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\ Study of Dropouts n Magnetic Film
y ERNEST W. FRANCK
Recording and playback losses caused by coating imperfections are evaluated quantitatively. A new technique is described, called "Micropolishing" which physically removes the mound or "nodule" type of imperfection.
JL HERE ARE so many ways in which nagnetic sound tracks resemble photo;raphic tracks that motion picture :ngineers have learned to use the new nedium in a remarkably short time. )ne soon feels equally at home with a >hotographic slit or a gap in a ring-type lead. Many problems, such as azinuth troubles, are common to both systems. In making photographic tracks, \harp focus determines resolution. In a nagnetic system, resolution depends on \ood contact of the film with the record 'lit. In most parallels of this nature, •he magnetic track is found to be less :ritical than the photographic, although here are some exceptions. Our present ubject concerns one of these exceptions. To show how sensitive a magnetic i-ecording system is to small separations !)etween the film and the heads, some jneasurements were made on a Westrex
'resented on October 10, 1952, at the
Society's Convention at Washington, D.C.,
>y Ernest W. Franck, Magnetic Materials
3iv., Reeves Soundcraft Corp., Spring
lale, Conn.
This paper was received on January 22,
953.)
RA 1497 Recorder. Small nonmagnetic shims of various thicknesses were placed between the head gap and the film. This caused a separation between the film and the head equal to the shim thickness, and the loss for each spacing was measured.
Figure 1 shows the loss at various spacings up to 1 mil at a frequency of 7000 cycles/sec. The loss shown is the difference in playback level caused when the shim is in position and lifts the film from the head by the shim thickness. The lower line shows 7000-cycles/sec losses where the bias current is set to give peak 1000-cycles/sec response, in this case 20 ma. The upper line shows 7000-cycles/sec losses when the bias is increased to 33 ma and indicates a considerable reduction at the higher bias. At a spacing of 1 mil the increase in bias has reduced the loss from 30 db to 19 db.
In Fig. 2 are shown similar results at 1000 cycles/sec. The 1000-cycles/sec recording loss is seen to be the same as the 7000-cycles/sec recording loss in Fig. 1 when the bias current is 20 ma. The recording loss at 1000 cycles/sec is also
April 1953 Journal of the SMPTE Vol. 60
507