Abstract:
Computers commonly connect to remote computers and networks using the telephone lines. A telephone line is composed of a positive line (Ring) and a negative line (Tip). Because there is no guarantee, in general, which line on a two-conductor telephone jack will be positive and which will be negative, diode bridges are often used to assure correct polarity of devices coupled to the telephone lines. Commonly computers connect to telephone lines using modems, which plug into the computer. The modems often contain two jacks, one to connect the modem to the telephone line, the other to provide an outlet for a telephone. Commonly both jacks are wired in parallel. Embodiments of the inventions provide for wiring the telephone jacks in opposite polarity, or switching polarity via jumpers. A method is provided to detect a telephone plug in the jack of incorrect polarity. A user may be instructed in advance or prompted to switch jacks or jumpers if necessary and the need for the diode bridge is eliminated.

Description:
FIELD OF THE INVENTION 
     The present invention relates to communication systems, methods and devices used to communicate data between computers and in particular embodiments to telephone line coupled modem systems, methods and devices. 
     BACKGROUND OF THE INVENTION 
     The need to communicate between distant computers has led to the use of telephone lines for data communication. The telephone lines are a natural choice for communications because of their ubiquitous nature and ability for dedicated instantaneous transmission between points. Modems are often used to communicate data between computers across a telephone line. A modem is a device that accepts digital data (for example, from a computer) and uses the data to modulate an analog signal for transmission across a telephone line. At the receiving end of the transmission another modem converts the analog information sent by the first computer and modem to digital data by demodulating the analog signal. The process of MODulating a signal on the sending end and DEModulating the signal on the receiving end is how the term “MODEM” was derived. 
     FIG. 1A illustrates a block diagram of a telephone modem coupled to a telephone line. In FIG. 1A the modem system  101  functionally comprises two blocks. The first block  103  is the actual telephone modem, which includes a telephone line interface circuit or Data Access Arrangement (DAA)  105 . The second block  111  is the plain old telephone system (POTS)  111  which both accepts information from and provides information to the DAA  105  portion of the telephone modem  103 . The telephone modem  103  commonly couples to the telephone system  111  via two telephone line terminals commonly denominated Tip or “T”  107  and Ring or “R”  109 . The data access arrangement  105  provides the interface between the telephone modem  103  and the analog telephone system  111 . The DAA  105  is typically an isolated Analog Front End (AFE), which the telephone modem uses to interface to the analog telephone system  111 . 
     From the early days of telephone modems and telephone line equipment in general isolation is required between the telephone modem system  103  and the telephone system  111 . The purpose of this requirement is to decouple any difference voltage potential between the telephone modem  103  and the telephone system  111 . Furthermore, the isolation protects the user of the telephone modem from such things as lightning strikes within the telephone system  111 , which could be destructive to the system and fatal to the user without adequate isolation. A transformer, such as  125  illustrated in FIG. 1B, were commonly used to address this isolation requirement. Typically, at least one driver, such as illustrated in FIG. 1B as  113 , drives the transformer. Additionally, each driver circuit typically includes a resistor such as  121 , which are used to set the impedance of the DAA seen by the phone line. The same transformer  125  may also be used for reception of signals. Signals are commonly coupled from the transformer  125  into a circuit known as a hybrid  119  and then further coupled into a receive amplifier  117 . Generally, the function of the hybrid circuit  119  is to couple signals received from the telephone system to the receive amplifier  117  often after cancelling as much as possible of any transmit signal injected to the telephone line transmit buffer  113 . 
     In the early days of modem development, the transformer  125  was used to carry DC “loop” current  129  from the telephone line as well as AC communication signals to and from the telephone line. Transformers with windings that carry DC current as well as AC signals are sometimes called “wet” transformers. The DC loop current  129  conducted by a wet transformer functions to inform the telephone system  111  that the modem is ready to communicate AC signals to and from a central office (CO) communications are impending. The process of causing a DC current in the telephone line is commonly referred to as going off-hook or seizing the telephone line. The magnitude of the DC current  129  used to inform the telephone system  111  that lines  109  and  107  are being seized is generally between 20 to 100 milliamps, depending on the distance of the modem system to the (CO). 
     Generally wet transformers used in modems had a limiting resistor  127 , to set the DC resistance of the modem seen from the telephone line within specified limits. A typical current limiting resistor (e.g.  127 ) has a value of, for example, about 150 ohms and is typically placed in series with a primary winding of a transformer  125 , which also commonly has a resistance of about 150 ohms. The addition of the transformer winding resistance and resistor resistance results in an additive DC resistance of approximately 300 ohms. The telephone line system  111  is, thus, presented with this 300 ohms resistance when a user goes off-hook. 
     An example arrangement for a wet transformer to provide off-hook current is shown in FIG.  2 . The arrangement includes a relay  201 , in series with the current limiting resistor  127 . The closing of relay contact  201  couples the serial combination of the primary transformer  125  and resistor  127  to the telephone system. An advantage of a wet transformer system is that its primary winding is not polarized. Therefore, while FIG. 2 shows one example in which the resistor  127  side of the transformer  125  is coupled to the tip-line, coupling the resistor  127  to the ring side of the transformer would work equally well. Many early low-speed modems were configured with wet transformers. 
     However, wet transformers tend to exhibit nonlinear operation when DC current flows through the primary winding which can be problematic for higher speed modems. Because a transformer is a mechanical device, it is subject to such variations as magnetization, temperature variations and varying permeability. In addition, as the amount of current passing through its primary winding changes, so does the permeability of the transformer&#39;s core. Modem systems generally function by detecting phase differences in incoming signals. As the speed of modem transmission increases above 2400 baud, modem systems became less tolerant of the distortion introduced by wet transformers, and wet transformers became less practical and more expensive to build than “dry” transformers for a specified linearity characteristic. FIG. 3A illustrates a dry transformer arrangement. 
     In FIG. 3, a direct current (DC) blocking capacitor  301  prevents DC from passing through the telephone line side primary winding of transformer  305 . Because no DC passes through the primary of transformer  305 , the linearity of the transformer  305  can be substantially improved over the wet transformer system, for the same physical size and cost. The dry transformer system does not inherently provide a path for the off-hook DC current however, and, so, another method is needed to provide the DC current for signaling the telephone system of an off-hook condition. 
     To draw off-hook current, a system referred to as an electronic inductor (EI)  303  was included with the dry transformer arrangement. An electronic inductor  303  has the ability to conduct off-hook DC current but appear as high AC impedance. It is beneficial for the electronic inductor  303  to appear as a high AC impedance so that the electronic inductor  303  does not contribute AC loading to either the transformer  305 , or to the telephone system  311 . The AC impedance value of the electronic inductor  303  is important because, in modern high-speed modems, an AC bandwidth from 10 hertz to 3.4 kilohertz is commonly desired and any impedance in parallel with the telephone line may affect the bandwidth of the signal and ultimately the performance of the system. Therefore, the electronic inductor must exhibit high impedance at frequencies from 10 hertz to 3.4 kilohertz and such as a coil, a passive inductor is not practical to use because its impedance is 2π times the frequency of a signal, times the inductance. In order to use a passive inductor to provide high impedance to a 10-hertz signal, the inductance would be unacceptably large, and the physical size of the inductor impractical. 
     FIG. 4 illustrates an electronic inductor circuit  303 . The DC current conducted by the electronic inductor is conducted primarily through the transistor  407  and resistor  405 . The voltage at the junction  411  of resistors  401  and  403  determines the level of current through the transistor  407 . The voltage at junction  411  is applied to the base of transistor  407 , thereby holding the emitter of transistor  407  at a voltage equal to the voltage at  411  minus the voltage drop across the base-emitter junction of the transistor  407 . Because the emitter-based voltage of transistor  407  is relatively constant, resistor  405  will determine the current through transistor  407  and consequently the current drawn by the electronic inductor  303  based on the bias voltage set at node  411 . In order to keep the AC impedance of the electronic inductor high, a large capacitor  409  is provided at the base of transistor  407 . To block AC signals, the series combination of capacitor  409  with the Thevenin equivalent of resistors  401  and  403  with capacitor  409  is the predominant factor determining the frequency response. Common exemplary values for resistor  407  and capacitor  409  are 50 KΩ and 10 uf, respectively. The serial RC combination of  401  and  409  provides high equivalent AC impedance for very low frequencies, such as 10 hertz. 
     Electronic inductor  303 , however, still exhibits problems relating to the polarities on the lines  307  and  309 . Transistor  407  is operational as an electronic inductor in FIG. 4 only if line  307  is positive (Ring) and line  309  is negative (Tip). In common telephone wiring, there is no guarantee which line is positive. Accordingly, a common approach is to use a diode bridge  501  to guarantee correct polarity, as shown in FIG.  5 . Typical telephones and other telephone equipment in general also commonly use diode bridges to ensure correct polarity. By using a diode bridge  501 , the ring signal, which is by definition positive, is directed into the collector of transistor  407  of the electronic inductor  303 , or the positive terminal of EI  303 . 
     Although some modern applications do not use transformers, most DAA circuits commonly include polarized circuit components, for instance transistors. Because the DAA circuits are polarized, diode bridges are commonly used to insure correct polarity. Diode bridges, however add to the overall circuit cost, and can also contribute to circuit noise. 
     SUMMARY OF THE DISCLOSURE 
     Accordingly, to overcome limitations in the prior art described above, and to provide other advantages that will become apparent upon reading the present specification, preferred embodiments of the present invention relate to telephone line coupled devices, such as modem DAAs, which dispense with the need for a diode bridge. 
     A preferred embodiment of the present system involves a system for providing correct polarity signals without the use of a diode bridge. 
     In particular embodiments, the present invention provides a method and apparatus for switching incorrect polarity signals presented to a DAA. 
     A popular method for providing a modem is through the installation of a modem card into a personal computer. Modem cards inserted into a personal computer typically have two telephone jacks on one card. When installing a modem card, the user commonly connects one of the telephone jacks to the telephone outlet in the wall (or wall jack). The other telephone jack often provides a connection for a telephone, which may have previously been coupled to the wall jack and was disconnected to allow the modem card to be coupled to the wall jack. Such dual connectors on a personal computer modem card are commonly labeled “phone” and “wall,” “phone” and “line” or some other similar nomenclature. Although labeled separately, the telephone jacks on the modem card are often internally coupled together interchangeably such that there is no electrical difference between jacks. Embodiments of the present invention, which may be utilized in personal computer modems, provide for two telephone jacks wired in opposite polarity or a single jack whose polarity may be switched. Embodiments of the present invention provide that the tip signal on one jack will be the ring signal on the other jack and visa versa. In embodiments having 2 oppositely wired jacks, by plugging the telephone line into the correct jack, a correct polarity may be instituted in a DAA circuit or any polarized circuitry, without the use of a diode bridge. In embodiments having a jack with a switchable polarity if it is incorrect the correct polarity may be assured without the use of a diode bridge. Embodiments of the present invention provide, in addition to oppositely wired input jacks or switchable polarity, a method for insuring that the jack polarity is correct. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, in which consistent numbers refer to like elements throughout. 
     FIG. 1A is a block diagram of a common telephone modem interconnected to the telephone system. 
     FIG. number  1 B is a combination circuit and block diagram illustrating an arrangement of circuitry as may be used to provide a DAA function. 
     FIG. number  2  is a circuit diagram illustrating a method for providing off-hook current to a telephone system, using a “wet” transformer. 
     FIG. number  3  is a combination circuit and block diagram illustrating the use of a dry transformer to connect to a telephone system. 
     FIG. number  4  is a circuit diagram illustrating an exemplary circuit, which may be used to provide an electronic inductor function within a DAA. 
     FIG. number  5  is a circuit and block diagram illustrating an electronic inductor coupled through a diode bridge, to provide proper polarity connection to a telephone system line. 
     FIG. 6A is a graphical illustration of a prior art modem card as may be inserted in a personal computer. 
     FIG. 6B is a graphic illustration of a modem card according to an embodiment of the invention, as may be inserted in a personal computer. 
     FIG. 6C is a graphic illustration of a modem card with one phone jack according to an embodiment of the invention. 
     FIG. 7 is a combination block and circuit diagram illustrating circuitry used to detect incorrect polarity according to an embodiment of the present invention. 
     FIG. 8A is a combination block and circuit diagram illustrating circuitry according to an embodiment of the present invention. 
     FIG. 8B is a circuit diagram of an altered portion of the circuitry presented in FIG. 8A according to an embodiment of the present invention. 
     FIG. 9 is a graphic illustration of circuitry illustrating problems encountered with the use of a diode bridge. 
     FIG. 10 is a circuit diagram of a typical DAA. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. Other embodiments may be utilized as structural changes may be made without departing from the scope and inventive concepts of the present disclosure. 
     Accordingly, embodiments of the present invention relate, generally, to telephone line coupled devices. However, for the purposes of simplifying this disclosure, preferred embodiments are described herein with relation to computer modems and, in illustrative embodiments, to circuit cards which provide modem capabilities to personal computers. The examples disclosed are intended to illustrate the inventive aspects of this disclosure, and not to limit the invention to the illustrative embodiments disclosed. 
     FIG. 6A is a graphical illustration of a common, prior art modem card. The modem card  611  is configured for insertion into a slot within a personal computer. The modem card  611  commonly contains two jacks designed to accept telephone type plugs. These two jacks are labeled  615  and  617  in FIG.  6 A. Typically, the two telephone jack receptacles,  615  and  617 , are provided with labels such as “wall” and “phone,” line and “phone” or other similar nomenclature. Commonly there is no electrical difference between the jacks and they are wired in parallel. Telephone jacks  615  and  617  generally comprise two electrical connections, although they may comprise more than two. Jack  615  comprises electrical connections  625  and  627  and jack  617  comprises electrical connection  629  and  631 . 
     A typical telephone plug  601  for engaging and electrically connecting to any one of the jacks  615  and  617 , is illustrated in FIG.  6 A. The telephone plug  601  comprises a set of wires  603  and  605  coupled to contacts  607  and  609  respectively. Commonly a key  600  is used to orient and retain a plug  601  within a jack (e.g.  615 ). When the telephone plug  601  is inserted into jack  615 , wire  603  is electrically coupled to contact  625  and thereby to line  307 . In like manner wire,  605  is electrically coupled to contact  627  and thereby line  309 . 
     If the same plug  601  is inserted into the second telephone jack  617 , wire  603 , is electrically coupled to contact number  629  and thereby coupled to wire  307 . Additionally, wire  605  is electrically coupled to contact  631  and then further coupled to conductor  309  within the card. In other words, the telephone plug  601  may be inserted into either one of the telephone jacks  615  or  617 , yet the electrical connection is the same. Even if circuit board layout constraints force the jacks to be wired oppositely, no distinction is made because current telephone line coupled equipment is designed to use any polarity. 
     FIG. 6B is a graphical illustration of a similar modem card, however, configured according to an embodiment of the invention. Modem card  613  also contains two telephone jacks  619  and  621 , each configured to accept a standard telephone plug  601 . When the telephone plug  601  is inserted into telephone jack  619  in FIG. 6B, wire  603 , which is coupled to contact  607  on the telephone jack  601 , is electrically coupled to contact  633 , which is further coupled to line  307 . Likewise, wire  605 , which is coupled to contact  609  on the telephone plug  601 , becomes electrically coupled to contact  635  and thereby further coupled into wire  309 . However, if the telephone plug  601  is inserted into telephone jack  621 , wire  603 , which is coupled to contact  607 , becomes further coupled to contact  637  and wire  309 . Wire  605 , which is coupled to contact  609 , becomes electrically coupled to contact  639  and thereby further coupled to wire  307 . 
     In other words, when telephone plug  601  is inserted into telephone jack  619 , wire  603  is coupled to conductor  307  and wire  605  is coupled to conductor  309 . When telephone plug is inserted into telephone jack  621 , however, wire  603  is coupled to conductor  309  and wire  605  is coupled to conductor  307 . By selecting either telephone jack  619  or  621 , the polarity of the telephone jack  601  may be reversed with respect to conductors  307  and  309  within the modem card. In this manner, one of the jacks  619  and  621  will have proper polarity relative to the polarity (tip or ring) of the wires  603  and  605 . 
     Jacks  619  and  621 , on modem card  613 , are functionally equivalent when coupled to a telephone. Telephones have diode bridges or other mechanisms within them to direct the negative and positive polarities of the telephone line correctly to the electronics of the telephone, in order to account for the possibility of reversed polarities. 
     By instructing a user of the modem card  613  to insert the telephone plug  601  into the proper telephone jack, either  619  or  621 , the need for the diode bridge within the modem connection could be eliminated. Instead of directing the polarity of the telephone line using a diode bridge, the user is directing the polarity of the telephone line by inserting the telephone plug into a jack wired for the corresponding polarity. The other jack may be used for the telephone. 
     One method to assure that the plug is inserted into the correct jack is to first allow the user to insert the plug into a random jack. Once the plug is inserted, polarity of the telephone line voltage may be determined and then the user may be informed, for example, to switch receptacles in the case that the polarity is incorrect. If the user selects the correct polarity, no correction is needed. If the user has a telephone plugged into the second jack and the incorrect polarity is selected, then the user must switch both the plug coupled to the telephone and the plug coupled to the telephone line. The telephone will operate equivalently when coupled to either jack. 
     Alternatively a single phone jack may be used. Polarity on a single phone jack may be switched through the use of inexpensive jumper wires on the modem card. The use of jumper wires to switch polarity is illustrated in the embodiment shown in FIG.  6 C. Jumper wires  649  and  651  are used to connect contacts  645  and  647  with lines  307  and  309 . In the illustration in FIG. 6C, contact  645  is coupled to line  309  and contact  647  is coupled to line  307 . By reversing the position of jumper wire contacts  653  and  655  and jumper wire contacts  657  and  659 , the jack  643  polarity may be reversed from the polarity illustrated in FIG.  6 C. Similarly by reversing the position of jumper wire contacts  657  and  659  the jack  643  polarity may be reversed from the polarity illustrated in FIG.  6 C. 
     The polarity of the line voltage may be determined using any suitable circuits. An example of a circuit for determining line voltage is illustrated in FIG.  7 . The telephone line  701  is coupled to a voltage divider comprising resistors  703  and  705 . In the present exemplary embodiment resistor  703  is 6.8 megaohms and resistor  705  is 330 kilo ohms. Those skilled in the art will realize that the resistor values may be adjusted depending on the application. The junction of the divider network  707  is coupled into an analog-to-digital (ADC) converter  711 . Voltage divider point  707  is referenced with respect to point  709 . If the telephone line  701  is coupled so that point  713  is positive (ring) with respect to point  709  (Tip), ADC  711  will register a positive voltage. If, however, point  713  is negative (Tip) with respect to point  709  (ring) then the ADC  711  will read a negative voltage or 0 volts, if the ADC range cannot be negative. If the ADC  711  reads negative voltage or 0, it will have detected an incorrect polarity. The user then must switch the telephone plug from the telephone line to the opposite jack. Alternatively, the user may simply try the modem after inserting the telephone plug. If the modem does not work, for example if no dial tone is detected, the user may be instructed to switch the jacks coupled between the wall and the modem card. 
     FIG. 8A is an illustration of an electronic polarity detection circuit, such as that in FIG. 7, contained within an Application Specific Integrated Circuit (ASIC)  809 . The ASIC  809 , along with other functions, integrates electronic inductor circuitry as depicted in FIG.  4  and the ADC  711 . Information may be communicated from the ADC  711  within the ASIC  809 , back to the user system. The user system, represented by the system side device  821 , may receive communications from the ASIC  809  through lines  813  and  811 . The system side device  821  may be informed if the voltage is incorrect or correct through measurements from the ADC  711 , which are provided to the system side device  821 . The device  821  may be provided with a suitable user display or other interface for advising the user of the measurement. For example, the device  821  may provide a signal to a personal computer so that an appropriate message may be displayed to the user. Alternatively, the device  821  may provide an indicator such as an audio feedback, or a LED (Light Emitting Diode) indicator to inform the user of the correctness or incorrectness of the phone line polarity. The ADC  711  can be as a comparator (one bit ADC) with a fixed voltage reference. 
     If the approach of attempting to use the modem to determine if the telephone line polarity is correct is taken and there may be no ADC  711  present, the circuit configuration of FIG. 8A may exhibit a problem with a reverse current in transistor  407 . Such problem arises when line  309  is positive with respect to line  307 . Transistor  407  may also conduct current in the reverse direction. That is transistor  407  may conduct current from  309  to  307 . This current, depending on a variety of factors, may provide problems such as excessive on-hook current, on-hood current is generally limited to 10 micro-amps. 
     One solution to the reverse conduction problem is illustrated in FIG.  8 B. In FIG. 8B, a diode is inserted in series with the electronic inductor output transistor  407 . By inserting diode  819  in series with the electronic inductor, the electronic inductor conducts current only in the correct direction, or does not conduct at all. The protection diode  819  will, thus, inhibit transistor  407  from operating in the reverse direction. Diode  819  could be an LED type visible to the user on the computer modem card, for example, which in itself would indicate a correct polarity if LED  819  turns on when the telephone line is connected (current flows through the LED) or an incorrect polarity if the LED  819  remains off (reverse biased) after the telephone line is connected. 
     Alternatively, diode (or LED)  819  could be located in series with the emitter of transistor  407  (or resistor  405 ). In this position, diode (or LED)  819  would not be required to have high reverse-voltage rating because the voltage between the base of transistor  407  and ground is limited the circuitry of ASIC  809 . 
     In addition to cost savings associated with eliminating the need for a diode bridge, embodiments of the invention can improve modem performance and help reduce modem radiated and conducted emission for Electro-Magnetic Interference (EMI) compliance. One reason for performance improvement is illustrated in FIG.  9 . 
     FIG. 9 is a part schematic and part block diagram of a system containing a diode bridge  901 . In an arrangement as shown in FIG. 9, a difficulty arises when capacitor  907  is added from node  909  (+ terminal) of bridge  901  to chassis ground  915 , and capacitor  907  is added from node  911  (− terminal) of bridge  901  to chassis ground  915 . Capacitors  907  and  905  may be required to facilitate EMI compliance or improve signal balance between chassis ground and Tip and Ring, respectively. Chassis ground  915  is also the chassis of the PC where the modem card is installed, and can be referred to simply as PC chassis. 
     Capacitors  917  and  919  are typical of most traditional modem designs, and are generally added from Tip  307  and Ring  309  to PC chassis  915 , to suppress EMI emission from the modem external jack. To be effective, however, such capacitors must be physically located as close as possible to the external jack where the telephone line is coupled. This layout configuration prevents EMI radiation from being coupled to the modem telephone cord. 
     Silicon-based DAA&#39;s, however, typically use an integrated line side device  903  on the isolated side of the DAA system and receive digital clocks and data from the system side device  921  through capacitors  923  and  925 , for example. Under these conditions, capacitors  905  and  907  are often essential to provide a short return path to digital noise injected from the system side to the line side device, if placed at strategic locations on the modem printed circuit board layout. EMI capacitors  917  and  919  could not serve this purpose because they are coupled to nodes  307  and  309  “before” the diode bridge, as opposed to nodes  909  and  911  “after the diode bridge,” and their physical location must be close to the external jack and cannot be arbitrary. 
     When adding capacitors  907  and  905  on the line side device, a hum noise problem may result on all telephone equipment connected in parallel with the modem system in a household, for example. In this context, hum noise is defined as noise heard on telephone equipment in a household, when a modem is permanently connected in parallel to the same telephone line. This effect generally occurs only if there is a noisy difference of potential or “electrical noise” between the telephone line and the chassis of the PC where the modem is installed. 
     To understand how this electrical noise translates into hum noise on telephone equipment connected in parallel with the modem, consider the simplified diagram of a typical DAA shown in FIG.  10 . The circuit shows only components directly involved in the generation of hum noise, specifically diodes  913  and  927  of diode bridge  901 , the only two diodes forward-biased in the bridge for a given polarity of the telephone line voltage; resistor  703 - 705  between+terminal  909  and the − terminal  911  of bridge  901 ; and capacitors  907  and  905  from chassis ground  915  to + terminal  909  and − terminal  911  of bridge  901 , respectively. EMI capacitors  917  and  919  do not take part in the generation of hum noise, but they are shown for explanation purposes. 
     Resistor  703 - 705  represents the equivalent series effect of resistors  703  and  705 , respectively. This resistance typically provides bias to the electronic inductor while the modem is off-hook (primary function), and may allow an ADC to read the telephone line voltage while the modem is on-hook to implement various modem features such as Line-In-Use, for example. 
     Capacitors  907  and  905  can be added to the modem circuit to reduce EMI emission and to balance any noise injected into the telephone line from PC chassis  915 . Generally, electrical noise can be present on PC chassis  915  with respect to the telephone line when earth ground, i.e., the power outlet ground, is noisy with respect to the telephone line, a condition often encountered in realistic installations. 
     In reference to FIG. 10, noise currents between earth ground  915  and Ring  307  or Tip  309  can flow in and out of EMI capacitors  917  and  919  at the same time, resulting in a simultaneous or “balanced” current flow into Tip and Ring (this assumes that capacitors  917  and  919  are approximately equal in value, or “balanced”). Since a telephone connected on the same telephone line as the modem detects only the voltage differential between Tip and Ring during operation, the presence of “simultaneous” noise currents injected into Tip and Ring from the modem does not cause a telephone user to hear any noise in the telephone handset. Therefore, the presence of capacitors  917  and  919  does not cause hum noise on a parallel telephone. 
     On the other hand, the presence of capacitors  907  and  905  can cause severe hum noise on telephone equipment coupled in parallel with the modem, whether these capacitors are balanced or not. The mutual interaction of resistor  703 - 705  with capacitors  907  and  905  is responsible for hum noise on the telephone line. This noise is a result of noise current imbalance from the PC chassis into Tip and Ring, respectively. For example, noise current  107  can flow from Ring  307  through diode  913  into capacitor  907  (to PC chassis  915 ), but cannot flow out of capacitor  907  back into Ring  307  because diode  913  would be reverse-biased. Similarly, noise current  103  can flow in only one direction from PC chassis  915  into capacitor  905 , through diode  927  to Tip  309 . 
     If resistor  703 - 705  were not present, capacitors  907  and  905  would get charged and noise currents  101  and  103  would stop flowing after the initial charge-up. However, the presence of resistor  703 - 705 , which is essential in certain modem architectures to reduce overall cost, creates a discharge path  105  for capacitors  907  and  905 , as shown in FIG.  10 . Since current path  105  allows capacitors  907  and  905  to discharge, noise currents  101  and  103  can continue to flow in and out of PC ground, respectively. The resulting effect is that current flows either through Ring  307  (current  101 ) or Tip  309  (current  103 ), but never through both Tip and Ring simultaneously because these currents have opposite polarities, as depicted in waveforms  107  and  109 . The unbalanced current flow results in differential noise between Tip and Ring, which can be detected as hum noise by a telephone coupled in parallel with the modem on the same telephone line. 
     This hum noise is a relatively loud disturbance on the telephone line, which is composed of 60 Hz hum and harmonics and represented by voltage waveform  111 . The very nature of this noise makes it virtually impossible to design a filter between modem and telephone line to suppress or even reduce the effect. For example, an LC tank circuit configured as a notch filter would require component values in the order of 1H inductance and 10 uF capacitance, which is prohibitive as far as cost and physical size are concerned. Furthermore, there is no guarantee that harmonics of 60 Hz would be suppressed sufficiently, even if such a filter were used. Lastly, since the quality factor (Q) of a hypothetical filter would be very low, the entire low-frequency audio band of the modem could be compromised, resulting in severe modem performance degradation. 
     Since the circuit responsible for this effect is virtually independent of the modem functionality, hum noise persists whether the PC is powered on or off, provided that the PC chassis is noisy with respect to the telephone line and the modem is on-hook. When the modem goes off-hook, the hum noise ceases because diodes  913  and  927  are forward-biased with a large loop current compared to noise currents, which makes the diodes virtually conductive in both directions with respect to small noise currents. In this case, noise current injection in and out of the telephone line is balanced, as for the case of capacitors  917  and  919 , and the hum noise effect disappears. The effect only occurs while the modem is on-hook because the current through diodes  913  and  927  is less than 10 uA, which lets noise currents in the range of micro-amperes flow forward but not backwards through the diodes. The on-hook current through the diodes cannot be increased to more than 10 uA to reduce hum noise, because of PTT regulations. 
     It is clear from the above description that removing diode bridge  901  from a typical silicon-based DAA architecture would is very beneficial to reduce EMI emission and eliminate negative side effects associated with a modem in a typical installation environment. 
     The foregoing descriptions of exemplary embodiments of the present disclosure are presented for the purpose of illustration and description. They are not intended to be exhaustive nor to limit the inventive concepts to the embodiments disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not within this detailed description, but rather by the claims appended hereto, which appear below.