Method and system for line isolation

A method and apparatus for transceiving data across an isolation boundary in a communication network between a user device on a user side of the isolation boundary and a line-side associated with a communication network such as a telephone network. Transmit data on the user side and receive data on the line-side are pulse modulated using known techniques such as pulse position and pulsewidth modulation methods. The resultant pulse modulated data streams then digitally traverse the isolation boundary via an isolation device such as an optocoupler. Upon completion of the traversal of the isolation boundary, the traversed pulse modulated data is integrated to form an approximation of the original transmit and receive data, respectively, without added distortion from a traversal of the isolation boundary by analog data.

BACKGROUND OF THE INVENTION 
1. The Field of the Invention 
This invention relates generally to line isolation of electronic circuitry. 
More particularly, this invention relates to isolation circuitry for use 
in interfacing user equipment with telephonic or other networks. 
2. Present State of the Art 
Traditional custom equipment that interfaces with custom networks or 
systems may employ a variety of interface standards. However, interface 
devices such as end user equipment that interfaces with established 
networks, such as the telephone network, are required to employ an 
established interfacing standard. One such standard requires end user 
equipment to provide electrical isolation between user equipment and the 
telephone network. Such isolation requirements stem from regulatory 
agencies such as the Federal Communications Commission and other 
counterpart regulatory agencies in other countries. A fundamental purpose 
for such regulations is to provide safety and protection to the 
established telephone network. Such protection prevents end user equipment 
from accidentally injecting or accepting into the protecting network and 
the user/consumer telephone network extraneous or hazardous signals. 
While specific regulations and requirements may vary from country to 
country, established circuits which facilitate the isolation requirements 
of most regulatory agencies have become commonplace. One such circuit 
component is a Direct Access Arrangement (DAA). As shown in FIG. 1, DAA 
112 provides the most proximate interface of user equipment with a 
telephone network 104. As an exemplary configuration of user equipment, 
FIG. 1 illustrates a host 100 which may take the form of a personal 
computer or other interfacing equipment. As many types of end user 
equipment generate digital data, a modem 102 is required to modulate and 
demodulate digitized data into an analog format capable of propagation 
through telephone network 104. An exemplary modem 102 is further comprised 
of a host interface 106 generally taking the form of an ASIC integrated 
circuit or other programmable or discrete circuitry for compatible 
exchange of data with host 100. Most modern digital signal processors 
(DSP) such as DSP 108 provide, among other things, the functionality 
required for modulating and demodulating digital data. A CODEC 110 
provides a transformation between digital data and analog data for data 
transmitted from host 100 and translation of analog data to digital data 
for data dispatched by telephone network 104 to host 100. The 
functionality and composition of host interface 106, DSP 108, and CODEC 
110 are generally known in the art and need not be further discussed. 
As data generated by host 100 and destined for telephone network 104 must 
exit the end user equipment and travel on the telephone network, the 
illustrative modem 102 must incorporate the isolation requirements as 
dictated by regulatory agencies. In the present illustration of FIG. 1, 
DAA 112 provides the isolation requirements for protecting telephone 
network 104 from extraneous and injurious signals. To prevent such 
extraneous and injurious signals from passing from host 100 to telephone 
network or to endanger the uswer 104, an isolation boundary 202 is 
established wherein the signals generated by user equipment 100 must be 
coupled across isolation boundary 202 to telephone network 104. 
In FIG. 2, a traditional DAA 200 is depicted as implementing the required 
isolation boundary 202. As described above, isolation boundary 202 forms a 
partition between a user side connected to a host, such as a personal 
computer, and a line-side connected to the network, such as a telephone 
network. Traditional DAA implementations have incorporated on the user 
side a user side signal interface 204 to provide access to transmit and 
receive signals coupled to a hybrid circuit 216 which, due to the 
combining of transmit and receive signals on the line-side, requires that 
the transmit signal be cancelled or removed from the receive signal. 
Isolation boundary 202 is then established across a transformer 208 which 
provides the required coupling between the user side and the line-side. 
Because of the inherent nature in transformer coupling, the user side and 
the line-side may float with respect to a potential across isolation 
boundary 202. Traditional implementations of the line-side circuitry in 
DAA 200 include bypass capacitors 210, a hold circuit 212, and a diode 
bridge 214 for generating tip and ring signals 206 characteristic of a 
telephone network. Hold circuit 212, among other things, incorporates an 
off hook switch for providing the DC current path necessary to notify a 
telephone network of an off-hook condition and a ring detector for 
notifying a user of an incoming communication. Other isolation boundary 
implementations include capacitively coupling the analog transmit and 
receive signals across the isolation barrier by providing a pair or pairs 
of capacitors that differentially couple analog transmit and receive 
signals from a user side across the isolation barrier to a line-side. 
While modem isolation implementations of the past have generally been 
transformer based, there have also been other methods such as linear 
optical solutions that use an active region of an optical isolator for 
transfer of analog signals across the isolation boundary. While 
transformer designs were feasible for physically larger designs, 
transformer designs incorporated into smaller integrated products 
compromise the linearity and also generate insertion loss and a degraded 
frequency response as transformers are reduced in physical size. Linear 
optical approaches are also feasible, but require gain adjustments and 
tuning for variations in gains associated with optical isolator production 
runs. Furthermore, linear optical isolators are more expensive than 
standard optocouplers. 
Thus, what is needed is a method and system for providing an isolation 
boundary between a user side and a line-side for equipments interfacing 
with a defined network, such as a telephone network, that require an 
isolation boundary. Furthermore, what is desired is a method and system 
for providing an isolation boundary utilizing isolation components that 
lend themselves to economical integrated designs without subjecting 
themselves to the degraded affects associated with miniaturized magnetic 
components. 
SUMMARY AND OBJECTS OF THE INVENTION 
It is an object of the present invention to provide a method for 
transceiving data across an isolation boundary in a communication network 
between a user device on a user side of an isolation boundary and a 
communication network on a line-side of the isolation boundary. 
Another object of the present invention is to provide a method for 
digitally crossing an isolation boundary in a communication network 
thereby minimizing distortion associated with traversal of the isolation 
boundary. 
A still further object of the present invention is to provide a method for 
optically coupling digital data across the isolation boundary to minimize 
coupling between a user side and line-side of the communication network. 
Yet another object of the present invention is to provide a data access 
arrangement (DAA) for digitally transmitting data across an isolation 
boundary separating a first side such as a user side and a second side 
such as a line-side. 
A still further object of the present invention is to provide an isolation 
circuit for pulse modulating data for digital transmission across an 
isolation boundary in a communication network. 
A yet further object of the present invention is to provide an isolation 
circuit for transceiving data between a user side and a line-side across 
an isolation boundary in a communication network by digitally conveying 
data via an optocoupler operating in the saturation region. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by the practice of the invention. The 
objects and advantages of the invention may be realized and obtained by 
means of the instruments and combinations particularly pointed out in the 
appended claims. To achieve the foregoing objects, and in accordance with 
the invention as embodied and broadly described herein, a method and 
system for transceiving data across an isolation boundary between a user 
device on a user side of the isolation boundary and a line-side associated 
with a communication network is provided. 
An isolation circuit provides an isolation boundary between a user device 
on a user side of the isolation boundary and a line-side associated with 
the communication network, such as a telephone network. The isolation 
circuit may be incorporated within a DAA or may comprise the entire DAA. 
Additionally, the DAA or isolation circuit may be further incorporated 
within a modem or other traditional information transformation device. A 
DAA may also be supported external to a modem such as when located 
intermediate between user equipment such as a computer and network 
equipment such as a telephone. 
An isolation boundary partitions the isolation circuit into a user portion 
and a line-side portion. A user side portion interfaces with user 
equipment and receives transmit data for coupling across the isolation 
boundary and for delivery to a communication network such as a telephone 
network. The user side portion of the isolation circuit also receives 
coupled or combined data and separates the receive data for delivery to 
user equipment. The transmit data is received at a transmit pulse 
modulator located prior to the isolation boundary and pulse modulates the 
transmit data. 
In the preferred embodiment of the present invention, pulse position 
modulation is employed by the pulse modulators. Pulse position modulation 
utilizes a constant pulsewidth and varies the frequency of the constant 
width pulses with the modulated analog signal. Other pulse modulation 
techniques may also be implemented in the present invention to form other 
alternate embodiments included within the scope of the present invention. 
In the preferred embodiment, the pulse modulator is implemented from a 
counter and other discrete components. 
The isolation circuit is further comprised of an optocoupler for isolating 
an input signal from an output signal thereby forming the isolation 
boundary. While the present invention may employ linear optocouplers, that 
is to say optocouplers capable of operation in the linear region, the 
present invention may also utilize optocouplers operating in the 
saturation region due to the digital nature of the transfer of information 
across the isolation boundary. Therefore, less expensive optocouplers 
operating in the saturation region rather than linear couplers employing a 
plurality of photodiodes wherein at least one photodiode monitors the 
linearity of the optical coupling may be employed. 
The output of the optocoupler provides an isolated pulse stream to a hold 
circuit comprised of a transmit integrator or line-side integrator to 
approximate the analog signal of the transmit data as reconstructed from 
the pulse modulated transmit data. The line-side integrator also couples 
to a hybrid circuit to separate the transmit and receive signals from 
their combined state into their component parts by removing the transmit 
signal from the combined signal to present a receive only signal to a 
receive pulse modulator. The isolation circuit is further comprised of a 
diode bridge for providing a reversible interconnection polarity for the 
tip and ring signals of the communication network. 
Now turning to the receive portion of the isolation circuit, a pulse 
modulator receives the analog receive data and performs pulse modulation 
techniques to form a pulse stream carrying the corresponding receive data 
information. The receive pulse modulator may be implemented similar to the 
transmit pulse modulator as discussed above. The pulse modulated receive 
data as modulated by the receive pulse modulator is received at the 
receive optocoupler for coupling across the isolation boundary. The 
receive optocoupler provides coupling between a light emitting diode an 
optically coupled photodiode, as disclosed above. 
The receive optocoupler couples to a receive or user side integrator to 
reconstruct an approximation of the receive data as originated from the 
computer network as line-side receive data. As described above for the 
line-side integrator, the user side integrator may be implemented, among 
other ways, as a low pass op amp filter or an RC network to form the 
receive data on the user side for presentation to the user equipment. 
These and other objects and features of the present invention will become 
more fully apparent from the following description and appended claims, or 
may be learned by the practice of the invention as set forth hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention embodies within its scope both methods and systems 
for electrically isolating user equipment from network equipment, such as 
a telephone network, as may be required by regulatory requirements. As 
described in FIG. 1, safety and other concerns dictate that user equipment 
interfacing with an established network, such as a telephone network, must 
provide electrical isolation between the electrical levels associated with 
user equipment and the electrical levels associated with network 
equipment. Such isolation forms an isolation boundary which requires that 
signals passing across the boundary be coupled in some form across the 
isolation boundary as opposed to passing directly through the isolation 
boundary. In the present invention, the isolation circuitry may be 
incorporated within a DAA as functionally configured and illustrated in 
FIG. 1. That is to say, in a configuration incorporating a host, such as a 
personal computer, with a network, such as a telephone network, modem 
functionality is employed for bridging the traditional digital domain of 
user equipment with the analog domain of traditional telephony. The 
present invention may be incorporated within the modem functionality or 
may be independently incorporated between the modem functionality and the 
network interface. In either embodiment, the isolation circuit of the 
present invention receives analog signals, traverses the isolation 
boundary and recreates an approximation of the analog signals as received. 
FIG. 3 depicts an isolation circuit for providing the isolation boundary 
between a user device on a user side of the isolation boundary and a 
line-side associated with the communication network, such as a telephone 
network, in accordance with the preferred embodiment of the present 
invention. As depicted in FIG. 3, the isolation circuit is incorporated 
within a DAA 300 wherein DAA 300 may be incorporated within a modem or 
other traditional information transformation device or DAA 300 may be 
external to modem functionality but intermediate to user equipment such as 
a computer and network equipment such as a telephone. 
Referring to FIG. 3, an isolation boundary 302 partitions the isolation 
circuit into a user portion and a line-side portion. A user side portion 
interfaces with s user equipment and receives transmit data 304 for 
coupling across isolation boundary 302 for delivery to a communication 
network such as a telephone network. The user side portion of the 
isolation circuit also receives coupled data and generates receive data 
330 for delivery to user equipment. In the present invention, transmit 
data 304 is received at a pulse modulator 306 for processing prior to 
coupling across isolation boundary 302. Pulse modulator 306 pulse 
modulates transmit data 304 to form pulse modulated transmit data. Pulse 
modulation is a signal processing technique wherein an analog wave form 
may be partitioned into a series of digital pulses or a pulse train 
representative of an approximation of the original analog signal. In an 
alternate embodiment, pulse modulation takes the form of pulsewidth 
modulation wherein a series of pulses of variable width and constant 
amplitude may be used to regenerate a synthesized wave form. Pulsewidth 
modulation employs a constant pulse period while generating a varying 
width of the pulse. That is to say, the width of the pulse or the energy 
under the pulse generally tracks the analog signal being modulated by 
increasing the pulsewidth as the analog signal increases in amplitude. 
The preferred embodiment of the present invention employs pulse position 
modulation techniques within pulse modulator 306. Pulse position 
modulation employs a constant pulsewidth, however, the frequency of the 
constant width pulses varies with the modulated analog signal. That is to 
say, analog signals having a higher amplitude are modulated using pulse 
position modulation techniques by incorporating an additional number of 
pulses resulting from a reduction of the spacing between successive pulses 
thus incorporating the analog information. Pulse position modulation 
techniques allow a common clock frequency to be employed while varying 
only the number of pulses transmitted. Other pulse modulation techniques 
may also be implemented in the present invention to form other alternate 
embodiments included within the scope of the present invention. Pulse 
modulator 306 may be implemented as a digital signal processor or other 
discreet components, such as an op amp and counter, or other 
implementations known by those of skill in the art. Furthermore, since 
pulse modulator 306 is incorporated on the user side, pulse modulator 306 
may be implemented using a portion of the user equipment signal processing 
capabilities, or signal processing capabilities resident within a modem. 
The isolation circuit of the present invention is further comprised of an 
optocoupler 308 for isolating an input signal from an output signal 
thereby implementing isolation boundary 302. Optocoupler 308 generally 
consists of a light emitting diode (LED) generally of the form of an IRLED 
optically coupled to a photodiode responsive to light emitted by the 
IRLED. The photodiode is generally proportionally responsive to a 
proportional amount of photo currents induced by the IRLED. While the 
present invention may employ linear optocouplers, that is to say 
optocouplers operating in the linear region, the present invention also 
allows optocouplers to operate in the saturation region because of the 
digital nature of the transfer of information across the isolation 
boundary. That is to say, in the present invention information is 
transferred across the isolation boundary in digital form (e.g., a series 
of pulses) rather than in linear format. Therefore, less expensive 
optocouplers operating in the saturation region rather than linear 
couplers employing a plurality of photodiodes wherein at least one 
photodiode monitors the linearity of the optical coupling may be employed. 
The output of optocoupler 308 provides to a hold circuit 310 the isolated 
pulse stream as originated by pulse modulator 306. Hold circuit 310 is 
comprised of a transmit integrator or line-side integrator 312, a hybrid 
circuit 314 and other functional elements not shown such as a ring 
detector and off-hook switch. Line-side integrator 312 may alternatively 
precede or be located outside the bounds of hold circuit 310, or as shown 
FIG. 3, be incorporated within the integrated circuitry of hold circuit 
310. Line-side integrator 312 responds to the pulse train generated by 
pulse modulator 306 and may be implemented as an op amp having a low pass 
filter configuration or as an RC network, although an RC network 
configuration generally is not as effective in filtering out the high 
frequency pulse resulting in noise aliasing down into the audio band of 
interest. Line-side integrator 312 functions to approximate the analog 
signal of transmit data 304 as reconstructed from the pulse modulated 
transmit data. 
Line-side integrator 312 couples to a hybrid circuit 314. While additional 
functionality may be incorporated within hybrid circuit 314, the primary 
role performed by hybrid circuit 314 is to separate the transmit and 
receive signals from their combined state into their component parts. 
Hybrid circuit 314 removes the transmit signal from the combined signal to 
present a receive only signal to a pulse modulator 320. The preferred 
embodiment for the isolation circuit, including hold circuit 310, is 
detailed in FIG. 4. 
A diode bridge 316 interfaces with hold circuit 310 for providing a 
reversible interconnection polarity for tip and ring signals 318 
interfacing with the communication network. The specific implementation of 
a diode bridge is not further detailed as specific implementations are 
know by those of skill in the art. 
Referring now to the receive signal as separated by hybrid circuit 314, a 
receive pulse modulator 320 receives the analog receive data. Pulse 
modulator 320 performs pulse modulation techniques upon the receive data 
to form a pulse stream carrying the corresponding data information. Pulse 
modulator 320 may be implemented similar to pulse modulator 306 as 
discussed above either by discreet components, such as op amps and 
counters, digital signal processor, or other techniques known by those of 
skill in the art. As discussed above, pulse modulator 320 may employ 
pulsewidth modulation techniques wherein variations in the width of the 
pulses contain the information to be modulated, or alternatively, pulse 
position modulation techniques may be employed wherein the width of the 
individual pulses remains constant while the frequency of the pulses is 
increased or decreased to encode the signal information. Alternatively, 
other pulse modulation techniques known by those of skill in the art may 
be employed to achieve similar results. 
It should be noted that in the present invention the line-side portion of 
the isolation circuit is powered by the loop current provided by the 
communication network. That is to say, the power associated with tip and 
ring signals 318 is tapped or utilized for powering the line-side portion 
of the isolation circuit. Those skilled in the art will appreciate that 
pulse modulator 320 receives as an input signal a generally smooth 
continuous analog signal. However, in the process of translating the 
receive analog signal into a pulse stream, pulse modulator 320 generates a 
pulse train having very high slew rate pulses. Such high slew rate pulses 
inject high frequency components or ripple onto the supply line powering 
the line-side circuitry. As discussed immediately above, the supply line 
powering the line-side circuitry is generated from the tip and ring 
signals 318 of the communication network. Therefore, without any 
additional conditioning of the circuitry supply lines derived from tip and 
ring signals 318, contamination of the tip and ring signals, and hence the 
communication network would result. The present invention contemplates 
such contamination signals and conditions the supply lines by 
incorporating an inversion 322 for providing a phase inversion of the 
pulse train for driving an inverse current sync 324. Inverse current sink 
324 provides a complementary current sink for conditioning the supply 
lines thereby mitigating the effect upon the tip and ring signals of the 
communication network. Inversion 322 and inverse current sink 324 may be 
implemented as a transistor in opposite phase with the pulse train as 
generated by pulse modulator 320. 
The pulse modulated receive data as modulated by pulse modulator 320 is 
received at optocoupler 326 for coupling across isolation boundary 302. 
Optocoupler 326 performs similarly to optocoupler 308 by providing 
coupling between a light emitting diode and an optically coupled 
photodiode. As disclosed above, optocoupler 326 need not operate in a 
linear region as required by prior art configurations of optocouplers. In 
the present invention, optocoupler 326 may operate in the saturation 
region due to the digital transmission of information across the optically 
coupled device. Because of this digital employment of an optocoupler, less 
expensive optocouplers may be employed as linearity need not be monitored 
or compensated. 
Optocoupler 326 couples to a receive or user side integrator 328. 
Integrator 328 receives the pulse modulated receive data as optically 
coupled by optocoupler 326 to reconstruct an approximation of receive data 
as originated from the computer network as line-side receive data. As 
described above for line-side integrator 312, user side integrator 328 may 
be implemented, among other ways, as a low pass op amp filter or an RC 
network to form receive data 330 for presentation to user equipment. 
FIG. 4 is a schematic diagram of the isolation circuit for pulse modulating 
data information across the isolation boundary, in accordance with the 
preferred embodiment of the present invention. As discussed above, 
isolation boundary 302 provides a partioning of user side functionality 
from the line-side functionality. Transmit data 304, is presented to a 
pulse modulator 306, herein shown as a pulse position modulator in 
accordance with the preferred embodiment. Transmit data 304 traverses 
bypass capacitors 332 for removal of the DC component of transmit data. Op 
amp 334 is further biased using a representative biasing network as shown. 
The biasing of op amp 334, as shown, transforms the differential transmit 
data input signals into a single-ended signal for additional processing. 
Resistor 336 provides feedback for setting the transmit gain of op amp 
334. Therefore, op amp 334 provides a dual end to single end converter. 
A counter 338, such as a 555 known by those of skill in the art, is driven 
by the single-ended transmit data signal to generate a pulse train 
comprised of pulse position modulated transmit data. A pulse position 
implementation of pulse modulator 306 provides a more simplified, and 
hence less expensive, implementation of a pulse modulator. It should be 
reiterated, however, that other pulse modulation techniques, such as 
pulsewidth modulation techniques, are considered to be within the scope of 
the present invention. Counter 338 provides a pulse generator wherein the 
density of the pulses is increased or decreased according to the transmit 
data to be pulse modulated. That is to say, for rising levels of transmit 
data, the pulse density on the output of counter 338 increases while the 
pulse density of pulses on the output for decreasing transmit data signals 
decreases according to a decreasing input transmit data signal. It should 
be reiterated that according to pulse position modulation techniques, the 
pulsewidth of the pulses remains constant while the density of the pulses 
changes as the input transmit data signal varies. Resistors 340 provide 
the bias for counter 338, that is to say, resistors 340 define the base 
frequency of operation by counter 338. 
The output of counter 338 carries the pulse position modulated transmit 
data to optocoupler 308 for traversal of isolation boundary 302. 
Optocoupler 308 may be comprised of a typical optocoupler known by those 
of skill in the art. It should be reiterated that optocoupler 308 need not 
be operated in the linear region and therefore does not require 
optocoupler 308 to be comprised of dual photoreceptors for linearity 
monitoring. In the preferred embodiment, optocoupler 308 is comprised of a 
single, light emitting diode (LED) and a single photoreceptor optically 
coupled to the LED. 
In FIG. 3, the optically coupled pulse modulated transmit data signal is 
shown feeding into pulse circuit 310 comprised of line-side integrator 312 
and 314, however, in FIG. 4 the circuitry for line-side integrator 312, 
hybrid circuit 314 and miscellaneous hold circuitry 310' are shown as 
distinct portions of the isolation circuit for clarity. Returning to FIG. 
4, the optically coupled modulated transmit data is processed by a 
line-side integrator 312 to transform the digitally modulated signal back 
to an analog signal. Line-side integrator 312 is comprised of an op amp 
342 and miscellaneous capacitors and resistors for generating a low pass 
filter or integrator. 
A transmit current driver 344 receives the integrated transmit data signal 
and provides the drive necessary for driving tip and ring signals via 
diode bridge 316. In the preferred embodiment, transmit current driver 344 
is comprised of an op amp 346 and a transistor 348 for providing the 
necessary drive levels required by tip and ring signals 318. Transmit 
current driver 344 further couples to hybrid circuit 314, which in the 
present invention, is located on the line-side of the isolation circuit as 
opposed to many prior art configurations that incorporate the hybrid 
circuit on the user side portion of the isolation circuit. Hybrid circuit 
314 isolates the line-side receive data from the combined transmit and 
receive signal by removing the transmit portion from the combined signal. 
Additionally, series resistor and capacitor 350 provides an AC termination 
to terminate the tip and ring signal to the standard termination values, 
which in this embodiment approximates 600 ohms, in accordance with United 
States telephony standards. 
The independent detailed functionality of hold circuit 310 (FIG. 3) is 
illustrated in FIG. 4 as hold circuit 310'. Zener diode 352 provides an 
inductive circuit thereby allowing the AC components to be exhibited 
across iso V+ an isoGnd. Hold circuit 310' is further comprised of a 
complementary Darlington pair 354, which in the present configuration, 
assists in the elimination of the transconductance of each transistor. 
When a relay 356 is closed resulting from an off-hook condition, such as 
occurs when placing or answering a call, complementary Darlington pair 354 
draws current between tip and ring 318 thereby notifying the telephone 
company's central office of an off-hooked condition. 
Line-side data receive signal 360 as separated by hybrid circuit 314, is 
processed through the line-side receive portion of the circuitry. An op 
amp 362, although not entirely necessary for the present configuration, 
provides additional signal drive for pulse modulator 320 thereby reducing 
loading of hybrid circuit 314. Op amp 362 is shown in the present circuit 
primarily because op amps traditionally are packaged in pairs, and 
therefore op amp 262 may be tied-off as a spare or incorporated into the 
isolation circuit as shown to provide incremental improvement to the 
overall circuit. 
Receive pulse modulator 320 is configured as a pulse position modulator, in 
accordance with the preferred embodiment. It is reiterated, however, that 
receive pulse modulator 320 may take other forms such as pulse with 
modulators or other pulse modulation techniques known by those of skill in 
the art. Receive pulse modulator 320 is comprised of an op amp 364 for 
providing the required drive levels necessary for driving counter 366. 
Counter 366 performs similarly to the operation of transmit counter 338 by 
increasing the density of the pulses generated in response to an 
increasing line-side receive data signal level and reducing the density of 
pulses generated corresponding to a reduction in the line-side receive 
signal. Resistors 368 provide the biasing for counter 366 corresponding 
with the biasing set by resistors 340 for the transmit pulse modulator. 
The pulse modulated receive data signal passes through inverse current sink 
324 wherein transistor 370 drives optocoupler 326 for optically traversing 
isolation boundary 302. Optocoupler 326 is comprised of an LED and a 
photoreceptor consistent with the description of transmit optocoupler 308 
described above. Inverse current sink 324 further provides a transistor 
372 which operates at 180.degree. out of phase with transistor 370. By 
operating out of phase with the modulator receive data transistor 370, a 
more consistent current drive level is exhibited on the line-side current 
draw from tip and ring signals 318. In the absence of out of phase 
transistor 372, receive data signals echo back on tip and ring signals 318 
through the central office equipment. Such echoes would be interpreted by 
the central office as transmit signal data resulting in a perceived echo 
or distortion of the receive data at the remote party. 
The pulse modulated receive data, as digitally coupled through the 
optocoupler, is then returned to analog form via receive or user side 
integrator 328. Receive integrator 328, like line-side integrator 312, is 
comprised of a low pass filter, implemented, in the preferred embodiment, 
using an op amp 374 to generate receive data 330. 
While the preferred embodiment has been illustrated in FIG. 4 as a pulse 
position modulation isolation circuit, such as that incorporated within a 
DAA, other modulation techniques, particularly pulsewidth modulation may 
be incorporated into alternate embodiments thereby providing the same 
functionality of digitally traversing the isolation boundary by 
incorporating a digital coupler such as an optocoupler for traversing the 
isolation boundary. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential characteristics. The described 
embodiments are to be considered in all respect only illustrative and not 
restrictive. The scope of the invention is, therefore, indicated by the 
appended claims rather than by the foregoing description. All changes 
which come within the meaning and range of equivalency of the claims are 
to be embraced within their scope.