Unix Backup and Recovery

Tape Drives

Before listing the various tape drive technologies, we will discuss the two main technological differences between different types of tape drives.

• The method that is used to record the data on the tape—helical scan or linear

• The method used to spool the tape itself—cartridge or cassette

Linear Versus Helical Scan

One of the best ways to illustrate the difference between helical scan and linear recording technologies is to look at a typical VCR, since it actually incorporates both technologies and illustrates an important point. Have you ever recorded and watched a movie on a non-hi-fi^[94] VCR using the Extended Play (EP) setting? When you play that tape, it sounds horrible. Yet if you were to record the same movie on the same setting with a hi-fi VCR, the audio sounds fine. Have you ever wondered why?

Figure 18-2. VCR tape path

Look at Figure 18-2. A VCR’s tape is brought out of the cartridge and wrapped around a rotating drum. As you can see in Figure 18-3, the drum is angled slightly and has recording heads on its side. (The rectangle sitting at an angle in Figure 18-3 represents the angled drum with its rotating recording heads.) As the tape is pulled slowly around the drum, the diagonally positioned recording heads write “stripes” of video data diagonally across the tape, as can be seen in the bottom of Figure 18-3. Although the tape is moving very slowly around the ...