Monolithic telephone subscriber line interface circuit

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.

Dependencies are inferred from the claim text.