DIGITAL MAGNETIC TAPE RECORDING

LEARNING OBJECTIVES

After completing this chapter, you'll be able to do the following:

• Describe the characteristics of digital magnetic tape recording and the difference between analog and digital recording.
• Describe each of the three formats for digital magnetic tape recording (serial, parallel, and serial-parallel).
• Define the following terms as they apply to digital magnetic tape recording: mark, space, bit-cell period, packing density, and bit-error rate (BER).
• Describe the eight most common methods for encoding digital data onto magnetic tape.
• 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.

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.

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:

• Low density - 200 to 1,000 bits per inch (bpi).
• Medium density - 1,000 to 8,000 bpi.
• 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?
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?