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Multifrequency Receiver

IP.com Disclosure Number: IPCOM000085839D
Original Publication Date: 1976-Jun-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Esteban, D: AUTHOR

Abstract

In modern telephone systems the dialing is performed by combining audio frequencies. Generally speaking, each number is coded by two sine waves the frequencies of which belong to a given set of frequencies. Multifrequency receivers (MFR) are thus needed to decode the dialing which in most cases is combined with noise. Most MFR's already known are bulky, expensive or not reliable.

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Multifrequency Receiver

In modern telephone systems the dialing is performed by combining audio frequencies. Generally speaking, each number is coded by two sine waves the frequencies of which belong to a given set of frequencies. Multifrequency receivers (MFR) are thus needed to decode the dialing which in most cases is combined with noise. Most MFR's already known are bulky, expensive or not reliable.

This is an inexpensive and reliable MFR based on an autocorrelation method. For instance, let the set of dialing frequencies be: 697 Hz 770 Hz Group A 852 Hz (Low frequencies) 941 Hz 1209 Hz 1336 Hz Group B 1447 Hz (High Frequencies) 1633 Hz

The dialing will be performed by combining together one frequency from each group.

The autocorrelation method which will be used in this MFR has been disclosed in the IBM Technical Disclosure Bulletin, Vol. 17, No. 5, October 1974, pp. 1380-1382 for pitch detecting. In the present application, however, taken into account is the fact that the frequencies to be detected belong to a given set of frequencies. In addition, by taking one frequency in each group of frequencies, the MFR is split into two similar parts, each one detecting one out of four frequencies.

In the article cited above, detection is performed by locating the similitude peaks between a block of N samples and preceding blocks, by shifting the autocorrelation window on a continuous basis (k = 1,2,3...). In the present application k will take only a few fixed values, for instance four values for MFR based on two times one out of four frequencies. Then the MFR may be designed as shown in the figure.

The X(n) input samples are fed to the MFR input at a rate of 16 KHz. They are applied to a first sign detector SX providing S(n) = Sign (X(n)), and to a second sign detector D(n...