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648 RICHARDSON'S BLUEBOOK OF PROJECTION
of the synchronizing impulses, no change in scanning would follow. At any frequency the signal impulses are purposely made too weak to actuate the grids of these scanning circuits.
(21) Additionally, the scanning impulses, when received, are applied not only to their own saw tooth oscillator tubes, but they are also connected with the gridcylinder of the cathode ray tube. In virtue of the fact that they are considerably stronger than the video frequencies they cut off the electron beam in the cathode ray tube entirely during the period of "fly back." In other words, when the beam has reached the end of one line it "goes out" and resumes again at the beginning of the next line. Similarly when it has reached the bottom of the field it is cut off and resumes at the top of the field.
(22) The reader will understand that only one method of achieving given results has been described here and even then only in outline. The gas-filled triode is not the only tube arrangement that can be used to generate saw tooth impulses; other tubes in other circuits are sometimes employed. There are cathode ray tubes in which deflection of the electron beam is secured by means of magnets mounted outside the tube instead of by means of vanes inside the tube. The basic outline of the superheterodyne receiver does not vary, but, as said, the oscillator circuit of that receiver may embody a separate tube or may be incorporated with the first detector circuit in a pentagrid converter arrangement. It is not possible to go into the vast number of details which permit the general methods outlined to be applied through patented differences in component parts and specific circuits. The space available allows only a general outline of the basic principles underlying the performance of theatre television equipment, and one example, at most, of how each of those principles is embodied in concrete apparatus.