GB2282911A

GB2282911A - Data tape formatting apparatus

Info

Publication number: GB2282911A
Application number: GB9420508A
Authority: GB
Inventors: Kevin L Miller
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1993-10-13
Filing date: 1994-10-11
Publication date: 1995-04-19
Anticipated expiration: 2014-10-11
Also published as: GB2282911B; DE4419761A1; GB9420508D0

Description

h 2282911 DATA RECORDING SYSTEM This invention relates to a data recording

system and to a method for positioning data tracks on a magnetic tape.

For low cost magnetic tapes used for data recording, one common format has longitudinal tracks written by drives that have a moveable magnetic head having one read/write gap which is stepped from track to track. Generally, for longitudinal track devices, the specification for each track position is an absolute position. Specifying an absolute track position facilitates tape interchange between drives and random access of tracks. Any drive can step its magnetic head to any track without having to search for the track.

Figure I (prior art) illustrates the QIC-80 standard, an example of a longitudinal track format which is commonly used for magnetic data tapes in the personal computer industry. Further information on the format illustrated in figure I may be found in Fle-xible-Disk-ControllerCot?ipatible Recording Fonnatfor Infonnation Interchange, available from Quarter-Inch Cartridge Drive Standards, Inc., 311 East Carrillo Street, Santa Barbara, CA 93101. Magnetic data tapes compatible with QIC formats are formatted by drives in a manner that is similar to formatting of flexible disks and hard disks. Tracks are written onto blank tapes by drives. Tracks are logically formatted into hierarchical levels of addressable blocks of data. The smallest addressable unit is a sector, each sector having an address and other overhead information recorded once during formatting and a data field which may be re-recorded many times. Once a track is formatted, the location of the track is fixed for any subsequent data recording. As illustrated in figure 1, the QIC-80 standard specifies 28 tracks on a tape which is 0.250 inches (6.35 mm) wide. Section 4. 1. 1 of the standard specifies the location of the centerline of each track relative to a recorded track location indicator. All even numbered tracks (forward direction) are specified as absolute distances from a first track location indicator 100 that is aligned with track 0 (104). All odd numbered tracks (reverse direction) are specified as absolute distances from a second track location indicator 103 that is aligned with track 3 (106). Track centerlines are spaced at.0085 inches (. 2159 mm) with track position tolerances of +.0010 inches, -.0012 inches (+ .0254 mm, -.0305 mm).

For QIC-80 compatible tapes, the track location indicators (100, 102) are written by the drive that formats the tape. The location indicators (100, 102) are specified as fixed frequency signals (7.35K flux reversals per inch) and are therefore referred to as reference bursts. The tape shifts position slightly when direction is reversed. Therefore, there are two separate reference bursts, one for each direction of tape travel. There is a gap between the reference bursts to ensure that a head read gap cannot straddle the two reference bursts and read a partial signal from each. The location accuracy of the reference bursts is specified relative to the tape overall centerline with a tolerance of +/-.0015 inch (0.0381 mm). The location of the tape centerline is typically mechanically determined relative to the edges of the tape which in turn are determined by driving the magnetic head into mechanical stops which are accurately located relative to tape edge guides.

Tape drives compatible with the QIC-80 format typically have a stepper motor head actuator geared so that multiple steps of the motor are required to traverse one track on the tape. For example, one commercially available drive with a stepper motor and lead screw arrangement requires 30 steps of the stepper motor to move the head.0085 inches (.2159 mm) from one track to the next. When a drive is powered on or when a tape is inserted, head position is initially calibrated by adjusting the head read gap position to the center of the appropriate reference burst. All subsequent head movement is typically performed open loop A by stepping the head actuator stepper motor the integral number of steps per track (30 in the example above) times the required number of tracks. Since any QIC-80 drive can format tapes, every QIC-80 drive must have a head positioning mechanism capable of absolutely locating tracks with the precision described above.

Magnetic media is subject to dimensional changes due to temperature and humidity changes. Head positioning mechanisms have inherent accuracy limitations which are further degraded by calibration inaccuracy, nonlinearities, and wear. As discussed above, tape position inevitably shifts with direction of travel. Media dimensional changes and various mechanical inaccuracies fundamentally limit the track density. For example, consider a hypothetical track density of ten times the QIC-80 standard (280 tracks instead of 28). The mechanical precision required to place a hypothetical 140 tracks with an absolute position tolerance of +/.0001 inches (.0025 mm) relative to one reference burst would require a much more expensive head positioning system than current low cost stepper motor and lead screw arrangements. Media dimensional changes and mechanical inaccuracies can be partially compensated by providing factory formatted tapes having embedded servo information within tracks (between sectors). An alternative solution is to provide factory written track position locators for every track. Either solution adds to the expense of the product. Embedded servo information requires real-time closed-loop head positioning that increases the cost of drives. Factory formatting or factory marking of track location increases the cost of magnetic media. Factory formatting or factory marking of track location might require multiple products, each formatted for a different standard, even though the unformatted tapes are identical. There is a need for tape drive systems having increased track density using low cost head positioning systems and unformatted tapes.

1 4 The present invention seeks to provide an improved data recording system.

Accordin4 to an aspect of the present invention, there is provided a data recording system comprising a magnetic tape having first and second tape edges; drive means for formatting the magnetic tape, wherein formatting includes writing the following: a plurality of track locators including a first track locator; a plurality of data tracks; the first track locator located relative to at least one of the tape edges; each track locator other than the first track locator located solely relative to the location of one adjacent previously written track locator; and each data track located solely relative to at least one track locator.

According to another aspect of the present invention, there is provided a method for positioning data tracks on a magnetic tape, wherein the magnetic tape has a first tape edge, the method comprising the following steps:

a. writing one or more track location markers positioned relative to at least one of the tape edges; b. writing a data track positioned relative to the track location markers of step a; c. if this is the first time step c is performed, writing one or more track location markers positioned relative to the track location markers written in step a; if this is not the first time step c is performed, writing one or more track location markers positioned relative to adjacent track location markers written the previous time step c was performed; d. writing a data track positioned relative to the track location markers of step c; and e. repeating steps of c and d.

It is possible with the invention to provide a magnetic tape data recording system with increased track density using relatively low precision (low cost) head 4a position control and blank (un-formatted) tapes. Each track can be individually located by location markers recorded by thd drive that formats the tape. The drive that formats the tape preferably positions each track relative to an adjacent track with an accuracy determined by the inherent accuracy of the head position actuator. As a result, absolute track position may vary substantially from drive to drive. when tapes are read or rewritten after formatting, compatible drives may use the track location markers to locate each track.

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which:

Figure 1 (prior art) is a plan view of a magnetic tape illustrating the track format specified by the QIC- standard. Figure 2 is a plan view of a magnetic tape illustrating track positioning in accordance with an embodiment of the present invention. 20 Figure 3 is a plan view of a magnetic tape having an alternative embodiment of track location markers. Figure 4 is a flow chart of the preferred method of relative positioning of markers and tracks.

M-e ptmary g_-al c)f th._ des=:kEd eltnjimmt is increased track density while using low cost head actuators and unformatted tapes. Since unformatted tapes are used, the preferred solution to the interchange problem is for the formatting drive to write track location markers for all tracks. Compatible drives must have head actuators capable of fine position adjustment (resolution) but need not have absolute position accuracy. Each drive other than the formatting drive must adjust head position at least once for each track. Track location markers eliminate the need for the formatting drive to have high accuracy absolute track positioning. Track location markers enable relative track positioning which in turn enables the formatting drive to use less precise (and therefore lower cost) head positioning compared to absolute track positioning.

Tape drives are typically used for archival data storage, hard disk backup, data exchange and other secondary mass memory applications. In contrast to magnetic or optical disk drives, tape drives are typically not used as random access devices. Data is typically stored or retrieved in long continuous blocks where data transfer rate within a block is important and access time to a specific block is less important. In typical applications for a tape drive, it is acceptable to search for a track or to adjust position at a track, as opposed to stepping open loop directly to a track. Therefore, in typical data tape applications, it is not necessary to have random open loop track changes without recalibration of track location.

In random access devices, the magnetic medium is typically formatted into logical blocks and data is written in a separate operation after formatting. To increase the data transfer rate in a tape drive for archival data storage, formatting and data writing both occur during the same operation. A track may be "written" in one operation which includes writing the formatting data and the track may be re-written in an operation which is limited to updating only tile data fields.

0 -6 is Therefore, in this specification, "formatting" refers to the first time a track is written and not necessarily to a separate operation.

Figure 2 illustrates one embodiment of a tape format with reference markers written by the formatting drive. Figure 2 illustrates 6 tracks numbered 05. In figure 2 as in figure 1, even numbered tracks are recorded in one direction and odd numbered tracks are recorded in the opposite direction. In figure 2, near the Beginning of Tape (BOT) 212, a reference burst (214, 216) is written during formatting for every other even numbered track. Near the End of Tape (EOT) 232, a reference burst (234,236) is written for every other odd numbered track. Each reference burst (214, 216, 234, 236) has a centerline (218, 222, 238, 242) aligned with the centerline of the corresponding track (200, 204, 234, 236). Reference bursts (214, 216, 234, 236) are preferably placed at least on each end of the tape as illustrated. Additional copies may also be placed within tracks at logical "volume" (fixed number of sectors) boundaries to improve track access time. Tracks without a reference burst (202, 208) have a centerline midway between two reference bursts. After formatting, a drive reading or re-writing a tape formatted as in figure 2 must locate the appropriate track by moving the tape to one end, stepping the head across the reference bursts (214, 216, 234, 236) while the tape is moving near one end, and adjusting the head to either the center of a reference burst (218, 222, 238, 242) or midway between two reference bursts (220, 240) as appropriate. In typical operation, the drive streams from end to end in a serpentine pattern so that time spent locating track position at each end of the tape adds little overhead to the overall transfer time.

If reference bursts (214, 216, 234, 236) were to be placed on every track, a reading head straddling two adjacent bursts would pick up some signal from each of two bursts. Therefore, the edges of the reference bursts could not be determined. Placing a reference burst on every other track ensures that a magnetic head reading the reference bursts generates distinctive maximum and minimum signal amplitude levels for determining reference burst edges and centerlines.

4 Generally, it is easier to detect the edges of reference bursts than to detect centerlines. Therefore, for example, a drive reading a tape after formatting will locate centerline 218 midway between reference burst edges 224 and 226, centerline 220 midway between reference burst edges 226 and 228 and so forth. Tile reference bursts (214, 216, 234, 236) may be a fixed frequency or any distinctive digital pattern.

In figure 2, the track centerline position for the first track (200) may be determined relative to tape edge guides in any of several manners which are well known and currently used by drives to locate reference bursts. For example, the magnetic head may be driven into mechanical stops, into mechanical switches or into light beam sensors, where the stops, switches or sensors are accurately located relative to tape edge guides. Alternatively, the head may be stepped across the width of the tape while writing to form a temporary "track" diagonally across the tape. Then, the temporary diagonal track is read while the head is again stepped across the width of the tape. The tape edges are then detected by observing where the signal disappears. For all tracks other than the first track (200), however, each track centerline position is determined by the inherent accuracy of the head position actuator of the formatting drive. That is, the formatting drive simply takes the appropriate integral number of steps per track. Track 1 (206) may be spaced with a gap between track 1 (206) and track 0 (200) to accommodate position shift with reversal of direction but track 1 (206) is still placed relative to track 0 (200) rather than at an absolute location. A subsequent reading or writing drive using a tape after formatting may have a slightly different amount of head movement for the same number of steps or a subsequent drive may have an entirely different gear ratio. To compensate, the subsequent reading or writing drive must locate the centerline of each track as described above. Specifically, for tapes formatted as in figure 2, track centerline positions are not determined by any absolute position accuracy relative to tape edges or to an overall tape centerline.

is Figure 3 illustrates an alternate embodiment of track location markers. In figure 3, tracks 200-210 with centerlines 218-222, 238-242 are the same as illustrated in figure 2. Track location markers 300-322 are reference bursts with edges (for example 326 and 328) adjacent to and either side of track centerlines. A magnetic head read gap is centered over a track centerline by adjusting head position until identical signal amplitudes result from consecutive markers. For example, to center a read gap on centerline 218, head position is adjusted until a signal amplitude from burst 300 is identical to a signal amplitude from burst 302 (and 304 and 306). Likewise, to center a read gap on centerline 220, head position is adjusted to equalize signal amplitudes from bursts 300, 310, 304 and 312. In the reverse direction, a read gap is centered on centerline 238 by equalizing signal amplitudes from bursts 316, 318, 320 and 322. Only a few reference bursts (300322) are illustrated. There are as many sequential reference bursts as necessary to enable the head position control system to converge on a track centerline before reading track overhead information.

A drive formatting a tape as illustrated in figure 3 must first locate tile center of the tape from edge of tape information as described above. Then, for example, reference bursts such as bursts 302 and 306 are written. Then the head is stepped one-half the track spacing to format track 0 (200). After returning to the BOT 212, the head is stepped onehalf the track spacing to write bursts 300 and 304 and then another onehalf track spacing to format track 2 (202). A gap 324 between pairs of bursts provides offset direction information to the head positioning mechanism. For example, for centerline 218, the head positioning algorithm knows that in tile forward direction, the first burst after a gap 324 (burst 304) is proximal to centerline 218 relative to the overall tape centerline and the second burst after the gap 324 (burst 306) is distal to centerline 218 relative to the overall tape centerline.

If a predetermined number of tracks is desired, a tape formatted as in figures 2 or 3 can have a predetermined number of tracks. The predetermined is number must be based on a worst-case (maximum) track width. If maximum tape capacity is desired, a formatting drive can maximize tape capacity by simply writing tracks until reaching a tape edge. A drive having a relatively narrow track width can write more tracks than the predetermined number determined by a worst case restriction.

Figure 4 is a flow chart illustrating the method of relative positioning of track location markers during formatting. At step 400, the process is initialized by locating the innermost marker and associated track relative to one or both tape edges (for example, figure 2, marker 214 and track 200). In steps 402 and 404, the mark-er(s) and the associated track are written. In step 406, the head is moved relative to the previously written markers. If maximum tape capacity is desired (step 408), new tracks are written until the edge of the tape is reached (step 410). If a predetermined number of tracks are desired, new tracks are written until a predetermined number of tracks is reached (step 414).

Other track location marker embodiments may be used. The important parameters are: (a) the drive which formats the tape xxald aim uTite the track location markers and (b) all compatible drives 9muld have hea3 pcsiticn Omtrol systems with sufficient resolution to center a head read or write gap over each track centerline. The resulting benefit is that high track density is obtained with blank (unformatted) tapes and without requiring high accuracy absolute head position control.

M-e fmWing descriptim is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of tile invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated.

The disclosures in United States patent application no. 08/135,714, from which this application claims priorit, and in the abstract accompanying this application are incorporated herein by reference.

Claims

CLAIMS What is claimed is: 1. A data recording system Comprising: a magnetic tape having first and second tape edges; drive means for formatting the magnetic tape, wherein formatting includes writing the following: a plurality of track locators including a first track locator; a plurality of data tracks; the first track locator located relative to at least one of the tape edges; each track locator other than the first track locator located solely relative to the location of one adjacent previously written track locator; and each data track located solely relative to at least one track locator.

1

2. A data recording system as in claim 1 further comprising: each data track having a track centerline; each track locator comprising at least one marker; each marker having two marker edges; and each track centerline located midway between two marker edges.

A

3. A data recording system as in claim I further comprising: each data track having a track centerline; each track locator comprising a plurality of markers; each marker in the plurality of markers having a marker first edge and a marker second edge; and each track centerline aligned with at least one mark-er first edge and aligned with at least one marker second edge.

4. A method for positioning data tracks on a magnetic tape, wherein the magnetic tape has a first tape edge, the method comprising the following steps: a. writing one or more track location markers positioned relative to at least one of the tape edges; b. writing a data track positioned relative to the track location markers of step a; c. if this is the first time step c is performed, writing one or more track location markers positioned relative to the track location markers written in step a; if this is not the first time step c is performed, writing one or more track location markers positioned relative to adjacent track location markers written the previous time step c was performed; d. writing a data track positioned relative to the track location markers of step c; and c. repeating steps c and d.

1 13

5. A method as in claim 4 wherein step e further comprises repeating steps c and d until a location of the track location markers in step c is at one tape edge.

1#

6. A method as in claim 4 wherein step e further comprises repeating steps c and d until a predetermined number of data tracks have been written.

7. A data recording system substantially as hereinbefor described with reference to and as illustrated in Figures 2 and 4 or Figures 3 and 4 of the accompanying drawings.

8. A method of positioning data tracks on a magnetic tape substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 4 or Figures 3 and 4 of the accompanying drawings.