Hook switch status glitch filter

Details Hook switch status glitch filter Hook switch status glitch filter

2000-03-27

2004-03-09

Bui, Bing (Department: 2742)

Telephonic communications

Terminal

Switch or switch actuator structure

C379S448000

Reexamination Certificate

active

06704416

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to analog glitch filter circuitry, and more particularly to a hook switch status glitch filter to prevent false dial pulses and false off hook indications at the central office.
BACKGROUND OF THE INVENTION
A telephone line, or subscriber loop, provides electrical and physical connection of a subscriber telephone to the central office. The telephone may be built using components which, by their nature, are such that they may create undesirable electrical transients. These transients can adversely interfere with the expected operation of the line equipment. For example, when an off hook subscriber dials a digit using a rotary dial pulse telephone, the subscriber opens, or breaks, and closes, or makes, the loop current thereby creating temporary on hook and off hook conditions. Sequences of these makes and breaks form a pulse train that indicates to the central office the digit dialed by the subscriber. On occasion, an extra, or false, pulse may be created by the telephone dial due to the nature of the handset relay. The central office dial pulse detector must ignore these false pulses to ensure that the proper dialed digit is interpreted.
Ringing transients may also be created during the application or removal of ringing power to the loop. These transients can be so severe that they cause the central office loop status detector to falsely detect an off hook condition on the line. When the false off hook is detected, the central office removes ringing from the line.
According to Telcordia standards document GR-506-CORE, “LSSGR: Signaling for Analog Interfaces”, Issue 1, June 1996, Section 10.1.4 Rejection of False Dial Pulses, the central office should ignore as a dial pulse any pulse of 2 ms or less, and, according to Section 14.1.4 Ring Trip, ignore transients of 12 ms or less as an off hook condition during the alerting state.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a line card circuit that will mask false pulses during digit dialing while maintaining the integrity of the dialed pulse train. Another object of the invention is to provide a line card circuit that will mask the shorter duration false off hook indications during ringing.
The invention of Applicant is a line card circuit that employs a pair of comparators arranged such that the output of the first comparator is coupled to an input of the second comparator through an RC circuit. The RC circuit is tuned such that short transients, such as false pulses and false off hook indications, passed by the first comparator are not passed by the RC circuit to the second comparator. The input of the first comparator is coupled to the subscriber line and the output of the second comparator is coupled to the switch's hook status detection circuitry.


REFERENCES:
patent: 3836730 (1974-09-01), Marshall
patent: 4075434 (1978-02-01), Merritt, Jr.
patent: 4636587 (1987-01-01), Zoerner

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