Line interface unit for voice and wide band signal coupling

A line unit interfaces voice band signals and wide band signals extending from D.C. into the voice band between an analog line and a digital telecommunication facility. The line unit includes an analog to digital converter, a control circuit for generating one of two control signals and a line circuit. The line circuit includes a hybrid circuit in a first signal path for carrying voice band analog signals, and a second signal path for carrying wide band analog signals from the analog line. The signals in either signal path are transmitted to the analog to digital converter for encoding by way of a first switch or a second switch connected in series with the first and second signal paths respectively. The first and second switches are alternately conductive in response to one or the other of the control signals respectively.

The invention relates to communication systems and more particularly to a 
line unit for interfacing an analog line with a digital telecommunication 
facility. 
A digital telecommunication facility for example a time division multiplex 
(TDM) pulse code modulation (PCM) telephone switching office, is well 
suited for passing and utilizing binary signal streams organized in a 
precisely defined format. Analog subscriber lines associated with the 
switching office are typically connected therewith by means of interface 
circuits; each interface circuit including analog and digital signal ports 
for providing voice band communication with the switching office. The 
aforementioned interface circuit is typically referred to as a line unit. 
The line unit includes both analog to digital, and digital to analog 
converters. The converters are coupled between a hybrid line circuit in an 
analog portion of the line unit and a digital access circuit or 
multiplexer in a digital portion of the line unit. One example of such an 
arrangement is disclosed in Canadian Pat. No. 1,051,998, entitled "A TDM 
PCM Communication System", issued on Apr. 3, 1978 to B. R. Barrett et al. 
An example of a line circuit suitable for use in a line unit is disclosed 
in U.S. Pat. No. 4,103,112, entitled "Telephone Line Circuit With 
Differential Loop Current Sensing and Compensation", issued on July 25, 
1978 to V. V. Korsky. 
In a line unit, functions related to the operation of the analog line such 
as the application and withdrawal of high level ringing signals, and the 
recognition of ON hook, OFF hook and dial pulsing states of the analog 
line, have typically been performed by dedicated circuitry in the line 
unit. This dedicated circuitry is somewhat similar in operation to that in 
past common control analog switching offices. This dedicated circuitry is 
typically autonomous in function, as for example in the performance of the 
withdrawal of ringing from the line, typically referred to as a ring trip 
function, or partially dependent in function upon a periodic poling or 
scanning function of the associated switching office, as for example in 
the recognition of dial pulse information. Signals communicating these 
functions to the switching facility have been routed via paths, space 
separated with respect to the voice band communication paths or channels. 
Recently, it has been recognized that an improvement in the digital 
telecommunication facility is achieved by sharing the voice band digital 
transmission path associated with an analog line, between the encoded 
representations of voice band signals from the analog line and encoded 
signals corresponding to direct current conducted by the analog line. In 
one example, a time shared ring trip function is disclosed in U.S. Pat. 
No. 4,007,334 entitled "Time Division Digital Local Telephone Office with 
Telemetering Line Unit", issued on Feb. 8, 1977 to H. S. McDonald. In this 
example, telephone voice band analog signals are coupled from the 
subscriber analog line to an analog input of an analog to digital (A/D) 
converter in a conventional manner via a first signal path including a 
hybrid line circuit and a low pass filter. A second signal path, referred 
to as a "telemetric" path includes a current senser, which is coupled to 
the line and which generates a signal representation of net line current. 
This signal representation is filtered by a low pass filter having a 
cutoff frequency of 40 Hz and is transmitted around the hybrid circuit, 
via a switch, to the analog input of the A/D converter. When the switch is 
turned on, essentially net line direct current signal representations are 
encoded by the A/D converter along with any voice band signals from the 
hybrid circuit for use in the digital telephone office. The low pass 
filter is required to provide frequency separation between the voice band 
signals and line current signal representation. As the first and second 
signal paths pass only signals in mutually exclusive frequency ranges, 
voice voice band analog signals, which would otherwise cause ambiguous 
results at the A/D converter, are prevented from also passing along the 
second path. The encoded signal from the A/D converter is conventionally 
multiplexed into the binary signal bit stream format of the operating 
digital office and is available to be routed to a digital circuit located 
remote from the line unit in the switching office. The digital circuit is 
time shared among a group of line units and controls the ring trip 
function of each of the line units. 
In a line unit with a resistive feed network similar to that disclosed by 
V. V. Korsky, energizing current having a wide frequency range from direct 
current through the voice band can be supplied via the resistive feed 
network. In an operating telephone system, analog line testing and 
maintenance includes energizing the analog line with a transient voltage 
and/or continuous AC signals. One such test, useful in determining the 
number of ringers associated with a telephone subscriber line, is the well 
known ballistic test. This test involves direct current and voice band 
frequencies. In this test the analog line loading response to the 
application of a transient voltage impulse is monitored. It would be 
convenient to be able to telemeter the analog line loading response to 
this test into the associated digital facility. The telemetric path 
disclosed by H. S. McDonald is, however, unsuitable for this use because 
it is essential that it be severely band limited in order to operate 
compatibly with the remainder of the line unit. 
In accordance with the present invention, a line unit is provided with a 
broad band telemetric path for bypassing signals from an analog line 
around a voice band hybrid line circuit. Signals from a sensing means and 
signals from the output of the hybrid line circuit are time switched by 
way of respective transmission switches which are controlled to couple 
either one or the other of the signals to an analog input of an analog to 
digital converter at any one time. The line unit provides for a common 
control ring trip function and additionally at no extra cost provides an 
essential link to facilitate test and maintenance procedures. 
The invention resides in a line unit for interfacing an analog line with a 
digital telecommunication facility. The line unit comprises an analog to 
digital converter having an analog input for receiving analog signals for 
encoding. A control means generates one of a plurality of control signals 
at any one time, in response to directions received in the line unit by 
way of the telecommunication facility. A line circuit, in the line unit, 
includes a first signal path having a hybrid circuit for connection to the 
analog line and for carrying a voice band analog signal, and a second 
signal path for carrying another analog signal. A first switch is 
connected in series between an analog output of the hybrid circuit in the 
first signal path and the analog input of the A/D converter. The first 
switch is responsive to a first one of the control signals to couple the 
analog signal from the hybrid circuit to the encoder. A second switch is 
connected in series between the second signal path and the analog input of 
the A/D converter. The second switch is responsive to a second one of the 
control signals to couple the other analog signal to the converter.

The telephone line unit in FIG. 1 is connected across a two-wire analog 
line 3 at analog line terminals 1a and 1b and connected to the digital 
telecommunication facility through a digital transmit and receive signal 
path 39. The line unit includes a line circuit having a transformer 10 
connected between a hybrid network 30 and the analog line terminals 1a and 
1b. The two-wire analog line 3 is connected between the line terminals 1a 
and 1b and a subscriber telephone 4. The transformer 10 includes a split 
primary winding with first and second half windings 12 and 13 
respectively. The first winding 12 is connected in series between ground 
and the terminal 1b via a resistor 17. The second winding 13 is connected 
in series between a source of d.c. power -V and the other terminal 1a via 
a resistor 18 and break contact portions of relay transfer contacts K1 and 
T1. The resistors 17 and 18 are of similar ohmic value, for example about 
200 ohms. At least one high level ringing signal source 5 associated with 
the telecommunication facility is normally connected for supplying 
energizing current to any of a plurality of line interface units. The 
ringing source 5 is connected to the resistor 18 through a terminal 6 and 
make contact portion of the relay transfer contact K1. A test signal 
source 7 associated with the telecommunication facility is also connected 
to supply energizing test current to any of the plurality of line 
interface units. The test signal source 7 is connected to the resistor 18 
through a terminal 8, a make contact portion of a relay transfer contact 
T1 and through the break contact portion of the relay transfer contact K1. 
The resistors 17 and 18 comprise a resistive feed network through which 
energizing current is coupled from any of the sources -V, 5 or 7 to the 
analog line 3. A capacitor 19 is connected between the first and second 
windings 12 and 13 and provides a coupling path for voice band analog a.c. 
signals between the two windings 12 and 13. Relay make contacts K2 and T2 
are connected across the first winding 12 and provide a low impedance 
coupling path across the primary winding in the event that either the high 
level ringing signal source 5 or the test signal source 7 is connected to 
the line unit. A switch, not shown is optionally inserted in series with 
the capacitor 19. In one example the switch is provided by a break contact 
of the relay T, which when operated eliminates the loading effect of the 
capacitor 19 during a period when the analog line 3 is energized from the 
test signal source 7. The transformer 10 includes a split secondary 
winding having first and second half windings 14 and 15 respectively. The 
secondary windings 14 and 15 in combination with the hybrid network 30 
provide a two wire--four wire hybrid circuit in a first signal path which 
carries voice band analog signals from the analog line 3 to an analog 
input 37 of an analog to digital (A/D) converter 36. The hybrid network 30 
includes a two-wire input port 31 connected to receive analog signals from 
a digital to analog (D/A) converter 35 via a voice band low pass filter 
33. The hybrid network 30 also includes a two wire output port which is 
connected to an input of a voice band low pass filter 34. An output of the 
low pass filter 34 is coupled to the analog input 37 of the A/D converter 
36 by way of transmission switch in the form of a field effect transistor 
(FET) 40. The FET 40 includes a source electrode s connected to the output 
of the filter 34, a drain electrode d connected to the analog input 37, 
and a gate electrode g used to control the conductivity of the FET 40 
between the source and drain electrodes. A digital access circuit 38 is 
connected to the digital transmit and receive signal path 39. The digital 
access circuit is also connected to a digital input port of the D/A 
converter 35 and a digital output port of the A/D converter 36. The 
digital access circuit 38 includes circuitry (not shown) which interfaces 
the input/output timing requirements and format of the telecommunication 
facility with the operating requirements of the converters 35 and 36. In 
one example, the converters 35 and 36 are compatible with eight bit PCM 
words. However, each word transmitted to the digital access circuit 38 
from the telecommunication facility contains at least nine bits, one of 
these bits being used to indicate that the remaining eight bits comprise 
either a PCM word for conversion to a corresponding analog signal level or 
a data word for use in supervisory control functions. In the case where 
the eight bits are designated as data by the ninth bit, this data is 
registered in the digital access circuit 38. Supervisory control functions 
are, for example, operation of either relays T or K via relay drivers 46, 
and control the state of a flip-flop 47 in the digital access circuit 38. 
The supervisory control functions are dictated by predetermined bits or 
combinations thereof in the data word. 
The line circuit further includes a sensing circuit represented by circuit 
elements 20 to 26 inclusive. An amplifier 20 in the sensing circuit 
operates to generate a signal proportional to the net current flow in the 
resistors 17 and 18 of the sensitive feed network. Similar to that 
described in the previously mentioned patent to V. V. Korsky, the output 
of the sensing circuit is coupled via a low pass filter 27 to a current 
regulator 28 which accordingly conducts a balancing current in a balance 
winding 16 in the transformer 10 to reduce d.c. flux in the core of the 
transformer 10. The proportional signal from the amplifier 20 is also 
connected to an ON-hook, OFF-hook and dial pulse detector 45. The detector 
45 generates binary signals corresponding to the ON-hook, OFF-hook and 
dial pulsing state of the line 3 for use in the digital access circuit 38. 
A second signal path carries the proportional signal from the amplifier 20 
and includes a second transmission switch in the form of a FET 42. The FET 
42 includes a source electrode s connected to the output of the amplifier 
20 in the sensing circuit, a drain electrode connected to the analog input 
37, and a gate electrode g used to control the conductivity of the FET 42 
between the source and drain electrodes. The conductivity of each of the 
FETs 40 and 42 is controlled in complementary fashion by Q and Q outputs 
of the flip-flop 47. Control signals from these outputs are coupled to the 
gate electrode g of the FETs 40 and 42 by control leads 41 and 43 
respectively. Alternately, the control signals may be provided by a single 
control lead connected directly to one of the gate electrodes and with the 
second control signal being provided via an inverter connected from the 
one gate electrode to the other gate electrode. 
In operation, with the control signal from the flip-flop 47 on lead 41 
being asserted, voice band signals from the line 3 are coupled through the 
first signal path in the line circuit by way of the FET 40 to the analog 
input 37 of the A/D converter 36. PCM signal representations of the analog 
voice band signal are generated in the converter 36 and interfaced with 
the associated digital telecommunication facility by the digital access 
circuit 38. Wide band signals, including frequencies extending from direct 
current through the voice band inclusive, are available from the second 
signal path for encoding by the A/D converter 36. In the event that either 
of the relays T or K is activated to connect the test signal source 7 or 
the ringing source 5, the flip-flop 47 is set to assert the control signal 
on lead 43 and not on lead 41. The wide band signals are coupled on the 
second signal path from the amplifier 20 to the analog input 37 of the A/D 
converter 36 through the FET 42. PCM signal representations of the wide 
band analog signal are generated in the analog to digital converter 36 and 
interfaced with the associated telecommunication facility by the digital 
access circuit 38 in a manner similar to that for the voice band analog 
signal. 
The encoded wide band signals are particularly useful in combination with 
the time shared digital circuitry (not shown), to which they are routed by 
the telecommunication facility to control the withdrawal of high level 
ringing signals in the event that the subscriber telephone goes OFF hook, 
and in the analyzing of line response to test signals. In one example of 
this function, wide band analog signals from the line 3 are encoded just 
prior to the connection of either of the test signal source 7 or the 
ringing source 5. These signals provide for the generation of a dynamic 
threshold adjustment, in the time shared digital circuitry, which is 
useful in correction for current leakage in the line 3 and various offset 
errors typically inherent in the analog circuitry portion of the line 
unit. 
Additional wide band signal paths in the line unit will provide for 
telemetering of various line unit operation conditions via the digital 
telecommunication facility. By this means it is envisaged, for example, 
that the voltage levels of operating voltage supply lines, for feeding 
operating currents to the line unit, are also from time to time encoded by 
the A/D converter 36 for transmission to a remotely located, time shared, 
digital maintenance monitor circuit.