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Compensating for Static Skew During Record Read Initialization

IP.com Disclosure Number: IPCOM000078748D
Original Publication Date: 1973-Mar-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 38K

Publishing Venue

IBM

Related People

Irwin, JW: AUTHOR

Abstract

For increased throughput in magnetic tape readback systems, it is highly desirable that each self-clocking track start reading data at the appropriate time. For example, in data formats where binary 1's are represented as flux transitions and binary 0's as absence of such transitions, the demarcation between the preamble and the data can be a binary 0. For example, in the illustrated format, the preamble consists initially of alternate 1's and 0's followed by a long string of 1's. Then, the marker signal, all 0's in all tracks, follows the string of 1's. The beginning of data in each of the tracks is denoted by a string of 15 1's followed by a 0.

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Compensating for Static Skew During Record Read Initialization

For increased throughput in magnetic tape readback systems, it is highly desirable that each self-clocking track start reading data at the appropriate time. For example, in data formats where binary 1's are represented as flux transitions and binary 0's as absence of such transitions, the demarcation between the preamble and the data can be a binary 0. For example, in the illustrated format, the preamble consists initially of alternate 1's and 0's followed by a long string of 1's. Then, the marker signal, all 0's in all tracks, follows the string of 1's. The beginning of data in each of the tracks is denoted by a string of 15 1's followed by a 0. When the string of 1's in the preamble consists of 30 or more 1's, random readback errors such as caused by tape-to-media separation, media defects and the like, can result in the readback system falsely indicating start of data in any one or more of the tracks. Such defects and dropouts appear as binary 0's, i.e., absence of transitions. Accordingly, it is desired to minimize this problem by gating readback signals (1's) to beginning-of-data detectors, not shown, which precede the marker signals by a minimum amount. Since each preamble in all of the tracks is of known fixed length, this minimum number of signals is known. Accordingly, when the electromagnetic transducer or head is disposed perfectly transverse of the tape, i.e., there is no static skew, then all of the signals from all of the tracks will be read substantially simultaneously. In such a situation, the minimum gating window can be employed for gating the string of 1's immediately preceding each marker signal to the respective beginning-of-data detectors in the readback system.

In practical magnetic tape readback systems, the tape-to-head relationship is not perfectly perpendicular. Each head has a different amount of linear or static skew indicated in the diagram by the dash line labeled Skew "A". As a result of such skew, which is unknown and random between various tape units, there is a time delay T between the leading track and the most-lagging track in a tape system. Generally, these are the two outside tracks 4 and 5. With the head skewed as shown, track 5 is the leading track; i.e., the gap in the head for scanning track 5 is most upstream and hence encounters signals from track 5 time T before signals from track 4 a...