Automatic Correction of Timing Errors in Magnetic Tape Recorders

A basic problem in magnetic recording is the preservation of accurate time relationships between reproduced bits of information. If information is recorded at a given rate, the reproduced signal rate will tend to deviate from this rate. Servo systems for controlling the mechanisms of the machine can maintain the reproduced signal rate equal to the recorded signal rate, averaged over a period of a few milliseconds. Mechanical servos cannot, however, prevent short-term deviations in reproduced signal rate, because of their relatively slow response. Electronically variable delay lines (EVDLs) have been developed which, because of their relatively fast response, can further reduce timing errors. This paper presents a mathematical analysis of the problem and of the requirements imposed on the EVDLs. Two devices for the accomplishment of electronically variable delay are then described, one for high-frequency signals, and one for low-frequency signals. Experimental results obtained by using these two delay lines are also presented.