Entities; Subgroups; Basic Group Transformation Summary; Subcode Information - Seagate STD124000N Product Manual

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Entities

Each entity includes an entity header and a processed record sequence. The
header is 8 bytes long and precedes the processed record sequence. By definition,
all processed records contained within an entity go through the same processing
algorithm; additionally this processing takes place only on unprocessed records of
equal length.
Entities can span basic groups as long as the header and the first 8 bits of the
processed record sequence are within the same basic group. Where an entity has
spanned multiple basic groups, the individual parts of the entity within the basic
groups are referred to as partial entities .

Subgroups

When the formation of a basic group is complete, the next step is to split the basic
group into 22 G1 subgroups of 17,468 bytes numbered from 0 through 17,467. Each
G1 subgroup is assigned a consecutive number from 1 through 22.
The bytes of each G1 subgroup are then randomized to form a G2 subgroup of
equal size.
Each G2 subgroup is then rearranged to form a G3 subgroup of 17,472 bytes. The
first 8,734 bytes are grouped into Track A of the G3 subgroup; the remaining 8,734
bytes are grouped into Track B of the same G3 subgroup.
Finally, each G3 subgroup is transformed into a G4 subgroup that contains an array
sign, a fragment number and a serial number. Each G4 subgroup consists of two
arrays of 96 fragments each. Each fragment is transformed into a main data
fragment . Each main data fragment consists of a fragment ID, an area ID, a frame
number, subcode bytes and fragment header parity bytes.

Basic Group Transformation Summary

Through the transformation process described above, each basic group is
transformed into 4,224 main data fragments before recording to tape.

Subcode Information

Subcode information specifies the running number of the basic group, the number of
separator 1s written since the logical beginning of tape (LBOT), the number of
separator 2s written since the LBOT, the number of records written since the LBOT
information about track contents and information about the history of the tape.

Subcode Location

Subcode information is written within each main data fragment header.
Chapter 6
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