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MAGNETIC TAPE LEARNING OBJECTIVES After completing this chapter, you'll be able to do the following: Describe the physical properties of magnetic tape in terms of:
PHYSICAL PROPERTIES OF MAGNETIC TAPE The three basic materials used to make magnetic tape are (1) the base material, (2) the coating of magnetic oxide particles, and (3) the glue to bind the oxide particles onto the base material. See figure 2-1. Figure 2-1. - Magnetic tape construction.
BASE MATERIAL The base material for magnetic tape is made of either plastic or metal. Plastic tape is used more than metal tape because it's very flexible, it resists mildew and fungus, and it's very stable at high temperatures and humidity. OXIDE COATING Oxide particles that can be magnetized are coated onto the base material. The most common magnetic particles used are either gamma ferric oxide or chromium dioxide. It's very important that these magnetic particles are uniform in size. If they're not, the tape's surface will be abrasive and will reduce the life of the recorder's magnetic heads. An ideal magnetic particle is needle-shaped. It's actual size depends on the frequency of the signal to be recorded. Generally, long particles are used to record long wavelength signals (low-frequency signals), and short particles are used to record short wavelength signals (high-frequency signals). GLUE The glue used to bond the oxide particles to the base material is usually an organic resin. It must be strong enough to hold the oxide particles to the base material, yet be flexible enough not to peel or crack. TYPES OF MAGNETIC RECORDING TAPE There are two basic types of magnetic recording tape in common use: analog and digital. Analog magnetic tape is used to record, store, and reproduce audio and instrumentation type signals. These signals are usually in a frequency band from very-low frequency (VLF) to 2.5 MHz. Digital magnetic tape is used to record, store, and reproduce computer programs and data. It's base material thickness is about 50 percent thicker than analog magnetic tape. This allows the digital tape to withstand the more strenuous starts and stops associated with digital magnetic recorder search, read, and write functions. Digital magnetic tape is also held to much stricter quality control standards. It's important not to have any blemishes or coating flaws on the tape's surface. Because, if you lost one digital data bit, your computer program or data would be bad. In contrast, losing one microsecond of an analog signal is not nearly as critical. Q.1 Magnetic tape is made of what three basic materials? TAPE ERRORS AND THEIR EFFECTS Four types of tape errors that will degrade the performance of a magnetic recording system are signal dropout, noise, skew, and level (signal amplitude changes). DROPOUT ERRORS Signal dropout is the most common and the most serious type of tape error. It's a temporary, sharp drop (50% or more) in signal strength caused by either contaminates on the magnetic tape or by missing oxide coating on part of the tape. During recording and playback, the oxide particles on the tape can flake off and stick to the recorder's guides, rollers, and heads. After collecting for awhile, the oxide deposits (now oxide lumps) break loose and stick to the magnetic tape. As the tape with the lumps passes over the head, the lumps get between the tape and the head and lift the tape away from the head. This causes the signal dropouts. Although oxide lumps cause most signal dropouts, remember that any contaminate (such as dust, lint or oil) that gets between the tape and the head can cause signal dropouts. NOISE ERRORS Noise errors are unwanted signals that appear when no signal should appear. They're usually caused by a cut or a scratch on the magnetic tape. It's the lack of oxide particles at the cut or the scratch that causes the noise error. SKEW ERRORS Skew errors only occur on multi-track magnetic tape recorders. The term skew describes the time differences that occur between individual tracks of a single magnetic head when the multi-track tape isn't properly aligned with the magnetic head. There are two types of skew errors: fixed and dynamic. Fixed skew happens when properly aligned magnetic tape passes an improperly aligned magnetic head. Dynamic skew happens when misaligned tape passes a properly aligned head. This type of skew is usually caused by one or more of the following:
LEVEL ERRORS Magnetic tape is manufactured to have a specified output signal level (plus or minus some degree of error). Level errors happen when the actual output signal level either drops or rises to a level outside the expected range. For example, if a magnetic tape is rated for 10 volts ( +/-10%), any output signal level below 9 volts or above 11 volts is a level error. Level errors are caused by an uneven oxide coating on the magnetic tape. This can come from either the original manufacturing process or from normal wear and tear. Some causes of level errors are permanent and cannot be removed by any means. For example, a crease in the tape, a hole in the oxide, or a damaged edge. Other causes of level errors are removable and may be cleaned off the tape. For example, oxide flakes or clumps, metallic particles, or dirt are removable. Q.5 What are four types of tape errors that can degrade a magnetic recording system's
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