Telephonic communications – Subscriber line or transmission line interface – Network interface device
Reexamination Certificate
1999-08-20
2003-04-22
Tieu, Binh (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
Network interface device
C379S093060, C379S387020, C379S390020, C379S399020
Reexamination Certificate
active
06553118
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for regulating DC current. Specifically, it relates to a telecommunication device for regulating the DC line current on a telephone line to conform to desired parameters.
BACKGROUND OF THE INVENTION
Telephone systems in countries throughout the world have unique system requirements that need to be met in order to legally sell and use telecommunication devices within their respective borders. One of the commonly known system requirements mandates that when a telephone line goes off-hook (i.e., when the telephone line is in use), the DC current level on the line must reach a certain level within a specified period of time and maintain that level until the call is completed. The DC current level on the line must stay at a certain level in order to be interpreted by the telephone system as an active line throughout the duration of the telephone call. The current rise time and maximum current level are also regulated to prevent damage to telecommunication equipment.
In order to hold a telephone line in the off-hook condition, a specified level of current must be drawn which relates to the voltage level on the line and conforms to a country's telecommunication requirements. The desired operating current is generally expressed on a graph of current-versus-voltage, known in the art as a load-line. The load-line represents a level of resistance for voltages on a current-versus-voltage graph, allowing a level of current to be determined for a given voltage.
FIG. 4
is an example of a current-versus-voltage load-line requirement to keep a telephone line in an off-hook condition. The slope of the load-line on a current-versus-voltage graph is the inverse of the line resistance.
Telephone systems develop a voltage which is a potential impressed on the telephone line between two terminals, commonly known as the tip and ring voltage. As seen in
FIG. 4
, the desired level of current to keep a telephone line in the off-hook condition can be achieved for a given voltage by setting an appropriate line resistance. The template illustrated in
FIG. 4
is representative of the parameters set forth by a country and varies from country to country. The parameters can even change within a country due to changes in a country's requirements (e.g., if a country updates their telecommunication system).
To conform to established requirements, consumer telephone equipment, such as computer modems and telephones, must be capable of setting the DC line current on a telephone line. One method that has been used to set the DC line current on a telephone line when the telephone line goes off-hook is to place an inductor in series with a resistor across a telephone line connection and then couple the voice circuits to the line through a capacitor. As shown in
FIG. 5
, a commonly known prior art circuit for setting DC line current comprises resistance R
DC
, capacitance C and inductance L. Inductor L is chosen to have an impedance over the 200 Hz to 4 kHz voice-band that is much larger than the impedance of the phone line and the capacitor-voice circuit combination. Virtually all the AC current flows through the capacitor and voice circuits. At DC, the capacitor looks like an open circuit and the inductor looks like a short circuit, so R
DC
sets the DC current level. The circuit of
FIG. 5
is less than optimal because of the inherently bulky nature and high cost of the inductor L, the amount of time for inductor L to charge, and the need to change circuit elements in countries with different off-hook current level requirements.
Another prior art approach that has been used to control the DC line current in a telephone system replaces the inductor L of
FIG. 5
with additional system components that are smaller and less expensive. The arrangement of components as shown in
FIG. 6
can be used to control DC line current and is commonly known in the industry as a gyrator. The prior art gyrator depicted in
FIG. 6
can be used to control DC line current without the use of an inductor L. The circuit in
FIG. 6
functions like a large inductor across the telephone line and can be used in place of the prior art circuit shown in FIG.
5
. The gyrator is implemented with many discrete components such as transistors, resistors, capacitors, and digitally controlled switches located close to the tip and ring telephone line interface. As shown in
FIG. 6
, the gyrator contains digitally controlled switches DCS
C
and DCS
R
used to switch different levels of capacitance and resistance into the gyrator circuit, respectively. By switching different levels of capacitance and resistance into the circuit, the time constant of the circuit can be changed, such that the transistors can be manipulated to provide the correct level of current on the telephone line within a specified period of time. The circuit allows different start up transient times and DC current levels to be adjusted in accordance with a user's specifications using a single circuit. The DCS
C
switches affect initial transient settling time and the DCS
R
switches affect the DC load-line. The adjustability of the circuit is established when the circuit components are installed at the time of manufacture. If the specifications change after manufacture, in order to change the device, components need to be physically changed within the device or an entirely new device needs to be installed.
Recently, a gyrator has been developed using digital processing technologies. By incorporating a gyrator into a digital device, the desired line current parameters can be achieved by adjusting parameters on a country by country basis in software. An example of a digital gyrator is disclosed and described fully in co-pending U.S. patent application No. 09/310,021 filed on May 11, 1999, entitled “Digital Gyrator,” having at least one common inventor and assigned to the same assignee as the present application (attorney docket Fischer 16-28-9), and is incorporated herein by reference.
A block diagram of the prior art gyrator is depicted in FIG.
7
. The gyrator depicted in
FIG. 7
is used to control the DC line current flowing between tip
80
and ring
81
at the interface between the data access arrangement (DAA)
74
and the telephone company central office
72
. The system controls the DC line current by first using the DAA
74
to generate an analog signal which represents the DC voltage between tip
80
and ring
81
. The analog signal is then converted to digital by the analog-to-digital (A/D) converter
82
located in the coder/decoder (CODEC)
76
. The resultant digital signal is then processes by the processor
78
which filters
86
and scales
88
the digital signal to achieve a digital DC current control signal, and combines the digital signal with a computer modem transmit (TX) signal
92
. The combined digital signal is then converted back to analog by the digital-to-analog (D/A) converter
84
. The resultant combined analog signal is then used to control a current source
94
which places a desired DC line current and an AC modem current onto the tip
80
and ring
81
interface between the DAA
74
and the telephone company central office
72
.
Although the digital gyrator
70
depicted in
FIG. 7
is capable of setting the DC line current on a telephone line in accordance with the specifications of various countries, a system error can occur in the resulting DC line current seen by the central office
72
between tip
80
and ring
81
that could be potentially problematic. The system error is inherent to the prior art digital gyrator
70
because, in order to control the DC line current with processor
78
, a DC feedback path between the DAA
74
and the processor
78
is shared with an AC feedback path. Separate A/D converters could be used for converting the DC path and the AC path, however, it is more feasible to use a single A/D converter
82
. Conventional A/D converters
82
often can accommodate only a small range of voltage at their inputs. In addition, present modem specifications
Fischer Jonathan Herman
Laturell Donald Raymond
Smith Lane A.
Witmer Steven Brooke
Agere Systems Inc.
Synnestvedt & Lechner LLP
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