Figure 20. Four-Head Cylinder Design - Seagate STD2401LW-S Product Manual

Theory of operations
advantage of this design is that a RAW check is performed immediately after the
data is written.
As mentioned earlier, the cylinder rotates rapidly (10,000 RPM) in the same direction
that the tape moves. The wrap angle of the tape on the cylinder is approximately 102
degrees. The combined movement of the tape and cylinder results in a relative head-
tape speed of 20.4 inches per second (ips).
Figure 20 illustrates a helix track, the four-head design and shows the 102 degree
wrap angle.
Direction of drum rotation:
Read Head B
Write Head A
Tape
Drum

Figure 20. Four-head cylinder design

The recorded tracks are written diagonally across the tape from bottom to top by
each write head. Because the head is wider than the track written, tracks overlap
with no tape space between them. In conventional recording, such overlap or even
proximity results in crosstalk (signals from adjacent tracks interfering with signals
from another track).
However, in helical scan recording, the heads are set at different azimuth angles so
that alternate tracks on the tape are written at alternate azimuth angles. (See Figure
21) Because the read head is set to the same angle as its corresponding write head,
it picks up a stronger signal from data written in the same azimuth angle as itself. So
it reads the track with minimal crosstalk. At the same time, the head is maintained
centered in the track by the timing tracking hardware and firmware.
6˚ Drum inclination angle
102˚ Angle of tape wrap
37
Write Head B
Read Head A
Tape
Direction
Track of one
recording head
across tape surface