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504 O. K. KOLB November
heads are connected to a mechanical assembly by means of which all movements necessary for adjustment are carried out.
Synchronization
Contrary to a photographic sound record, it is impossible on a magnetic record to ascertain whether or not sound has been recorded on the film merely by looking at it and should sound have been recorded it is impossible to see where the recorded signals begin or end.
The simplest way of obtaining an approximate idea of the beginning and end of sound passages on the film for the purposes of synchronization, editing and cutting is to rely upon an audible signal reproduced from the film. This was easily arranged on the existing photographic sound-editing machines by removing the optical system and exciter lamp and replacing these parts with a magnetic pickup head.
In certain cases it is more advantageous to provide the editor with a visible indication of the actual sound signals. This visible indication could be a registration of similar indication of the actual signals or a registration of their envelope or an indication derived from them.
In Fig. 1-C the envelope of the invisible magnetic sound signals contained in the left side of the sound record is indicated by the trace T on the right side. This visible registration of the sound signals can be effected in several ways, for example:
(a) by an inking method in which a small nib fed with indelible ink traces the envelope of the sound waves;
(b) by a stylus which engraves the visible indication on the white coating; or by
(c) a dry chemical process in which a stylus made of a special metal reacts with a chemical compound in the white coating shown in Fig. 1-C.
Method (c) has given the best and quickest results and has been used on film stock of the kind shown in Fig. 1-C in which the white coating consists of zinc oxide mixed with a nitrocellulose lacquer and coated on the film. The recording stylus is made of bronze or brass or a similar alloy and the reaction between the metal stylus and the coating forms the trace T. This latter process has the advantage that it is entirely dry and can be carried out at almost any speed.5
Figure 7 illustrates apparatus which has been evolved for the purpose of adding a visible signal indication record to a magnetic sound film ; this consists of a magnetic pickup head of the ring-shaped kind (recognizable in the photograph by the circular side plate held in position by screws) which is arranged in contact with the magnetic