Foreground subject-identifying apparatus

A foreground subject-identifying apparatus which comprises a first signal-generating means for producing a first signal having a plurality of frequencies falling within a prescribed range;

• second signal-generating means for sending forth a second signal having a higher frequency than the first signal;
• first frequency mixing means for mixing output signals from the first and second signal-generating means and modulating the second signal by the first signal to produce a modulated signal;
• first transmission and reception means for transmitting an output signal from the first frequency mixing means;
• second transmission and reception means for receiving a signal sent forth from the first transmission and reception means;
• coding means provided with at least one resonance circuit to generate a coded signal corresponding to the frequency of said resonance circuit;
• a nonlinear circuit provided with a nonlinear element, coupled between the second transmission and reception means and coding means, and adapted to detect a signal issued from the second transmission and reception means to supply the coding means with a signal corresponding to the first signal, and modulate a signal corresponding to the second signal by a coded signal from the coding means to deliver a modulated signal to the second transmission and reception means;
• first dividing means for dividing a signal received through the first transmission and reception means into two portions;
• second dividing means for dividing an output signal from the second signal generating means into two portions having a phase difference of (n+1/2).pi. (n is a positive integer);
• second frequency mixing means having first and second frequency mixers connected to receive at one input terminal respective output signals of the first dividing means and to receive at the other input terminal respective output signals of the second dividing means, thereby respectively producing output signals corresponding to the frequency of an output signal from the coded means;
• third frequency- mixing means having third and fourth frequency mixers connected to receive at one input terminal respective output signals from the first and second frequency mixers and to receive an output signal from the first signal generating means at the other input terminal first and second squaring means for squaring output signals from the third and fourth frequency mixers, respectively;
• an adder for adding together output signals from the first and second squaring means to generate a signal corresponding to the coded signal;
• and a coded signal-interpretting circuit for judging the coded signal from an output signal of the second frequency mixing means.