Abstract:
An analog subscriber unit adapter/interface which generates a low voltage ring-tone signal to an audio speaker is provided The adapter facilitates the use of an analog subscriber unit over an ISDN communications link and provides added functionality to ISDN devices. The SLIC simulator interface of the present invention is preferably provided as part of an ISDN modem, thereby giving added functionality to the modem by allowing analog phones to operate over an ISDN line. The interface operates by simulating the services and functions of an analog SLIC (Subscriber Line Interface Circuit) while converting and formatting the analog information to an ISDN compatible form, and vice versa. The SLIC simulator provides an audible ring-tone signal indicative of an incoming call by way of an audio speaker, thereby obviating the need for a high amplitude ring voltage signal to be sent to the subscriber unit. The need for a ring-trip circuit is therefore also obviated. The absence of a high amplitude ring voltage also permits circuit simplification and associated cost savings because the electrical isolation requirements recognized in the telecommunications industry are less strict for low-voltage circuits.

Description:
This application is a division of U.S. patent application Ser. No. 08/783,562 filed Jan. 15, 1997 abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to telecommunication systems and more particularly to an interface device for providing analog telephone functionality in connection with a digital telephone network. 
     In a traditional analog telephone system, each telephone or other communication device (“subscriber unit”) is typically interconnected by a pair of wires (“tip” and “ring” wires or, cooperatively, “subscriber lines,” “subscriber loop” or “phone lines”) through a series of equipment to a switch at a local telephone company office (“central office”). At the central office, the tip and ring lines are interconnected to a device known as a “subscriber line interface circuit” or “SLIC,” which provides required functionality to the subscriber unit. The switches at the central offices are interconnected to provide a network of switches thereby providing communications between, e.g., a local subscriber and a remote subscriber. 
     The SLIC is an essential part of the network&#39;s interface to individual analog subscriber units. The functions served by the SLIC include providing talking battery, ring voltage, ring trip, off-hook detection, and call progress signals such as ringback, busy, and dial tone. In many business office environments, the small-scale function of a central office is assumed by a PBX system, which, in turn, may include a number of SLICs to provide interconnected subscriber units with the required functionality. 
     Analog subscriber units generate and receive analog signals. While most modern telephone networks are digital and route digital signals from point to point, the subscriber units are still predominantly analog. This is possible because the analog signal generated by an analog subscriber unit is converted to digital form for transmission through the network, and is converted at the remote location back to analog form for transmission over the subscriber loop for reception by a remote analog subscriber unit. The analog signals generated and received by analog subscriber units generally take the form of voice frequency signals for end-to-end communications between local and remote subscriber units. Such signals generally represent human speech, or may be modulated signals which are treated by the phone network as if they were ordinary speech signals. 
     Other analog signals generated or received at the analog subscriber units are supervisory signals which are not intended for transmission to a remote terminal. Rather, they are designed to communicate with the network to enable functions such as call initiation, call progress indication, and call termination. These signals include those provided by, or through, a SLIC such as (i) “talk battery voltage” which provides power to the analog subscriber unit; (ii) “ring voltage” which is a relatively high voltage indicative of an incoming call, (iii) call progress tones such as dial tone, busy tone, and ringback tone. These various signals and tones will be described below. 
     The subscriber line interface circuit provides DC power, or “talk battery” power, along the phone lines to enable operation of circuitry in subscriber units connected to those lines. All telephone systems work on DC (direct current) power. Typically, the talking battery voltage on analog phone lines is between 5 (off-hook) and 48 volts (on-hook). Most telephone systems and PBXs are connected directly to an AC (alternating current) outlet on the wall and convert that AC power to the DC power required by the phone system. Telephone company central offices are also often driven by rechargeable lead acid batteries, which not only provide necessary power in the event of a power outage but also serve as a filter to smooth out fluctuations in the commercial power and remove the “noise” that power often carries. Talking battery must provide sufficient voltage to enable analog telephones to perform functions such as amplification and sound pickup as well as other modern phone functions such as DTMF keypads and speakerphone operation. The talking battery power supply should always be available to an analog telephone, in the event the phone is placed in an off-hook, or closed-circuit state. 
     A subscriber line interface circuit provides a ring voltage signal to the analog subscriber unit to cause the analog phone to ring in the event of an incoming telephone call. Analog phone systems recognize a ring voltage signal placed on the phone lines by the SLIC, and in turn generate an audible electronic or mechanical ring sound to alert the subscriber of an incoming call. In order to ensure that an analog phone will recognize the ring voltage signal, the ring voltage is required to be 70 to 90 volts (or 140 to 180 volts peak-to-peak AC) at a frequency of 17 to 20 Hz. Significantly, voltages of this magnitude may be harmful if accidentally applied to circuitry intended for operation at a lower voltage. Therefore, in the event of a malfunction, the SLIC is required to meet certain safety specifications and requirements relating to electrical isolation. 
     For example, European Standard EN 60950, entitled Particular Safety Requirements For Equipment To Be Connected To Telecommunication Networks, defines a hazardous voltage as a voltage exceeding 42.4 volts AC (peak) or 60 volts DC. Any components within a circuit that are connected to voltages above these limits must be electrically isolated (and in some cases, physically isolated) from the lower voltage components that are typically in connection with central office circuits. Components requiring isolation include the power supply and the tip and ring connections on the customer/subscriber side of the telephone network. 
     The power supply isolation requirement is typically satisfied by providing an additional secondary winding within the voltage transformer of the DC-to-DC converter which is used to generate the high amplitude ring voltage. The line isolation is typically satisfied by a line transformer. In general, however, these isolation requirements impose additional costs associated with the added componentry. 
     A SLIC also passes call progress tones such as dial tone, busy tone, and ringback tone to the subscriber unit For the convenience of the subscriber who is initiating the call, these tones are provided by the central office as an indication of call status. When the calling subscriber lifts the handset, or when the subscriber unit otherwise generates an “off hook” condition, the central office generates a dial tone and supplies it to the calling subscriber unit to indicate the availability of phone service. After the calling subscriber has dialed a phone number of the remote (answering) subscriber unit, the SLIC passes a ring back sound directed to the calling subscriber to indicate that the network is taking action to signal the remote subscriber, i.e., that the remote subscriber is being rung. Alteratively, if the network determines that the remote subscriber unit is engaged in another call (or is already off-hook), the network generates a busy tone directed to the calling subscriber unit. 
     The SLIC also acts to identify the status to, or interpret signals generated by, the analog subscriber unit. For example, the SLIC provides −48 volts on the ring line, and 0 volts on the tip line, to the subscriber unit. The analog subscriber unit provides an open circuit when in the on-hook condition. In a “loop start” circuit, the analog subscriber unit generates an off-hook condition by providing a termination, i.e., by closing, or “looping” the tip and ring to form a complete electrical circuit. This off-hook condition is detected by the SLIC (whereupon a dial tone is provided to the subscriber). Most residential circuits are configured as loop start circuits. Some countries, however, have other requirements. Germany, for example, requires a ground to be applied on an additional lead that acts as a control signal. 
     The SLIC must also be able to detect the off-hook condition during application of ring voltage. That is, when a call is incoming and the SLIC is providing the high amplitude ring voltage signal to the analog subscriber unit, the SLIC must be able to detect when the analog subscriber unit goes off-hook to answer the call. This is known as “ring trip.” The SLIC must immediately cease the ring voltage signal upon detection of the off-hook condition, and provide the analog subscriber unit with the voice channel signals originating from the distant end subscriber unit. 
     The SLIC must also pass Dual Tone Multi-Frequency (DTMF) signals generated by the analog subscriber unit to the network. This signaling format is a well known method of providing dialing information. Each number on a keypad array is represented by two separate tones, one tone identifying the column, and the other representing the row. Together, two tones uniquely identify a digit. These tones are passed along to the network by the SLIC. 
     A digital transmission system such as integrated services digital network (“ISDN”) provides digital information all the way to the subscriber unit. The central office uses special digital interface equipment on subscriber loops that provides ISDN services. The system also requires the presence of digital subscriber units to interpret this digital information. Although the central office equipment and the subscriber equipment is digital in nature, this digital information is typically transmitted along the same copper wires used by plain ordinary telephone service (POTS) from the central office to the customer premises. Basic ISDN service is known as 2B+D, for two Bearer (B) or subscriber channels, and one Data (D) channel for control and signaling data. The B channels are 64 K bits per second, and the D channel is typically 16 K bits per second. These data rates are significantly faster than rates of traditional analog modems that can provide data rates in the neighborhood of 28.8 K bits per second or 33.6 K bits per second. 
     A typical ISDN configuration is shown in FIG.  1 . The two-wire interface between the ISDN switch  10 , and the customer premises is known as a “U” interface  12 . An ISDN Network Terminator (“NT1”)  14  converts this two-wire interface to a four wire “S/T” interface  16 . The modulation format used between the central office and the NT1 is known as 2B1Q in North America, and 4B3T in Europe and Japan. The ISDN NT1  14  in turn demodulates the signal and passes the resulting digital information along the S/T bus  16  to a digital subscriber unit, also known as Terminal Equipment 1 (TE1) such as an ISDN phone  18 , or alternatively, to an ISDN modem  20  for providing a communications link between computer devices. 
     There are many such ISDN modems commercially available which allow computers to connect to other computers or to networks of computers, via ISDN communication lines. Analog devices are designated as TE2,  22 , and operate on what has been designated an R interface  24 . To operate on the S/T bus  16 , a TE2 analog device requires a terminal adapter (TA)  26 . Typically, a TA might utilize a SLIC integrated circuit to provide the necessary interface functionality. 
     In the United States, customers are typically provided with a U interface by the service provider, and customers must provide and maintain their own NT1 termination for interconnecting devices designed for S/T interfaces. Of course, for convenience, an ISDN modem designed for use in the United States may also include the circuitry necessary for connection directly to an ISDN U interface. One disadvantage of a device having a built-in NT1 interface is that it might prevent other devices from being connected to the S/T bus if the device does not provide external access to the S/T bus. 
     It is well understood that analog subscriber units of the type described above will not function in an ISDN environment without appropriate TA interface circuitry. Indeed, devices exist that provide an analog interface for ISDN communications links. There are TA&#39;s that provide RJ-11 telephone connection plugs and RS-232 interfaces. The present invention provides an improved interface with numerous advantages over the prior art devices. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved SLIC interface that generates an audible ring-tone signal by way of an audio transducer or speaker without generating a high amplitude ring voltage signal. The present invention also provides added functionality to ISDN devices. Specifically, it provides an analog subscriber unit adapter/interface to facilitate the use of an analog subscriber unit over an ISDN communications link. The interface of the present invention is preferably provided as part of an ISDN modem, thereby giving added functionality to the modem by allowing analog phones to operate over an ISDN line. The interface operates by simulating the services and functions of an analog SLIC (Subscriber Line Interface Circuit) while converting and formatting the analog information to an ISDN compatible form, and vice versa. The SLIC simulator provides an audible ring-tone indicative of an incoming call by way of an audio speaker, thereby obviating the need for a high amplitude ring voltage signal to be sent to the subscriber unit. The need for a ring-trip circuit is therefore also obviated. The absence of a high amplitude ring voltage also permits circuit simplification and associated cost savings because the electrical isolation requirements recognized in the telecommunications industry are less strict for low-voltage circuits. 
     An object of the present invention is to provide a low voltage ring-tone signal to be emitted from a speaker or audio transducer. 
     Another object of the present invention is to provide an analog interface to an ISDN communications device to allow the use of analog subscriber equipment over an ISDN communications link. 
     Another object of the present invention is to provide added functionality to an ISDN modem by, inter alia, facilitating the use of analog subscriber devices over ISDN lines. 
     Still another object of the present invention is to provide the above analog interface functionality through the economical use of portions of the ISDN modem. 
     Yet another object of the invention is to circumvent the need for compliance with industry electrical isolation specifications which govern the isolation of the ring voltage power supply and the remaining circuitry. 
     Another object of the present invention is to provide SLIC functions in an efficient and inexpensive manner. 
     A further object of the present invention is to obviate the need for a high amplitude ring voltage signal by utilizing the modem audio speaker to generate the audible ring-tone signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the present invention will be more readily appreciated upon reference to the following disclosure when considered in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts an ISDN communications network; 
     FIG. 2 shows a block diagram of a preferred embodiment of the SLIC simulator; 
     FIG. 3 shows a block diagram of the SLIC simulator within an ISDN modem; 
     FIG. 4 shows a block diagram of the audible ring-tone circuit; 
     FIG. 5 a  shows a schematic representation of a portion of a preferred embodiment of the SLIC simulator; and 
     FIG. 5 b  shows a schematic representation of a portion of a preferred embodiment of the SLIC simulator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A block diagram of a preferred embodiment of the SLIC simulator  30  is shown in FIG.  2 . The analog subscriber unit  32  connects to the SLIC simulator preferably via RJ-11 connector  34 . Feeding Bridge  36  provides talking battery voltage through the polarity protection circuit  38 . Off-Hook detection circuit  40  detects when the analog subscriber unit  32  generates an off-hook condition and provides an off-hook indicator signal on line  41  to the Digital Signal Processor (DSP) via the ASIC interface. The DSP, ASIC and Codec are shown as one functional block  54  for simplicity. An off-hook condition exists when subscriber unit  32  provides an electrical termination on the loop circuit  42 . Line isolation is optional, and when necessary or desired, is provided by Line Isolation circuit  44 . Op-amp Protection circuit  46  prevents the signals received from analog subscriber unit  32  from overdriving or damaging the operational amplifier within Audio/Data Receive circuit  48 . Receive circuit  48  is a two-wire simplex circuit that receives the analog signals generated by the analog subscriber unit  32  and provides the signal to the codec for A/D conversion. Audio/Data Transmit circuit  50  is a two-wire simplex circuit that receives analog signals from the codec and transmits them towards the analog subscriber unit  32 . The Balanced Hybrid circuit  52  interconnects the four-wire circuit comprising the two-wire Transmit and Receive circuits  48  and  50  to a two-wire duplex circuit  51 . The Balanced Hybrid circuit functions to prevent signals from the transmit circuit  50  from being fed back into the receive circuit  48 . 
     FIG. 3 shows the SLIC simulator  30  situated in an ISDN communications system with an ISDN modem. The ISDN network  60  provides a U interface  62 . The NT1  64  provides an S/T bus  66  to the ISDN Controller  65 . The ISDN Controller  65  communicates with the DSP microprocessor  68  via bus  67 . The ASIC interface  72  provides the ISDN Controller  65  with control information over bus  70 . The DSP microprocessor  68  communicates with the ASIC  72  over data bus  74  and and address bus  75 . The DSP  68  communicates with the Codec  76  over line  78 . The DSP therefore acts as a data interface circuit between the ISDN controller circuit  65  and the codec  76 . Line  78  is preferably a serial port connection, but may also be a parallel data bus, depending on the capabilities of the codec  76 . Additionally, as can be appreciated by someone skilled in the art, the databuses  67  and  74  may be a common data bus. 
     ASIC  72  provides a pulse width modulated (PWM) signal output on line  81 . This signal is generated by the DSP, and provided to the ASIC  72  via bus  74 . The PWM signal on line  81  is used to generate a simulated audible ring-tone signal. The PWM signal is filtered, or shaped, to generate a signal consisting of discrete tones. The signal is then amplified and provided to an analog speaker when the ISDN modem wishes to indicate to the user that an analog phone call is incoming. 
     FIG. 4 shows the DSP  68 , ASIC interface  72 , analog filter  90 , amplifier  92 , and audio speaker  94 . The PWM signal on line  81  is a 4 MHz pulse width modulated pulse train generated by the DSP  68  and passed through the interface ASIC  72 . The signal is shaped by the analog filter  90  which preferably includes a series of three cascaded RC filters. Specifically, the filters are a low pass, a high pass and another low pass filter having cutoff frequencies of 159 Hz, 159 Hz, and 1592 Hz, respectively. Alternatively, the filter circuit  90  may be implemented with active devices such as operational amplifiers. After filtering, the ring-tone signal on line  96  preferably contains essentially two frequencies, 350 Hz and 450 Hz. The tones are applied to analog power amplifier  92 . Amplifier  92  is preferably a simple operational amplifier such as an LM 386. The operational amplifier is preferably powered by a 5 volt power supply. The output of the amplifier  92  is a low voltage ring-tone signal (preferably no more than approximately 5 volts peak-to-peak) and drives an speaker  94  to produce the audible ring-tone signal. The ring-tone signal may be amplified to any appropriate analog voltage, but is preferably significantly lower than a typical ring voltage. Particularly, it is desirable that the ring-tone signal amplitude is lower than what the pertinent specifications define as a hazardous voltage, or low enough so as not to require additional isolation circuitry. Alternatively, a piezoelectric transducer may be used to provide the audible signal. The ring-tone signal may alternatively consist of unfiltered pulses, tones of alternating frequencies, or a signal which when emitted from an audio transducer, emulates a ringing sound. Still further, the transducer may alternatively provide a visual indication of an incoming call. A light emitting diode, for instance, may serve as a transducer to signal an incoming call. 
     The pulsing, or cadence, of the ring-tone signal to generate a periodic ring-tone signal is obtained by the DSP  68  periodically stopping the PWM signal on line  81 . Alternatively, the microprocessor may continue to generate the PWM signal on line  81  and provide the pulsing by way of a mute input on the power amplifier  92 . The mute signal would be applied periodically to result in a pulsed ring-tone signal. The microprocessor controls the mute input to power amplifier  92  by way of an input/output port, which is preferably routed through ASIC interface  72 . 
     Generating the audible ring-tone signal in this manner, rather than by providing a ring voltage directly to the analog subscriber unit, has numerous advantages. First, the need for a high amplitude ring voltage is eliminated. This means that significant circuit simplifications may be made because the isolation specifications no longer apply to the device. For example, many SLIC circuits provide a ring relay signal that controls a relay (which provides isolation) that applies a high amplitude ring voltage signal on the subscriber loop. In the present invention, no relay is required. Additionally, because there are no high voltages within the circuit the need for an isolated secondary winding within the voltage transformer, as discussed above, is not required. 
     Indeed, if the SLIC simulator is situated within a PC, the voltage transformer may be omitted entirely. A traditional SLIC circuit situated within a personal computer requires a DC-to-DC converter to generate the high amplitude DC voltage used to generate the high amplitude ring voltage because a PC power supply typically provides a maximum voltage of ±12 volts. The transformer within such a voltage converter would likely require additional isolation. The SLIC simulator of the present invention when situated within a PC does not need a DC-to-DC converter to generate a high voltage. 
     Furthermore, a ring-trip detector is no longer necessary in the SLIC simulator. Of particular importance in traditional SLIC circuits is the rapid removal of the ring voltage to prevent the ring voltage from generating a loud sound in the subscriber&#39;s handset. A typical ring-trip circuit must detect variations in loop impedance (typically from about 5 or 10 K ohms to about 200 ohms) in the presence of a high amplitude ring voltage. Therefore, in a traditional analog loop serviced by a standard SLIC, off-hook detection in the presence of a ring voltage required a more sophisticated off-hook detection circuit, or a separate ring-trip circuit. 
     In the circuit of the present invention, an off-hook condition generated by the subscriber unit in response to an audible ring-tone from the speaker can be detected by the simple off-hook detection circuit  104 . A sophisticated or specialize ring-trip detection circuit is no longer necessary because there is no ring voltage present on the subscriber loop to interfere with the off-hook detection circuit  104 . 
     The codec  76  operates to convert digital samples to analog signals (D/A conversion) and to convert analog signals to digital samples (AID conversion). In the preferred embodiment, microprocessor  68  provides the codec  76  with necessary binary data from the ISDN B channel(s), to generate the voice frequency analog signal which is to be sent to the analog subscriber unit  32 . Microprocessor  68  alternatively provides the data to the codec  76  via ASIC interface  72 . The data on the B channel provided to the codec is the data which originated at the remote terminal. The DSP also sends data to the codec to generate call progress tones in response to ISDN signaling information. 
     The preferred codec is a standard codec that utilizes pulse code modulation (PCM), also known as μ-law analog to digital (A/D) conversion in North America. (A slightly different format known as A-law is used in Europe and Japan). This type of A/D conversion is well known in the telecommunications art and is know to be based upon a logarithmic relationship between the PCM codewords and the analog voltages they represent. The logarithmic conversion function is approximated by linear segments of varying slope. The PCM codewords are 8 bits in length, where the most significant bit (MSB) represents the sign of the analog voltage. The next four bits represent the linear segment, and the last three bits represent the point on the specified linear segment. The standard sampling rate for a codec is known to be 8 KHz. 
     The codec  76  receives sequences of samples at an 8 KHz rate represented as PCM codewords from the microprocessor  68 , and converts the codewords back to a corresponding time-varying analog voltage signal for transmission through the SLIC simulator  30  to the analog subscriber unit  32 . The codec  76  may also be implemented using a standard A/D and D/A conversion which is based on a linear relationship between the codewords and the analog voltages, resulting in equally spaced quantization levels. Of course, the codecs of two communicating units should operate using the same conversion format. 
     The ASIC  72  communicates to a computer over bus  80 . Because the preferred embodiment is an internal ISDN modem card for use in a personal computer (PC), bus  80  is preferably a standard ISA bus. The bus  80  may also be an Enhanced ISA (EISA) bus, a PCI bus, an RS-232 serial bus (well suited for devices intended for external connection to a PC), or any other suitable data link. 
     The SLIC simulator, as shown in FIGS. 5 a  and  5   b , includes a Subscriber Unit Connection  100 , a Feeding Bridge  102 , an Off Hook Sensor  104 , and a Polarity Protection circuit  106 . Audio/Data Receive circuit  108  and Audio/Data Transmit circuit  110  are interconnected in a balanced hybrid configuration. The SLIC simulator also preferably includes a Line Isolation circuit  112 , and an Op-Amp Input Protection circuit  114 . Power is preferably provided by the personal computer in which the ISDN modem is installed. The SLIC simulator also relies on portions of the ISDN modem to provide certain aspects of the interface, as detailed below. 
     The Feeding Bridge circuit  102  provides the talking battery voltage to the tip and ring conductors  116  and  118 , respectively. Talking battery voltage is generated from the ±12 volts,  120  and  122 , respectively, from the power supply of the computer and is supplied to the tip and ring conductors  116 ,  118  through the Darlington pair transistors  124  and  126 . The transistors are biased on (in or near their linear region of operation) by the DC bias current through resistors  128 ,  130 ,  132 ,  134 ,  136 ,  138 ,  140  and  142 . When the subscriber unit is on-hook, the loop across tip  116  and ring  118  is not terminated, or has a high impedance, and little or no current flows through the transistors  124  and  126  to the loop circuit. The Feeding Bridge  102  reaches an equilibrium point where nearly the full 24 volts is applied across the tip and ring conductors  116  and  118  to the analog subscriber unit. 
     When the analog subscriber unit goes off-hook, current flows through the transistors  124  and  126  and through resistors  128 ,  144 ,  146 , and  142 . The increased voltage drops across  128  and  142  caused by the loop current change the biasing of transistors  124  and  126  by way of resistors  130 ,  132 ,  134 ,  136 ,  138 , and  140 , causing the transistors to conduct less, thereby increasing the voltage drops across their emitter-collector terminals. The increased emitter-collector voltage drops serve to limit the voltage (and thus the current) supplied to the subscriber unit. 
     The Feeding Bridge  102  thus reaches a new equilibrium point dependent upon the resistive value of the loop termination. The net result is that the Feeding Bridge  102  limits the current provided to the subscriber unit, while maintaining a sufficient taking battery voltage on the tip and ring conductors  116  and  118 . The Feeding Bridge  102  will also provide current limitation even in the event of a short circuit across the tip and ring conductors. This Feeding Bridge configuration  102  has been determined to provide sufficient power for termination resistive values within the expected range, and can maintain sufficient talking battery voltage at the subscriber unit, particularly when the wires  31  (FIG. 3) connecting the SLIC simulator to the analog subscriber unit is fairly short in length, thereby tending not to load the line significantly. 
     The Off-Hook Detector circuit  104  provides a sensing signal to the microprocessor  68  representative of the off-hook, or on-hook, condition of the analog subscriber unit. The sensing input  41  is preferably provided to the DSP microprocessor  68  through the interface ASIC  72 , or alternatively by direct interrupt connection to the DSP microprocessor  68 . The Off-Hook Detector circuit  104  includes the LED  150  and photosensitive Darlington transistor  152  on a single chip  154 , pull-up resistor  156 , and sense lead  41 . When the analog subscriber unit is on-hook, no current is flowing through the conductors  116  and  118 , and the Darlington transistor  152  is in the off state. The sense lead  41  is at a high voltage (+5 volts) due to the presence of pull-up resistor  156  connected to the +5 volt supply. When the analog subscriber unit generates a termination associated with an off-hook condition, current from the Feeding Bridge  102  begins to flow through the subscriber loop via tip and ring conductors  116  and  118 . The current flowing through LED  150  causes the Darlington transistor  152  to conduct, which in turn causes a low voltage, near 0 volts, to be applied on the sense lead  41 . 
     In the event the subscriber attempts to use a pulse dial subscriber unit, the off-hook detector may inadvertently generate an off-hook sensing signal. To prevent the functioning of the microprocessor from being diverted or interrupted, it can be programmed to ignore such inadvertent off-hook detection signals of a period corresponding to a pulse dial signal. 
     Audio/Data Transmit circuit  110  includes transmit op-amp  170  on U 23   172 , pins  5 ,  6 , and  7 , with differential inputs TXA− and TXA+ applied on lines  174  and  176 , respectively. The differential input signals TXA− and TXA+ are supplied from the codec  76  shown in FIG.  3 . The analog output of the Transmit circuit  110  is provided to the subscriber unit via the Balanced Hybrid circuit  52 . The transmitted analog signals include the voice frequency information signal such as human speech or traditional modem signals, but also include call progress tones generated by the DSP in response to ISDN signaling information. 
     Audio/Data Receive circuit  108  operates to receive analog signals generated by the subscriber unit  32 , and to pass the signals to the codec  76  for conversion to digital format for further transmission over the ISDN line. The Audio/Data Receive circuit  108  includes receive op-amp  180  on U 23   172 , pins  1 ,  2 , and  3 . The signal is received at inverting input of op-amp  180  through resistor  182 . The op-amp  180  provides the analog signal to the codec  76  for quantization via line  184 , and for subsequent transmission over the ISDN line. 
     Any signal transmitted from Transmit op-amp  170  will also be detected by the Receive op-amp  180  on the inverting input through resistor  182 , thereby creating an echo signal. Thus, a two-wire to four-wire Balanced Hybrid circuit is used to provide echo cancellation. The signal from Transmit op-amp  170  is also applied to the noninverting input of receive op-amp  180 . This is for the purpose of canceling the echo signal. For optimal cancellation, the resistor-capacitor network of  190 ,  192  and  194  is balanced to match (or to be proportional to) the line impedance of the subscriber loop. 
     Op-amp Protection circuit  114  functions to protect the operational amplifier from analog subscriber units that use pulse dialing techniques. Pulse dialing works by providing periodic short circuits between tip and ring conductors  116  and  118 . If such a short circuit condition were periodically applied to the conductors  116  and  118  by the subscriber unit, the Feeding Bridge circuit would limit the talking battery voltage to a relatively low level (approximately 6 volts) during these pulses. The resulting AC voltage that would be applied to the input of the receive op-amp would be approximately 16 volts, which exceeds the input range of the op-amp  180 . Note that the power rail inputs to op-amps  170  and  180  are ±12 volts. The two zener diodes  200  and  202  of Op-Amp Protection circuit  114  clamp the AC signal input to a maximum of approximately 6 volts. However, the zener diodes  200  and  202  still permit DTMF tones to pass through to the receive op-amp  180 . 
     Polarity Protection circuit  106  includes diodes  210 ,  212 , and  214 . These diodes function to prevent any reverse polarity voltages that may be inadvertently applied by the analog subscriber unit  32  from propagating through the SLIC simulator  30  circuitry. For example, if a user accidentally connects the RJ-11 connector  34  to a standard central office subscriber loop, the tip  116  will be at approximately zero volts and the ring will be at approximately −48 volts. In such a situation, diode  210  conducts to prevent current flow into the Feeding Bridge circuit  102 , thereby preventing damage to the Feeding Bridge  102 . If in the event the polarity is reversed on the loop tip and ring  116  and  118 , then the blocking Diodes  212  and  214  prevent current flow into the Feeding Bridge  102 . It should be noted that the subscriber line interface circuit of the present invention does not require lightning protection on the subscriber loop because the subscriber is located nearby in-house. 
     Line isolation is provided by DC blocking capacitors  220  and  222 . When desired, additional isolation is provided by transformer  224 , but generally the isolation is not required because of relaxed isolation requirements due to the absence of a high amplitude ring voltage. Capacitor  222  and jumpers  226  and  228  are only in place when transformer  224  is not included in the circuit. Conversely, jumper  230  is present only when transformer isolation is used. This is because sufficient DC blocking is provided by capacitor  220  when transformer  224  is used. 
     The DSP microprocessor  68  provides a DTMF tone recognition function. The DSP microprocessor analyzes the voltage samples (incoming codewords from codec  76 ) for the presence of DTMF tones. Thus, when a subscriber unit provides dialing information via DTMF tones, the DSP is able to convert the dialing information to the appropriate ISDN signaling for call initiation. The DSP microprocessor also provides the codec  76  with the codewords necessary to generate the appropriate supervisory signals such as dial tone, ringback tone, and busy tone. 
     Various ISDN modem components, including the DSP microprocessor  68 , ASIC interface  72 , and ISDN controller  65 , operate to provide appropriate B and D channel information to the ISDN network  60  and interpret incoming B and D channel information from the network  60  to thereby establish communications to the analog subscriber unit  32  in accordance with the pertinent ISDN industry specifications. As a result, the distant end subscriber unit need not have any special knowledge about the presence or capabilities of analog subscriber unit  32 . 
     The detailed description of the preferred embodiment is intended as an illustration, and not as a limitation, of the present invention. Thus, while variations and modifications of the invention will occur to those skilled in the art, it is to be understood that such modifications are within the scope of the invention as defined by the following claims: