We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
Separation in Mils 0.2 0.4 0.6 0.8 1.0
Fig. 1. Loss at various spacings up to Fig. 2. Loss at various spacings up tc 1 mil at a frequency of 7000 cycles/sec. 1 mil at a frequency of 1000 cycles/sec.
reduced when the bias current is increased from 20 ma to 33 ma. At this higher bias current the 1 000-cycles/sec loss is less than the 7000-cycles/sec loss. The greater loss at the shorter wavelength is perhaps due to a loss of gap definition.
Similar measurements were made of the losses which occur during playback when the film is separated from the playback head by small distances. A constant level was recorded on the film, and the same nonmagnetic shims were used to separate the film from contact with the head by known distances. The results are shown in Fig. 3. It will be noted that the 1 000-cycles/sec playback loss is comparatively small but the 7000-cycles/sec loss is large, as would be expected from the geometry involved.
If a magnetic film were to have some coating imperfection, such as a nodule, or a piece of embedded dirt, the film would lift from contact with the record head during recording and from contact with the playback head during playback with the result that the combined recording and playback losses are experienced. These losses, although of only momentary duration may be so large that they are
very noticeable. A separation of 0.0002'; in., for instance, will cause a loss of Hi db during recording, and an 8-db loss] during playback at 7000 cycles/sec.) when the bias is 20 ma (peak output atj 1000 cycles/sec), or a total loss of 19 db.: With the bias increased to 33 ma the) loss would be reduced to only 11 db.l Similar losses at 1000 cycles/sec would; be only half as much because the playback loss is much less. With this;! sensitivity to separation of film from the head gap in mind it is easy to understand the severe requirements for flatness and smoothness of a magnetic track.1 Any small coating defect which lifts the<i film from the gap gives a momentary i drop in output. This effect is commonly referred to as a "dropout."
It is fortunate that -^-in. tape products ' came into general use before magnetic! film products because a small coating imperfection causes much less trouble on thin, flexible tape base than it does on! thick and comparatively rigid film base, i The reason for the difference is illustrated in exaggerated scale in Fig. 4.
On the upper left of Fig. 4 is shown a ( section through ^-in. tape as it passes i
508
April 1953 Journal of the SMPTE Vol.60