Loudspeaking telephone set having an anti-Larsen device for inst

Details Loudspeaking telephone set having an anti-Larsen device for inst Loudspeaking telephone set having an anti-Larsen device for inst

1986-09-02

1988-02-02

Schreyer, Stafford D.

Telephonic communications

Substation or terminal circuitry

For loudspeaking terminal

H04M 908

Patent

active

047232773

ABSTRACT:
A loudspeaking telephone set incorporating an anti-Larsen device which upon detecting instability controls the insertion of an attenuator in either or both of the listening and transmission channels. Such device incorporates a detection circuit (6) for detecting a start of instability, consisting of two analogue multiplier circuits (11), (15), two low-pass filters (18), (20), and two absolute-value convertors (19), (21), these elements being arranged in two parallel chains. One of the multiplier circuits (11) receives at its inputs the amplified signal from the receiver (4) and from the microphone (3). The other multiplier circuit (15) receives the same signals, except that one of them is phase-shifted approximately 90.degree. at the Larsen frequency, by a phase-shift circuit (16). At the output of the absolute-value convertors (19), (21), the signals are added together by an adder (23) which delivers at its output an attenuation control signal after passing through a threshold comparator (24).
From among all the signals present in the telephone set, the detection circuit (6) performs effective selection of those which correspond to a start of instability.

REFERENCES:
patent: 4484034 (1984-11-01), Ferrieu

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