Control device for a hand-free alternate operating telephone set

Details Control device for a hand-free alternate operating telephone set Control device for a hand-free alternate operating telephone set

1989-12-27

1991-01-29

Ng, Jin F.

Telephonic communications

Substation or terminal circuitry

For loudspeaking terminal

379388, 370 321, H04M 160

Patent

active

049892428

ABSTRACT:
A control device for a hands-free telephone set automatically controls microphone and amplifier gains so that a feedback loop has less than unity gain to avoid circuit instability and resultant self-oscillation. An emission channel includes a microphone, a signal compressor and a controllable attenuator. A reception channel includes a signal compressor, an adjustable attenuator and a loudspeaker. The combination of the emission and reception channels form an amplification loop whereby the output of the reception channel is acoustically coupled to the input of the emission channel while the output of the emission channel is coupled to the input of the reception channel through a common telephone line. To avoid circuit oscillation, a circuit initially sets the gain of the loop to a predetermined value slightly less than unity (0 db) and subsequently maintains the loop gain constant by maintaining the sum of the compressor and attenuator gains at a fixed value.

REFERENCES:
patent: 3925618 (1975-12-01), Kato et al.
patent: 3952166 (1976-04-01), Kato et al.
patent: 4484034 (1984-11-01), Ferrieu et al.
patent: 4571461 (1986-02-01), Uno et al.
patent: 4741018 (1988-04-01), Potratz et al.
patent: 4843621 (1989-06-01), Potratz
patent: 4879745 (1989-11-01), Arbel
patent: 4894862 (1990-01-01), Defretin et al.
B. Gilbert, "A Precise Four-Quadrant Multiplier with Subnanosecond Response", IEEE Journal of Solid-State Circuits, vol. SC-3, No. 4 (Dec., 1968), pp. 365-373.

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