Helical Scan Technology

Tape recording based on helical scan is the same type of technology as that used in VHS video-tape recorders. As the data is written in diagonal stripes across the tape, more data can be packed onto the tape than just with parallel bands up the tape. However, data access on helical scan is slower that linear, so it tends not to be used for applications which need high performance.

In a helical scan tape system, both the tape heads and the tape move. The heads are arranged around a cylindrical drum, and the tape is wrapped halfway around this. The drum contains two read heads and two write heads, arranged alternately. The read heads verify the data written by the write heads. The tape moves slowly, in the opposite direction to the spin of the drum. The drum is tilted at about 5 degrees to the drum. Although the tape moves relatively slowly, the drum spins fast, so the relative tape speed is high. The cylinder head is tilted slightly in relation to the tape, and spins at 2,000rpm. Data is written in diagonal tracks across the width of the tape.

The second write head writes data at a 40 degree angle to the first one. The data is read back by the correct head, because the two tracks a recorded with different magnetic polarisations. The criss-cross pattern packs more data onto the tape, enabling helical scan systems to achieve very high data densities. A directory of files is stored in a partition at the front of the tape, or in a file on the hard disk.
There is a large surface contact area between the tape and the drum, and this means the tapes are prone to wear and tear. It is difficult to see how helical scan can scale up like linear tape, as it would be difficult to fit more pairs of heads to the rotating drum.

Several implementations of Helical Scan exist, the principle ones are Exabyte, AIT and DAT, with Storagetek's Redwood used for top quality applications. See the product comparison page for details.

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