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Servo Error Detection using Pointers

IP.com Disclosure Number: IPCOM000088378D
Original Publication Date: 1977-Jun-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 4 page(s) / 66K

Publishing Venue

IBM

Related People

Gardner, HJ: AUTHOR [+2]

Abstract

Fig. 1 depicts a rectified readback servo signal which is associated with a data stripe on a length of flexible media. The servo signal is used primarily for aligning a rotating read head with the data stripe. A threshold is set at the 50% amplitude level. At point 10 a counter (not shown) begins to count up until a sync bit is detected at point 12. The counter then counts down to point 14. The difference between the up count and the down count is a measure of head track alignment. In order to check the information derived from servo burst 16, the procedure is repeated in servo burst 18.

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Servo Error Detection using Pointers

Fig. 1 depicts a rectified readback servo signal which is associated with a data stripe on a length of flexible media. The servo signal is used primarily for aligning a rotating read head with the data stripe. A threshold is set at the 50% amplitude level. At point 10 a counter (not shown) begins to count up until a sync bit is detected at point 12. The counter then counts down to point 14. The difference between the up count and the down count is a measure of head track alignment. In order to check the information derived from servo burst 16, the procedure is repeated in servo burst 18. If the difference in count 1 (which is derived from servo burst 16) and count 2 (which is derived from servo burst 18) exceeds a predetermined value, this is called a validity check and signifies that either one or both bursts have a defect. A "servo dead" is flagged and the error count is not used. With this scheme, both bursts must be good to produce a usable count (AND function).

Fig. 2 depicts a scheme whereby a pointer is created by checking the electrical signals for defects in each servo burst. This scheme changes the "AND" function for producing a usable count to an "OR" function for producing a usable count. As the servo signal is read for each burst, the signal is checked for defects and a pointer is stored for any burst that has a defect. If a validity check occurs following the procedure enumerated above, the pointers are observed to determine which servo burst is bad. Then the count for the good burst is used, and no servo dead is flagged. As depicted in Fig. 2, a sample-data voltage control system with an output ramp, which follows the peaks of the full-wave rectified readback signals, provides error signals within a controlled loop which are used as pointers to determine if the input is good in the region of the 50% threshold.

The output 20 of the sample-and-hold circuit 22 is a measure of the difference between the peaks of the readback signal and the ramp at each sampling interval. If a dropout occurs, the control loop does not respond instantly; thus, that error signal is a measure of the size of a dropout. If the error signal on terminal 24 exceeds the error referenced on terminal 26, a pointer occurs on terminal 30 which indicates a defect. The input signal to integrator 32 is a measure of the slope of the ramp. This is compared to a slope reference in comparator 34. If the difference between the ramp slope and the slope reference exceeds a predetermined value, a slope pointer is crea...