Information Technology Reference
In-Depth Information
on the tape and allows faster access speeds.
Magnetic tapes have an extremely high capacity and are relatively cheap. Data is saved in
a serial manner with one bit (or one record) at a time. This has the disadvantage that they are
relatively slow when moving back and forward within the tape to find the required data.
Typically, it may take many seconds (or even minutes) to search from the start to the end of a
tape. In most applications, magnetic tapes are used to back up a system. This type of applica-
tion requires large amounts of data to be stored reliably over time but the recall speed is not
important.
The most common types of tape are:
•
Reel-to-reel tapes - the tapes have two interconnected reels with an interconnecting tape
which is tensioned by tension arms. They were used extensively in the past to store com-
puter-type data but have been replaced by the following three types (8 mm, QIC and DAT
tapes).
•
8 mm video cartridge tapes - this type of tape was developed to be used in video cameras
and is extremely compact. As with videotapes the tape wraps round the read/write head in
a helix.
•
Quarter inch cartridge (QIC) tapes - a QIC is available in two main sizes: 5.25 inch and
3.5 inch. They give capacities of 40 MB to tens of GB.
•
Digital audio tapes (DAT) - this type of tape was developed to be used in hi-fi applica-
tions and is extremely compact. As with the 8 mm tape, the tape wraps round the
read/write head in a helix. The tape itself is 4 mm wide and can store several GBs of data
with a transfer rate of several hundred kbps.
6.10.1 QIC tapes
QIC tapes are available in two sizes: 5.25 -inch and 3.5 -inch. The tape length ranges from
200 to 1000 feet, with a tape width of 0.25 -inch. Typical capacities range from 40 MB to tens
of GB. A single capstan drive is driven by the tape drive. Figure 6.6 illustrates a QIC tape.
Drive belt
Capstan
drive
Access
door
Figure 6.6
QIC tape
6.10.2 8 mm video tape
The 8 mm video tape is a high specification tape and was originally used in video cameras.
These types are also known as Exabyte after the company that originally developed a back
up system using 8 mm videotapes. They can be used to store several GBs of data with a
transfer rate of 500 kbps. In order to achieve this high transfer rate the read/write head spins
at 2000 rpm and the tape passes it at a relatively slow speed.
