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1950 MAGNETIC SOUND FILM 505
coating on the film on which sound has already been recorded. This head picks up the recorded signals and the induced currents are fed to an amplifier, rectified, and applied to an electro-mechanical device located adjacent to the magnetic head. This device carries a stylus, in a somewhat similar manner to a gramophone needle, at the end of the arm shown protruding from the casing. The device is energized when the rectified signal currents are fed to it and the stylus, which is of the metallic alloy kind described above, traces a visible signal indication of the type shown in Fig. 1-C corresponding to the adjacent magnetically recorded sound signals. From this trace it will be seen that periods of silence are indicated by a straight, unmodulated line while sound passages are indicated by modulation of the
Fig. 7. Apparatus for adding a visible signal indication record to the magnetic sound film.
line. By these means the beginning and end of words and sound passages can be easily and accurately located whereby the editing and precise cutting of the magnetic sound record is greatly facilitated.
Joints
During the editing and cutting of the magnetic sound films a certain amount of trouble was experienced at first when making the joints as it was found that steel scissors and film splicers having steel plates and cutters often tended to magnetize any such joints made with them.
Magnetization caused in this way is only partly removed by running the film over the usual erasing head as the latter loses contact near and on the joint and therefore effects only a partial erasure.
Two ways have been evolved for overcoming this difficulty.