Quantum DLT 2000 Handbook page 129

Helical Scan Systems
The next two backup systems we'll discuss are 4mm DAT (digital
audio tape) and 8mm tape. Both of these systems are based on
helical scan technology developed in the early days of television to
store video images. This is virtually the same technology used in
your home VCR. With QIC and DLTtape systems, the tape is
drawn rapidly past a fixed read/write head. With helical scan sys-
tems, both the tape and head move.
The tape in a helical scan system is pulled from a two-reel car-
tridge and wrapped halfway (more in some systems) around a
cylindrical drum containing the read/write heads. The read/write
cylinder is tilted and spins at a rapid rate in one direction as the
tape is drawn slowly past in the opposite direction. In this way,
blocks of data are recorded diagonally across the tape.
By alternating the angle of the data from track to track, a helical
system can actually overlap tracks. When reading from the tape,
the head that matches the recorded data's angle reads the strongest
signal. This technique enables helical scan systems to achieve very
high data densities.
A serious drawback to the helical system design is the complicated
tape path. Because the tape used in helical systems must be pulled
from a cartridge and wrapped tightly around the spinning read/write
cylinder, a great deal of stress is placed on the tape. It's interesting to
note that the combination of slow moving tape and the rapidly spin-
ning read/write head results in a relative tape speed that's actually
greater than that in a DLTtape system. This fact results in even
greater tape stress in helical scan systems. In addition, the recording
surface of the tape is drawn over numerous rollers, resulting in addi-
tional wear and increasing the likelihood of errors.
QUANTUM DLTtape HANDBOOK 11.7