GB377276A - An improved method of and means for electro-magnetically recording acoustic signals - Google Patents

Abstract

377,276. Electromagnetic soundrecording apparatus. DAHMEN, K., 29, Dorotheenstrasse, Dusseldorf, Germany. April 22, 1931, No. 11913. [Class 40 (ii).] Comprises a method for the electromagnetic recording of sound in which a plurality of different sound records are superimposed upon each other by producing a pair of records of each signal simultaneously and causing predetermined wave-form variations of the mean recording intensity of one while varying the mean recording intensity of the other inversely to the first, different wave-form variations of mean intensity being allocated to the different pairs of records. The record carrier consists of two pre-magnetized steel bands 1 which move over guide rollers 2 at constant speed, and above the bands 1 are arranged two electromagnets 11, 12. The electromagnets 11, 12 are moved towards and away from the bands 1 by the operation of oppositely-disposed cams 9, 10 which are driven by a shaft 7 rotated by a pulley 8. A lead 22 of the recording circuit is connected to the coils 3, 4 of the electromagnets 11, 12 through two variable resistances 13, 14, the sliding contacts 15, 16 of which are relatively displaced by an angle of 180‹. The other end of the coils 3, 4 is connected to the lead 21 of the recording circuit. When a record is being made, the contacts 15, 16 are rotated uniformly by sheaves 17, 18, so that the current strengths in the coils 3, 4 will be varied to follow a wave-form curve, the variations of current in one coil being the exact inverse of that in the other coil. The total mean intensitv of the magnetic influence acting on the bands will be constant however. At the same time, the magnets 11, 12 are alternately moved towards and away from the bands 1 to cause further inverse variations of the magnetic intensity. By suitably choosing the speeds at which the cams 9, 10 and sliding contacts 15, 16 are rotated records may be produced which vary in mean recording intensity in accordance with curves having predetermined characteristics. Reproduction of each recording may then be effected by rotating the cams 9, 10 and sliding contacts 15, 16 at the same speeds as were employed during the recording of the particular signal to be reproduced, the magnets 11, 12 then serving as reproducing devices which feed a loud-speaker through an amplifier. It is stated that, in these circumstances, the other recordings carried by the bands 1 will not be reproduced to an extent sufficient to render the required reproduction confused.