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6.1 Introduction
CHAPTER 6
TAPE FORMATS
TAPE FORMATS
The Python OAT drives with data compression capabilities use the ANSI Digital Data Storage (DDS) and
Digital Data Storage Data Compression (DDS-DC) tape formats to record uncompressed and compressed
data on Digital Audio Tape (DAT) media. The DDS-DC format is a superset of the DDS format, thus
ensuring compatibility and allowing the Python DDS-DC drives to write compressed and uncompressed data
to the same cassette.
As a superset of the DDS format, the DDS-DC format adds support for in-drive data compression and
decompression. The features of this format were conceived for use with lossless compression algorithms
based on substitution; however, the format is not necessarily limited to those algorithms.
The
implementation of data compression in Python drives uses the advanced DCLZ (data compression
LempelZiv) algorithm.
The industry standard DDS-DC format has been approved by the DDS Manufacturers Group, ANSI, and
ECMA.
6.1.1 DDS Format
The DDS format supports OAT devices as computer peripherals. The objectives of the DDS format are:
o
To maximize storage capacity and perfonnance
o
To facilitate data interchange
o
To provide compatibility with existing storage command sets
o
To provide extremely fast random access
o
To take advantage of the helical scan recording method
o
To augment the inherent error-detection and error correction capabilities of the DAT technology
Achievement of these objectives is shown in the following list of advantages provided by the DDS format
o
Maximizes capacity and transfer rate.
o
Provides compatibility with both l/2-inch tape and QIC command sets.
o
Supports SCSI sequential command set functions.
o
Supports random read functions.
o
Supports both variable-length and fixed-length records.
6-1
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