In modern technology, it is frequently necessary or desirable to monitor various operating conditions for industrial processes and to retain a record of this information which can be analyzed or compared if the need should arise. For this purpose, various recording devices adapted to inscribe a visible curve on a record medium, such as an elongated strip chart, a circular chart or the like, driven at a given rate of speed so that variations in the monitored condition as a function of time are recorded in visible form have been developed.
With increasing emphasis being placed on the operation of industrial processes by remote control with a maximum of automation, it has become necessary to transmit the operating parameters for the given processes to remote locations, and curve scanning and transmitting systems, which will be referred to here as a "curve line reader," have been developed for this purpose. Such a system typically utilizes a television camera tube on the face of which is projected an optical image of a given portion of the recording medium to be read, and this optical image is repeatedly scanned by the electron beam of the camera tube at the usual high scanning rate along a substantially linear locus, extending at right angles to the length of the medium to be scanned. Since the optical density of the visible curve line differs from the background density of the medium, a pulse will be created in the video signal each time the beam crosses the curve line. Thus, the video signal gives an electronic representation of the location of each precise segment of the curve scanned by the electronic beam and from these signals, the original curve can be recreated at a remote point or fed either as analong or digital values to an electronic computer serving to process the data as may be desired.
In has been found that curve line readers of this type provide a satisfactory indication of the curvature of the curve line being scanned so long as the direction of the curve line generally follows the direction of the relative movement between the recording medium and the camera tube, i.e. where all of the segments of the curve line extend at a significant angle relative to the scanning locus of the video tube. However, where the curve includes segments of a direction substantially parallel to the video scanning direction of the electron beam, as would occur when a sharp or instantaneous variation of significant magnitude in the process variable has been recorded, the representation of the position of such curve segments becomes inaccurate. In this case, the "apparent width" of the curve line being transmitted becomes greatly extended or expanded although the actual width of the line inscribed by the recorder does not in fact change, due to the coincidence between the directions of the electron scanning beam and the curve line segment being scanned. Since the diameter of the scanning electron beam has a certain finite magnitude and the leading and trailing edges of an almost parallel line segment being scanned only gradually intrude into and out of the path of the scanning beam under such circumstances, the rise and fall of the video signal pulse created at the crossing of the line becomes increasingly gradual or "dull," making identification of the precise geometrical position of the curve from the video signals difficult.