Abstract: An audio frequency converter apparatus which includes an input amplifier, first threshold sensor, connected to an output of the input amplifier, for cutting off a voltage supplied by the input amplifier at a threshold, after rectification, and second threshold sensor, connected to the output of the input amplifier, for adding at a threshold the voltage supplied by the input amplifier after rectification. The second threshold sensor has a time constant which is at least equal to that of the first threshold sensor. A voltage controlled oscillator is connected to the output of the first threshold sensor to provide an oscillating output having a frequency that decreases with an increase in the output of the first threshold sensor.