Abstract: 1. A magnetic disc coder-decoder comprising a rotatable magnetic disc mea said disc means being adapted to have a code comprising a plurality of tone bursts recorded thereon, means for rotating said disc means, means for taking said tone bursts from said disc means and transmitting said code in the form of pulses of radiant energy, means for receiving transponder reply pulses in the form of coded infrared flashes, means for converting said infrared flashes to tone bursts, means for recording said tone bursts on said disc means, a coincidence detector, means for taking said tone bursts from said disc means and means for applying said tone bursts to said coincidence detector, said coincidence detector providing a single pulse output only upon simultaneous application thereto of a predetermined number tone bursts.