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ARCHIVE PYTHON DDS-DC OAT DRIVES---PRODUCT DESCRIPTION MANUAL
1.5 OAT Technology Overview
First developed for the audio electronics market, DAT technology was first applied in computer peripherals
in the late 19808.
Unlike traditional magnetic tape audio cassette products, DAT technology proves
inherently reliable through the helical scan recording method, which provides a high recording density with
a very low error rate. All DAT products, including computer implementations, use the helical scan recording
method. This recording method has been used in professional video tape recorders (VTRs) since 1956 and in
home video cassette recorders (VCRs) since 1974. In 1986, DAT products using helical scan technology
were first developed for audio applications. DAT consumer products are specifically designed for digital
audio recording and playback and compete with such products as analog audio cassette decks and compact
disk (CD) players.
1.5.1 Helical Scan Recording
Helical scan recording was originally developed as method of efficiently recording high-quality television
signals on a relatively slow moving tape. It requires that both the tape and the recording head move
simultaneously. This recording method results in an extremely high recording density, far higher than can be
achieved with stationary-head devices such as 1I2-inch open-reel or 1/4-inch cassette tapes. (See Chapter 8,
"Helical Scan Recording -- Four-Head Design" for additional information.)
In helical scan recording, both the read and write heads are located on a rapidly rotating cylinder or drum.
The cylinder is tilted at an angle in relation to the vertical axis of the tape. As the tape moves horizontally, it
wraps around the part of the circumference of the cylinder (90
0 )
so that the head enters at one edge of the
tape and exits at the other edge before the tape unwraps.
The horizontal movement of the tape in combination with the angular movement of the cylinder causes the
track to be recorded diagonally across the tape rather than straight down its length. The resulting recorded
track, nearly one inch, is approximately eight times longer than the width of the tape.
1.5.2 Tape Formats
Archive Python DDS-DC drives are designed to use the industry standard DDS and DDS-DC tape formats.
These two formats are summarized in the following text.
DDS Tape Format
This standard fonnat was codeveloped by DDS manufacturers to support DAT devices as computer
peripherals. The objectives of DDS are to maximize storage capacity and performance; to facilitate data
interchange; to provide compatibility with existing tape storage command sets; and to provide extremely fast
random access. The DDS format also takes advantage of the helical scan recording method and the inherent
error correction capability of the DAT technology to augment error detection and correction.
The format consists of a finite sequence of data groups with each data group being a fixed-length recording
area. A data group is made up of 22 data frames and 1 ECC frame; each frame is made up of two helical
scan tracks. The advantages of the fixed-length data group is that ECC is easily generated, and buffering
requirements are simplified. (See Chapter 6, "Tape Formats", for additional information.)
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