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DAT
drives use a technology called Helical-scan. Using 4-mm DAT
tapes, the drives record large amounts of data onto the very-slow-moving
tape. Magnetic read/write heads are mounted on a rotating drum,
with an axis of rotation at 6 degrees from the perpendicular.
DAT
drives have two heads for reading and two for writing. The tape
wraps 90 degrees around the drum's circumference, and the heads
move in a spiral motion from the bottom to the top of the tape.
The drum rotates at 2000 rpm, and the 4-mm tape moves slowly
in the same direction at 8.15 millimeters per second (or 0.32
inch per second). The diametrically opposed heads describe portions
of a helix on the tape. This manner of writing and reading data
is distinctly different to DLT.
Each track is written diagonally from top to bottom. The heads
are wider than the written tracks, so each new track overlaps
the previous one, wasting no tape between tracks. Overlapping
tracks would normally result in cross talk between adjacent
tracks when reading data from the tape, but the device minimizes
cross talk by angling the heads 20 degrees relative to the data
track (the azimuth angle) and in opposite directions from each
other.
When data is read from the tape, the read head receives a much
stronger signal from data written to the same azimuth angle.
Angling permits very close packing of tracks and very high data
densities. ATF (Automatic Track Finding) circuitry keeps the
head centered on the track by balancing the weaker signals from
adjacent (i.e., off-azimuth) tracks.
In the early days of DAT's evolution, DAT devices used one of
two proposed low-level formatting standards: DDS, developed
jointly by HP and Sony, and Data/DAT, developed by Hitachi.
DDS offers slightly more storage capacity and faster sustained
transfer rates than Data/DAT. DDS devices write data sequentially,
appending data to the existing information, and they can read
data randomly, beginning at any point.
Data/DAT
drives can overwrite existing data files in place, reducing
the inefficiencies of multiple copies of the same file. However,
the market appears to have standardised on DDS and a few extensions
of DDS. All the DAT drives sold by Discovery use the DDS standard.
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DDS
- Digital Data Storage Formats
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DDS-1
The DDS format represents a modification
of the DAT technology. Unlike the continuous data stream
that's produced by the DAT format, DDS constructs a sequence
of fixed-capacity groups on the tape. DDS packs up to 2
GB on a 60-, 90-, or 120-meter tape running at the same
speed as DAT. The best-case scenario for a DAT recording
is a sustained transfer rate of 183 Kbps to fill a tape
during 2 hours of transfer time. |
DDS-2:
This was developed to allow data transfer
to occur in SCSI-2 burst mode. DDS-2 doubles the density
of DDS, but it maintains full DDS functionality and ensures
backward compatibility. |
DDS-DC:
Similar to DDS-2, DDS-DC was established
to include a data-compress ion standard. It allows uncompressed
data to be stored in a way that maintains full DDS functionality
and ensures backward compatibility with existing DDS drives.
DDS-DC is not supported by Discovery. |
DDS-3:
An extension of the DDS format allowing
125 metre tapes to be used and increasing the capacity to
12GB native and 24GB compressed. DDS-3 drives read and write
to DDS-2 media, providing a painless upgrade path. |
DDS-4:
A next generation extension of the DDS format
increasing the capacity to 40B native and 80GB compressed
with a 2:1 compression ratio. DDS-4 drives read and write
to DDS-2 and DDS-3 media, providing a painless upgrade path. |
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Compression
As with all compression ratios, caution should
be taken when calculating the required capacity. A 2:1 is an
average compression ratio - different data compresses at various
ratios. If data has already been compressed, using additional
compression techniques may even result in larger file sizes
rather then smaller ones.
Please
read our compression guides and explanations in Technical
Information for further details.
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