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Transmitter-receiver with automatic alternation control

Image Number 3 for United States Patent #4326293.

Prior to each voice activity period, the transmitter supplies a digital preamble for coding the speech signal levels and during these periods high frequency signals modulated in substantially constant amplitude, but transmits nothing other than said preamble and said periods. On reception, a signal detector and logic elements control a switch to obtain only speech signals at the input of an attenuator. The preamble decoding circuit controls the attenuation value of the attenuator in order to resupply to the receiver output signals of the same relative level as those applied to the transmitter input.

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