Logic for a Digital Servo System

Digital storage of information 0 1 photographic emulsion is char1 acterized by the large amounts of information that can be stored 0 1 a 1 small physical area.1 This same advantage also implies the need for exceptionally high precision in the access facilities. A memory system of this type has been developed which uses a cathode ray tube to interrogate simultaneously a group of photographic plates.2'3 4 The access problem in this store is to position an electron beam in accordance with a binary number to an accuracy of a fraction of a thousandth of an inch with microsecond positioning times. The binary address calls for digital circuitry, while the high accuracy implies the use of feedback techniques. The positioning technique adopted uses a fraction of the storage capacity to indicate, in parallel digital form, the present position of the 1 2 THE B E L L SYSTEM T E C H N I C A L J O U R N A L , JANUARY 1959 cathode ray spot. This digital indication is then compared with the binary number representing the desired position. The comparison is performed in circuitry called the digital servo logic. The output of the servo logic is an analog error signal which drives the electron beam to eliminate the positional error. The use of this feedback technique permits the beam position to be determined by the mechanical edges of bar patterns on a group of photographic plates. This relaxes the mechanical tolerances on the optical system and reduces the need for high precision in the electrical circuits.