Ground start detection in telephone loop signalling circuit

Claim 1

A loop signalling circuit capable of operating in the presence of spurious signals, for replicating signalling functions on a telephone line when serially connected between a central office and a remote terminal operating in either a loop start or a ground start configuration, comprising: low-pass filter means for passing signals from the remote terminal below the frequency of said spurious signals; first detector means for detecting when the line current on the remote terminal side of the telephone line exceeds a preselected value; second detector means for detecting signals from the low-pass filter means when the line current exceeds the preselected value; a clock generator which is synchronized on each occurrence that the first detector means detects current in the line that exceeds said preselected value; latch means set to the output state of the second detector means on each occurrence of a pulse signal from the clock generator; and control means responsive to the latch means for applying a ground start impedance on the central office side of the telephone line.