Detection of multifrequency tone signals
US5333191A · kind A · utility
Assignee
Inventor
Key dates
| Filing date | Apr 22, 1993 |
| Grant date | Jul 26, 1994 |
| Priority date | — |
| Expiry date | Apr 22, 2013 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH04L27/30
- WIPO fieldTelecommunications
- WIPO sectorElectrical engineering
Abstract
A method of operating a digital signal processor to detect DTMF tones in a digital voice telephone system in which the digitally encoded signals appearing on the telephone channel are decimated to compress the spectrum to be monitored for the appearance of call signalling tones. The signals received in a decimated block are "correlated" or convolved with one another on a forward and backward time-shifted basis and each forward and backward correlation product is summed to form the elements of a 5.times.5 modified covariance matrix. The modified covariance matrix exhibits the desirable property that its eigenvectors will be symmetric. Since all eigenvectors of the modified covariance matrix are orthogonal and the eigenvectors associated with the signal span the signal subspace, the signal subspace is orthogonal to the eigenvector associated with the noise. The dot product of the noise eigenvector with the signal subspace is set to zero. The roots of the resultant polynomial identify the frequencies of the DTMF tones, if in fact the same were present in the received signal. The noise and signal eigenvectors of the modified covariance matrix are more quickly and efficiently determined…
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.