Detecting signals within a passband on a telephone line
US4540855A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | May 14, 1984 |
| Grant date | Sep 10, 1985 |
| Priority date | — |
| Expiry date | May 14, 2004 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH04M1/82
- WIPO fieldTelecommunications
- WIPO sectorElectrical engineering
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. 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 into a periodic but asymmetric pattern in the digital output signal and declare pick-up when lock is lost…
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.