PB 151401

^ecknlcal ^vT,ot& 42

ANALOG-DIGITAL

CONVERSION EQUIPMENT FOR

ELECTROCARDIOGRAPHIC DATA

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March,

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards; the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems; in- vention and development of devices to serve special needs of the Government; and the development

of standard practices, codes, and specifications. The work includes basic and applied research,

development, engineering, instrumentation, testing, evaluation, calibration services, and various

consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of the Bureau or when the

Bureau's imique competence is required. The scope of activities is suggested by the listing of

divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers. These papers appear either in the Bureau's own series of publications or in the journals

of professional and scientific societies. The Bureau itself publishes three periodicals available from

the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary

and preliminary reports on work in progress; and Basic Radio Propagation Predictions provides

data for determining the best frequencies to use for radio communications throughout the world.

There are also five series of nonperiodical publications: Monographs, Applied Mathematics Series,

Handbooks, Miscellaneous Publications, and Technical Notes. Information on the Bureau's publications can be found in NBS Circular 460, Publications of the

National Bureau of Standards ($1.25) and its Supplement ($1.50), available from the Superintendent

of Documents, GK)vemment Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS

technical ^vZote

42

APRIL 1960

ANALOG-DIGITAL CONVERSION EQUIPMENT FOR

ELECTROCARDIOGRAPHIC DATA

L. Taback

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $1.25
TABLE OF CONTENTS

Page

J.O INTRODUCTION I

2.0 SYSTEM REQUIREMENTS 2

3. OPERATING PROCEDURE 3

4.0 COMPONENTS 3

4. 1 Analog Tape Recorder 3

4.2 Operator's Control Unit and Display 4

4. 3 Tape Control 5

4. 4 Format Control 6

4. 4. 1 Diode Matrix 6

4. 4. 2 Ring Counters 8

4.4.3 Bi -Stable Multivibrator 8

4. 4. 4 Dual Delay Multivibrator -. 8

4.4. 5 Time -Base Generator and Clock

Control 8

4. 4. 6 Character and Word Drivers .... 8

4. 4. 7 Line Driver 9

4. 4. 8 Beam-Former 9

5. DIGITAL TAPE RECORDER 9

5. 1 Tape Drive Assembly 9

5. 2 Tape Tension and Servo Control Assembly. . 10

5. 3 Control Circuit Assembly 10

6. DIGITAL "WRITE" ELECTRONICS 10

7.0 MULTIPLEXER 10

8.0 ANALOG-DIGITAL CONVERTER 11

9.0 BUFFER 11

10.0 PROGRAMMING AND DATA CORRELATION . . . 11

11.0 ACKNOWLEDGEMENTS. . . 12

APPENDIX A - Data Format • 13

APPENDIX B - Design of Delays in Y and Z

Amplifiers 14

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I.IST OF ILLUSTRATIONS

Figure 1 Overall Block Diagram

Figure 2 Picture of Assembled Equipment

Figure 3 Operator's Control Unit (Schematic)

Figure 3a Amplifier K2W (Schematic)

Figure 3b Amplifier (AMP 1)

Figure 3c Trigger (Trig)

Figure 3d Delay Miiltivibrator (DMV)

Figure 3e Driver (D)

Figure 3f Driver Flip-Flop (DFF)

Figure 3g Multivibrator (MV)

Figure 4 Patient Identification Switch Arrangement

Figure 5 Preamplifier and Limiter

Figure 6 Audio Amplifier

Figure 7 Tape Control (Block Diagram)

Figure 8 Tape Control (Circuitry)

Figure 9 Tape Control Flip-Flop

Figure 10 Format Control (Block Diagram)

Figure 11 Diode Matrix

Figure 12 Ring Counters

Figure 13 Bi -Stable Multivibrator

'Figure 14 Dual Delay Multivibrator

Figure 15 Time Base Generator and Clock Control

Figure 16 Character and Word Driver

Figure 17 Line Driver

Figure 18 Beam Former

Figure 19 Head Driver

Figure 20 Analog Gate

Figure 21 Trigger Generator

Figure 22 Buffer

Figure 23 Word Format

Figure 24 K2W Operational Amplifier

Figure 25 Amplifier with Delay

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ABSTRACT

A corrected orthogonal 3 -lead system has been used to record

electrocardiograms directly from patients at Veterans Hospitals,

using three FM channels of magnetic tape. A pilot facility has been

designed and assembled by NBS to permit a medical technician to

inspect these on an oscilloscope and select a significant cardiac cycle.

This is automatically sampled at millisecond intervals and the

numerical values are stored in digital form on magnetic tape

acceptable to an electronic computer. Upon writing various progranns

for the digital computer, the cardiac researcher will have a flexible

tool for objective analysis of large quantities of biological data by a

variety of possible criteria.

IV -
ANALOG-DIGITAL CONVERSION EQUIPMENT

FOR ELECTROCARDIOGRAPHIC DATA

by

L. Taback

1.0 INTRODUCTION

Under a Veterans Administration research program, the National

Bureau of Standards has designed and assembled a pilot facility for

monitoring electrocardiographic data taken in analog form on magnetic

tape and converting it to a digital magnetic tape format for automatic

analysis by a digital computer. There are several good reasons for

taking this approach to the study of heart signals.

(1) The method will make possible a comipletely objective analysis,

by eliminating the effect of variations in interpretation by individual

observers.

(2) Fidelity of the data is maintained both in accuracy and in

dynamic range. Conventional methods of cardiographic analysis are

conducive to large errors in amplitude measurements and also large

errors in time relationships. In addition, many of the paper records

do not have a wide enough frequency band and consequently lose the

higher -frequency components in the signals.

(3) The method -will permit the convenient storage of large

amounts of data. Equally important, it provides a quick and efficient

means for retrieving old records in a form suitable for comparison.

(4) The collection and storage of data on magnetic tapes in a

form suitable for direct input to high speed computers will make it

convenient to perform adequate statistical studies on large quantities

of data. The diagnosis of cardiac and circulatory conditions from an

electrocardiogram is still largely empirical, and it appears desirable

to set more accurate limits for normality and abnormality. There is

a need to investigate new criteria for evaluating the data and to performi

evaluation studies on the effectiveness of these criteria. The use of a

digital computer provides a flexible tool for this purpose and provides

a means of applying sophisticated mathematical techniques completely

impractical by hand methods.

The raw data for this pilot study have been collected in Veterans

Administration hospitals by recording electrocardiographic signals on. magnetic tape. For this project, VA physicians have an electrode

system such that three orthogonal signals are generated; these

represent the three components of the heart vector. The method

chosen for recording these signals was wide -deviation FM. A fourth

channel using direct recording was reserved for voice identification of

1
the patient. The records for approximately 100 patients can be

recorded on a single 12 -inch reel. In this fashion large quantities of

data from various field hospitals may be accumulated conveniently.

The conversion of the data into a form suitable for analysis by a

digital computer is accomplished by a medical technician at a central

processing installation for which the equipment herein described is a

prototype. The data, as originally recorded, is continuous in nature.

It is put into the desired digital form by sampling each of the three

analog values at 1 msec intervals. Since it has been planned to make

prototype studies at the National Bureau of Standards, where an IBM

704 computer is available , the format chosen was for this machine.

2. SYSTEM REQUIREMENTS

The central processing facility is designed to accept the original

magnetic tape recordings and to produce another magnetic tape

containing this information in the required numerical form. While the

prototype facility was initially aimed at the electrocardiograph problem,

it was expected that the number and kinds of information would increase.

For this reason the equipment was designed to be expandable. Only

three channels are presently used for ECG data, but provision has been

made for increasing the number of channels by simply adding "plug -in

units. In order to maintain adequate fidelity for the digital electrocardiogram, an equivalent bandwidth of to 200 cps is considered

necessary. For this reason a sampling rate of 1, 000 samples/sec was

chosen so as to provide 5 samples per cycle at the highest frequency of

interest.

Since it is expected that this equipment will be operated by a

medical technician, it is imperative that the controls be kept simple

and as few as possible. In addition they should be so designed as to

minimize operator errors. Provision must also be made for correction

of errors, if they do occur.

Means must be provided for selecting and monitoring one or more

specific waveforms, from the lO-second sample usually taken from

each patient. In addition, provision must be made for including

calibration information on the original analog recording, as the raw

data will be received from many sources. The accuracy requirements

for medical data are generally in the order of a few percent.

Conversion of analog data into 7 or 8 binary digits would generally

suffice; however, since analysis will here require a certain amount of

pattern recognition, it is felt that the resolution of the converted data

should be somewhat better. For this reason, the ECG values are

represented by 10 binary digits. In anticipation of adding other

continuous variables, the format provides two bits in each word to

identify the type of data the word contains. (See Appendix A. ) Patient

identification is inserted by maiiual setting of a 6 -digit number.

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3.0 OPERATING PROCEDURE

Figure 1 shows a block diagram of the equipment that has been

designed and assembled for this purpose. The X, Y, Z components

of the heart signal are re -played from the play back heads on the

Analog Tape Recorder. The operator listens for the patient's

identification number on the voice channel, and enters it on his Control

Unit by positioning a 6-digit switch on the front panel of the latter.

This Control Unit contains a high-speed 4-channel electronic switch

which permits viewing the three components simultaneously on the

Display oscilloscope. The three wave -forms viewed correspond to that

portion of the sequence of heart cycles that will be numerically recorded

if so ordered by the operator. This Unit also provides adjustments for

selecting the beginning and end of the heart complex desired. This is

easily done in a matter of seconds with the aid of the Display. When

the operator has selected the stretch of wave -forms he wishes to

digitize, he presses the "record" key on his Control Unit. The Tape

Control, which receives the "start" signal produced thereby, brings

the digital tape up to proper speed. After a few milliseconds delay,

to produce proper gap lengths between recordings, a "run" signal is

sent to the Format Control which drives the Digital Write Electronics

in the Digital Tape Recorder.

The Format Control consists of a diode selector matrix

sequenced by two ring counters and their associated clock circuitry.

It is designed to accept parallel binary digits and deliver them serially

on six parallel channels; a seventh channel is used for timing. This

unit continues to operate as long as the "run" signal is applied. Since

the Analog -Digital Converter is to be time -shared among three signals,

an analog delay must be provided for two of these. All three are read

simultaneously from the analog magnetic tape, and the X-signal is

converted immediately. The Y signal is amplified and delayed l/3 mis

before it is presented to the A-D Converter. Similarly, the Z-amplifier

is designed to delay the Z -signal for 2/3 ms. The counters in the

Format Control provide control signals for the Multiplexer; this

consists of a three -channel transistorized analog switch. The Multiplexer produces a trigger signal to initiate the operation of the A-D

Converter, whose outputs are fed to a Buffer for driving the diode

tnatrix, as they are switched from X to Y to Z. Figure 2 is a picture

of the assembled equipment.

About 1,000 cardiac complexes of some 800 points each can thus

be stored on one 2, 400 -foot reel of digital tape. Programs are being

written for carrying out on the IBM 704 Computer the processing and

printout of this digitized data, after collating it with patient histories

fed from the Card Reader.

4. COMPONENTS

4. 1 Analog Tape Recorder

This unit is a conunercially available analog tape recorder,

containing three wide -band FM channels for the ECG waveforms and

-3-
one direct recording channel for patient identification. On the FM

channels it has a frequency range from dc to 2. 5 kc at a tape speed

of 15 ips. This unit has been modified slightly to provide an additional

output by including an additional head connected to a fifth playback

amplifier. Its position on the tape is several inches ahead of the normal

read head, so that it produces a signal earlier in time; from this a sync

pulse has been obtained for controlling the sweep circuits of the Display

unit.

4. 2 Operator's Control Unit and Display

Figure 3 shows the circuitry of the Operator's Control Unit.

The input signals are X, Y, Z, trigger, and voice from the analog tape

unit. The inputs X, Y, and Z are fed into Anap 2, Amp 3, and Amp 4,

which are Type K2W, as shown on Figure 3a. These amplifiers provide

a gain of 7, thus producing an output of 10 volts for approximately 1. 5

volts input. The 10-volt level is full scale for the A-D converter.

Amplifiers, 3 and 4 are designed (see Appendix B) to delay their input

signals to allow time for conversion. All three outputs appear at output

connectors from which they are cabled to the multiplexer. They are

also fed to drivers D4 and D5 which feed an electronic switch for display

purposes. Centering controls are provided for each of the signals; this

is done by shifting the d-c output levels of D4 and D5. Ther^ are four

channels available in the electronic switch, but one has been suppressed.

Each signal is fed to the collector of a gating transistor. The emitters

of the three transistors are paralleled and provide the vertical

deflection of the Display scope.

Only one transistor at a time is turned on. The gating of the

transistors is sequenced as follows. A free -running multivibrator

MV-1 oscillates at about 20 kc. Its output is fed to a bi-stable flipflop CFF-1 which divides the frequency by two. The outputs of MV-1

and CFF-1 are fed to drivers D6 and D7. Since each of the two units

may be in two possible states, there are four combinations possible

altogether. The outputs of the drivers feed and-gates which select

the four possible conditions, and the outputs of the and-gates are used

to turn on the transistorized electronic switch. Each time MV-1

changes states, a positive pulse is produced by the bridge circuit made

up by CR 9, 10, 11, 12. These pulses are used to blank the display

during the switching time from one channel to the next.

The input TRIG^n is fed into amplifier AMPl. This signal comes

from the analog tape unit and is generated from the X channel by the

extra head which is several inches ahead of the "read" head. It therefore occurs earlier in time, but is similar in form to the X

component of the heart vector. The output of AMPl drives a Schmitt

trigger (TRIG 1). This circuit changes state each time its input signal

exceeds a fixed level and returns to its original state when the input

signal falls below this fixed level. An indicator is provided on the front

panel to indicate whether or not the circuit is triggering. A sensitivity

control is provided for the operator so that he can adjust the triggering

level for optimum. The output of the trigger is sent to DMV 1. This

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delay is also variable and under control of R2

.

The sweep signal for the Display is derived from the internal

timing mechanism of DMV2 and buffered by driver DZ. By adjusting

R2, the turn-on time of DMV2 is set to occur just ahead of the electrocardiogram, and by adjusting R3, DMV2 is set to turn off just after its

completion. The output of DMV2 is used to gate the recording cycle,

so that the operator can see exactly what is being recorded. That is

accomplished by feeding the output of DMV2 via driver D3 to one input

of a 2 -input and-gate consisting of CR 13, and CR 14. The oth^r input

to the and-gate is supplied by flip-flop DFF2. The output of the andgate is buffered by driver D3 and fed to flip-flop DFFl. The output of

DFFl produces the start/stop signal for the system. DFFl can only be

triggered by the leading edge of the output of DMV2 if DFF2 has been

set by the "record" key. Thus, if the "record" key is operated during a

display, DFFl will not be energized until the start of the following

display, thereby preventing the recording of partial complexes.

The and-gate behaves as an "or" for negative signals so that

DFFl is always reset at the trailing edge of the output of DMV2, whether

DFF2 is set or not. At the end- of a recording cycle the trailing edge of

the output of DFFl is used to reset DFF2. A "ready" light and "record"

light are provided on the front panel to indicate the status of these two

flip-flops. The detailed circuitry of the various plug -in units is shown

in Figures 3b, 3c, .... 3g.

This Control Unit also contains provisions for patient identification

by manually setting a 6 -digit number on panel switches. Their

arrangement is shown in Figure 4. Each switch provides 1-2-4-8 BCD

code representing digits from zero to nine. In addition, provision is

made for deleting records, tagging calibration records and for putting

a "logical tape end" signal on the digital tape after the last recording

has been made. The details of thie circuitry are shown in Figure 4.

The voice signal from the Analog Tape Recorder is fed into an

audio amplifier and speaker so that the operator may hear the

physician's spoken identification of the patient. In addition, a

monitoring signal from the Digital Tape Recorder is fed through a preamplifier and limiter to the audio amplifier (Figures 5 and 6). It is

helpful to be able to monitor the digital tape, as a means of locating the

last digital record of a previous recording session, and as an indication

that a normal recording is being made.

4. 3 Tape Control

Figure 7 is a block diagram of the Tape Control. The

sequence of operations is initiated by a positive pulse at the EXT

START termiinal. This signal actuates delay multivibrator DMVl. The

output of DMVl is fed to or -gate OR 1 which in turn energizes

amplifier AMP 1 and delay multi -vibrator DMV2. The output of AMP 1 provides a signal which causes the digital tape transport to start naoving

-5-
tape. AMP 1 also provides a pulse which is fed to the head driver flipflops. The output of DMV2 provides a signal turning on these flipflops in preparation for making a recording. It must be noted that

DMV2 is direct -coupled in such a way that a delay occurs only for the

turn-off signal, and not for the turn-on signal.

Since the PRESET signal and the HEAD CURRENT GATE signal

occur simultaneously, the head driver flip-flops are set to their

reference direction. When DMVl returns to its initial state, after the

few milliseconds delay necessary to get the tape up to speed, and to

provide proper gap lengths on the tape, a pulse coincident with the

trailing edge of its output is fed to flip-flop FFl causing it to change

state. The output of FFl and DMVl are both fed to an and -gate in the

Format Control. The output of this and-gate controls the operation of

this unit. The Format Control will operate as long as any of its four

inputs to the and-gate are "down". Thus, operation starts with the

trailing edge of DMVl and will be kept running by FFl. The output of

this and-gate is inverted by INVl and is then fed to or -gate ORl to keep

the tape running and maintain the HEAD CURRENT GATES on. A

recording cycle is ended by applying a negative pulse to the EXT STOP

terminal.

This signal resets FFl to its initial state. The Format Control

will then complete its scan when the WORD COUNTER and CHARACTER

COUNTER return to their initial positions, and the output of the andgate will rise. This signal is then inverted by INVl and causes the

output of or,-gate 1 to fall. INVl also produces a pulse at this time

which is delayed by DMV3 and is us ed to reset the head driver flipflops to the reference state. This is necessary to maintain clean tape

gaps between recordings. When the output of or -gate 1 falls, AMPl

caus es the tape transport to shut off. Head current is maintained for

a few milliseconds by the action of DMV2 until all motion of the tape has

ceased. This too is a precaution to keep the tape gaps clean.

Detailed circuitry is shown in Figures 8 and 9.

4. 4 Format Control

The Format Control is effectively a 64-position 6 -pole

commutator switch, as shown in Figure 10. Of the possible 384

possible lines to the diode matrix, only 8x36 are needed to supply the

six output channels. The commutator sequentially connects all six

outputs to successive groups of six inputs. The data on any one output

channel consists of 48 bits per scan, one for each of six characters in

each of eight words. Details of the various circuits are discussed

below.

4. 4. 1 Diode Matrix.

The cross-bar switching arrangement shown schematically

in Figure 11 is made up of a number of and- and or- gates. The input

gates are supplied by static voltages (either or +20 v). Data

-6-
appearing at the output of channel 1 are received from the input lines

connected to the first and-gate in each of the rows and columns of the

matrix. Character drivers supply positive -polarity pulses to all eight

gates in a particular column, and word drivers supply positive -polarity

pulses to all eight gates in a particular row. The first character in

the first word is selected by the simultaneous occurrence of a pulse in

the character column 1 and in row 1. The second character in the first

word is selected by the coincidence of a pulse in column 2 and in row 1. The character driver pulses are advanced from column 1 to column 6

in sequence and then returned to character number 1; the word "readout" pulses are advanced from word position 1 to word position 8. This

cross-bar selection continues until all 48 characters in the matrix have

been read out.

The sequence of operations is started by introducing a negative

pulse from the Tape Control into the time -base generator and clock

control. This input is one of four to an and-gate. When the and-gate is

"up" (this requires coincidence of all four inputs), it inhibits the

operation of the time -base generator. Before the "start" pulse is

introduced, both counters are in their "1" position. The and-gate goes

"down" with the negative -going "start" pulse, allowing the time-base

generator to operate. The time -base generator is held in operation by

a flip-flop in the Tape Control which is set by the "start" pulse. The

output pulses from the time -base generator drive a dual delay multivibrator whose purpose is to produce pulses of proper width and

amplitude. The output of the first half of the dual delay multivibrator

provides the necessary time for triggering the A-D Converter, making

a conversion, and setting up the numerical value in the Buffer. The

output of the second half serves the following three functions :

(1) It produces an output on the six information channels by

energizing the output and-gates preceding the line drivers

(see Figure 11). The first read-out corresponds to the

first character of the first word, since the counters are

in these states at the start.

(2) It provides the output on the seventh or sprocket channel.

(3) The trailing edge of the pulse operates the character multivibrator which advances the character ring -counter.

Each output pulse connects the six output channels to successive

6-bit characters in the diode matrix, the character counter being

advanced at the end of each character readout. When the character

counter goes thjsDugh a complete cycle and returns to the "1" position,

it provides a signal which energizes the word multivibrator which then

advances the word ring -counter. This sequence of operations continues

until a stop signal originating in the operator's Control Unit resets the

clock control flip-flop in the Tape Control. However, the clock control

will continue to operate until the word and character counters return to

their initial positions, causing the and-gate to come "up". Thus, each

recording must consist of an integral number of 8 -word blocks.

-7-
^* ^' ^ Ring Counters

Figure 12 shows a schematic diagram of the "magnetron"

beam switching tube used as a counter for characters or wovds. This

tube is capable of maintaining a beam of current on any one of the 10

targets arranged in a ring. It can be switched from one target to the

next by driving alternate grids with a push-pull voltage from the plates

of a conventional flip-flop circuit. Since only eight positions are needed

for the word counter, two targets have been eliminated by electrically

preventing the beam from forming on them. The character counter is

constrained to six positions. The potential at any target is

approximately 250 volts when off, since there is no target current, and

is approximately 120 volts when on.

4.4.3 Bi-Stable Multivibrator

Figure 13 shows the schexnatic diagram of the multivibrators

MVl and MV2 used to advance the character and word counters,

respectively. They are conventional flip-flops in which only one or the

other tube is permitted to conduct. Each input pulse causes both tubes

to change states. The outputs are taken from the plates and are fed to

the grids of the ring coixnter tubes. The character multivibrator is

triggered by the trailing edge of the delay multivibrator output pulse,

and the word multivibrator is triggered by the negative pulse at the No.l

target of the character ring counter when it returns to the No. 1 position.

4. 4. 4 Dual Delay Multivibrator

Figure 14 shows the schematic diagram of .the dual delay

multivibrator DMVl and DMV2. Input trigger pulses from the time base

generator produce standardized output pulses. The second DMV is

triggered by the trailing edge of the first DMV. The cathode follower

outputs provide the necessary power for operating the associated

circuits.

4. 4. 5 Time "Base Generator and Clock Control

Figure 15 shows the schematic diagram of this unit. The

circuit consists of a blocking oscillator which is turned on or off by the

output of a four -input and-gate. When all three inputs are "up" (+20v),

the common cathode is also "up". This bias is sufficient to keep the

blocking oscillator from operating » When any of the three inputs is

"down" (0 volts), the blocking oscillator becomes free -running,

producing negtitive pulses at a frequency determined by the time

constant in the transformer secondary.

4. 4. 6 Character and Word Drivers

There are eight character drivers and eight word drivers,

all identical, in the Format Control. Their purpose is to energize with

the proper wave -form the proper row and coluinn in the diode matrix.

Figure 16 shows the schematic diagram of one such driver. Its input is

-8-
obtained from a target of one of the ring counters. When this target is

"off" the net voltage to the input transistor is positive. The input

transistor operates as an emitter -follower; its output is positive and of

sufficient magnitude to turn the output transistor "on". The output is

then held at ground potential. When the counter advances to an "on"

position, the input voltage falls and the net voltage to the input

transistor is negative. This negative voltage also appears at the output

of the emitter -r follower and is sufficient to cut off the output transistor.

The collector voltage will then rise until it is limited by the diode at

+20 volts. This arrangement is capable of handling large signal

currents with little power dissipation in the output transistor. When the

output is "down", the output voltage is approximately 0. 4 volt, and at

100 ma load current the dissipation is only 40 mw. When the output is

"up" (+20v), all the load current is carried by the bumper diode at a

dissipation which is again small because of the small forward drop of

this diode.

4. 4. 7 Line Driver

Figure 17 shows the schematic of the output -line driver

used in the Format Control. It is a dual cathode follower whose purpose

is simply to shift the d-c level of the output pulses and to provide the

required power level for properly driving the associated equipment.

4. 4. 8 Beam-Former

When the Format Control is initially turned on, it is

possible to find one or both of the counter tubes in the "cut-off"

condition. The automatic beam-former, shown schematically in

Figure 18, senses the presence or absence of cathode current in the

counter tubes. If one or both are "off" it energizes the relay. When

the relay closes, its contacts ground the "1" spade of each counter.

This causes each to form a beam on its "1" target As soon as the

beams are formed, the relay is automatically de -energized and the

counters resume normal operation.

5. DIGITAL TAPE RECORDER

The digital tape transport is a commercially available 7 -channel

magnetic tape transport using l/2-inch tape. A detailed description can

be obtained from the Ampex Corporation on the Model FR 200. To

summarize, it contains the following three major subassemblies:

5. 1 Tape Drive Assembly

This portion is responsible for proper motion of the tape.

It consists of a drive motor, capstan, and pinch rollers with their . associated solenoids.

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5. 2 Tape Tension and Servo Control Assembly

This mechanism is used to maintain constant tape tension

during operation of the transport. It consists of two take -up arms,

the take-up and supply motors, with a switching arrangement and

associated circuitry, which provides the servo control function.

5. 3 Control Circuit Assembly

This provides proper switching and interlocks for turning

power on and off, forward and reverse run, and manual playback. It also provides the necessary solenoid amplifiers through which the tape

transport can be controlled either manually or electronically.

6. DIGITAL "WRITE" ELECTRONICS

The ^'write" electronics consists of seven transistorized flip-flops

capable of supplying enough magnetizing current to the "write" heads to

fully saturate the magnetic tape. The "write" heads are center -tapped

and are connected to the collectors of the transistors so that

approximately 30 ma flow in one or the other half of the winding,

depending on which half of the flip-flop is conducting. Figure 19 is a

schematic diagram of one of the head drivers. A transistor gate is used

so that head current can be turned on and off as needed. Head current

is allowed to flow only while writing; this precaution is necessary to

prevent inadvertent erasure of a previous record.

To insure that head current flows in the reference direction when

the "write" gate is energized, a "preset" signal is used. This signal

is a positive pulse fed to the flip-flop through its associated diode which

causes the flip-flop to start always in the reference direction. After

the "write" gate is energized, the flip-flop will accept inputs from the

Format Control. Steering diodes are used to couple these signals to

the bases of the flip-flop, causing them to change state for each

successive input pulse. In order to maintain the magnetic tape

reference level in the gaps between blocks of information, a "reset"

signal is provided at the end of each block of information which causes

the flip-flops to return to the reference state.

7. MULTIPLEXER

The Multiplexer (presently three -channel) consists of several

plug -in units, one for each information channel and one additional unit

for generating a trigger to initiate the operation of the Analog-toDigital Converter. Figure 20 is a schematic of one analog-gate plug-in

unit. CRl and CR2 are used to provide a two-input and-gate in

conjunction with pull-up resistor Rl. The output of this and-gate is fed

through CR3 to the trigger generator (Pin 1, Fig. 21). Thus CR3 and

the corresponding diodes of other plug-in units form an or -gate feeding

the trigger unit in conjunction with the pull-down resistor Rl of the

trigger generator. The output of the and-gate is amplified and inverted

by T 1 which then turns on T2. Thus, the collector of T2 approaches

-10-
the emitter voltage of +10 volts whenever both inputs to the and-gate are

present. The emitters of all the analog -gate plug -in units are tied

together to form an output line. Thus, any information channel ma.Y be

selected to appear on the output line by properly energizing its

associated gating circuitey. The signals for energizing those gates are

generated by the character and word drivers, as described under

Format Control.

The output of the or -gate feeds one input of a two -input and-gate

consisting of CRl and CR2 in the trigger generator (Fig. 21). The

second input is a narrow pulse which occurs at the clock rate of the

Format Control. The output of this gate is amplified by Tl and inverted

by T2 to provide the trigger signal for the A-D Converter. Thus, for

each channel of information a trigger is generated at the proper time.

The capacity of this unit can, therefore, be increased or decreased

simply by adding or removing analog -gate plug -in units.

8. ANALOG-DIGITAL CONVERTER

The A-D Converter is a commercially available unit capable of

making 45, 000 independent analog -to -digital conversions per second.

Each conversion produces ten binary bits plus sign to represent the

input analog value. For details of this unit, the instruction manual for

the DATRAC Model B-611 of EPSCO, Inc. should be consulted.

9.0 BUFFER

The Buffer consists of plug -in transistorized amplifiers which

accept the outputs of the A-D Converter and provide the necessary

power for driving the Format Control. Figure 22 is a schematic of one

of the plug-in units. Each unit contains two amiplifiers. The A-D

converter produces a -20v signal for a binary "one" and Ov for a "zero"

When a "one" is fed to the input of the amplifier the transistor is turned

"On" producing a +20v at the output. When a "zero" is applied, the

transistor is held off, and the 33k pull-down resistor tied to -150v

causes the output to go negative. However, the output is held at volts

by biimper diodes 1N67 connecting the collectors to ground. Thus, the

signals are inverted to make them compatible with the Fornrat Control,

as well as amplified. In order to keep the polarity of the sign bit the

same as in the IBM 704, an inverter circuit (Figure 24) has been

provided for the sign channel.

10.0 PROGRAMMING AND DATA CORRELATION

In order to use the digitalized data for statistical studies, the

Veterans Administration plans to complement the ECG informiation

by additional historical and clinical information about the patient. This

will be done by entering data such as name, sex, age, height, weight,

record of illnesses, and clinical findings on an IBM punched card, or

cards, together with patient identification number as recorded on the

digital magnetic tape. In cases where naore than one recording of a

patient has been made, the cards will carry the additional record

-11-
numbers and their associated dates. Studies may then be made based

on age groups, weight groups, etc. or combinations of these parameters,

by merging the tape and card information. Whenever additional data is

acquired or computed the new information may be added to the punch

card under computer control. To improve computer access time, this

card information could be stored on another magnetic tape. The

punched cards may also carry the information in printed form. These

cards may be kept in numerical, or alphabetical order for the purpose

of maintaining readable records. Another advantage of keeping the

records in this form is the availability of equipment for sorting and

collating the information rapidly and automatically.

The program will also include provision for calculating the proper

calibrations for the digitized electrocardiogram. At each station where

data is collected a 10 cps 2 mv peak-to-peak square wave will be

recorded on the analog magnetic tape. During playback at the central

processing installation, this is noted by the operator who then tags the

record with the special code for calibration (see Appendix A - Data

Format). When the data is read into the digital computer the

calibration code causes it to calculate the calibration constant for each

channel which it then applies to all succeeding data until a new

calibration is noted.

The digital program will also contain a subroutine for discarding

any record which is followed by a record marked with a "delete" code.

This provides the operator with a naeans of eliminating records which

are improperly made as a result of a poor analog record or a manual

error.

1 1 . ACKNOWLEDGEMENTS

Acknowledgement is due to Dr. Hubert V. Pipberger of the

Veterans Administration Hospital, Washington, D. C, for inspiring

and supporting this program, and to Dr. H. L. Mason of the National

Bureau of Standards who gave much of his time throughout the project.

The author is also indebted to John Marlin, Thomas Childs, and

Thomas Nolan of NBS for their contributions during the construction

and checkout phases of the equipment.

In order to test the effectiveness of the overall scheme, a program

for the IBM 704 computer was prepared by Mrs. Ethel Marden of NBS.

This program provided a printout of the nrimerical data from which

plots could be made. When this plot was compared with an enlarged

photograph of the original data, it was found to overlap within the line

width of the original recording.

Machine programs for statistical studies based on experimental

diagnostic techniques are being prepared by Dr. R. J. Arms of NBS.

-12-
APPENDIX A

DATA FORMAT - FIGURE 23

To provide a means for keeping track of patients in a large

statistical study, it was decided to provide a six-digit patient

identification number. Each decimal digit is repres'ented in l-E-4-8

binary coded deciinal (BCD) notation which uses four bits in each of six

words, as shown in Figure 23. However, when BCD representations

for the numbers 10 to 15 appear in these cells, they serve to tag the

word with "Calibration", "Delete block" and "Tape end" markers. In

addition, a two-bit code is recorded in bits 34, 35 of each word to

identify the type of information it contains. This is necessary since

four kinds of data will be included in future work:

(1) Electrocardiogram

(2) Ballistocardiogram

(3) Phonocardiogram

(4) Arterial Pressure

The location of these types of information within the block will

depend on the sampling rates required. If a recording rate of 2,000

words per second is assumed, and the respective requirements are for

(1) ECG at 1, OOO/sec, (2) ballistogram at 500/sec, (3) phonocardiogram at 250 /sec, and (4) arterial pressure at 250/sec, the 8 -word

block might take the following form:

1st word Electrocardiogram

2nd word Arterial Pressure

3rd word Electrocardiogram

4th word Ballistocardiogram

5th word Electrocardiogram

6th word Phonocardiogram

7th word Electrocardiogram

8th word Ballistocardiogram

13-
APPENDIX B

Design of Delays in Y and Z Amplifiers

In order to provide signal delay in the Y and Z amplifiers, a network has been synthesized to approximate e~^^^ as follows:

g-2Ts ^ e""^^ ^ 1-Ts + T^sVz

e^*^® 1 + Ts + T^s^/2 .... As an approximation only the first two terms of the series are used.

The operational amplifier of Figure 24 has the following transfer

function:

^<^1^^2^ ., ^f % " R^ + R^ ®1 " "RY"

®2

Simplifying

Rr R I

"I" R^ "^ RiRo/Rr

% ^ R^ R^ ®1 " ®2

R. aR^

Then letting ^- = a and R. = r- , we have

e = a 2e, - eo 12

By modifying input e , of Figure 24 by an RC network as shown in

Figure 25, with

RC = T, e- = e, , we have

1 - Ts

e = a. 4 — e

,

o 1 + Ts 1

For gain of 7, R- is chosen = 700 Kohms

R. = 100 Kohms

.'. R2 = r- X 100 k = 116. 7 Kohms

Thus, the overall transfer function of the amplifiers AMP 2 and

AMP 3 provides a flat amplitude response with a delay equal to 2T. By

choosing the proper values of R and C the required delay in each

channel is obtained.

At a delay of 2/3 ms (required in the Z channel) this network is

within 5% accuracy out of 200 cps. For the Y channel where the

delay required is only 1/3 ms, the network is within 5% out to 400 cps.

-14-
S<

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CALIBRATE^ DELETE xTTTm

NORMALi SPAREJ r^^-

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SPECIAL CODES 84 2 1 1±

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CHARACTER READY

START, FROM TAPE CONTROL

WORD READY

FIGURE 15. TIME BASE GENERATOR AND CLOCK CONTROL
IN UO

- I50V68 K

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-hl50V 7

4- 20 V 9

GROUND 3

20V

2 DRIVERS PER PACKAGE

note:

"^ 2N37

1.8 K

7 MAY BE USED TO REPLACE 2NI02. 2.7K CHANGED TO

WITH 2N377.

FIGURE 16. CHARACTER AND WORD DRIVER
50 V

IN

IN 2 (u\

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OUT

OUT 2

INI -10

OUT I - I IN2 -II

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FIL - 5

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FIGURE 17 LINE DRIVER

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2- " I " SPADE OF

WORD COUNTER

3- GND

7 - + 150 V DC

10- CATHODE OF

CHARACTER COUNTER

11- CATHODE OF

WORD COUNTER

FIGURE 18. BEAM FORMER
OUT

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RECORD )=L

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NOTE:

ALL TRANSISTORS 2NII2 OR 2N4I4.

ALL DIODES IN67.

FIGURE 19. HEAD DRIVER
FIGURE 20. ANALOG GATE

IMAR. PULSE

+ 150V

+ I0V

FIGURE 21. TRIGGER GENERATOR
-I50V

+ 150 V (EXT.)

^AA/^ 1

+ 20 V

-150 V

-f 150V (EXT.)

1- IN 1 2- OUT 1 3- -150 V

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7- -150 V

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OUT

+20 V

EXT. EPSCO BIAS RES: TO H50

68 K- 1,4,7, 8» 9 SIGN

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FIGURE 22. BUFFER
S 1 2

DATA

9 10 II 12 21 22 23 24 33 34 35 R«M^^ m^B^ B_a^

DATA

4 BITS

FOR BCD DIGIT OF

PATIENT IDENTIFICATION

NUMBER (6 WORDS REQUIRED)

OR SPECIAL CODES

*

"y

DATA TAG

TAG CODE DATA TYPE

00 ARTERIAL PRESSURE

1 PHONOCARDIOGRAM

1

1 1 BALLISTOGRAM

ELECTROCARDIOGRAM

* SPECIAL CODES

10- SPARE

Oil- SPARE

1 00 - DELETE BLOCK

I I - TAPE END

I I - SPARE

I I I - CALIBRATION

FIGURE 23. WORD FORMAT
'2 —^\AA^

FIGURE 24. K2W OPERATIONAL AMPLIFIER

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FIGURE 25. AMPLIFIER WITH DELAY

USCOMM—
U.S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front

cover. WASHINGTON, D.C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

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Metrology.

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Radicals Research.

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Energy Radiation. Nucleonic Instrumentation. Radiological Equipment.

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Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas Liquefaction. Radio Propagation Physics. Upper A.tmosphere Research. Ionospheric Research. Regular Propagation

Services. Sim-Earth Relationships. "VHF Research. Radio Warning Services. Airglow and Aurora. Radio

Astronomy and Arctic Propagation.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Research. Radio Noise,

Tropospheric Measurements. Tropospheric Analysis. Propagation Obstacles Engineering. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High Frequency Impedance Standards. Electronic Calibration Center. Microwave Physics. Microwave Circuit Standards. Radio Communication and Systems. Low Frequency and Very Low Frequency Research. High Fre- quency and Very High Frequency Research. Ultra High Frequency and Super High Frequency Research. Modulation Research. Antenna Research. Navigation Systems. Systems Analysis. Field Operations.
NBS:

•J.