Brian P Mccarthy

US Patent:

53331915, Jul 26, 1994

Filed:

Apr 22, 1993

Appl. No.:

8/051189

Inventors:

Brian M. McCarthy - Lafayette Hill PA

Assignee:

InterDigital Technology Corporation - Wilmington DE

International Classification:

H04M 900

US Classification:

379386

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.