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Separation in Mils 0.2 0.4 0.6 0.8 1.0
§ 10
7,000 cps
I is
20
25 L
"ig. 3. Losses during playback with film eparated from head by small distances.
>ver a head. If the coating is flat and ree from bumps, good contact with the lead is achieved. At the upper right is ,een the film counterpart, with a flat, .mooth coating in good contact with the lead. The two lower views show what lappens when there is a small bump in :he coating. The tape, because it is nore flexible is lifted from good contact •lose to the bump only, leaving other parts )f the tape width in fairly good contact. The film, however, being too rigid to pend as the tape does, is lifted from the lead over the entire track width.
A similar effect can be imagined along :he length of the film with the result that :he film, because of its rigidity, remains out of contact with the head longer than ;:he more flexible tape whenever a bump masses over the head.
While in most uses of ^-in. tape these 'dropouts" are not troublesome, there ire some tape applications where they lire serious.
Telemetering applications, and appli:ations using amplitude-modulated high requency are sensitive to small dropouts A^hich would not affect most audio .vork. Also, in computing machines
ere seven narrow tracks are used on [--in. tape to record binary system Dulses, the tracks are so narrow that a
Tape
Film
Fig. 4. Results with thin, flexible tape base and comparatively rigid film base.
bump like that shown at the lower left of Fig. 4 could cause complete loss of a pulse in one or even two channels, causing an error.
The need for good, uninterrupted head contact forced the manufacturers of magnetic products to take unusual precautions against dirt, undispersed clumps of magnetic powder, lint particles and the like. Enormous progress has been made during the past few years, as tape and film were used more and more in critical applications. Even after all dirt was eliminated, and special precautions were taken against dirt contamination in the coating process, there were still occasional dropouts. These were traced to a type of formation in the coatings usually referred to as "nodules." These are typically spherical in form and have no detectable nucleus. Unlike other defects which cause dropouts, they could not be filtered out — not because they were too small — but because they form in the coating during the drying process. Attention to drying conditions has helped to minimize them, but we have never been able completely to eliminate them, and have never seen any magnetic coating without them.
Ernest W. Franck: Dropouts in Magnetic Film
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