Abstract :
In recent years many designs of electromagnetic cutter-head for laterally-modulated recording on nitrate-coated discs have appeared, but the accounts published in the technical Press have generally been confined to reports on their performance, rather than on the theory of their design. In the paper an attempt has been made to set out a rather fuller statement of the fundamental principles of design of a recording head, including both the elementary theory of the vibratory system and the design of the electromagnetic system. The design of a balanced-armature type of unit is examined, and it is shown that there is no simple method of calculating the optimum values for the dimensions of the moving parts, since there are a number of variables, each one of which affects the optimum values of all the others. The causes of non-linear distortion are analysed, and it is shown that, by making certain approximations, it is possible to calculate the effect of the relative dimensions of the various parts of the system on distortion. As a practical example, the design and performance of a recording head are discussed. It is shown that the limitations set by the magnetic circuit make it impracticable to obtain modulation velocities of the order of 15 cm/s at 400 c/s unless the fundamental resonant frequency of the mechanical system is made less than 3 000 c/s.
