Switching circuit for cancelling the direct sound transmission from the loudspeaker to the microphone in a loudspeaking telephone set

Claim 2

A loudspeaking telephone set comprising a transmitting channel including a microphone, a receiving channel including a loudspeaker, gating means connected in each of said channels, a source of high frequency signals, means for coupling said high frequency signals to said gating means to enable and disable the same at the high frequency rate, and means for controlling the frequency of said source so that the time duration of an odd multiple of half wavelengths of the source frequency equals the direct acoustic delay between the loudspeaker and the microphone.

Claim 3

A loudspeaking telephone set as defined in claim 1 including means for disabling the frequency control means in the absence of speech energy in said receiving channel.

Claim 4

A loudspeaking telephone set as defined in claim 2 wherein the signals of said source are of a frequency greater than 10 kHz., whereby the modulation of speech produced by the gating means is not objectionable to a local user.

Claim 5

A loudspeaking telephone s et as defined in claim 1 wherein the frequency of said source is controlled so that the time duration of a half wavelength of the source frequency equals said direct acoustic delay.

Claim 6

In combination, a transmitting telephone channel including a microphone, a receiving telephone channel including a loudspeaker, gating means connected in each of said channels, a source of high frequency switching signals, means for coupling said switching signals to said gating means to switch the same on and off at the high frequency switching rate, and means for controlling the frequency of said source so that the pulses of direct sound energy from the loudspeaker arrive at the microphone just at the time that the transmitting channel is switched off by said gating means.

Claim 7

In combination, a transmitting telephone channel including a microphone, a receiving telephone channel including a loudspeaker, gating means connected in each of said channels, a source of high frequency switching signals, means for coupling said switching signals to said gating means to switch the same on and off at the high frequency switching rate, and means for controlling the frequency of said source so that the pulses of direct sound energy from the loudspeaker arrive at the microphone just at the time that the transmitting channel is switched off by said gating means.

Claim 8

A loudspeaking telephone set comprising a transmitting channel including a microphone, a receiving channel including a loudspeaker, a pair of gates respectively connected in the transmit and receive channels, means for producing square wave signals at a high frequency rate, means for coupling said square wave signals to said gates to simultaneously switch the same on and off at the high frequency rate, means for monitoring the transmit channel for pulses of sound energy that appear during the on periods of the gate connected therein, means coupled to the monitor means and responsive to a differential energy distribution of said sound energy in said on periods to generate a control signal indicative of the same, and means for coupling said control signal to the high frequency signal producing means to alter its frequency such that the time duration of an odd multiple of half wavelengths thereof equals the direct acoustic delay between the loudspeaker and the microphone.

Claim 9

A loudspeaking telephone set comprising a transmitting channel including a microphone, a receiving channel including a loudspeaker, a pair of gates respectively connected in the transmit and receive channels, means for producing square wave signals at a high frequency rate, means for coupling said square wave signals to said gates to simultaneously switch the same on and off at the high frequency rate, means for monitoring the transmit channel for pulses of sound energy that appear during the on periods of the gate connected therein, means coupled to the monitor means and responsive to a differential energy distribution of said sound energy in said on periods to generate a control signal indicative of the same, and means for coupling said control signal to the high frequency signal producing means to alter its frequency such that the time duration of an odd multiple of half wavelengths thereof equals the direct acoustic delay between the loudspeaker and the microphone.