Bell Telephone Laboratories

At&t Information Systems Inc.

Kent Vincent Mina

Victor Bernard Lawrence

Clair A Buzzard

There is disclosed a multifrequency tone distribution arrangement for a communication system having a common time division multiplexed bus in which digital samples of tone frequencies are supplied from a central signal generating circuit to system port circuits via selected time slots. Each port circuit automatically accepts selected signals from the common bus for conferencing to produce multifrequency digit tones and service tones.

Multifrequency tone distribution using a conferencing arrangement

In U.S. Pat. No. 3,906,199, issued to R. B. Kieburtz and K. V. Mina on Sept. 16, 1975, limit cycle noise is reduced in a second order recursive digital filter by a circuit which randomly inhibits the rounding signal in one of the digital multipliers disposed in the filter. It has been found, however, that in certain instances, this approach results in even longer transients. In accordance with the present invention, random inhibiting is constrained to instances in which limit cycle noise is reduced, by monitoring, with suitable logic circuitry, the signals present in the various feedback loops of the filter. Generally, it has been found that the inhibiting circuit should be allowed to operate only if D.sub.1 and D.sub.2 satisfy predetermined magnitude and polarity relationships, where D.sub.1 .ident. a version of the filter output signal Y(n) that is removed in time by one sample interval, and D.sub.2 .ident. a version of the filter output signal Y(n) that is removed in time by two sample intervals. In accordance with another aspect of the invention, DC and half sampling rate limit cycles are eliminated by monitoring three consecutive values of D.sub.1 or, alternatively, the present values of Y(n), D.sub.1 and D.sub.2. If the same magnitudes are detected, the magnitude of the next D.sub.1 is reduced by one.

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Colts Neck, NJ, United States of America

Kent Vincent Mina