Abstract:
A full duplex infrared communication system suitable for use in connection with a telephone system is disclosed. The communication system includes a base station and a remote wireless set. High quality audio is provided through use of an FM sinusoidal current driven signal transmission between the base station and the remote wireless set. Further, transmission is made over bands which do not harmonically overlap. For example, the base station may transmit at 250-430 1 KHz and the remote wireless set may transmit at 1.45-1.63 MHz. Further, adverse effects of cross-talk and signal reflection are reduced by using transmission carrier signals for the base station and the remote wireless set having different wavelengths. For example, the base station may use LEDs which emit signals at a wavelength of 940 nm while the remote wireless set may use LEDs which emit signals at a wavelength of 880 nm. LEDs with slow turn-on and turn-off times, e.g., approximately 1 microsecond, are used in the Base TX to minimize harmonic radiation and Base RX desensitization. The remote TX LEDs are current driven, and have much faster turn-on and turn-off times than the Base TX LEDs, however its radiated fundamental and harmonics are much higher in frequency than the remote RX frequency and do not effect it during normal operating conditions. Further, the system does not require optical gain (lens, parabolic reflectors) and is omnidirectional.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     Wireless communication systems for interconnection with telephone systems are known. Such systems include a fixed transceiver which is connected to a telephone line and functions to relay signals to and from a remote device. Wireless systems allow substantial freedom of movement during a telephone conversation since the user is not limited by a fixed connection such as a telephone cord. However, some wireless systems provide relatively poor audio reproduction quality. More particularly, background noise and electromagnetic interference are significant sources of poor audio quality. Further, since the remote devices in such wireless systems are typically battery powered, power consumption is an important concern. It is therefore desirable to have a wireless communication system which provides energy efficient operation, high quality audio, security, and which reduces interference between closely spaced units. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a wireless communication system for use in connection with a telephone system includes a first transceiver with a first transmit circuit and a first receive circuit, said first transceiver being connected to the telephone system; and a second transceiver with a second transmit circuit and a second receive circuit, wherein the first and second transmit circuits transmit signals and the first and second receive circuits receive the signals from the second and first transmit circuits, respectively. In further accordance with the present invention, distinct signal transmission bands and carrier signal wavelengths may be employed to further improve audio quality. 
     Improved audio quality is provided by the communication system of the present invention with several advantageous features. Improved audio quality is provided by using FM sinusoidal current signal transmission between the base station and the remote wireless set. The FM sinusoid current signal transmission is advantageously resistant to the electromagnetic interference which negatively effects some known systems. Improved audio quality is also provided by transmitting at frequencies which avoid excessive harmonic overlap. For example, the first transceiver, which may be a base station, may transmit at 250-430 KHz while the second transceiver, which may be a remote wireless set, may transmit at 1.45-1.63 MHz. Improved audio quality is also provided by sending transmissions from the first transceiver with a carrier signal having a different wavelength than that with which transmissions are sent from the second transceiver, thus reducing cross-talk and reflection problems. LEDs which emit signals at a wavelength of 940 nm may be employed by the base station while the remote wireless set may employ LEDs which emit signals at a wavelength of 880 nm. 
     In addition to the above advantages, infrared communication offers inherent advantages insofar as IR signals are generally confined to the room in which they are transmitted. As such, IR systems provide a measure of privacy which is not ordinarily available in wireless communication systems. Further, systems can be used in close proximity without interference, for example in adjacent rooms. The present invention also reduces receiver desensitization problems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features and advantages of the present invention will be apparent from the Detailed Description of the Preferred Embodiment and the Drawing, in which: 
     FIG. 1 is a block diagram of an infrared communication system including the base unit and remote unit; 
     FIG. 2 is a block diagram of the remote unit; 
     FIGS. 3 and 4 are expanded block diagrams of the remote unit; 
     FIGS. 5 and 6 are detailed schematic diagrams of the remote transmitter; 
     FIGS. 7 and 8 are detailed schematic diagrams of the remote receiver; 
     FIG. 9 is a block diagram of the base unit; 
     FIGS. 10 and 11 are expanded block diagrams of the base unit; 
     FIG. 12 is a detailed schematic diagram of the base transmitter; 
     FIGS. 13 and 14 are detailed schematic diagrams of the base receiver; 
     FIGS. 15 and 16 are detailed schematic diagrams of the base telephone interface; and 
     FIGS. 17 and 18 are detailed schematic diagrams of the power and control systems. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides in its preferred embodiment a wireless remote telephone which communicates via infrared link to a stationary relaying device. Referring to FIGS. 1,  2  and  9 , the system includes a base station  10  and a remote wireless set  12 . The base station  10  has a regulated power supply  14 , an infrared (IR) transmitter  16  with IR LEDs  20 , a telephone headset amplifier  22 , a host telephone  24  and an IR receiver  26  with PIN diodes  28 . The wireless set has an IR receiver  32  with PIN diodes  34 , a headset  36 , an IR transmitter  38  with IR LEDs  40 , and a battery  42 . 
     The base station  10  and wireless set  12  provide full duplex voice exchange and control signal transmission over an IR link for controlling access to PBX or local exchange telephone lines via a host phone. Control signals switch the headset (Base ON) or handset (Base OFF). Such signals may be used for controlling access to local phone lines with the addition of a hybrid to the base unit and a dialer to the remote unit. The base station transmitter receives audio signals  44  from the telephone lines for transmission through either IR diodes  20  or an optional coaxial cable  45 . An optional coaxial cable  45  is connected to a remote IR transmitter for increased coverage in the same or another room. Signals transmitted by the IR diodes  20  are received by the set of PIN diodes  34  within the receiving environment, typically a single room or a single room and a limited area outside such room as for example through an open doorway, via direct and wall, ceiling and floor illumination. The signals from the host phone  24  are provided from a handset jack thereof and are processed in the headset amplifier  22  for use by the transmitter  16 . Similarly, the headset amplifier  22  receives incoming audio signals from the base IR receiver  26  via PIN diodes  28  or optional coaxial cable  46  from a remote receiver. The noise squelch selects the remote receiver based upon the best signal to noise ratio. The telephone headset amplifier  22  may also be provided with a switching circuit for selecting appropriate microphone characteristics expected by the host telephone  24  so that incoming and outgoing signals are appropriately conditioned for the particular type of host telephone  24 . The telephone headset amplifier  22  may also include a switching circuit to permit switching between the remote wireless and the standard handset normally associated with the host telephone. 
     The headset  36  includes ear pieces  36 ′ and a microphone  36 ″ for reproducing signals from the receiver  32  and applying speech to the transmitter  38 , respectively. Outgoing signals are processed in the remote wireless set  12  by transmitter  38  for application to emitting IR diodes  40 . 
     The remote wireless set  12  IR receiver  32  processes signals from the base station with IR sensitive devices such as PIN diodes  34 . A rechargeable battery  42  powers both the receiver  32  and transmitter  38 , including diodes  40 , and can be recharged when the remote wireless set  12  is placed in a charge cavity  43  which is associated with and powered from the base station  10 . 
     The system provides improved audio quality with several features. First, audio quality is improved by using frequency modulated sinusoidal current driven signal transmission between the base station and the remote wireless set. Second, audio quality is improved by transmitting at frequencies which avoid excessive harmonic overlap. The base station transmits at 250-430 Khz and the remote wireless set transmits at 1.45-1.63 Mhz. Audio quality is also improved by applying Dynamic Noise Reduction to the output of the base receiver, which provides the transmit (mic) signal for the host phone. This filters out noise caused by weak IR signals to the base receiver. Finally, audio quality is improved by sending transmissions from the base station with a carrier having a different wavelength than that with which transmissions are sent from the remote wireless set, thus reducing cross-talk. LEDs which emit signals at a wavelength of 940 nm are used by the base station while the remote wireless set uses LEDs which emit signals at a wavelength of 880 nm. Also, the base station LEDs  20  are relatively slow (1 microsecond ON/OFF time) and are continuously driven. 
     FIGS. 3 and 4 are block diagrams of the remote wireless set  12 , and FIGS. 5,  7  and  8  illustrate detailed circuit components corresponding to those labeled in FIGS. 3 and 4. 
     The following description will be referenced primarily to the elements of the block diagrams of FIGS. 3,  4 ,  5 ,  6 ,  7  and  8  where particular attention is directed to circuit-level components. 
     The remote wireless set includes circuitry for transmitting and receiving both audio and control signals via frequency modulated IR pulses. Audio signals from the headset microphone  36 ″ (FIG. 1) are applied at an input  50  (FIG. 3) to a 6 Db per octave preemphasis  54 . The output of the preemphasis  54  is applied to an amplifier  56 . The output of the amplifier  56  is applied to a high pass filter  60  with a low-frequency cutoff point of 300 Hz for tone control purposes. The low-frequency cutoff prevents low frequency components in the voice signal from passing further into the circuitry and interfering with tone control signals, which are added for tone control purposes as described below. The tone control signals are applied between a mute function  52  and a subsequent amplifier  64 . A mute switch  53  is provided to silently short circuit the signal path to ground when desired. The amplifier  64  includes an automatic gain detect and control circuit (AGC)  80  which is of particular value for FM signal modulation. The output of the amplifier  64  is applied to a voltage controlled oscillator (VCO)  68  which comprises an asymmetrical oscillator which is frequency modulated by the signal from the amplifier  64 . A channel select system  70 , illustrated for two channels, but operable for as many channels as desired, is illustrated in particular detail in FIGS. 5 and 6. The VCO  68  includes a deviation adjustment potentiometer  67  which is connected to the output from the amplifier  64 . The VCO  68  provides a narrowband FM output which is applied to a power amplifier/switch  72 , thereby driving the transmission LEDs  40  with a frequency modulated current signal. The LEDs  40  operate typically in the 880 nm optical output range and are driven in a 1.45-1.63 Mhz frequency band. 
     Referring to FIGS. 3 and 7, the remote wireless set receives incoming signals  106  from the base station with an IR sensitive device such as a tuned PIN diode detector  104 . The PIN diodes  34  are receptive to the 940 nm IR wavelength corresponding to the base station transmission LEDs  20  (FIG.  1 ), and are arranged in a tuned parallel circuit. A channel selector  108  operates to select a predetermined receive signal in the 250-430 Khz frequency band in which the frequency modulated sinusoidal current driven signal from the base station is transmitted. The PIN diodes  34  are physically arranged for the broadest possible view. More particularly, two PIN diodes are tilted at 30-45 degrees for better left and right side views. 
     The signal received by the PIN diodes  34  is processed to provide an audio signal. The PIN diodes drive a high speed JFET  194 . The output from the JFET  194  is applied to a limiting preamplifier  110  which includes a limiting circuit. A parallel tuned circuit is used at the output of preamplifier  196  to filter out harmonics generated when strong signals cause the preamplifier to go into limiting. 
     Referring to FIGS. 3 and 8, the output from the preamplifier  196  is applied to an integrated circuit  199  which includes FM detection amplification and filtering circuitry to be described as the remainder of the receiver section of the receive station. In particular, the output of the preamplifier is applied to a mixer  112  within IC  199 , which is typically a Motorola MC3367DW. A signal from a low-frequency channel selected local oscillator  114  is also provided to the mixer in IC  199 . The output of the mixer  112  is provided to a 455 Khz intermediate frequency ceramic bandpass filter  116 , the output of which is in turn reinjected into the IC at an amplifier stage  118 . 
     Referring to FIGS. 4 and 8, the output from the amplifier stage passes through a further 455 Khz ceramic filter  122 , and is reinjected into an internal amplifier  124  in the IC  199 . A quad detector  128  which includes a 3 Khz low pass filter is also provided within the IC. The output of the detector from the IC is applied through an audio amplifier  132  which provides deemphasis in a deemphasis circuit. A tone and loudness control  136  is provided for personal tailoring. An AGC feedback circuit  134  associated with the audio amplifier  132  provides automatic gain and loudness control and limits Sound Pressure Level to a safe level as is desirable in such an FM system, and for substantially adjustment-free operation. This is a slow attack AGC which reduces the power output to a comfortable level when there is a continuous tone or noise, which can occur if the user walks out of the room. 
     Referring again to FIGS. 3,  4 ,  5  and  6 , the power supply and tone-control circuits are shown to include unswitched +4.8 volt DC which is supplied to a DC power switch  86  with a 4 second OFF delay  85 . An ON-OFF power switch  102  is operative to turn system power ON and OFF. A +4.8 volt switched output of the DC power switch  86  is applied to a 4 volt DC regulator  90 . Unswitched, switched and regulated voltages are applied as indicated throughout the circuits illustrated in the referenced FIGS. Unswitched power is utilized for the LEDs  40 , which only conduct upon input from the power amplifier switch  72 . The regulated +4 volts is applied to a 0.3 second ON timer  92 . The 0.3 second ON timer generates a 0.3 second ON signal when power is applied which is supplied to a NOR gate  96  at power ON. The output of NOR gate  96  is applied to an OR gate  97  whose output is supplied to a 147 Hz oscillator  94 , which produces a tone. The oscillator  94  is thus triggered to produce a tone (here, an ON signal) at turn-ON for 0.3 seconds. 
     The output of oscillator  94  provides tone control signals  71  to the remote transmitter through a tone control gate  84 . The tone control gate  84  is activated by the output of the NOR gate  96  to inject a tone into the transmission path prior to amplifier  64 . An AGC inhibit circuit  82  is also activated by the NOR gate  96  and disables the AGC circuitry  80  to allow a strong control tone to be transmitted. The tone control signals in the remote unit switch the base unit transmitter ON for headset mode and OFF for handset mode. The signals could also be used for “operator flash.” 
     Referring to FIGS. 4,  6 ,  7  and  8 , the IC  199  includes a low-battery detector  100 . The low-battery detector has an output which indicates a low battery by switching on a 0.4 second low-battery timer  98  which provides a 0.4 second signal through OR gate  97  to activate the 147 Hz tone oscillator  94 . The resultant tone is applied to the audio amplifier  132  to provide an indication in the user&#39;s headphone of the battery condition. 
     A parallel resonant circuit including C 140  and L 106  may be added to the output circuit of RF amplifier Q 104  in the remote receiver to eliminate “birdies” caused by mixing of F 1  and F 2  harmonics with harmonics of the receiver local oscillator. The circuit is tuned approximately midway between channel frequencies F 1  and F 2  and has a Q of approximately 4-5. The circuit significantly reduces harmonics of F 1  and F 2  generated when strong signals cause the RF amplifier to go into limiting. 
     FIGS. 9,  10 ,  11 ,  12 ,  15 ,  16 ,  17  and  18  illustrate the base station. The base station functions to relay signals between the remote wireless set and a local telephone system. A connecting jack  152  provides connection between the host telephone and the base station. The base station accesses the local telephone system through the host telephone. A set of relay contacts  150  control connection of the host phone to either the host phone handset or to the base station (headset mode). As such, the user may use either the host telephone or the remote wireless set for telephone communication. Switch  186  provides equalization for carbon and electronic microphones and appropriate loading therefor. 
     In order to relay signals, the base station is operative to convert audio signals from the local telephone system into frequency modulated current driven signals which are transmitted to the remote wireless set. Audio signals from the local telephone system enter the base station through connecting jack  152  and relay contacts  150 , and are applied to a transformer  156 . The output of the transformer is applied to an amplifier  158  through an amplitude clipping circuit  157 . The amplitude clipping circuit  157  functions to limit abnormally high level signals to prevent overloading and disabling of the AGC circuit. An AGC circuit  160  provides AGC control feedback around the amplifier  158 . The output of amplifier  158  is applied through a preemphasis network  162  including an amplifier with preemphasis in the negative feedback for compatibility with FM transmission. The output of the preemphasis circuit  162  is applied to an amplifier  164 . The output of the amplifier  164  is applied to a voltage controlled oscillator (VCO)  168 , the frequency range of which is determined by a channel selector circuit  172  with switch selectable resistors which provide current control of the 12 volt power supply to the VCO  168 . R 274  and C 248  in amplifier  158  provide high frequency boost for better receiver audio high frequency response. Additionally, for the first three of the seven channels shown, a shunt circuit  165  is provided to reduce gain for the other channels and thereby accommodate lower deviation sensitivity on the first three channels in the modulated output. The VCO  168  has a temperature compensating capacitor  169  which is applied to PIN  7  of an exemplary LM 566 CN component used as the VCO. The output of the VCO is applied to a buffer amplifier  174  and subsequently to an active low pass filter  178  with a 650 Khz low-frequency cutoff point. An optional satellite transmission source input  176  can be supplied at this point if desired. The output of the filter  178  is applied to a power amplifier  180  which includes a further 650 Khz low pass filter. The power amplifier  180  drives the transmission LEDs  20 . A power shunt can optionally be included for increasing the range of the emitted IR signals. 
     As shown in FIGS. 10,  11 ,  13  and  14 , incoming frequency modulated signals received by the base unit are converted starting at  192  to audio signals and relayed to the local telephone system. The incoming frequency modulated signals are received on tuned PIN diodes  28  which are connected to channel selecting tuneable inductors  194  which control the center resonant receive frequency. A high pass filter capacitor  195  connects the PIN diodes  28  to a high impedance buffering amplifier FET  197  which is capacitively coupled to a preamplifier  196  having limiting diodes  196 ′ to control amplitude and prevent mixer overload. The limited output of the preamplifier  196  is applied to an MC 3367 IC 113, within which the remaining FM detection circuitry is provided. In particular, the output of the preamplifier  196  is applied to the input of a mixer  198 . Frequency tuning inductors in a channel selector system  200  are connected into the IC  113  to select the appropriate LO frequency for the mixer. The output of the mixer  198  is applied through a ceramic filter  202  at the mixer output of the IC and reinputted to the IC at an amplifier  204 . The amplifier output is applied through a further ceramic filter  205 , with a 455 Khz center frequency. The output of the ceramic filter is amplified in an amplifier  208  and supplied to a quad detector  210 . Within the IC, the quad detector  210  provides FM detection. The output of the quad detector is reinserted through a low pass filter  212 . The output of the low pass filter  212  is applied to a TX amplifier  218  which also has de-emphasis. The output of the amplifier  218  is applied to the Dynamic Noise Reduction (“DNR”)  219  which drives transmitter line driver  221  which feeds a local telephone system through a transmission line level adjust  188 , the mode select switch  186  and relay contacts  150 . 
     Referring to FIGS. 10 and 11, the output of the 3 Khz low pass filter  212  is additionally utilized for a control tone. The output of the 3 Khz low pass filter  212  is applied through a preamplifier  244 , a 147 Hz bandpass filter  246  and a 147 Hz tone decoder  248  for tone discrimination. The output of the tone decoder  248  is returned to a NAND gate  214  which shuts off input to the amplifier  218  upon the presence of a tone and a zero voltage level from the decoder  248 . 
     A noise squelch detector  240  is provided for improved audio quality. Squelching disables audio signal transmission to the telephone line when a very noisy signal or absence of a signal is sensed. The output of the quad detector  210  is applied to a 10 Khz bandpass filter and amplifier  236 . The output of the bandpass filter and amplifier  236  is applied through a potentiometer  238  to the noise squelch detector  240 . The output of the noise squelch detector  240  is applied to a comparator  242  with a threshold set by a reference. 
     The output of the comparator is then applied to the input of audio amplifier  218  to provide squelching. 
     A Dynamic Noise Reduction (“DNR”) IC (U  202  on the Base Telephone/Interface pc board, LM1894 by NSC) is inserted between the base IR receiver and the transmitter line driver to reduce low level noise. DNR is a non-complementary noise reduction system which can provide over 13 Db of noise reduction as configured. DNR operation is dependent upon two principles: that the audible noise is proportional to system bandwidth (decreasing the bandwidth decreases the system noise) and that the desired signal is capable of “masking” the noise when the signal to noise ratio is high. DNR automatically and continuously changes the system bandwidth in response to the amplitude and frequency content of the audio to nearly eliminate the audible noise. Companders and expanders may be added to both links if higher signal to noise ratios are desired. 
     Referring to FIG. 14, a parallel resonant circuit  255  tuned to 455 Khz including C 58  and L 18  may be added to the mixer output of the base unit receiver. The purpose of this circuit is to eliminate spurious responses of the 455 Khz ceramic filters. 
     Referring again to FIGS. 10 and 11, the base station is responsive to tone control signals which are transmitted from the remote wireless set. More particularly, the base station can be turned ON and OFF by control signals from the remote wireless set. Tone control signals which modulate the IR transmitter from the remote wireless set are received by the PIN diodes at  192  and after detection and filtering propagate to the pre-amplifier  244 , then to bandpass filter  246  and then to tone decoder  248 . The output of the tone decoder  248  is applied to a NAND gate  250  along with the output of the comparator  242 . The output of the NAND gate is applied to a long tone decoder  254 , whose output is applied through an OR gate  256  to a DC switch  262 . Regulated +12 volts system power interruption detected by a power ON-OFF sensor  260  is also applied to the OR gate  256 . When the remote is inserted in the base charge cavity, charge current is sensed by charge current sense  257  and this is also applied to NAND gate  250 . 
     With respect to the specific circuitry shown, the SCR within the long tone decoder  254  is unlatched in response to a long tone signal power interruption or remote charge current. The SCR switches off the regulated +12 volts to the VCO  168  and relay  154  through a power control transistor. The base station thus effectively turns OFF in response to the long control tone from the remote wireless set power interruption or remote unit charging. 
     Referring to FIG. 17, the application of +12 volts to the base station power and control circuit and decoding of a short “ON tone” burst activates relay contacts which enable communication access through the base station and the remote wireless set rather than the normal telephone handset. A power supply provides power through a transformer  272  to a voltage doubler  274 , and from there through a regulator  276  to back bias the base station PIN diodes. Additionally, a regulator  278  provides +12 V regulated. This is further applied through a regulator  280  to generate +4 V regulated. The output of regulator  278  is also used to power a charger circuit  281  through a current detector  259  which generates a signal to the OR gate  256  (FIG. 10) and switches the base transmitter off automatically when the remote unit is inserted in the charge cavity. 
     Referring to FIGS. 10,  11 ,  17  and  18 , the base station turns ON in response to a short control tone from the remote wireless set. The presence of a short control tone from NAND gate  250  is detected through the charge rate indicator  252 . 
     Transistor  253  and transistor  251  are switched ON and latched ON in response, thereby switching ON the +12 volts to the base station and turning the base station ON. Capacitor  260  differentiates the leading edge of +12 V when AC power is switched ON, and the leading edge is used to ensure that the base transmitter is switched to OFF when AC power is switched to ON or interrupted. 
     The base station also turns OFF in response to placement of the remote wireless set in the charging cavity. A signal yy indicating the flow of a charge current in the charge cavity  43  is generated when the remote wireless set is placed in the charging cavity  43 . The signal yy is applied to the base of transistor  258 , which operates to cut the +12 volts to the base station. More particularly, transistor  258  unlatches the transistor equivalent ( 251 ,  253 ) of an SCR by grounding the base of transistor  253 . Hence, the base station is turned OFF automatically by placing the remote wireless set in the charging cavity. 
     Referring to FIGS. 5 and 6, the remote wireless set may also include an input  270  (XX), which is operative to switch the remote wireless set OFF when it is placed in the charging cavity. The input  270  is triggered upon placement of the remote wireless set in the charging cavity and is applied to the base of DC power switch  86  and thereby functions to control application of +4.8 volts unswitched to the remote wireless set through the switch  86 . Switching the remote wireless set OFF when it is placed in the charging cavity advantageously allows the remote wireless set to obtain a full battery charge and conserve power. 
     Referring generally to FIG. 1, it should be noted that the IR receiving PIN diodes are preferably isolated from the IR transmitting LEDs on both the remote and base units. While total isolation is not necessary, weak “birdies” and actual audio feedback from a demodulated signal can be prevented by forming a partition between, and using different windows for, the PIN diodes and LEDs. Further, the IR LEDs should have an angle of incidence to the window of at least 30 degrees so that most of the IR signal will pass through the window and not reflect off of the window surface. It has been found that an angle of incidence of approximately 40 degrees is optimum for achieving minimum loss through the window and a broad radiation pattern for the remote unit. 
     It should be understood that the invention is not limited to the particular embodiments shown and described herein, and that various changes and modifications may be made without departing from the spirit and scope of this novel concept as defined by the following claims.