1975 Volume 27 Issue 5 Pages 425-436
The paper describes a data conversion and editing system devised to handle large amounts of data produced on film by recording magnetometers used in array studies. The field data are in the form of five analog traces on 35mm photographic film. The film is scanned by a fine light spot obtained from a flying spot scanner cathode ray tube. The spot position and intensity are controlled and corrected by dedicated electronic hardware. The intensity of the light transmitted through the film is measured and converted to digital form suitable for recording in I. B. M. compatible magnetic tape format. At this point the data may be recorded on an incremental tape transport, without further modification, to form a minimum cost system. Our system was expanded to use part of the time available on a local PDP-11 computer, which was already controlling a continuous data link to the University of Alberta Geophysical Observatory. The optical scanning is controlled asynchronously as the lowest priority task. A first level editing procedure uses centroid calculation and noise rejection to reduce by a factor of about 13 the number of data points recorded. The reduced and buffered data records are written synchronously in I. B. M. compatible form on magnetic tape. The final editing, including the sorting of crossed traces, is done at the I. B. M. 360-67 installation by direct operator interaction via a graphic terminal and lightpen. While the system was devised for the specific magnetometer array task the basic hardware could be used quite generally for reliable and economic conversion of analog traces on 35mm film to a computer compatible digital form. The low-cost practical limitations produce a system with a definition of -106 points/square frame with a 64 gray level acceptance scale for the transmitted light.