Ring trip detector for a subscriber line interface circuit

A ring trip detector arrangement for use in a subscriber line interface circuit which couples a two-wire subscriber line with the transmitting and receiving branches, respectively, of a four-wire line in a telecommunication exchange, said subscriber line interface circuit including a two-to-four wire hybrid circuit having an off-hook detector;

• a ring generator for producing an AC ring voltage of given frequency for ringing a subscriber set connected to said subscriber line;
• and a ring relay for selectively connecting said hybrid circuit to said subscriber line when in a first position and connecting said ring generator to said subscriber line when in a second position;
• said ring trip detector arrangement comprising, in combination:
• (a) means for sensing the current flowing through said subscriber line when said relay is in said second position;
• (b) a low-pass filter coupled to said sensing means for selectively passing frequencies below said given frequency, said low-pass filter shifting the phase of its output signal with respect to its input signal at said given frequency;
• (c) a threshold detector coupled to said sensing means and to said low-pass filter for producing a first signal representing an off-hook condition of the subscriber set when said output signal of said low-pass filter exceeds a prescribed threshold;
• (d) means, having inputs for receiving, respectively, said first signal and a second signal that is synchronized with zero crossings of said ring voltage, for producing a synchronized off-hook signal at the next, subsequent zero crossing of said ring voltage after said first signal indicates the presence of said off-hook condition;
• and
• (e) means having an input for receiving said synchronized off-hook signal for switching said relay to said first position.