Abstract: In the transmitter, a reference signal, produced by a voltage-controlled oscillator whose frequency is controlled by the battery voltage, is applied together with the signal to be transmitted to the transmission light emitting diode. In the receiver, the signal received is applied to a high-pass filter delivering the useful signal and to a low-pass filter reforming the reference signal. The same is compared with a predetermined voltage and the error signal is used for polarizing the reception diode and thereby maintaining constant the overall gain of the transmission channel. The output of the filter is also applied to a device controlling the voltage of the battery by measuring the frequency of the signal applied thereto.