Browse Prior Art Database

Window-Shifting Mechanism in Data Separator

IP.com Disclosure Number: IPCOM000040369D
Original Publication Date: 1987-Nov-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 3 page(s) / 130K

Publishing Venue

IBM

Related People

Cunningham, EA: AUTHOR [+4]

Abstract

The mechanism effectively shifts the window in which a bit is read from a disk file by its head so as to assure that the bit is correctly read. The mechanism consists of means which utilizes a method of rapidly establishing a new definition of the window center provided by the VFO (variable frequency oscillator) controlling the position of the window with respect to disk rotation in which a certain bit is read and the method of employing this window-skewing action for error recovery. The block diagram is a representation of a window-skewing scheme implemented in the VFO of a typical disk reading system. The control bits determine the amount and direction of timing window shift, as shown in Table 1.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 53% of the total text.

Page 1 of 3

Window-Shifting Mechanism in Data Separator

The mechanism effectively shifts the window in which a bit is read from a disk file by its head so as to assure that the bit is correctly read. The mechanism consists of means which utilizes a method of rapidly establishing a new definition of the window center provided by the VFO (variable frequency oscillator) controlling the position of the window with respect to disk rotation in which a certain bit is read and the method of employing this window-skewing action for error recovery. The block diagram is a representation of a window-skewing scheme implemented in the VFO of a typical disk reading system. The control bits determine the amount and direction of timing window shift, as shown in Table
1. Selecting the proper control lines prior to a read operation or, as in the case of the data recovery application, a re-read, conditions the data standardizer to compare data pulses against non-centered window edges. The result of the non- centered detection window edges is better performance for the error site containing a single shifted data bit. The technique of interest is the employment of the window skew circuitry to aid in recovery from the most dominant error type, herein single shifted bit errors. The proper amount of window skew must be determined. This number seeks to strike a balance between sufficient skew to recover the single bit in error and excess skew that would detrimentally raise the probability of other data bits being read in error. Once the value of skew has been designed into the VFO, all that remains is to determine the place in the total data recovery procedure (DRP) that the "re-read with shifted window" step is to be employed.

(Image Omitted)

Employing the VFO Window Shift or "Data Strobe Offset" hereof in conjunction with a re-read operation as a portion of the DRP can greatly enhance the overall effectiveness of the procedure. This step is specifically designed to assist in recovering from time shift errors. Referring to Table 2, upon detection of an error through a miscompare of the freshly calculated ECC value to that written with the data, the DRP is initiated. For this procedure it was decided that rereads and headshifts be performed prior to the Shift Window Step. After headshifts have been tried, continued miscomparing of the ECC bytes would invoke the Shift Window Step in which the programmable VFO Window or data strobe is offset to recover from a pos...