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Apparatus and method for reading helically recorded tracks and rereading tracks as necessary
| Details |
Inventors: Georgis, Steven P.; Pisciotta, E. Christopher;
Assignee: Exabyte Corporation (Boulder, CO)
Primary Examiner: Psitos; Aristotelis
Assistant Examiner: Garland; Steven R.
Attorney, Agent or Firm: Griffin Branigan & Butler
A helical drive system (10) reads tracks (18) on a storage medium (12), including tracks which appear as distorted tracks. Each track (18) comprises a plurality of blocks of data, with each block having unique block-identifying information. A track-following servo system (84) causes a read head (16C) to traverse predetermined original azimuthal paths (P0) across the storage medium (12) for reading the blocks recorded along each of the original azimuthal paths (P0). A controller (50) uses the block-identifying information to determine whether any blocks expected to be read during the traversal of the original azimuthal paths (P0) were not read. If expected blocks were not read, the controller (50) repositions the storage medium (12) and directs that the medium (12) be re-read with the head (16C) positionally offset until the expected blocks are actually read. During the re-read attempts, the read head (16C) is positionally offset to traverse sets of modified azimuthal paths (P1, P2, P3, . . . P10 ). Each set of modified azimuthal paths (P2, P3, . . . P10) is offset with respect to the original azimuthal paths (P0) relative to a direction of medium travel (15), thereby providing opportunities to read blocks which would not otherwise be read due to distortion of the tracks (18). In one mode of the invention, the controller (50) dynamically selects from among a plurality of offset degree values based on the historical success rate of the differing offsets in reading blocks which formerly were missing blocks. |
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DETAILED DESCRIPTION OF THE DRAWINGS A helical-scan drive system 10 for recording on and reading magnetic tape 12 is illustrated in FIGS.
1 and 2.
A drum 14 is angularly oriented with respect to the edges and direction of travel 15 of magnetic tape 12.
As indicated in FIG.
2, drum 14 has heads 16A, 16B, and 16C mounted thereon to establish a physical relation between the heads on the drum 14 and the tape 12.
Head 16A is a write (or recording head); head 16B is a servo head; and, head 16C is a read (or playback) head.
By the illustrated arrangement, data is recorded on the tape 12 at an angle with respect to the direction of travel 15 of the tape 12, and hence, as indicated in FIG.
1, is recorded as discrete tracks or stripes 18 when the drum is rotated at a high speed relative to the speed of the tape 12.
As indicated in FIG.
2, the tape is preferably moved at a rate of 10.
89 millimeters per second while the drum is rotated at 1800 rpm, the tape speed being controlled by the rotational speed of capstan 19.
It is to be realized, however, that the operating speeds as set forth are by way of example and the invention is not meant to be limited thereto.
FIG.
3 shows a plurality of discrete tracks 18.
sub.
1, 18.
sub.
2, 18.
sub.
3, and 18.
sub.
4 properly recorded on tape 15.
Being properly recorded, each of the tracks 18 extends in a straight line for a predetermined length (depicted by line 22) at an a angle 24 (also known as a "track angle" or "stripe angle") relative to the direction of tape travel (indicated by arrow 15).
The centerlines of adjacent tracks are separated by a track pitch, represented by reference numeral 26 in FIG.
3.
The tracks are separated by inter-track guard bands 28.
In the illustrated embodiment, the a angle 24 is on the order of 4.
9 degrees; the track pitch is on the order of 31 micrometers.
FIG.
3 also shows a format of tape 12 usable with the present invention.
The tape 12 is block oriented, with eight data blocks B0.
sub.
x, B2.
sub.
x, .
.
.
B7.
sub.
x being provided per track 18, tracks x=1, 2,
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I/O Systems 
