1980-06-17
1983-05-31
Popek, Joseph A.
328138, H04M 150
Patent
active
043862390
ABSTRACT:
A signal recognition circuit (10) is disclosed which is capable of distinguishing between the digit tones used for dialing in multifrequency tone telephone sets, and spurious signals at the same frequencies, such as noise and voice. The discriminator comprises, in cascade, an AGC amplifier (11), an adjustable threshold comparator (12), and a signal validation circuit (13). The operation of the discriminator is based upon the fact that the true dialing tones have envelopes of constant magntiude whereas the spurious signals tend to have pronounced envelope variations. By the appropriate adjustment of the comparator threshold, these variations show up as gaps in the comparator output signal, which are readily detected by the validation circuit.
REFERENCES:
patent: 2974281 (1961-03-01), Feldman
patent: 3537001 (1970-10-01), Friend
patent: 3539731 (1970-11-01), Legedza
patent: 3582565 (1971-06-01), Beeman et al.
patent: 3795775 (1974-03-01), Cowpland
patent: 3875347 (1975-04-01), Alaily
patent: 4091243 (1978-05-01), Mizrahi et al.
G. T. Kraemer et al., "The Family of Touch-Tone", Bell Laboratories Record, vol. 43, No. 7, Jul./Aug. 1965, pp. 283-287.
Bell Telephone Laboratories Incorporated
Popek Joseph A.
Sherman S.
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