UNITED STATES EPARTMENT OF IOMMERCE

UBLICATION NBS TECHNICAL NOTE 739

A Universal

Dropout Tester

for Magnetic Storage Media
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation2—Nuclear Radiation2—Applied Radiation 2—Quantum Electronics3—

Electromagnetics3—Time and Frequency3—Laboratory Astrophysics3—Cryogenics3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

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Evaluation and Application4. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

4 Part of the Center for Building Technology.
• •"---, j* " —"• « M l*^» *»—' » «-« " ^« —

1972

-A Universal Dropout Tester

for Magnetic Storage Media

[2.

Sidney B. Geller

Information Processing Technology Division

Center for Computer Sciences and Technology

National Bureau of Standards

Washington, D.C. 20234

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U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acting Director,

Issued September 1972
National Bureau of Standards Technical Note 739

Nat. Bur. Stand. (U.S.), Tech. Note 739, 32 pages (Sept. 1972) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:739). Price 35 cents.
TABLE OP CONTENTS

PAGE

1. INTRODUCTION 1

2. FEDERAL SPECIFICATIONS 2

2.1 Computer Magnetic Tape 2

2.2 Instrumentation Magnetic Tape 4

2.3 Magnetic Tape Cassettes 4

3. MEASUREMENT SYSTEM THEORY AND DESIGN 4

3.1 Circuit Description 4

3.1.1 Computer Tape Measurement Section . 5

3.1.2 Instrumentation Tape Measurement

Section 8

4. SYSTEM CALIBRATION AND OPERATION 9

4.1 Calibration Method 9

4.1.1 Calibration of the Computer Tape

Measurement Section 9

4.1.2 Calibration of the Instrumentation

Tape Measurement Section 13

4.2 Operation of the Measurement System. ... 16

4.2.1 Operation of the Computer Tape Testing System 16

4.2.2 Operation of the Instrumentation

Tape Testing System 17

5. COMPONENT LIST 18

6. REFERENCES 19

LIST OF FIGURES

Figure

Number

1 Block diagram of the dropout system .... 3

2 Signal waveforms in computer tape

measurements 20

3 Signal waveforms in instrumentation tape

measurements 21

4 Rectifying amplifier circuit, RA-1 22

iii
LIST OP FIGURES (continued)

Figure

Number

5

6

7

8

9

Pulse control PC-1 and OA-7 circuits.

C-ROS-1 threshold detection circuits.

LI-PG-1 pulse generator system. . . . SP-DP dropout counting gates

Signal waveforms in system calibration

Page

23

24

25

26

27

iv
A UNIVERSAL DROPOUT TESTER FOR

MAGNETIC STORAGE MEDIA

Sidney B. Geller

This Technical Note describes a signal dropout detection and counting system which was designed to make

measurements on 1/2 inch magnetic computer tape, magnetic instrumentation tapes and magnetic cassette tapes

as required by existing and potential Federal Specifications. The calibration and operational procedures

for the system are described in detail.

Key words: Dropouts, cassette tapes; dropouts, computer magnetic tapes; dropouts, instrumentation magnetic tapes; dropouts, measurement system; magnetic

storage media; signal amplitude, dropouts.

1. INTRODUCTION

This Technical Note describes a signal dropout detection and counting system which was designed to make measurements on 1/2 inch magnetic computer tape, magnetic instrumentation tapes and magnetic cassette tapes as required by

existing and potential Federal Specifications. The system

was developed in the Performance Measurements Laboratory at

the NBS Center for Computer Sciences and Technology. It is

presently in operation at the GSA - Magnetic Surfaces Laboratory as an integral part of its Quality Products List

testing facility for instrumentation tape.

This measurement system is used:

(a) On 1/2 inch magnetic computer tape: to detect

and to count the number of signal pulses or pairs of pulses

in a reproduced test pulse train (all "l's") whose amplitudes drop below a preset reference level under specified

test conditions.

(b) On magnetic instrumentation tape: to detect and

to count dropout "windows". A window is the specified

length of time that the test sine wave signal reproduced

from the tape must fall below a preset drop-out reference

level. Each such decrease in signal amplitude which persists for a period of time exactly equal to a "window"

length constitutes a dropout. The dropout count continues
to mount as long as the test sine wave signal amplitude

remains below the reference level. No count is produced by

a signal decrease which exists for less time than a full

window length under the specified test conditions.

(c) On magnetic tape cassettes: similar to (a) above.

The instrument has been designed so that it can be used

as a general instrument in a magnetic media laboratory.

For example, it was used in a study of the effects of magnet fields on recorded computer tapes [1]1. Its flexibility

is due to such features as an internal pulse generator

which remains under incoming signal control until the signal

drops below approximately 1% of the reference (100$) signal level.

This generator continues to produce dropout pulses for

counting even if the incoming signal has fallen to a zero

amplitude. It does this by automatically going into a freerunning mode which has been preset to the exact repetition

rate as the incoming test signal.

2. FEDERAL SPECIFICATIONS

2.1 Computer Magnetic Tape

The operation of the computer tape measurement section

of the dropout system is based upon Interim Federal Specification WT-0051B (March 1969) in which a dropout is defined in three classes:

(a) Class 1-7 Track-800-bits per inch (NRZI)

(b) Class II - 9 Track-800-bits per inch (NRZI)

A dropout (s) in Class I and II occurs when the amplitude

of any one of the reproduced pulses falls below 50% of the

amplitude of the reference (100$) signal level. Each such

occurrence is counted and recorded as an individual dropout.

(c) Class III - 9 track 1600 bits per inch (PE)

A dropout in Class III occurs when the amplitude of any

two consecutive reproduced pulses fall below 35% of the

amplitude of the reference (10058) signal level. Each such

pair of consecutive pulses is counted and recorded as a

Figures in brackets indicate the literature references on

page 19.
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2.2 Instrumentation Magnetic Tape

The design of the instrumentation part of the system is

based upon the Interim Federal Specification WT-001553

(December 1970) which defines an instrumentation tape dropout as follows:

(a) A dropout is defined as a 6.0dB (or greater)

reduction in output signal amplitude from a reference level

for a period of 10 microseconds at the 1.0 mHz recording

frequency or for a period of 20 microseconds at the recording frequency of 200 kHz. These periods were referred to

as dropout "windows" in section lb.

(b) Signal losses of 6.0dB (or greater) which extend

over a 10 or 20 microsecond period shall result in

additional dropout counts for each additional 10 or 20

microseconds during which the signal loss persists. For

example, at the 1.0 mHz recording frequency a loss of signal amplitude of 6.0dB for 35 microseconds results in 3

dropout counts. However, a signal loss for 9 microseconds

should result in no dropout counts.

2.3 Magnetic Tape Cassettes

At the present time there is no Federal Specification

for magnetic tape cassettes, however, the standards which

are being written by the industrial committees indicate that

the dropout system will also be able to perform with the

tape cassettes. To date, successful tests have been made

using various cassette transports operating at speeds

varying from 4 to 12 inches per second with cassette tapes

recorded with pulse trains at 1600 frpi. This resulted in

operating frequencies from approximately 3kHz to 10 kHz.

3. MEASUREMENT SYSTEM THEORY AND DESIGN.

3.1 Circuit Description.

Figure 1 shows a block diagram of the entire dropout

measurement system with some of the switches that are used

when changing from the instrumentation "INSTR" measurement

mode to the computer "COMP" mode. Enclosed in each block

is the figure number of the circuit diagram which corres4
ponds to that block. Figures 2, 3 and 9 show typical signals which appear at various points in the system for computer and instrumentation tests respectively.

3.1.1 Computer Tape Measurement Section

(a) The test signal from the computer tape transport read channel amplifier is fed into the measurement

system via the "GAIN" control R]_ (figs. 1 and 4). R]_

feeds the signal simultaneously into amplifiers OA-1 and

OA-2 of the rectifying amplifier RA-1 (fig. 4) via switch

SW-1. The signal is inverted and clipped and the output

from OA-2 is reinverted by OA-3. The outputs OA-1 and

OA-3 are summed by amplifier OA-4 which produces a fullwave rectified version of the input test signal. The

system is calibrated so that a 1.0 volt peak-to-peak input

signal from R-|_ is adjusted by Rcj for a 6.0 volt base-topeak rectified output signal from OA-4. This is the

nominal 100$ signal level for the system (fig. 2b).

(b) The "PULSE ADJUST" control R2 in the feedback

loop of OA-3 can be adjusted for the following 2 test modes:

1. The output pulses from OA-4 (point H) are made

equal in amplitude even if the positive and negative test

signal peak amplitudes are unequal at the input to R-j_.

2. The relative amplitudes of the pulses at the

outputs of OA-4 are adjusted to be the same as they

appeared at the input to R]_. This is the usual test mode.

(c) The output from OA-4 (fig. 2b) is now injected

both into the pulse control eircuit PC-1 (fig. 5) and into

the comparator - retriggerable one-shot circuit C-ROS-1

via SW-3 (fig. 6). The output from. PC-1 controls the

repetition -rate of the pulses which are produced by

the pulse generator LI-PG-1 (fig. 7). This control action

is described in section 3.1.1.f2.

C-ROS-1 is the threshold setting and sensing circuit

:

it detects when the test signal amplitude has fallen below its

preset dropout signal level. For example, when the 100$

signal level at the output of OA-4 is set for-6.0 volts

base-to-peak then for Class I and II testing a drop-out

occurs when the input to comparator C-l falls below 3.0 volts

base-to-peak and for Class III testing a dropout occurs when

the C-l input falls below 2.1 volts base-to-peak.
(d) The threshold of comparator C-l is adjusted

with the "COMP ADJUST" control R3 so that it switches its

output state towards ground whenever any portion of the input test signal from OA-4 exceeds that threshold. Figures

2b and 2c show these signal relationships. Note that the

outputs of C-l are either a positive voltage level (dropout)

or a negative going pulse train (no dropout). C-l is able

to make these decisions on a pulse by pulse basis at all of

the specified test frequencies.

(e) The output from C-l is injected into the

retriggerable one-shot ROS-1 via diode D4 (fig. 6). The

"ROS-ADJUST"potentiometer R4 sets the cycle time of ROS-1

into the correct relationship with the driving pulse train

from C-l. When this pulse train appears at the ROS-1 input

then its output remains in the ground state (fig. 2d).

However, when the C-l output pulse train ceases due to a

dropout (s) then ROS-1 senses the absence of the next expected

incoming pulse (s) and can be adjusted to switch its output

level to approximately + 2.50 volts dc until the pulses reappear. ROS-1 can switch states in response to the absence

of alternate pulses in the negative going input train,

from C-l as shown in figure 9c, d and e.

(f) The output voltage levels from ROS-1 are now

used to control the operation of the linear integrator,

pulse generator system LI-PG-1 (fig. 7) which operates in

2 modes as follows

:

(1) Free-running mode (switch SW-8 "open"):

The linear integrator stage OA-6 which is driven by

the T1-T2 circuit begins to produce a negative going voltage ramp V^r across the capacitor (s) Cr in response to a

positive voltage from ROS-1 (fig. 3d). This occurs during

a dropout event. The voltage ramp which drives comparator

C-2 continues to grow in amplitude until it reaches the

preset negative C-2 triggering level and causes the output

to switch from ground to a positive voltage level. This

positive voltage is fed into the T5-T5 current switch

which is located in the (OA-6)-(C-2) feedback loop. The

collector of T5 becomes negative and drives the gate transistor T2| into conduction. T^ now discharges the ramp

capacitor (s) Cr back towards ground. This in turn causes

C-2 to switch its output voltage back towards ground which

causes the collector of T5 to return to its quiescent

positive voltage state. The result of this regenerative

feedback action between OA-6 and C-2 has produced a pulse

with a width of approximately 0.40 microseconds at the outputs of both comparator C-2 (point S and the single pulse
gate (SP) output (fig. 8). The rate at which these pulses

are generated depends upon:

(a) The linear-integrator time constants as set by

the (total) ramp capacitance Cr and the "LI-TRIM" control

Rt- and

(b) The comparator C-2 triggering level as set by the

"FREQ ADJUST" control R5. Variations in CR and R^ alter

the slope of the linear voltage ramp Vq^ and thereby alter

the length of time which is required to reach the C-2

trigger point. This varies the pulse regeneration rate and

can be made to coincide with the pulse rate of the Incoming

test signal from OA-4. The separation between the pulses

can also be adjusted to be equal to one window length for

use in instrumentation tape testing.

The pulse generator system LI-PG-1 will continue to

produce a pulse train so long as ROS-1 remains at its

positive (dropout) voltage level. At the instant that the

dropouts terminate then ROS-1 returns to its ground state

and T2 applies a negative voltage into the base of To which

discharges Cp>. Vqj^ remains near zero potential until the

next dropout appears.

(2) Controlled generator mode (figs. 5 and 7):

When the amplitude of the computer tape test signal

out of OA-4 is greater than approximately 1% of its nomial

6.0 volt (100$) level then the frequency of the LI-PG-1

pulse generator is controlled by the test signal. This is

done as follows: the signal from OA-4 is inverted and

shifted by OA-5 (fig. 5). The rectification of the input

signal in RA-1 has produced sharp cusps at the signal

crossover points (fig. 2b). When these cusps are inverted

and level shifted by OA-5 and the "C-2 Control" potentiometer Ry then sharp positive output control pulses are

produced as shown in figure 2f. These pulses are injected

into comparator C-2 at point Y, figure 7 and override the

control of the ramp voltage Vcr. This causes C-2 to

trigger the regenerative cycle at the rate of the Incoming

test signal. This pulse control is retained until the test

signal falls to less than 1% of its nominal level. When

this occurs LI-PG-1 returns to its free-running mode and

continues to produce pulses at the preset rate. This

assures a dropout count even with a complete loss of signal which is often caused by a defective tape surface area.

(g) The output pulses from comparator C-2 (point S)

and ROS-1 (point P) are fed into the series T7-T3 "AND"

gate (fig. 8). The output from transistor T2 is differen-
tiated and also fed into the gate. . This differentiated

pulse is produced from the leading edge of the pulse on

the collector of T2 which is produced at the onset of the

dropout interval. It is always counted as the first dropout pulse in a string of dropout pulses (fig. 2g). All

of the remaining pulses are produced by the action of LIPG-1 as described in section 3.1. If.

For Class I and II computer tapes the output from

point SP (fig. 2g and 8) is fed to a counter unit (see

section 5) which records the total number of dropouts that

are measured on a test tape. For Class III tape testing

each two successive dropouts are given single counts.

This is accomplished with the binary "toggle" circuit TG1 and the T]_q-T]_i "AND" gate. The output appears at

point DP (figs. 2h and 8) and is fed to the counter unit.

3.1.2 Instrumentation Tape Measurement Section

As defined in section 2.2 an instrumentation tape

dropout does not consist of individual pulse counts; instead it consists of time interval counts or "window"

counts. In addition no count must appear if a dropout

interval persists for less than a full window length in

duration.

(a) The use of an AGC input circuit as shown in

figure 1 is optional. One such device is used in the GSAMagnetic Surfaces Laboratory dropout model [2]. When an

AGC circuit is not used in the signal path, then R^ attenuates the input signal level to 1.0 volt peak-to-peak at

point T (fig. 1). It is then fed directly to the input

of OA-7. If an AGC circuit is used it is adjusted to

provide an unmodulated 1.0 volt peak-to-peak output signal.

(b) OA-7 (fig. 5 amplifies the AGC or R]_ output

signal to 6.0 volts peak-to-peak (fig. 3a). 3.0 volts

base-to-peak is the 100$ signal level for the system while

+1.5 volts base-to-peak is the dropout threshold for the

instrumentation tape measurements.

(c) The signal output from amplifier OA-7 is fed into

the C-l comparator circuit which has been adjusted to

switch its output state when the input signal amplitude passes through +1.50 volts (fig. 3b). The operation of C-l

and ROS-1 is identical to that described in sections 3.1. Id

and e; their outputs are shown in figures 3b and c.

(d) The pulse generator system LI-PG-1 is controlled

by the output from ROS-1 in the same manner as described

in section 3.lLlfl. LI-PG-1 is operated in the freerunning mode for instrumentation tape dropout measurements

8
and is adjusted with Cp, R5 and Rg to produce pulses which

are spaced one window length apart. The signal control

pulses from PC-1 (point X) are switched out of the C-2

circuit with switch SW-8 in this operating mode.

(e) The pulses from C-2 (point S) and ROS-1 (point P)

are fed into the T7-T8 "AND" gate (points R and Q, fig. 8).

For instrumentation tape measurements the differentiated

signal into point V, figure 8 is switched out by SW-7.

Therefore the first dropout pulse will not appear at point

SP before the end of the first "window" interval. This

occurs 10 ys after C-ROS-1 detects the beginning of the

dropout interval in 1.0 mHz testing; or after 20 ys in the

case of 200 kHz testing. The output from point SP is fed

into a counter device which records the total number of

dropouts over the length of tape which is being measured.

Note that in figures 3d and e that when LI-PG-1 was

set for a 10 ys pulse spacing (window) that no SP output

was obtained from the T7-T8 gate for the 4 ys dropout

interval immediately following the second dropout pulse

in the 24 ys dropout interval: Vqo has been returned to

the ground state by transistor T3 (fig. 7) before C-2 was

triggered for an additional pulse at SP.

Experiments were performed with LI-PG-1 operating in

the controlled frequency mode at the 1.0 mHz rate. A

divide-by-ten integrated circuit was used to produce the

10 ys windows. Additional development is required for this

approach.

4. SYSTEM CALIBRATION AND OPERATION

4.1 Calibration Method

4.1.1 Calibration of the Computer Tape Measurement Section

Table 1 gives the initial settings for the "PREQ ADJUST"

control Rg and switches SW-4, SW-5 and SW-6 at the various

operating densities. The "LI-TRIM" control Re is set to

approximately 1500 ft (3.25 turns: CW). Set switch SW-1

into "COMP" (fig. 1).

After these settings are made proceed as follows:

(a) Connect an ac sine wave generator* to the Input

point A (fig. 1). Switch SW-10 into "CAL" position.

(b) Connect the counter unit* to point SP (fig. 8).

*These instruments are listed in section 5.0.

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(c) Connect an oscilloscope to point SP (fig. 8).

(d) Set the ac generator output to approximately 3.0

volts peak-to-peak at the operating frequency given in Table

1. Reduce the ac input signal to zero by turning the "GAIN"

control Ri fully counterclockwise. Now adjust the freerunning pulse repetition rate from LI-PG-1 with the "PREQADJUST" control Rg : check the pulse spacing and the counter

frequency at point SP using Table 1. For example, if the

test is to be made on a computer tape which has been recorded at 3200 frpi at 112.5 ips then the pulse spacing at point

SP is set to 2.78 ys and the counter (in the "Frequency"

position) should read 360 kHz due to the rectifying action

of RA-1 (fig. 2b).

Note: If the tape has been recorded at densities which

vary slightly from the tabulated values, then the freerunning pulse rate as well as the ac generator frequency

should be set equal to the recorded frequency.

(e) After the free-running pulse rate has been set,

adjust R^ so that the signal amplitude at the center tap

is 1.0 volt peak-to-peak at point T (fig. 4).

(f) Adjust the "PULSE ADJUST" control Rp and the

OA-4 gain control Rg (fig. *0 until the rectified output

from OA-4 (point H) has equal amplitude peaks whose values

are 6.0 volts base-to-peak as shown in the first 4 pulses

of figure 2b. With this adjustment the relative amplitudes

of the actual signal peaks as read from the tapes will

remain as they appear at the system input. This is the usual

test procedure.

(g) Adjust the "C-2 CONTROL" Ry and R12 in PC-1

(fig. 5) until the pulses at the OA-5 output (point X)

are approximately 1.0 volt peak-to-peak (fig. 2f). Close

switch SW-8: this applies these PC-1 control pulses to

comparator C-2 in LI-PG-1 (fig. 7).

(h) Adjust the comparator C-l (fig. 6) dropout

threshold level as follows:

(1) Reduce the signal from OA-4 with "GAIN" control

Rl to:

(a) 3.0 volts base-to-peak: this is the 50% level

in Class I and II computer tape testing.

(b) 2.1 volts base-to-peak: this is the 35$ level

used in Class III computer tape testing.

11
(2) Now adjust the "COMP-ADJUST" control R3 (fig. 6)

until an unsteady negative going pulse train appears at the

output of C-l (point TP-1). Increase the signal amplitude

slightly with R^ and note if the pulse train becomes steady.

Readjust R3 and R^ until C-l just produces the pulse train

when it is triggered with a 3.0 volt (or 2.1 volt) base-topeak input signal. Figure 9b shows the output from C-l when

the OA-4 signal amplitude is slightly greater than the 50%

(or 35%) threshold level.

(i) Adjust the retriggerable-one-shot ROS-1 as follows:

(1) Close the "SET ROS" switch SW-2 in RA-1 (fig. 4).

This produces the pulse pattern shown in figure 9c. Adjust

the "GAIN" control R-]_ until the peak amplitude of the smaller

of the two pulses is slightly less than 50% (or 35%) - the

other pulse will be greater than 50% (or 35%) as shown in

the figure. The output from comparator C-l (point TP-1)

will now appear as shown in figure 9d as alternate C-l output pulses.

(2) Place a scope probe at the output of ROS-1 (point

P) and set ROS-1 with the "ROS-ADJUST" control R^ as follows:

(a) Figure 9e shows the ROS-1 output pulses as they

approach the correct adjustment. The SP output from the

T7-T8 gate consists of pulse pairs (fig. 9f).

(b) Continue adjusting R4 so that the leading edge of

each ROS-1 pulse moves to the right (fig. 9g) until each

SP pulse on the left merges with the one on the right as

shown in figure 9h. Now, for example at 800 frpi the counter

should read 45kHz because every alternate pulse in the 90kHz

pulse train Is a dropout.

(c) Reduce the input signal amplitude with the "GAIN"

control R-, until both peaks are below 50% (35%) in amplitude:

the counter should read 90kHz. Reduce the input signal amplitude to zero: the free-running counter should continue to

read 90kHz.

(3) Check the DP count at the output of T10 (fig. 8):

this should be exactly one-half of the SP count.

The system is now calibrated for making computer tape

dropout measurements according to Interim Federal Specifications WT-0051B (March 1969). These same calibration techniques:

are used for setting up the system for digital cassette dropout testing.,

12
4.1.2 Calibration of the Instrumentation

Tape Measurement Section

Table 1 gives the initial settings for the "FREQ-ADJUST"

control Rg and switches SW-4, SW-5 and SW-6. Switch SW-8 in

the PC-1 circuit (fig. 5) is opened since LI-PG-1 operates

only in the free-running mode for instrumentation tape testing. Set the "LI-TRIM" control Re to approximately 1500ft

(3.25 turns: CW)

.

The calibration proceeds as follows:

(a) Connect an ac sine wave* generator to the system

input (point A, fig. 1). Switch SW-10 is in the "CAL"

position.

(b) Connect the counter unit* to point SP (fig. 8).

(c) Connect an oscilloscope to point SP (fig. 8).

(d) Switch SW-1 is set into the "INSTR" position:

The ac signal can be either:

(1) Attenuated by the "GAIN" control Ri to 1.0 volt

peak-to-peak and then fed directly into OA-7, or

(2) Fed into an AGC amplifier (optional) whose output

is adjusted for a steady state output level of 1.0 volt

peak-to-peak. This AGC output signal is then fed directly

into OA-7 (fig. 1). An AGC amplifier is employed in the

GSA-Magnetic Surfaces Laboratory model [2] •

(e) Adjust the gain of OA-7 with potentiometer R]_]_

for a signal output of 6.0 volts peak-to-peak at point I

(fig. 3a and 5). Switch SW-9 is open during this adjustment.

(f) Switch SW-9 is now closed and potentiometer R-,q

(fig. 5) is adjusted so that the output voltage at point

I is reduced to 3.0 volts peak-to-peak. In this way, the

50% level of any input signal can be obtained by activating

SW-9.

(g) The C-l threshold level and ROS-1 are adjusted as

follows

:

Method I: (1) The dropout threshold is adjusted by

the "COMP-ADJUST" control R3 (fig. 6) in the same manner as

described in section 4.1. In with the exception that the 50%

* These instruments are listed in section 5.0.

13
level is set for +1.5 volts base-to-peak. If control R^

feeds OA-7 directly then the input signal can be varied by

R]_ about the dropout level. If an AGC amplifier is used

then the 50% level can be set by activating switch SW-9.

(2) The adjustment of ROS-1 proceeds as follows:

(a) Set the ac sine wave generator frequency to

exactly one half of the test frequency, i.e., 500kHz for

the l.OmHz test frequency and 100kHz for the 200kHz test

frequency.

(b) Set switch SW-1 into the "COMP" position. The

input signal is now rectified by RA-1 so that the positive

peak pulse spacing is corrected to l.Ous and 5.0ys for the

l.OmHz and 200kHz test frequencies respectively.

(c) Close the "SET ROS" switch SW-2 (fig. 4). This

produces the pulse pattern shown in figure 9c. Adjust R]_

so that each smaller peak lies below the 50% level (i.e.

below +1.5 volts).

(d) Set the "ROS-ADJUST" control R4 so that the

output signal from ROS-1 (point P) is adjusted to switch

at the peak of each dropout pulse as shown in figure 9e.

The ROS-1 output signal returns to ground when the input

pulse amplitude goes above the 50% level.

(e) Note: The threshold of comparator C-l can

also be set using an actual tape signal by activating switch

SW-9 as described in section 4.1. 2f and adjusting the "COMPADJUST" control Ro to achieve an unsteady pulse train at

the C-l output point TP-1.

Method II: (1) C- ROS-1 can also be adjusted through

the use of a sine wave generator which is capable of being

modulated* by a signal which produces an envelope that goes

above and below the 50% level (fig. 3a). The modulated sine

wave frequency is set equal to the test frequency and the

modulated signal is passed either through an AGC circuit or

directly through OA-7. C-l and ROS-1 are then adjusted as

described in sections 4.1.1 and 4.1.2. The C-l adjustments

are made so that the negative pulse train stops with the

first modulated peak which falls below the 50% level (figs.

3a and 3b). The ROS-1 adjustment is made so that it switches

on that same peak and returns to ground with the first peak

that exceeds the 50% amplitude level (fig. 3c).

* These instruments are listed in section 5.0.

14
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15
(h) Reduce the input signal to zero amplitude with

"GAIN" control R]_. This sets the pulse generator circuit

LI-PG-1 into the free-running mode. Now set the pulse rate

as described in section 4.1. Id. For example, as indicated

in Table 1; at the 1.0 mHz test frequency the pulse spacing

is set to 10 ys and the counter should read a frequency of

100 kHz.

The calibration is completed for instrumentation tape

dropout testing according to the Interim Federal Specification

WT-001553 (December 1970).

4.2 Operation of the Measurement System

4.2.1 Operation of the Computer Tape Testing System

(a) The switches in the system should be set according

to Tables 1 and 2. Note that switches SW-4, SW-5 and SW-6

are set according to Table 1 at the various densities and

machine speeds.

(b) Make the following connections:

(1) Read amplifier output into SW-10 (fig. 1).

(2) Counter unit and oscilloscope to point SP (fig. 8).

(3) Oscilloscope on point H (fig. 4).

(c) Start the tape transport and adjust the read

amplifier signal output level for approximately 3.0 volts

peak-to-peak from the test tape.

(d) Set the pulse counter into the "FREQUENCY" position.

Reduce the signal output from OA-4 (point H) with the "GAIN"

control R-, to slightly less than 3.0 volts or 2.1 volts baseto-peak according to the test mode and measure the dropout

count at point SP.

(e) Turn R]_ fully counterclockwise and adjust the

"FREQ ADJUST" control Rg to produce the same frequency count

at SP as in (d) above. The free-running frequency is now

equal to the test frequency. The count at point DP (fig. 8)

should now be equal to one-half of the SP count.

(f) Readjust R-, until the signal output from OA-4 (point

H) is 6.0 volts base-to-peak (100$ level).

16
(g) (1) Rewind the tape back to the beginning.

(2) Set the counter unit into the "COUNT" position.

(3) Start the tape transport.

(4) Zero and start the counter unit. The system is in

operation.

Note: During normal operating procedures, the calibration of the unit can be quickly checked with the actual tape

signal input. For example, the "SET ROS" switch SW-2 can be

closed and the input signal reduced with R]_ as described in

section 4.1.11.

4.2.2 Operation of the Instrumentation

Tape Testing System

(a) The switches in the system should be set according

to Tables 1 and 2. Note that switches SW-4, SW-5 and SW-6

are set according to Table 1 at the various densities.

(b) Make the following connections:

(1) Read amplifier output into SW-10 (fig. 1).

(2) Counter unit and oscilloscope to point SP (fig. 8),

(3) Oscilloscope on point I (fig. 5).

(c) Turn the "GAIN" control Rj_ fully counterclockwise

and adjust the free-running frequency from LI-PG-1 at point

SP to produce the "window" pulse spacing as given in Table 1,

(d) Start the tape transport and adjust Ri until the

signal amplitude at point I (fig. 5) is 6.0 volts peak-topeak. If an AGC amplifier is used, adjust Ri until the

normal input signal level for that amplifier is achieved.

( e ) (l) Rewind the tape to the beginning.

(2) Set the counter unit into the "COUNT" position.

(3) Start the transport and permit it to get up to

speed.

(4) Zero and start the counter unit. The system is

in operation.

17
5. COMPONENT LIST

The mention of specific equipments or components in

any portion of publication is not to be construed as an

endorsement of these items by NBS to the exclusion of other

equivalent devices. There is considerable latitude in the

choice of semi-conductor components, potentiometers, recording, reproducing and measuring devices. The following is a

tabulation of some of the devices that are used in the NBS

dropout measuring system:

Transistors T(Numbers in Figures) Type

2N3646

2N3638

1,7-H

2-6

NPN Switching

PNP Switching

Diodes D(Numbers in Figures) Type

1N276

1N916

1N751A

1,2,3,13

4,5-8,9-12

D

Z

Ge Switch

Si Switch

Zener

Amplifiers (Numbers in Figures) Type

PP45U OA-l-to OA-7 C)perational Amplif

(Solid State)

Integrated Circuits Numbers in Figures

MC1710CG

SN7472N

MC9601

C-l,C-2

TG-1

ROS-1

Type

Comparator

J-K flip-flop (Toggle)

Retriggerable one-shot

Instruments

Counter unit

Sine wave generator

Modulated sine wave generator

Data generator

Model

CMC 600A

1202

606A

Datapulse 201

Manufacturer

Computer Measurements

Donner Scientific Cc

Hewlett-Packard

Systron-Donner Co.

18
6. REFERENCES

[1] Geller, S.B., The Effects of Magnetic Fields on Magnetic

Storage Media Used in Computers; NBS Technical Note 735,

July 1972.

[2] NSA, XR-3-80A Magnetic Tape Analyzer System, Manual

IB- 5 (March 1966).

The assistance of George A. Wilson of GSA with the

construction and testing of the GSA-MSL dropout unit and the

assistance of Amory W. Ericson with the construction and

testing of the NBS unit is gratefully acknowledged. The

efforts of John A. Warme in the preparation of the illustrations and drawings in this NBS publication is gratefully

acknowledged.

19
volts

-0.5

R| (point T)

(a

+ 6.0

+ 3.0 - + 2. 1 -

A 100% level

\

\ - \~ 50% level (Class IS II) --\-35%levellClass III)

\

\ OA-4 (point H)

(b)

+ 3.0 —

,

+ 2.5

v 1/ V

I1-^. dropout -

region

-•—adjusted

switching

point

C-l (point TP-I) (C

ROS-I (point P) Id)

-V

NT

(controlled mode)

(e)

r^-i -Ai i ir*-i 1 m^- r^- ^- i-^- ~ I.OV

-5.0

1st differentiated pulse -

PC-I (point X) (f )

SP

to dropout counter ig

-5.0

DP

to dropout counter (h)

TIME (not to scale )-

Figure 2. Signal waveforms in computer tape measurements.

20
volts

(a)

OA-7 (point I )

+ 3.0 -i C-l (point TP-D

lb)

ROS'I (point P) (C)

'CR Id)

(free-running mode)

-5.0

TIME (not to scale)

SPto (e)

dropout counter

Figure 3. Signal waveforms in instrumentation tape measurements

21
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50% (CLASS I d II) 35%(Ct_ASS III) OA-4 Ipoinl H)

to)

+3.0 C"l (point TP-D (b)

+ V

OA-4 (point H) lC )

+ 3.0 —

I

u

"LI

C-l (point TP-I) (d)

+ 2.5 1 ROS-l(pointP) (e)

SP to dropout counter (f )

ROS-I (point P) (g)

5.0

TT

I

I 4-li

I ' iJ

TT

If

u

i I

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i.i

SP to dropout counter (h

)

TIME (not to scale)

Figure 9. Signal waveforms in system calibration.

27
FORM NBS-114A (1-71) 2. Gov't Accession No.

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-739

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A UNIVERSAL DROPOUT TESTER FOR

MAGNETIC STORAGE MEDIA

5. Publication Date September 1972

6. Performing Organization Code

7. AUTHOR(S) 8. Performing Organization

Sidney B. Geller

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. Project/Task/ Work Unit No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as No . 9 .

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.)

This Technical Note describes a signal dropout detection and counting

system which was designed to make measurements on 1/2 inch magnetic

computer tape, magnetic instrumentation tapes and magnetic cassette

tapes as required by existing and potential Federal Specifications.

The calibration and operational procedures for the system are described in detail.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Dropouts, cassette tapes; dropouts, computer magnetic tapes; dropouts, instrumentation magnetic tapes; dropouts, measurement system; magnetic stora,gP mpHin; &j gn»i- _»w r

i i t.nH..

18. AVAILABILITY STATEMENT dropouts

Hcl UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

21. tfo. OF PAGES

32

9. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

35 cents

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