Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – Of hybrid or echo suppressor or canceller
Reexamination Certificate
1998-02-12
2001-02-06
Kuntz, Curtis A. (Department: 2743)
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
Of hybrid or echo suppressor or canceller
C379S008000, C379S027010, C324S525000, C324S527000
Reexamination Certificate
active
06185280
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention deals generally with the transmission of signals through transmission lines and more specifically to the transition between two-wire and four-wire transmission lines.
BACKGROUND OF THE INVENTION
In a typical switch in a telecommunications system there is an interface between a two-wire transmission line and a four-wire transmission line. The two-wire line provides a connection between the telephone company's central office and the telephone company's customers. The four-wire line is part of the customer's local phone system. One of the functions of the telephone switch is to connect the telephone company's lines and the customer's phone system. Among other considerations, this involves matching the impedance of the customer's switch to the impedance of the telephone company's line. There are chips available which can be programmed to provide a specific impedance between two-wire and four-wire transmission lines. If the impedance of the telephone company's two-wire line is known, then this type of chip can be used to connect the two-wire line to the four-wire line with minimum signal loss and echo.
A limitation in this capability is that the expected line impedance may not equal the actual line impedance. As a result, the two-wire line impedance for which the customer's telephone system and switch are designed will not be correct and when the customer's switch is connected to the telephone company's two-wire transmission line there will be an impedance mismatch. In some cases there will be an echo on the four-wire side due to a mismatch in the impedances of the two-wire and four-wire lines. The mismatched impedance will often attenuate the telephone signals due to the inefficient power transfer across the transmission line interface. Generally, echo signals will also be caused by the mismatched impedance.
This problem may arise if a system is designed for the impedance of a specific two-wire line but it is installed instead on, or moved to, a different two-wire line. In some situations the line impedance may be measured incorrectly or it may be difficult to accurately measure the line impedance.
When a programmable matching chip is used, selecting the wrong impedance can cause other problems in addition to signal attenuation or echo. If the wrong two-wire line impedance is used to generate the matching filter coefficients, that could make the transfer characteristics of the matching circuit significantly worse than they would be if a nominal impedance value was used to generate the filters. On the other hand, use of a nominal impedance value may produce too simple of a matching circuit, thereby significantly distorting the transmitted signals. In some cases, the signal performance can degrade to such an extent that switch hardware, such as a modem, will not work when the wrong impedance is selected.
SUMMARY OF THE INVENTION
These and other problems of the prior art are solved by a system and method which takes advantage of the known characteristics of the telecommunications switch and the matching circuit to determine the actual impedance of a telephone line. The present invention allows a system to dynamically determine the line impedance during installation or operation. Once the two-wire line impedance is known, the system can then reconfigure the front end hybrid matching circuit to match the switch to the two-wire line impedance as precisely as possible in order to optimize the signal transfer between the two-wire and four-wire lines, thereby reducing signal loss and echo.
The present invention operates by transmitting a test signal, such as a white noise signal, over the two-wire telephone line and then recording the echo signal that is reflected from the two-wire line. In the preferred embodiment, the echo signal is synchronously correlated to the transmitted test signal in order to measure any phase delays in addition to the amplitude of the echo signal.
A transfer function is generated by first cross-correlating the test signal input and the echo signal to derive the impulse response for the system. The system impulse response is then Fourier-transformed to derive the transfer function of the system comprising the combination of the customer's telecommunications switch and the telephone company's two-wire line. The transfer function of the telephone switch can be measured using a network analyzer or some other test equipment or it can be calculated from the design specifications. Once the transfer functions of the system and the switch are known, then the transfer function of the two-wire line can be derived by solving the circuit equations for the system. The impedance of the telephone line can be derived from the two-wire line transfer function.
Once the telephone line impedance is determined by this method, the switch's hybrid matching circuit can be modified precisely match the telephone line impedance. In switches with a programmable matching chip, the circuit can be reprogrammed to match the switch to the actual two-wire impedance. Other matching circuits may need to be manually modified to achieve an impedance match to the telephone line.
A technical advantage of the invention is that many existing systems, such as a private branch exchanges (PBX) or voice response units (VRU), already use the programmable matching chip described herein. Therefore, those systems can be adapted to use the present invention. The existing systems would simply need to be modified to apply the method of the present invention to determine the exact line impedance. Once the line impedance is calculated, then the matching circuit can be programmed to match this impedance.
Another technical advantage of the invention is that no additional hardware is required to properly match this impedance because the matching chip which is already installed in a great number of these systems is programmable and may be adapted to employ the present invention.
A further technical advantage of the invention is that this algorithm can be used with any number of systems in order to determine the line impedance. Therefore, even in systems with difficult to adjust, hard-wired impedance matching networks, the present invention is useful for determining the line impedance and for detecting problems that may be corrected by hardware or software modifications.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
REFERENCES:
patent: 5577097 (1996-11-01), Meek
patent: 5802169 (1998-09-01), Frantz et al.
patent: 5815568 (1998-09-01), Trump
patent: 5881130 (1999-03-01), Zhang
patent: 5917853 (1999-06-01), Greenblatt
patent: 6058162 (2000-05-01), Nelson et al.
Jarboe Steve
Lin Zhihong
Fulbright & Jaworski L.L.P.
InterVoice Limited Partnership
Kuntz Curtis A.
Tieu Binh K.
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