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MAGNETIC SOUND FILM 497
High-Frequency Bias for Magnetophone Tape
However, further developments during World War II, especially in Germany by Braunmuhl and Weber,2 who applied the high-frequencybias method of recording to the oxide tape, changed the situation materially, as they produced a magnetic sound record greatly improved and far superior to the old d-c3 or even a-c4 biased Telegrafone and the Magnetophone, d-c operated until that time. It was now possible to obtain a quality comparable and even superior to any other recording means with a frequency response up to 20,000 cycles/sec and with the tape running at the reasonable speed of about 30 in./sec.
Application to the Motion Picture Industry
The motion picture industry now started to take an interest in these new developments in magnetic recording and the possibilities of introducing magnetic sound into the industry were examined in different countries.
In Great Britain immediately after World War II an examination of the advantages of the application of magnetic sound to the film industry was made. Investigations at that time revealed that a very high quality could be obtained from magnetic sound record carriers at a considerably reduced speed. At the speed of standard 35-mm film a frequency range up to 10,000 cycles/sec was obtained while at 16-mm speed up to 7,000 cycles/sec was obtainable and even at 8-mm speed up to 3,500 cycles/sec could be reproduced.
In addition to the excellent frequency response a very good signalto-noise ratio was obtained, being at least 10 db better than the best photographic sound film recorded with a ground noise reduction system.
Nevertheless, a lot of development work had to be undertaken subsequently to enable the new type of sound recording to be introduced into the studios. This work consisted mainly of two parts : firstly, the manufacture of perforated film base with the new magnetic coating ; and, secondly, the provision or adaptation of apparatus for the recording and reproduction of the new type of sound record carrier.
Magnetic Film Stock
Figure 1 shows three different types of magnetic film coatings (A, B and C) which have been evolved for use in the motion picture industry. In type A (Fig. 1) the coating is applied over the entire width of the film. For this purpose large sheets of a suitable non