Dial tone detector
US4737984A · kind A · utility
Assignee
Inventor
Key dates
| Filing date | Dec 1, 1986 |
| Grant date | Apr 12, 1988 |
| Priority date | — |
| Expiry date | Dec 1, 2006 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH04M1/82
- WIPO fieldTelecommunications
- WIPO sectorElectrical engineering
Abstract
A dial tone detector is provided possessing a precise definition of a passband. In one example, virtually infinite roll off is achieved in less than 5 hertz. An incoming analog signal of at least a predetermined amplitude is limited to produce square wave pulses which are counted in a first counter over a period of time determined by a periodic RESET signal. A READ signal occuring just before the RESET signal causes either a valid count state or an invalid count state to be stored depending upon the value of the count in the first counter. When a valid count state is in storage, a second counter counts timing pulses while the valid count state persists, whereby in the second counter a count of greater than a predetermined number indicates the presence of dial tone.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.