Apparatus for detecting pick-up at a remote telephone set

United States Patent [19]

Szlam et al.

[li] Patent Number: 4,477,698 [45] Date of Patent: Oct. 16,1984

[54] APPARATUS FOR DETECTING PICK-UP AT A REMOTE TELEPHONE SET

[75] Inventors: Aleksander Szlam, Marietta; Chester P. Quinn, Chamblee, both of Ga.

[73] Assignee: Melita Electronics Labs, Inc., Atlanta, Ga.

[21] Appl. No.: 415,711

[22] Filed: Sep. 7,1982

[51] IntCl.* H04M1/26

[52] US. CI 179/90 BD

[58] Field of Search 179/5 R, 5 P, 6.02,

179/6.14, 90 BD; 375/120, 119

[56] References Cited

U.S. PATENT DOCUMENTS

3,445,601 5/1969 Whitelyetal 179/6

3,777,065 12/1973 Oalian et al 179/6 TA

3,899,645 8/1975 Brafman 179/5 R X

4,028,493 6/1977 Brennemann et al 179/5 R X

4,160,125 7/1979 Bower et al 179/6.02

4,188,510 2/1980 Bower etal 179/90 BD

4,201,896 5/1980 Bower et al 179/18 B

4,356,348 10/1982 Smith 179/1 MN

4,405,833 9/1983 Cave etal 179/90 BD X

4,412,299 10/1983 Huffman 375/120 X

4,418,318 11/1983 Swagertyetal 375/120 X

OTHER PUBLICATIONS John Snyder, "Digital Phase-Locked Loop Finds

Clock Signal in Bit Stream", Electronics, vol. 52, No. 18,
Aug. 30, 1979, pp. 126-130.

Primary Examiner—Keith E. George
Attorney, Agent, or Firm—Jones & Askew

[57] ABSTRACT

An improved combination of a hardware and digital signal processing filter for detecting pick-up of a telephone call, solely through audio information on the telephone line, is disclosed. The apparatus employs a high gain band pass filter (28) with no automatic gain control, the output of which goes to a window comparator (30). The output from the window comparator (31) goes to a digital high pass filter (32) and from there to an integrator 37 for providing a digital output signal (40) indicative of the presence or absence on the telephone line of a signal exceeding a predetermined magnitude within the filter pass band. The digital signal is then processed by an intelligent digital filter having a set of predetermined threshold values of durations for states of the digital output signal, by which determinations of pick-up are made. The digital filter is adaptive and learns the durations high and low states of the digital output signal as they occur, subsequently checking for deviations from previously learned valid values. The digital filter includes a digital phase lock loop which will lock onto a periodic but asymmetric pattern in the digital output signal and declare pick-up when lock is lost.

3 Claims, 11 Drawing Figures