Abstract:
An electronic circuit for connecting a standard analog telephone to the analog microphone and loudspeaker connections on a personal computer and for separating the talk signal from the combined talk/listen signal occurring at the telephone, effectively minimizing undesirable echo.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an interface for coupling a standard telephone to a computer for internet telephony. 
   Based on the increasing use of personal computers (PC&#39;s) and the internet, a new communications medium is in use. Known as Internet Telephony, Computer Telephony or VOIP (voice over internet protocol), it allows users to communicate with one another using standard personal computers connected to the internet through Internet Service Providers. The communication can include voice (audio), data and video. Voice communication over the internet can be achieved computer-to-computer, computer-to-phone, or phone-to-phone. 
   To facilitate this new form of communication, new services and software products such as Net2phone™, Netmeeting™, Dialpad™ and many others are now available. When a call is placed from and/or received by a personal computer, the voice signal normally travels via the computer&#39;s sound card or equivalent. 
   The most common user interface available for PC voice communication is a microphone and loudspeakers, which usually connect to the sound card or equivalent in a personal computer. A user talks into the microphone, and listens through the loudspeakers. 
   Because the microphone senses both the voice of the user and the sound coming from the speakers, and because the internet connection usually introduces some finite delay in transmission, a user may hear the echo of his own voice. To mitigate this annoyance, some users make use of a headset instead of loudspeakers and microphone. The headset connects to the PC sound card in place of the microphone and speakers. Use of a headset does not prevent the headset user from hearing his own echo, but it reduces the echo for the other user. If both users in a connection use headsets, echo is reduced for both. 
   While the use of headsets ran reduce echo, they may not be the communications devices of choice for many people. For those PC users who would like to use a standard telephone, including a cordless phone, there is no existing simple and inexpensive interface device allowing interconnection between a telephone and a personal computer sound card while satisfactorily eliminating echo. For those users who prefer to use a headset, the use of this invention with a cordless telephone allows a standard headset to be used as a cordless headset. 
   Two products have been recently introduced to the market for the purpose of connecting a standard telephone to a PC under the tradenames Internet Phonejack™ and Hi-Phone™. Both of these products connect to a standard analog telephone, but do not provide an analog output to the PC. Both products digitize the telephone signal before passing it to the PC. The Internet Phonejack™ is a plug-in card that is installed into the PC data bus. The data bus is strictly digital, and has no provision for the connection of an analog signal. The Hi-Phone™ is external to the PC, but communicates with it via a digital serial port connection. 
   Both of these products have analog-to digital and digital-to-analog signal processing functions between the telephone and PC. Because both of these products communicate with the PC in digital formats, both require the installation of additional software drivers in the PC. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a simple and inexpensive interface that allows users to make use of a standard telephone when placing or receiving telephone calls over the internet using a personal computer. 
   The invention achieves the above and other objects by the provision of an interface that includes means for separating the talk signal from the combined talk/listen signal occurring at the telephone, effectively minimizing undesirable echo. 
   In further accordance with the invention, the interface operates completely on analog signals. 
   In order to allow the normal use of the PC&#39;s microphone and speakers when the telephone is not being used, a switching function is provided. The switching function automatically disconnects the microphone and speakers whenever the telephone is off-hook, and restores the microphone and speakers to their normal connection whenever the telephone is on-hook. Lamps are also provided for indicating activity in the talk and listen sections of the telephone circuit. 
   While the concept of echo reduction and/or cancellation is not new, the instant invention is unique because it reduces the echo at its source. Other echo reduction/cancellation techniques address the echo after it leaves the source, necessitating far more complex and expensive strategies. Most rely of the synthesis of a “pseudo echo” which is then subtracted from the actual echo. 
   This invention permits a standard analog telephone to be connected directly to the analog microphone and loudspeaker jacks on a personal computer. It permits a user to plug any standard telephone into the interface, connect the interface to a personal computer by plugging connectors of the interface into the speaker and microphone inputs of and sound card, and use the telephone as he would the microphone and loudspeakers usually attached to his personal computer. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a schematic diagram of one preferred embodiment of the telephone-to-computer coupling circuit of an interface according to the invention. 
       FIG. 2  is a schematic diagram of a complete interface according to the invention containing the circuit of FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A standard telephone combines a microphone and earphone connected in series. Because the standard telephone connection uses only two wires, the electrical signal at the voice signal connection is a composite of both the microphone (talk) and earphone (listen) signals. If connected directly to the “mic” and “spk” jacks on a standard PC sound card, this mixing of signals leads to undesirable feedback and echo for the other user. This invention incorporates electronic circuitry to eliminate the feedback and reduce the echo by separating the microphone and earphone signals into two separate and distinct circuits, resulting in improved sound characteristics. 
     FIG. 1  is a schematic diagram of one preferred embodiment of the telephone-to-computer coupling circuit of an interface according to the invention. This circuit couples a standard telephone  1  to the sound card or other voice signal interface, of a computer  3 . 
   Standard telephone  1  has two connection lines, T 1  and T 2 . Line T 1  is connected to the positive side of a Direct Current (DC) power source  2 , which provides a voltage in the range of 6 to 12 volts, D.C. The negative side of power source  2  is connected to electrical ground. 
   A resistor R 2  is connected between line T 2  and ground. The signal on line T 2  contains both the “talk” and “listen” components of the telephone audio signal. Line T 2  is also connected to a capacitor C 1  and a resistor R 5 . The other side of capacitor C 1  is connected to the speaker output  4  of the sound card personal computer  3 . Speaker output  4  has left and right channels which are connected together, as shown. 
   Audio signals appearing at speaker output  4  are conducted through capacitor C 1  and are delivered to line T 2  of telephone  1 . Capacitor C 1  decouples any D.C. component of speaker output  4  and allows only the Alternating Current (A.C.) component of the signal to pass through. 
   Resistor R 5  is connected between line  12  and a terminal  5 , the non-inverting input, of a differential, or operational, amplifier (op-amp) U 1 B. A resistor R 7  is connected on one side to terminal  5  of op-amp U 1 B, and the other side of resistor R 7  is connected to ground. Resistor R 7  reduces the amplitude of the signal at terminal  5  of U 1 B. 
   Speaker output  4  is also connected to a capacitor C 2 . The other side of capacitor C 2  is connected to one side of a resistor R 4 , and the other side of resistor R 4  connected to a terminal  6 , the inverting input, of op-amp U 1 B. 
   Resistors R 10  and R 15  are connected to capacitor C 2  and resistor R 4  on one side and to ground on the other side, reducing the amplitude of the signal passing to the inverting input of op-amp U 1 B. Resistor R 15  and trimming resistor R 10  scale the amplitude of this signal. The resistance values of resistors R 10  and R 15  are chosen to assure that the amplitudes of the signals applied to the inverting and non-inverting op-amp inputs are properly matched, ie., have substantially equal magnitudes in both AC amplitude and DC offset. 
   A feedback resistor R 6  is connected between input terminal  6  and output terminal  7  of op-amp U 1 B in the usual way for configuring amplifiers. Resistors R 8  and R 14  carry the output signal from output terminal  7  of op-amp U 1 B to capacitor C 3 . Capacitor C 3  de-couples the D.C. component from the signal before it is passed to computer  3 . 
   The other side of capacitor C 3  is connected to microphone input  20  of the sound card of personal computer  3 . The left and right audio channels of the microphone signal are connected together, as shown. 
   A further description of the signals at S 1 , S 2  and S 3  will help to clarify the operation of the on-amp. Signal S 1  is the same signal appearing at telephone line T 2 . This signal is a composite of the “talk” and “listen” portions of the telephone signal. Signal S 2  is the A.C. component of the “listen” signal and is applied with the same amplitude to both inputs  5  and  6  of op-amp U 1 B. Therefore, signal S 2  does not contribute to the op-amp output signal. Op-amp U 1 B subtracts the “listen” signal S 2  from the “talk+listen” signal S 1 , so that signal S 3  is a pure “talk” signal. In this way, the “talk” and “listen” signals, although superimposed at the telephone, are separated from one another at personal computer  3 . 
   Connected in this way, the mouthpiece of telephone  1  is substituted for the PC microphone and the telephone ear-piece is substituted for the PC speaker. 
   The following table lists one exemplary and nonlimiting set of component values which have been employed in a practical embodiment of the invention which was found to operate in the desired manner. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Component Value 
               Units 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               R2 
               1K 
               ohms 
             
             
                 
               R4 
               100K 
               ohms 
             
             
                 
               R5 
               100K 
               ohms 
             
             
                 
               R6 
               100K 
               ohms 
             
             
                 
               R7 
               100K 
               ohms 
             
             
                 
               R8 
               20K 
               ohms 
             
             
                 
               R10 
               1K 
               ohms 
             
             
                 
               R14 
               100 
               ohms 
             
             
                 
               R15 
               220 
               ohms 
             
             
                 
               C1 
               0.1 
               mfd. 
             
             
                 
               C2 
               0.1 
               mfd. 
             
             
                 
               C3 
               1 
               mfd. 
             
             
                 
               U1B 
               LM358 
               op-amp 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 2  shows one example of a complete interface according to the invention with mic and speaker switching, a regulated power supply and “talk” and “listen” indicators. 
   In this circuit, a regulated power supply is provided in place of a battery. Relay  1  is provided for the purpose of switching the PC microphone input between the microphone and telephone circuits. 
   The interface includes six connectors J 1 -J 6 , for connecting the interface to a computer and a standard telephone, which may be a cordless phone. Connector J 1  is a speaker jack to which the plug of a computer speaker system will be connected. Connector J 2  is a microphone jack to which the plug of a computer microphone will be connected. Connector J 3  is a power supply jack, or plug, which will be connected to an electric power supply, such as a conventional household wall reccptacle transformer. Connector J 4  is a standard phone jack which will be connected the standard telephone receptacle in place of a connector which would normally connect the telephone to the local telephone system. Connector J 5  is a speaker plug that will be plugged into the speaker output of the computer&#39;sound card. Connector J 6  is a microphone plug that will be plugged into the microphone input of the computer sound card. 
   When the relay coil is not energized, the microphone is connected directly to the PC microphone input. When the relay coil is energized, the left and right channels of the PC microphone input are shorted together, and connected to the output of U 1 B. 
   Similarly, Relay  2  is provided for the purpose of switching the PC speaker output between the speaker and telephone circuits. When the relay coil is not energized, the speaker is connected directly to the PC speaker output When the relay coil is energized, the left and right channels of the PC speaker output are shorted together, and connected through capacitors C 1  and C 2  to telephone  1  and U 1 B, respectively. 
   When telephone  1  is off-hook, current flows through the telephone and a DC voltage is present across resistor R 2 . This voltage is passed along to a second op-amp U 1 A, which is another op-amp used as a voltage comparator. 
   A voltage divider consisting of resistors R 9  and R 12  creates a fixed reference voltage at the inverting input of U 1 A. Whenever the phone is off-hook, the DC voltage at the non-inverting input of U 1 A is greater than the reference voltage, and the output of U 1 A goes positive, causing a transistor Q 1  to allow current to flow, powering the coils of relays  1  and  2 . Thus, when the phone is off-hook, relays  1  and  2  switch off the microphone and speakers and connect the telephone to the PC. When the phone is placed on-hook, it is disconnected from the PC and the microphone and speakers operate normally. 
   The non-inverting input of a further op-amp U 2 A of a talk signal indicating unit is connected to receive talk signals at the output of U 1 B. The inverting input of U 2 A is grounded. U 2 A is an op-amp used as a voltage comparator. Whenever the talk signal is above zero volts, a light emitting diode (LED) D 3  is illuminated, to give an indication of talk activity to the user. 
   Similarly, the PC speaker output is passed to the non-inverting input of another op-amp U 2 B in a listen signal indicating unit, causing a LED D 4  to illuminate whenever the listen signal is above zero volts. 
   In an interface according to the invention, component values are chosen to provide compatibility between signal levels in the computer, microphone, speakers and telephone. Capacitors are used in the conventional way to isolate a signal&#39;s DC level from its AC component. 
   It will be apparent to those skilled in the art that transistor networks or other solid state components may be substituted for relays  1  and  2 . Similarly, alternative electronic circuits may be used to perform the functions of op-amps U 1 A, U 1 B, U 2 A and U 2 B. 
   Power may be supplied to the interface and the telephone from a sound card game-port, USB port, or other source on the PC. In addition, switching between the connection of the sound card to the interface and directly to a microphone and speaker system may be performed by a manual switch instead of relays. The circuitry of an interface according to the invention may be incorporated into a PC sound card, USB audio device or a telephone. 
   The instant invention does not need to synthesize any signal that is not already present in the telephone or PC. It does not require digital signal processing, delay lines, monitoring of echo, or dynamic adjustment techniques typical of prior art inventions. This invention does not require any hardware to be installed into the computer, and does not require any additional software to be installed into the PC. Because there is no digitization of signals in this invention, it can be implemented using simple and inexpensive electronics. 
   Although the instant invention does not require any special software to be installed into the PC, there are some specific situations where it may be desirable to use the invention in conjunction with complementary software. 
   One form of such complementary software could be configured to recognize the dialing (DTMF) tones generated by a standard telephone, and interpret those DTMF tones in such a way that the PC places a call to the number dialed. Examples of such software are marketed by GlimaDyne under the designation WINDTMF™ and by RA Solutions, Inc. under the designation WINTONE 2.0™. 
   Another complementary software application could generate voice messages to be played through the telephone for the purpose of commercial advertising, or for providing instructions to the user. One example of such software is available from Nullsoft under the designation Winamp™. 
   While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof the accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
   The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.