Flat servo bursts for arcuate track scanner

Details Flat servo bursts for arcuate track scanner Flat servo bursts for arcuate track scanner

2000-07-24

2001-09-04

Sniezek, Andrew L. (Department: 2651)

Dynamic magnetic information storage or retrieval

General processing of a digital signal

Data in specific format

C360S064000, C360S076000

Reexamination Certificate

active

06285519

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to tape drives having an arcuate scanner, and more particularly to information formats and servo control techniques of the scanner heads based on signals within the information format, to accurately control the arcuate scanner during reading and writing operations.
DESCRIPTION OF RELATED ART
A number of magnetic tape drive systems have been developed to provide mass data storage, for example for personal computer systems. One emerging technology providing high density storage, preferably on quarter inch magnetic tape, utilizes arcuate scanning. With this type of scanning, read and write scanner heads are mounted near the periphery of a circular planar surface and rotated thereon about an axis passing through the center of the circular surface and through the plane of a longitudinally-moving tape. In writing data on a tape, arcuate scanners produce a sequence of arcuately-shaped tracks which are transverse to the longitudinal axis of the tape.
Examples of arcuate scanning tape drives are described, for example, in: U.S. Pat. Nos. 2,750,449; 2,924,668; 3,320,371; 4,636,886; 4,647,993; and 4,731,681.
International Application WO 93/26005 to Lemke et al. discloses an example of an arcuate scanning tape drive for computer data storage, and the disclosure thereof is expressly incorporated herein entirely by reference. In the Lemke et al. arcuate scanning tape drive, a number of scanner heads are provided around the periphery of the circular planar surface. As the scanner rotates and the tape moves past the rotating scanner surface, the read and write heads alternately pass over the tape. The operation of the scanner is commutated or switched from “write” to “read” to alternately activate the appropriate operation through alternate ones of the scanner heads.
To effectively read and write data in arcuate tracks on a longitudinally moving tape requires (1) writing the tracks in an agreed format, position and alignment on the tape, and (2) corresponding positioning and alignment of the read heads during the read operation to locate and recover the data written on the tracks. In an arcuate scanner of the type described by Lemke et al., there are a number of variables which effect both the read operation and the write operation. These include tape speed, rotational speed of the scanner head and orientation of the scanner head with respect to the tape. Several of these variables are effected by external factors. For example, if there is some vibration of a scanner during the writing operation, it may be difficult to align the head with the data tracks during a subsequent read operation, particularly if the read operation is performed by a different scanner.
The above cited Lemke et al. document discloses the most effective technique proposed in the past for controlling the relevant variables during reading and writing operations by an arcuate scanner. In the Lemke et al. system the servo functions employ low frequency servo information detected at the beginning and end of each scan. The low frequency servo information indicates the alignment of the scanning path traced by transducers with respect to adjacent tracks.
FIG. 9
illustrates one arcuate information format used by Lemke et al. and the relationship of servo bursts within that recorded format to one of the servo scanning heads R
0
. As illustrated, servo burst segments are recorded in servo burst regions adjacent to the tape upper edge and the tape lower edge by alternating write transducers. These are denoted, respectively, as the first and second servo regions. The servo bursts are written by write transducers that Lemke et al. identify as ‘even’ transducers W
0
and W
2
but not by ‘odd’ write transducers identified as W
1
and W
3
. The servo bursts are written by turning on the even write transducers earlier than the odd write transducers after passing the upper tape edge and by turning off the odd write transducers earlier than the even transducers as the lower tape edge is approached. The servo bursts comprise alternate frequencies which are at equal amplitudes in an unequalized channel and are generated from a system clock.
The even and odd write heads record data at different azimuth angles with respect to the longitudinal axis of the recording tape and the read heads read data at corresponding azimuthal angles. Lemke et al. identify the azimuth of the even heads as a ‘CW’ azimuth and identify the azimuth of the odd heads as a ‘CCW’ azimuth. W
0
, (CW azimuth) engages the tape upper edge and writes track
0
consisting of a servo band of f
0
. After delay for passage through the first servo region, this write head writes track
0
containing data D
0
. Next write head W
0
writes a servo burst consisting of a band of f
0
in the second servo region.
Write head W
1
has a CCW azimuth. This head engages the upper edge of the tape. At this time, the tape has advanced a distance corresponding to one data track width or pitch. Writing with this transducer is delayed until W
1
has passed the first servo region, then W
1
, which overlaps track
0
, overwrites track
0
with track
1
consisting of data D
1
with no gap between track
0
and track
1
. This leaves a trimmed data track
0
with a width equal to one data track width. Track
1
ends at the upper edge of the second servo region. At this point, data tracks
0
and
1
are bracketed between upper and lower servo bursts comprising of servo frequency f
0
.
At the time that head W
2
begins crossing the tape, the tape has moved a total distance corresponding to two times the data track width. W
2
with a CW azimuth begins tracing its arc from the upper edge of the tape to write track
2
consisting, initially, of a band of f
2
in the first servo region. Write head W
2
then writes track
2
including data D
2
. The outer edge of W
2
overlaps track
1
so as to overwrite it, beginning in the data field, with track
2
data, ensuring no gap between track
1
and track
2
. This leaves a trimmed data track
1
of one pitch width. W
2
appends a servo burst consisting of f
2
in the second serve region.
When the tape has moved another track pitch, the write head W
3
with a CCW azimuth engages the upper edge of the tape and writes only a data track, in the manner described above with regard to the write operation by head W
1
. When W
0
with CW azimuth again engages the upper edge of the tape, the cycle described above is repeated.
A tape speed servo loop operates during playback to ensure head/track alignment. This is a sampled servo which receives servo information when the even-numbered read transducers, R
0
and R
2
, read the servo bursts in the leading segments of recorded tracks. In the first servo region of the tape, there are only even tracks (
0
and
2
) of frequency f
0
and f
2
, written by write transducers W
0
and W
2
, respectively. Servo frequency f
0
is written by write transducer W
0
, while servo frequency f
2
is written by write transducer W
2
. Now, assume that read transducer R
0
has just passed the upper tape edge on its counterclockwise scan of track
0
as shown in FIG.
26
A. If transducer R
0
is accurately positioned, substantially ¾ of its width will be on the servo track written by transducer W
0
, while ¼ of its width will be on the servo burst in a track written by transducer W
2
. Accordingly, with proper positioning, the read transducer R
2
while traversing the servo bursts at the beginning and end of each scan will generate a servo signal comprising the servo frequencies f
0
and f
2
in the ratio 3f
0
:1f
2
. These two frequencies are discriminated and their amplitude ratio is used to determine the magnitude of a servo error signal. Any other ratio is discriminated and controls the servo to change the tape speed so that the above ratio is obtained. The identical process occurs when read transducer R
2
is reading its tracks, in which case the ratio of the servo burst in the first servo region is 3f
2
:1f
0
.
In actual practice, the azimuth of the servo/data tracks w

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