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Digital Encoding Scheme With Error Detection

IP.com Disclosure Number: IPCOM000036940D
Original Publication Date: 1989-Nov-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Conway, JL: AUTHOR [+2]

Abstract

In disk files it is usual to encode digital data, such as guardbands and index, in the servo pattern as well as Servo-ID marks (SIDs) and the Position Error Signal (PES). There are various methods of coding such digital data and it is desirable that the file electronics used to decode the data is simple and low cost. Also important is that errors due to disk defects or electrical noise can be reliably detected, to avoid giving false information to the file servo controller.

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Digital Encoding Scheme With Error Detection

In disk files it is usual to encode digital data, such as guardbands and index, in the servo pattern as well as Servo-ID marks (SIDs) and the Position Error Signal (PES). There are various methods of coding such digital data and it is desirable that the file electronics used to decode the data is simple and low cost. Also important is that errors due to disk defects or electrical noise can be reliably detected, to avoid giving false information to the file servo controller.

A simple scheme is to have a series of pulses recorded on the disk, which are separated by either a 'short' gap or a 'long' gap. Digital '0' and '1' can be coded as 'short' and 'long'. A Delta-V detector converts each pulse on the disk into a logic pulse. It is then a simple matter to use a high speed clock and a counter to distinguish between 'longs' and 'shorts'.

One disadvantage of this scheme is that if several bits of data are needed, then the total length of the field will depend on the values encoded (because '0' and '1' are different lengths). Another problem is that errors due to disk defects (which generally give missing pulses) or electrical noise (which gives extra pulses) are not detected. The scheme described below shows how these problems can be overcome. Implementation

The digital coding scheme is modified as follows. A '1' is coded as a 'long' as before, but a '0' is coded as two consecutive shorts, giving a simple FM code. Thus '1' and '0' both take the same time, which is nominally 555 ns. The data is arranged in 6-bit fields, of which the last bit is a parity bit. Errors are detected in two ways. First, there is a conventional parity check on the 6 bits. Secondly, additional logic is added to the long-short detector to check for invalid sequences of pulses.

The detector consists of a clock and a counter. The clock i...