Compaq ProLiant 1000 DDS-3 Enhancing DAT Reliability, Capacity, and Performanc - Page 6

cont'd, timed tracking

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ECG036/1297 7(&+12/2*< %5,() (cont'd) ... Tape tracks ATF area Data area Tape Edge Read/write head Tape tracks Figure 4: By reading ATF information from tracks immediately left and right of the primary track it is reading, the head centers its position on the track. The Compaq 12/24 GB DAT Drive does not use embedded servo tracking with ATF. Instead, it uses a timed tracking scheme that frees the ATF areas for user data, thus increasing the storage capacity. Timed tracking is a track-following servo format that provides servo input by measuring the time required for read or write heads to scan from the tape edge to a known reference point on the track. These reference points are equally-spaced "timing marks." By referencing from the edge of the tape, the drive is able to measure the time it takes to reach the timing mark (Figure 5). Any deviation in time indicates a deviation from the track centerline. If the timing is too early, the tape speed increases to center the head on the track; if it is too late, the tape speed decreases. This process increases efficiency and performs head/track alignment more frequently than the embedded servo format. The timed tracking format does not require ATF storage on the track, so more of the track is available for storing user data. Timing mark Tape Edge Early Late "On-time" Figure 5: Timed tracking servo. 

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Tape Edge
Tape tracks
ATF area
Data area
Read/write head
Tape tracks
Figure 4: By reading ATF information from tracks immediately left and right of the
primary track it is reading, the head centers its position on the track.
The Compaq 12/24 GB DAT Drive does not use embedded servo tracking with ATF.
Instead,
it uses a
timed tracking
scheme that frees the ATF areas for user data, thus increasing the
storage capacity.
Timed tracking is a track-following servo format that provides servo input by measuring the
time required for read or write heads to scan from the tape edge to a known reference point on
the track.
These reference points are equally-spaced “timing marks.”
By referencing from the
edge of the tape, the drive is able to measure the time it takes to reach the timing mark (Figure
5).
Any deviation in time indicates a deviation from the track centerline.
If the timing is too
early, the tape speed increases to center the head on the track; if it is too late, the tape speed
decreases.
This process increases efficiency and performs head/track alignment more
frequently than the embedded servo format.
The timed tracking format does not require ATF
storage on the track, so more of the track is available for storing user data.
Tape Edge
Early
Late
"On-time"
Timing mark
Figure 5: Timed tracking servo.