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# Voice Switch for Loudspeaking Telephone Set

IP.com Disclosure Number: IPCOM000088256D
Original Publication Date: 1977-May-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 40K

IBM

## Related People

Ballatore, D: AUTHOR

## Abstract

Shown are two circuits designed to be incorporated in a loudspeaking telephone set to reduce the Larsen effect.

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Voice Switch for Loudspeaking Telephone Set

Shown are two circuits designed to be incorporated in a loudspeaking telephone set to reduce the Larsen effect.

In the transmission and reception channels T and R connected to the telephone set, two linear elements with variable gains G1 and G2 are varied through a gate arrangement under the control of levels of transmitted and received signals so that the condition G1.G2 = 1 is met.

A first circuit having a gain G1 = Vo(1)/Vi(1) = r/(r+R) is shown in Fig. 1a and a circuit having a gain G2 = Vo(2)/Vi(2) = (r+R)/r is shown in Fig. 2A. If resistor r in both circuits is varied by the same amount, gains G1 and G2 are changed and the condition G1.G2 = 1 is met.

Fig. 1b and 2b show the implementation of variable resistor r, comprising a gate arrangement having four matched diodes D1 to D4 and two current sinks S1 and S2 which change the biasing current I(b) through the diodes.

The resistance in the arrangement of the Figs. 1b and 2b equivalent to resistor r is proportional to: 1/I(b) absolute value of 1 - (i/I/2/(b)) where i is the signal current. Consequently, r is equal to a constant if i << I(b).

By varying current I(b), resistor r changes, causing gains G1 and G2 to be modified.

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