Positioning Accuracy; Timing Tracking - Seagate STD124000N Product Manual

Chapter 6
Page 52

Positioning Accuracy

The DDS-3 specification reflects a tighter requirement for positioning accuracy when
compared to previous standards.
The DDS-3 standard specifies the position reference point of all tracks to be 1.906
mm from the tape reference edge.
The position reference point of any adjacent 12 tracks, or positional reference point
variance, must be within 1.0 µm of the mean position reference point of all tracks
within the described continuum.
This compares to a position reference point variance of 6.6 µm as allowed in the
DDS-2 format standard.

Timing Tracking

Playback tracking in the previous DDS-2 and DDS formats was accomplished
through the use of automatic track following (ATF). The ATF method required that
special low frequency tracking signals be recorded at both ends of each data track
recorded to tape. The ATF method reduced overall format efficiency and also
required special analog hardware circuitry to read and process these ATF signals.
The DDS-3 format departs from the ATF method, using instead, a more
sophisticated timing tracking system to ensure accurate tracking throughout the
tape. timing tracking control is performed within the firmware by calculating the
elapsed time from a fixed point in the rotational arc of the head cylinder to a
reference point (sync signal or parity signal) recorded within the track area.
Advantages of the timing tracking include a 50 percent increase in format efficiency
over DDS-2, more accurate tracking due to separate tracking error calculations for
each record head and more flexible tracking because tracking can be performed
using data detected anywhere along the length of the track.
With timing tracking, capstan servo control is performed by monitoring the time from
the cylinder PG signal until the recorded track data begins; that is, until the first sync
or parity data is detected.
When the playback head is perfectly on track, this time is measured as T0. When
the head is off track by any amount, the measured time T1 is longer or shorter than
T0, depending on whether the tape is moving too fast or too slow. The capstan
servo can then use this time difference to correct the tape speed and to bring the
heads back on track.
Tape Formats
DAT Drives