the signal is sent through a detector and stereo demodulation (within the integrated circuit of IF amplifier  54 ) resulting in right and left channel audio signals. 
     In order to catch the modulated RF signal transmitted by transmitter  10 , a certain reference frequency is chosen to lock the local oscillation in the receiver  36  of the system. Thereafter, the mixer circuit  52  will output the 10.7 MHz signal. For example, if a 912.5 MHz signal is transmitted, a control circuit outputs a suitable frequency so that a 901.8 MHz VCO frequency is outputted which results in the mixer circuit  52  outputting a useable 10.7 MHz signal (the difference between the 912.5 MHz and 901.8 MHz signals). Accordingly, receiver  36  is tunable by changing the reference frequency produced by the control circuit. 
     With reference to  FIG. 2 , the outputted right and left channel audio signals are then fed into a right and left channel EF AMP  42 . After being amplified, the right and left channel signals are fed into an audio amplifier  44  through a filter network  46 . Audio amplifier  44  amplifies the right and left channel signals to drive a pair of electroacoustic transducers or a pair of speaker elements with a set of wireless headphones. Audio amplifier  44  is a monolithic integrated circuit for use with stereo audio amplification. 
     Receiver  36  can receive three frequency signals by pressing a switch (not shown). The switch is connected to a CPU  48 . When a signal switch is detected, CPU  48  begins to scan the RF signal transmitted by transmitter  10 . Depending on its scanning result, CPU  48  selects a relevant crystal, or a proper reference frequency, to match to transmitter  10 . CPU  48  is also employed to mute audio amplifier  44  when no signal is received. 
     Equivalent elements can be substituted for the ones set forth above such that they perform in the same manner in the same way for achieving the same result.