Selective automatic audio threshold circuit

Claim 1

An electronic system for controlling a radiophone transmitter whereby voice signals are transmitted in regard to a predetermined signal-plus-noise to noise ratio comprising in combination, a microphone positionable in a noise environment wherein random and periodic energy burst noise due to voice exist for detecting and converting such noise into electrical signals, an audio frequency amplifier provided with automatic gain control means having the input thereof coupled to said microphone and the output thereof coupled to the modulator of said radiophone transmitter and to a second audio frequency amplifier isolating a detector from said modulator, said audio frequency amplifier in response to electrical signals generated by the microphone in response to random noises producing a constant signal output through said second audio frequency amplifier to the input of said detector whereby said detector output voltage is maintained at its normal value and in response to periodic energy burst signal to apply through said second audio frequency amplifier a varying signal to the input of said detector whereby the output voltage thereof falls to half its normal value, a first Schmitt trigger having its input coupled to the output of said detector and responsive to the decrease in output voltage thereof to switch to the OFF state condition whereby a trigger voltage is produced at its output, a differentiator coupled between the output of said first Schmitt trigger and a normally saturated inverting amplifier whereby the output of said first Schmitt trigger is differentiated and fed to said inverting amplifier wherein differentiated fall of the output of said first Schmitt trigger appears at the output of said inverting amplifier as a pulse of constant width, and an integrator coupled between the output of said inverting amplifier and the input of a second Schmitt trigger whereby each said pulse of constant width is applied to the input of said second Schmitt trigger as a unit step voltage, said second Schmitt trigger having its output coupled to the transmit switch of the radiophone transmitter whereby when after a predetermined number of successive said unit step voltages are so applied to the input of the second Schmitt trigger it goes into an OFF state condition providing a trigger at the output thereof to said transmit switch whereby the radiophone transmitter is turned on. CM,2Nvention in accordance with claim 1 wherein said automatic gain control is a peak operating type which holds the noise level signal at the output of the audio frequency amplifier constant.