Introduction to Magnetic Recording

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Chapter 1

Introduction To Magnetic Recording

Learning Objectives

Learning objectives are stated at the beginning of each chapter. They serve as a preview of the information you are expected to learn in the chapter. The comprehension check questions placed within the text are based on the objectives. By successfully completing those questions and the associated NRTC, you show that you have met the objectives and have learned the information. The learning objectives for this chapter are listed below. After completing this chapter, you will be able to do the following: 1. Describe the history and purpose of magnetic recording.

2. State the prerequisites for magnetic recording. 3. Describe a magnetic recording head, how it’ s constructed, and how it operates.

Introduction

Have you ever wondered how a whole album of your favorite music got onto one of those little cassette tapes? Or, what about computer floppy disks; have you ever wondered how they can hold 180 or more pages of typed text? The answer to both of these questions is magnetic recording. Magnetic recording devices seldom get much attention until they fail to work. But without magnetic recording, recording your favorite television show on a video cassette recorder would be impossible, portable tape players wouldn’ t exist, and you wouldn’ t be able to get money from an automated bank teller machine at two o’ clock in the morning.

Now what about the Navy? Could it operate without magnetic recording? The answer is definitely no. Without it: • Computer programs and data would have to be stored on either paper cards or on rolls of paper tape. Both of these methods need a lot of storage space, and they take much longer to load into and out of the computer. • There wouldn’ t be any movies to show or music to play on the ship’ s entertainment system when the ship is at sea and is out of range for television and radio reception. • Intelligence-collection missions would be impossible since you couldn’ t store the collected signals for later analysis.

As you can see, magnetic recording plays a very important part both in our Navy life and in our civilian life. 1-2

History Of Magnetic Recorders

In 1888, Oberlin Smith originated the idea of using permanent magnetic impressions to record sounds. Then in 1900, Vladeniar Poulsen brought Mr. Smith’ s dream to reality. At the Paris Exposition, he demonstrated a Telegraphone. It was a device that recorded sounds onto a steel wire. Although everyone thought it was a great idea, they didn’ t think it would succeed since you had to use an earphone to hear what was recorded. It wasn’ t until 1925, when electronic amplifiers were developed, that magnetic recording started to receive the attention it deserved.

The best magnetic recording is the one that produces an output signal identical to the input signal. It didn’ t take long to realize that the magnetism generated during the recording process didn’ t vary directly to the current which caused it. This is because there’ s a step in the magnetism curve where it crosses the zero point and changes polarity. This step causes the output signal to be distorted when compared with the input signal. Figure 1-1 shows this step. Figure 1-1.—Magnetic recording without bias voltage.

In 1907, Mr. Poulsen discovered a solution to this problem. He discovered dc bias. He found that if a fixed dc voltage were added to the input signal, it moved the input signal away from the step in the magnetism curve. This prevented the input signal from crossing the zero-point of the magnetism curve. The result is an output signal exactly like the input signal. Figure 1-2 shows this process. 1-3 Figure 1-2.—Magnetic recording with dc bias voltage. Unfortunately, dc bias had its problems. Since only a small portion of the magnetism curve was straight enough to use, the output signal was weak compared with the natural hiss of the unmagnetized tape passing the playback head. This is commonly called poor signal-to-noise ratio (SNR). We’ ll explain SNR in more detail later.

From the beginning, the U.S. Naval Research Laboratories (NRL) saw great potential in magnetic recording. They were especially interested in using it to transmit telegraph signals at high speed. After electronic amplifiers were invented around 1925, W.L. Carlson and G.W. Carpenter at the NRL made the next important magnetic recording discovery. They found that adding an ac bias voltage to the input signal instead of a fixed dc bias voltage would • reproduce a stronger output signal • greatly improve the signal-to-noise ratio • greatly reduce the natural tape hiss that was so common with dc bias To make ac bias work, they used an ac frequency for the bias voltage that was well above what could be heard, and a level that placed the original input signal away from both steps in the magnetism curve.

This resulted in two undistorted output signals that could be combined into one strong output. See figure 1-3. 1-4 Figure 1-3.—Magnetic recording with ac bias voltage. Until 1935, all magnetic recording was on steel wire. Then, at the 1935 German Annual Radio Exposition in Berlin, Fritz Pfleumer demonstrated his Magnetophone. It used a cellulose acetate tape coated with soft iron powder. The Magnetophone and its "paper" tapes were used until 1947 when the 3M Company introduced the first plastic-based magnetic tape.

In 1956, IBM introduced the next major contribution to magnetic recording—the hard disk drive. The disk was a 24-inch solid metal platter and stored 4.4 megabytes of information. Later, in 1963, IBM reduced the platter size and introduced a 14-inch hard disk drive. Until 1966, all hard disk drives were "fixed" drives. Their platters couldn't be removed. Then in 1966, IBM introduced the first removable-pack hard disk drive. It also used a 14-inch solid metal platter. In 1971, magnetic tape became popular again when the 3M Company introduced the first 1/4-inch magnetic tape cartridge and tape drive. In that same year, IBM invented the 8-inch floppy disk and disk drive. It used a flexible 8-inch platter of the same material as magnetic tape. Its main goal was to replace punched cards as a program-loading device.

The next contribution to magnetic recording literally started the personal computer (PC) revolution. In 1980, a little-known company named Seagate Technology invented the 5-1/4-inch floppy disk drive. Without it, PCs as we know them today would not exist. From then on, it was all downhill. Magnetic tape became more sophisticated. Floppy disks and disk drives became smaller, while their capacities grew bigger. And hard disk capacities just went through the roof. All of the major hurdles affecting magnetic recording had been successfully cleared, and it was just a matter of refining both its methods and materials.

Q-1. Why did the early inventors of magnetic recording find it necessary to add a fixed dc bias to the input signal? Q-2. How does dc bias added to the input signal correct the distortion in the output signal? 1-5 Q-3. Why does adding dc vice ac bias voltage to the input signal result in a poor signal-to-noise ratio (SNR)? Q-4. What are three advantages of adding an ac bias voltage to the input signal instead of adding a fixed dc bias voltage? Q-5. Why does using ac vice dc bias voltage result in a stronger output signal?

Prerequisites For Magnetic Recording

To perform magnetic recording, you need three things: 1. An input signal you wish to record. 2. A recording medium. (This is a recording surface that will hold the signal you wish to record.) 3. A magnetic head to convert the input signal into a magnetic field so it can be recorded. If any one of these are missing, magnetic recording cannot take place. Input Signal An input signal can come from a microphone, a radio receiver, or any other source that’ s capable of producing a recordable signal. Some input signals can be recorded immediately, but some must be processed first. This processing is needed when an input signal is weak, or is out of the frequency response range of the recorder.

Recording Medium A recording medium is any material that has the ability to become magnetized, in varying amounts, in small sections along its entire length. Some examples of this are magnetic tape and magnetic disks. These are thoroughly discussed in chapter 2 of this module. Magnetic Heads Magnetic heads are the heart of the magnetic recording process. They are the transducers that convert the electrical variations of your input signal into the magnetic variations that are stored on a recording medium. Without them, magnetic recording isn’ t possible.

Magnetic heads actually do three different things. They transfer, or record, the signal information onto the recording medium. They recover, or reproduce, the signal information from the recording medium. And they remove, or erase, the signal information from the recording medium. MAGNETIC HEAD CONSTRUCTION.—A magnetic head is a magnetic core wrapped with a coil of very thin wire (see figure 1-4). The core material is usually shaped like the letter C, and is made from either iron or ceramic-ferrite material. The number of turns of wire placed on the core depends on the purpose of that specific head. The gap in the core is called a head gap. It's here that magnetic recording actually takes place. We'll go into more detail of magnetic head construction in chapter 3.

1-6 Figure 1-4.—Magnetic field distribution around the head gap. MAGNETIC HEAD OPERATION.—Whether you're recording on magnetic tapes or disks, all magnetic heads operate the same way. When an electric current passes through the coil of a magnetic head, magnetic field lines associated with the electric current follow paths through the core material. When the magnetic fields get to the head gap, some of them spread outside the core to form a fringing field. When a recording medium is passed through this fringing field, it is magnetized in relation to the electric current. This is called magnetic recording. Figure 1-4 illustrates this process.

Q-6. What three things are required to perform magnetic recording? Q-7. What is the meaning of the term recording medium as it pertains to magnetic recording? Q-8. What are the three functions of the magnetic heads on a magnetic recording device? SUMMARY This chapter briefly covered the historical development of magnetic recording principles and devices. The following is a summary of important points in the chapter. The BEST MAGNETIC RECORDING is one that produces an output signal that is identical to the input signal. However, a step in the magnetic curve causes the output signal to be distorted.

In 1907, DC BIAS was added to the input signal to remove the distortion in the output signal. But the dc bias caused a weak output signal with a poor SNR. Around 1925, the NRL used AC BIAS to reproduce a stronger output signal and greatly improve the SNR. To perform magnetic recording, you need (1) an INPUT SIGNAL, (2) a RECORDING MEDIUM, and (3) a MAGNETIC HEAD. A RECORDING MEDIUM is any material that can become magnetized in varying amounts (such as magnetic tape and disks). MAGNETIC HEADS are used to (1) record the signal onto the recording medium, (2) reproduce the signal from the recording medium, and (3) erase the signal from the recording medium.

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Answers To Questions Q1. Through Q8.

A-1. Because a step in the magnetism curve where it crosses the zero point and changes polarity causes the output signal to be distorted. See figure 1-1. A-2. The dc bias moves the input signal away from the step in the magnetism curve. This prevents the input signal from crossing the zero-point of the magnetism curve. See figure 1-2. A-3. With dc bias, the SNR is poor because only a small portion of the magnetism curve is straight enough to use, thus the output signal is weak compared with the natural tape hiss. A-4.

a. Reproduces a stronger output signal. b. Greatly improves the SNR. c. Greatly reduces the natural tape hiss. A-5. Because an ac bias voltage of the proper frequency and level places the input signal away from both steps in the magnetism curve. The result is two undistorted output signals that are combined into one strong output. A-6. a. An input signal. b. A recording medium. c. A magnetic head. A-7. A recording medium is any material that has the ability to become magnetized, in varying amounts, in small sections along its entire length.

A-8. a. Record the signal onto the recording medium. b. Reproduce the signal from the recording medium. c. Erase the signal from the recording medium.