Helical Scan Recording-Four-Head Design; Figure 33. Four-Head Design - Seagate STD124000N Product Manual

Theory of Operations
Helical Scan Recording—Four-Head Design
Product Manual
In helical scan recording, the heads are positioned opposite one another on a
cylinder, which is tilted approximately 6 degrees from the vertical plane and rotates
counterclockwise at 4,000 revolutions per minute (rpm). At the same time, the tape
moves slowly (0.430 inches per second in DDS-3 mode) in a horizontal path around
part of the cylinder. This simultaneous motion of cylinder and tape results in the
head traveling across the width of the tape in a helix-shaped motion.
The cylinder is designed with four, long-life ferrite heads—two read and two write
heads. These heads are set opposite one another with a rotation sequence of: write
A, read B, write B, read A (or write A new, read B old, write B new, read A old). The
advantage of this design is that a RAW check is performed immediately after the
data is written.
As mentioned earlier, the cylinder rotates rapidly (4,000 RPM) in the same direction
that the tape moves. The wrap angle of the tape on the cylinder is approximately
102 degrees. This wrap angle and the slow tape speed minimize tape and head
wear. The combined movement of the tape and cylinder results in a relative head-
tape speed of 243 inches per second (ips).
Figure 33 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 33. Four-Head Design

6˚ Drum inclination angle
102˚ Angle of tape wrap
Chapter 8
Write Head B
Read Head A
Tape
Direction
Track of one
recording head
across tape surface
Page 75