Arrangement in a subscriber line interface circuit

Details Arrangement in a subscriber line interface circuit Arrangement in a subscriber line interface circuit

1998-04-02

2001-02-27

Hudspeth, David R. (Department: 2741)

Telephonic communications

Subscriber line or transmission line interface

For line length compensation

C332S112000

Reexamination Certificate

active

06195429

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to subscriber line interface circuits and, more specifically, to an arrangement for determining the line voltage in a subscriber line interface circuit.
BACKGROUND OF THE INVENTION
To be able to adjust hybrid interface parameters and gain in a telephone circuit by means of a microprocessor on a line interface board in order to obtain good echo cancellation for telephone lines of different lengths as well as for certain test purposes, it is necessary to know the length of the respective telephone line.
One way of estimating the length of the line is to measure the line voltage. i.e. the voltage that appears across the line and the telephone set when a connection is established.
It is known to generate a pulse having a length corresponding to the line voltage. To get information about the line voltage, the length of this pulse is then measured by the microprocessor on the line interface board.
When a pulse length is to be measured, it is difficult to get at good resolution since the measurement period will be determined by the pulse length. The instruction cycle of the microprocessor has to be short relative to the shortest pulse length of interest and, moreover, the microprocessor cannot do anything else during that time.
SUMMARY OF THE INVENTION
The object of the invention is to bring about an arrangement in a subscriber line interface circuit for generating a signal for determining the line voltage without occupying the microprocessor unnecessarily.
This is attained by means of the arrangement in accordance with the invention in that a sawtooth wave having an amplitude corresponding to the difference between a first and a second voltage, e.g. the voltages of the A-wire and the B-wire, respectively, is generated and, in its turn, converted to a pulse train related to the line voltage.
In accordance with the invention, the pulse train is related to the line voltage by its pulse repetiton frequency and/or by its mark-space ratio. Depending upon the actual application, these different relationships offer different advantages. Generally, by generating a continuous pulse train and measuring its pulse repetition frequency and/or its mark-space ratio, it will be possible to freely choose the measurement period. Thus, a greater flexibility is obtained in accordance with the invention.


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