output terminal 54 appears the signal formed by the first polar coordinate quadratic curve system the output i of the latch 48 is supplied to a read only memory hereinafter called rom 30 for producing a square root and of which the output j is latched to a latch 49 and then produced at an output terminal 55 this output j is the key signal generated on the basis of the second ellipse system moreover the output e x of the latch 43 is supplied to a delay circuit 50 for phase matching an output k thereof is supplied to a subtractor 51 where it is subtracted from the output j of the latch 49 and a subtracted output l is produced through a latch 52 since the delay circuit 50 also has a function to double the level of data the output l of the latch 52 becomes j k j 2e this output l is multiplied by 1 3 by a rom 31 so that an output m 1 3l is produced through a latch 53 and then delivered to an output terminal 56 this output m is the key signal generated on the basis of the hyperbola system mentioned before the outputs i j and m produced at the aforesaid three output terminals 54 55 and 56 respectively are supplied to the inside side of a selector 57 and the selector 57 is controlled by a control signal from the microcomputer to permit the key signal based upon a desired system to be produced at an output terminal 58 in.