Most phase encoding systems for magnetic recording are fairly intolerant of variations in head-to-medium speed. Because of the high cost of the sophisticated equipment for assuring constant speed, a great deal of effort has been expended toward developing encoding systems which are less sensitive to speed changes. Most such systems employ two tracks and hence require dual electronics.
U.S. Pat. No. 3,720,927 (Wolf) concerns a time-ratio encoding system which requires only one track of the recording medium and yet is quite insensitive to speed changes. Binary 1's and 0's are recorded sequentially along the track, each bit having a constant time period represented by a first voltage level during the initial part of the time period and a second voltage level for the remaining part of the time period. To represent a 1, the first voltage level is maintained for more than one-half of the time period; to represent a 0, the first voltage level is maintained for less than one-half of the time period. The data is decoded by counting up until a transition from the first to second voltage level and then counting down at the same rate until the next transition to the first voltage level. A positive count indicates a 1 and a negative count indicates a 0.