Digital Magnetic Tape Recording

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Chapter 7

Digital Magnetic Tape Recording

Learning Objectives

After completing this chapter, you’ ll be able to do the following: 1. Describe the characteristics of digital magnetic tape recording and the difference between analog and digital recording. 2. Describe each of the three formats for digital magnetic tape recording (serial, parallel, and serial- parallel). 3. Define the following terms as they apply to digital magnetic tape recording: mark, space, bit-cell period, packing density, and bit-error rate (BER). 4. Describe the eight most common methods for encoding digital data onto magnetic tape.

5. Describe the characteristics and use of the following categories of digital magnetic tape recorders: (1) computer-compatible, (2) telemetry, and (3) instrumentation.

Introduction To Digital Magnetic Tape Recording

This chapter introduces you to digital magnetic tape recording. It describes (1) the three formats for digital magnetic tape recording, (2) the eight methods of encoding digital data onto magnetic tape, and (3) the configuration differences between the three types of digital tape recorders. Until now, you’ ve learned about magnetic tape recording from an analog point-of-view. That is, the signal you record and reproduce is the actual analog input signal waveform. In digital magnetic tape recording, the signal you record and reproduce is, instead, a series of digital pulses. These pulses are called binary ones and zeros. These ones and zeros can represent one of three types of data: (1) data used by digital computers, (2) pulsed square-wave signals, or (3) digitized analog waveforms.

The digital magnetic tape recording process stores data onto tape by magnetizing the tape to its saturation point in one of two possible polarities: positive (+) or negative (-). The saturation point of magnetic tape is the point where the magnetic tape is magnetized as much as it can be.

Digital Magnetic Tape Recording Formats

There are three digital magnetic tape recording formats: serial, parallel, and serial-parallel. Each of these is described below. Figure 7-1 shows each of the three formats as they apply to recording an eight-bit binary data stream. 7-2

Serial Digital Magnetic Tape Recording Format

This is the simplest of the three digital magnetic tape recording formats. It’ s usually used when recording instrumentation or telemetry data. In this format, the incoming data pulses are recorded onto a single recorder track of the magnetic tape in a single, continuous stream. Figure 7-1A shows how this looks. Figure 7-1A.—Digital magnetic tape recording formats.

Parallel Digital Magnetic Tape Recording Format

In this format, the incoming data pulses come in on more than one input channel and are recorded side-by-side onto more than one tape track. The data pulses across the width of the magnetic tape are related to each other. Figure 7-1B shows how this looks. This format is usually used to store computer data. Figure 7-1B.—Digital magnetic tape recording formats. 7-3

Serial-Parallel Digital Magnetic Tape Recording Format

This format is more complex. It takes a serial input stream of data pulses, breaks them up, and records them on more than one recorder track. When the tape is reproduced, the recorder recombines the broken-apart data into its original form. Figure 7-1C shows how this looks. The serial-parallel format is usually used in instrumentation recording when the input data rate is high. Figure 7-1C.—Digital magnetic tape recording formats.

Digital Magnetic Tape Recording Definitions

Before we describe the methods for encoding digital data onto magnetic tape, let’ s define the following terms: Mark: The voltage state of a digital one (1) data bit. It’ s also sometimes called true. Space: The voltage state of a digital zero (0) data bit. It’ s also sometimes called false. Bit-cell period: The time occupied by a single digital bit. Packing density: The number of bits per fixed length of magnetic tape per track. There are three categories of packing density: 1. Low density—200 to 1,000 bits per inch (bpi).

2. Medium density—1,000 to 8,000 bpi. 3. High density—8,000 to 33,000 bpi. Bit-error rate: The number of bits within a finite series of bits that will be reproduced incorrectly. Q-1. In digital magnetic tape recording, the series of recorded digital pulses can represent what three types of data? Q-2. What three formats are used for digital magnetic tape recording? Q-3. What format of digital tape recording is normally used to store computer data? 7-4 Q-4. What format of digital tape recording takes a serial input stream of data pulses, breaks them up, and records them on more than one data track?

Q-5. What format of digital tape recording is normally used to record instrumentation or telemetry data?

Digital Magnetic Tape Recording Encoding Methods

This section describes how digital data is electrically encoded onto the magnetic tape. The following paragraphs describe the eight most common digital data encoding methods. 1. Return to bias (RB) 2. Return to zero (RZ) 3. Non-return to zero (NRZ) and these four variations of the NRZ method: a. Non-return-to-zero level (NRZ-L) b. Enhanced non-return-to-zero level (E-NRZ-L) c. Non-return-to-zero mark (NRZ-M) d. Non-return-to-zero space (NRZ-S) 4. Bi-phase level

Return-To-Bias (Rb) Encoding

The RB encoding method uses magnetic tape that is pre-set to one of the two polarities (+ or -). This pre-sets the magnetic tape to all zeros. Digital ones are then recorded onto the magnetic tape by magnetizing the tape in the opposite polarity. After each one pulse, the tape returns to its original bias condition. Figure 7-2 shows the magnetic tape preset to a negative bias condition. It also shows how the digital data word 0100110001 is stored onto the magnetic tape using the RB encoding method. Figure 7-2.—Return-to-bias (RB) digital encoding method.

This method has a serious drawback: It requires an external clocking signal to read the zeros stored on the tape. 7-5

Return-To-Zero (Rz) Encoding

The RZ encoding method uses magnetic tape that is normally in a neutral condition (the tape is not biased positively or negatively). A digital one is recorded as a positive-going pulse: a digital zero is recorded as a negative-going pulse. The magnetic tape returns to its neutral state in between pulses. Figure 7-3 shows the magnetic tape in its neutral state. It also shows how the digital data word 0100110001 is stored onto the magnetic tape using return-to-zero encoding. Figure 7-3.—Return-to-zero (RZ) digital encoding method.

Non-Return-To-Zero (Nrz) Encoding

The NRZ encoding method is, by far, the most widely used. It’ s accurate, simple, and reliable. It does not return the magnetic tape to its neutral state in between pulses. The magnetic tape is always in saturation, either positively or negatively. The polarity of the saturating signal only changes when incoming data changes from a zero to a one and vice versa. Figure 7-4 shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ encoding method. Figure 7-4.—Non-return-to-zero (NRZ) digital encoding method.

7-6 There are four widely used variations to the basic NRZ encoding method. Each of these is described in the following paragraphs. Non-Return-To-Zero-Level (NRZ-L) Encoding In NRZ-L encoding, the polarity of the saturating signal changes only when the incoming signal changes from a one to a zero or from a zero to a one. Figure 7-4 also shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ-L encoding method. Note that the NRZ-L method looks just like the NRZ method, except for the first input one data bit. This is because NRZ does not consider the first data bit to be a polarity change, where NRZ-L does.

The NRZ-L encoding method isn’ t normally used in higher density (over 20,000 bpi) digital magnetic recording. This encoding method is sometimes called the non-return-to-zero-change (NRZ-C) encoding method. Enhanced Non-Return-to-Zero-Level (E-NRZ-L) Encoding This encoding method takes the basic NRZ-L data and adds a parity bit to it after every seven incoming data bits. The polarity of the parity bit is such that the total number of ones in the eight-bit data word will be an odd count. Figure 7-5 shows how the digital data word 0100010 is stored onto the magnetic tape using the E-NRZ-L encoding method.

Figure 7-5.—Enhanced non-return-to-zero-level (E-NRZ-L) digital encoding method. Before the parity bit is added, the original incoming data is compressed in time. This is done so that when the parity bit is added, the eight-bit data word takes up the same amount of time as the original- seven bit data word. When the tape is reproduced, the parity bit is taken out. This encoding method works very well in high density (up to 33,000 bpi) magnetic tape recording. And, it offers an extremely good bit-error rate of 1 error per 1 million bits.

Non-Return-to-Zero-Mark (NRZ-M) Encoding The NRZ-M encoding method is probably the most widely used encoding method for 800-bpi digital magnetic tape recording. In this method, the polarity of the saturating signal changes when the incoming signal is a one. An incoming zero would not change the polarity of the saturating signal. NRZ-M offers better protection from error than straight NRZ. In NRZ-M, there’ s a one-to-one relationship between incoming data and polarity changes. If one data bit is lost, only that one bit is lost.

7-7 Whereas, in straight NRZ, if one bit is lost, all of the bits that follow will be exactly the opposite in polarity from what they should be. Figure 7-4 also shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ-M encoding method. Non-Return-to-Zero-Space (NRZ-S) Encoding The NRZ-S encoding method works just like NRZ-M encoding, with one exception. Instead of the saturating signal changing polarity when the incoming data signal is a one, it changes when the incoming data signal is a zero.

Bi-Phase Level Encoding

The bi-phase level encoding method records two logic levels for each incoming data bit. When an incoming data bit is a one, bi-phase level recording records a zero-one. When an incoming data bit is a zero, bi-phase level recording records a one-zero. This encoding method helps to overcome any low-frequency response problems that the magnetic tape recorder may have. Figure 7-6 shows how the digital data word 101000111001 is stored onto magnetic tape using the bi-phase encoding method. Figure 7-6.—Bi-phase level digital encoding method.

Bi-phase encoding requires exactly twice the bandwidth of NRZ-L. That’ s why it’ s mostly used in medium-density digital magnetic tape recording. In fact, this encoding method is probably the most widely used encoding method for 1600-bpi digital magnetic tape recording.

Digital Magnetic Tape Recorder Uses

As you already know, digital magnetic tape recorders are used to store and retrieve digital data. These recorders fall into one of three categories, (1) computer compatible, (2) telemetry, and (3) instrumentation.

Computer-Compatible Digital Tape Recorders

Computer-compatible digital tape recorders store and retrieve computer programs and data. They’ re usually multi-tracked tape recorders with at least two, and up to nine, tracks for data. They use either 1/4" or 1/2" magnetic tape on either reels or cartridges.

Telemetry Digital Tape Recorders

Telemetry digital magnetic tape recorders are more commonly called wideband recorders. They’ re used for recording radar signals and other pulsed square-wave type signals with a bandwidth of 500 kHz to 2 MHz. They’ re also multi-tracked tape recorders that have either 14 or 28 tracks for data. They use 1" magnetic tape on either aluminum or glass reels. 7-8

Instrumentation Magnetic Tape Recorders

Instrumentation digital magnetic tape recorders are used to record other special signals with a bandwidth of less than 500 kHz. They, too, are multi-tracked recorders, normally with 7 tracks for data. They use 1/2" magnetic tape on metal or glass reels. Q-6. Which of the eight methods for encoding digital data onto magnetic tape is most widely used because it’ s accurate, simple, and reliable? Q-7. Which digital data tape encoding method presets the magnetic tape to all zeros and then records digital ones onto the tape?

Q-8. Which digital data encoding method records a digital one as a positive pulse and a digital (zero) as a negative pulse and returns the tape to neutral between pulses? Q-9. Which method of digital data encoding does NOT return the tape to neutral between pulses but, instead, saturates the tape positively or negatively as the incoming data changes between zero and one? Q-10. What are the four widely used variations of the NRZ encoding method? Q-11. Which digital data encoding method helps overcome a tape recorder’ s low-frequency response problems by recording two logic levels for each incoming data bit?

Q-12. Digital magnetic tape recorders used to store and retrieve digital data fall into what three categories? Q-13. What category of digital tape recorder is used for recording pulsed square-wave signals with a bandwidth of 500 kHz to 2 MHz? Q-14. What category of digital tape recorder is used to record special signals with a bandwidth of less than 500 kHz? SUMMARY Now that you’ ve finished chapter 7, you should be able to describe (1) the characteristics of digital magnetic tape recording, (2) the three formats for digital magnetic tape recording, (3) the eight methods for encoding digital data onto magnetic tape, and (4) the characteristics and uses of the three types of digital magnetic tape recorders. The following is a summary of important points in this chapter: Digital magnetic tape recorders record a SERIES OF DIGITAL PULSES called binary ones and zeros. These digital pulses can represent (1) data used by digital computers, (2) pulsed square-wave signals, or (3) digitized analog waveforms.

Three FORMATS FOR DIGITAL MAGNETIC TAPE RECORDING are serial, parallel, and serial-parallel. There are EIGHT COMMONLY USED METHODS FOR ENCODING digital data onto magnetic tape. The non-return-to-zero (NRZ) method and the four variations of the NRZ method are most commonly used. 7-9 THREE CATEGORIES OF DIGITAL MAGNETIC TAPE RECORDERS are (1) computer- compatible, (2) telemetry, and (3) instrumentation.

Answers To Questions Q1. Through Q14.

A1. a. Data used by digital computers. b. Pulsed squarewave signals. c. Digitized analog waveforms. A2. (1) Serial, (2) parallel, and (3) serial-parallel. A3. Parallel digital magnetic tape recording. A4. Serial-parallel digital magnetic tape recording. A5. Serial digital magnetic tape recording. A6. Non-return-to-zero (NRZ) encoding. A7. Return-to-bias (RB) encoding. A8. Return-to-zero (RZ) encoding. A9. Non-return-to-zero (NRZ) encoding. A10. a. Non-return-to-zero level (NRZ-L). b. Enhanced non-return-to-zero level (E-NRZ-L).

c. Non-return-to-zero mark (NRZ-M). d. Non-return-to-zero space (NRZ-S). A11. Bi-phase level encoding. A12. a. Computer-compatible digital tape recorders. b. Telemetry digital tape recorders. c. Instrumentation digital tape recorders. A13. Telemetry digital tape recorders. A14. Instrumentation digital tape recorders.