Monolithic telephone subscriber line interface circuit
First Claim
1. In a supervision circuit of a subscriber line interface circuit coupled to a subscriber line and having a filter, the improvement comprising:
- means for detecting dialing pulses with a predetermined positive--going threshold and a predetermined negative--going threshold of approximately one-half said positive--going threshold; and
a clamping amplifier for clamping positive--going signals on said subscriber line to approximately 1.5 times said predetermined positive--going threshold.
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Accused Products
Abstract
The present invention is an improved subscriber line interface circuit which allows fast detection of an off-hook signal in the presence of a ringing signal during an answer mode while also permitting fast detection of dialing pulses during a calling mode. A programmable filter is used in the supervision circuit of the SLIC to allow the cutoff frequency of the filter to be varied so that the 20 Hz ringing signal will be attenuated during a ringing sequence and dialing pulse rates up to 20 Hz will be passed by the filter during the calling mode. A clamping amplifier is used to clamp the received signal to a maximum of 1.5 times the loop threshold current. This eliminates the large variations in the rise and fall times of the pulse dialing signal due to variations in the loop current caused by varying impedances of the telephone line. The filter is programmed by using an analog switch to bypass certain filter elements. The analog switch is a voltage-follower amplifier which has a current source which is disabled when the switch is to be opened.
65 Citations
9 Claims
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1. In a supervision circuit of a subscriber line interface circuit coupled to a subscriber line and having a filter, the improvement comprising:
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means for detecting dialing pulses with a predetermined positive--going threshold and a predetermined negative--going threshold of approximately one-half said positive--going threshold; and a clamping amplifier for clamping positive--going signals on said subscriber line to approximately 1.5 times said predetermined positive--going threshold. - View Dependent Claims (2)
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3. In a supervision circuit of a subscriber line interface circuit having means for detecting an off-hook condition of a subscriber and dialing pulses, the improvement comprising:
a programmable filter having a first mode for attenuation of a ringing signal and adaptable to a second mode, responsive to detection of said off-hook condition, for providing said dialing pulses to said detecting means without significant attenuation. - View Dependent Claims (4, 5, 6, 7, 8)
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9. In a supervision circuit for a subscriber line interface circuit the improvement comprising:
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means for detecting dialing pulses with a predetermined positive--going threshold and a predetermined negative--going threshold of approximately one-half said positive--going threshold; clamping amplifier means for clamping a signal from a subscriber line to approximately 1.5 times said predetermined positive--going threshold; a programmable filter having a first mode for attenuation of a ringing signal from said subscriber line and adaptable to a second mode, responsive to detection of an off-hook condition, for passing said dialing pulses without signficant attenuation; and an analog switch for short-circuiting a portion of said filter upon detection of said off-hook condition.
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Specification