NATIONAL BUREAU OF STANDARDS REPORT

3634

TECHNICAL DETAILS

OF

PRINT READER DEMONSTRATOR

by

M. L. Greenough

and

C. C. Gordon

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

MBS [PROJECT MBS REPORT

0107-20-5515 August 20, 1954 3634

TECHNICAL DETAILS

OF

PRINT READER DEMONSTRATOR

by

M. L. Greenough and C. C. Gordon

Electronic Instrumentation Section

Electricity and Electronics Division

developed through

Data -Proces sing Systems Division

for

Department of the Navy

Bureau of Supplies and Accounts

BuSandA 56782

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, re Approved for public release by the

unless permission Is Director of the National Institute of

25, d, c, Such per Standards and Technology (NIST)

cally prepared If t On October 9, 2015

in part, is prohibited

iandards, Washington

[port has been specifieport for its own use,

TABLE OF CONTENTS

Section Title Page

1.

INTRODUCTION ..... 1

2. THEORY OF OPERATION - GENERAL

2.1 Optical System

2.2 Electronic Circuitry . » . 2 .3 Indicator

2 *I|. Operating Cycle

3. THEORY OF OPERATION - DETAILED ...................

3.1 Optical System .q... ...... ...... ...... ....... 3.2 Electronic Circuitry, Indicator

I;. CONCLUSIONS - Threshold and Error Level Adjustments

REFERENCES ....a.

TABLES

LIST OF ILLUSTRATIONS

Figure Title

1. Block Diagram of Print Reader Demonstrator . 2. Relative Timing - Full Period

3. Relative Timing - Frame Period

ii. Relative Timing - Field Period

Magnified Type written Letter and Associated Master

Character

6. Example of Television - Type Presentation for Checking Orientation of Image Dissector

7* Detailed Block Diagram

8 . Circuit Diagrams

(a) Gating and Indicator Circuits

(b) Scanning Circuits

(c) Recognition Circuits

(d) Image Dissector Circuits

9. Clipper Threshold Level vs Bias Supply Voltage

10. Photograph of Print Reader Demonstrator

1

6

6

9

16

19

20

i

H

w

n

1. INTRODUCTION

The general method of recognition for identifying typewritten

characters used in the Print Reader Demonstrator is an outgrowth

of the system developed by J. Rabinow. It is what might be called

a s canned-comparison system. The memory is in the form of optical

masks, one for each letter to be identified, plus a few control

patterns. Each mask is a photographic negative image of an individual

letter, i.e., it is transparent in the area where black ink would

be on the letter itself. Sequentially the masks are superimposed

over the unknown letter, and the mask area is scanned in a regular

pattern. During the scanning raster, areas wherein white paper is

visible through the transparent portions of the masks are summed up.

When the integrated total is below a threshold value upon completion

of the raster this mask then must be the correct one. Output indication is then provided to identify the particular mask.

In operation, a telephone-type selector switch moves in phase

with the mask changing. Upon recognition, this switch ceases to

move farther, leaving one indicator lamp lighted to show the identified character. The time required for recognition is about two seconds

per character. The Demonstrator is currently set up to identify all

of the alphabetic (upper case only) and numeric characters of Elite

type.

2. THEORY OF OPERATION - GENERAL

The important elements of the Print Reader Demonstrator ares

1) Optical System

2) Electronic Circuit ry

3) Indicator

The Optical System generates electrical signals from the visual

appearance of the letters under study. The Electronic Circuitry provides the necessary sweep current to the Optical System. In addition it carries out the necessary analysis of the output signals

from the Optical System. Finally the Indicator provides the identification of the unknown letter of interest.

2.1 Optical System

The important elements in the Optical System in sequence are

indicated in the block diagram, Figure 1, and are the following?

1
Illuminator

Unknown letter

Pick-up Lens

Deflector

Masks of letters in vocabulary

Projection Lens

Image Dissector Tube

Synchronizing Pulse Generator

The film handling portion of the optical assembly is a modified

Revere type 16 mm. motion picture projector® Relatively few modifications were necessary to adapt the unit for this service. The stand

and film sprockets were removed directly, while the lamphouse was

replaced with a mounting for the Pick-up Lens. The speed control is

set to mm the motor at 18 to 20 frames per second. Additions to

the projector are the Deflector Vanes described below and a bracket

for holding the Vocabulary Belt.

Light reflected by the paper and unknown letter is focussed through

the Deflector onto the masks by the Pick-up Lens. From here it is

focussed on the photo-sensitive surface of the image dissector television pickup tube.

The masks are essentially photographic negative images of each

letter in turn, located as successive frames of 16 mm. motion picture

film in an endless belt. There are 26 letters, 9 numerals, one opaque and one blank frame. The film-handling assembly is a modified

movie projector running at about 20 frames per second. Film motion

is of course intermittent; the masks are held stationary for about

two thirds of the time.

A synchronizing pulse generator is provided whose function is

to generate a pulse each time a new film frame has been brought into

position. The pulse is derived through the use of a slotted disk on the projector motor shaft, in conjunction with a lamp and photocell.

The Deflector is a transparent vane which is employed to provide

a small, slow horizontal shift from left to right of the unknown with

respect to each mask. Thus it allows a certain amount of horizontal

misplacement of the unkncrwnletter.

2.2 Electronic Circuitry

A block diagram of the equipment is shown in Figure 1. The major

subdivisions of the Electronic Circuitry are:

1) Gating circuits

2) Scanning circuits

3) Recognition circuits

3)

h)

5)

6 )

2
Control of the overall cycling of the scanning and recognition circuitiy is the function of the Gating circuits. In the

block diagram of Figure 1, this comprises the Master Flip-Flop,

Start-Alphabet Pulse Generator and Electronic Switch.

The Scanning circuits generate the various sweep voltages

required to scan the mask area on the Image Dissector. In Figure 1

this includes the Frame Flip-Flop, Vertical Scan Oscillator and

Deflection Amplifier, the Horizontal Scan Generator and Deflection

Amplifier, and lastly the Field Counter.

The Recognition circuits analyze the signals from the Optical

System to determine when a mask matches the unknown character. In

the diagram this includes the Clipper, the Error Integrator, the

Coincidence Circuit, and the Amplifier and Shaper Circuits.

2.3 Indicator

The basic elements of the Indicator are;

1) Stepping Switch

2) Stepping Switch Actuator

3) Indicator Lights

The Stepping Switch is arranged through its Actuator to start

from Home position when the first letter mask in the Vocabulary Belt comes into place. The Switch moves one position each time a new

mask appears, and is left in position after recognition occurs. It

remains in position until returned to the Home position when the

operator pushes the Reset Switch.

The Indicator lights are energized through the contacts of

the Stepping Switch. "When this Switch stops moving, one light

is therefore turned on. The Indicator lights are marked with the

appropriate characters. For convenience with the recognition

method employed, the Indicator Panel contains the lights in the same order as on the Vocabulary Belt.

2 .It Operating Cycle

2,ha Initial Conditions

These are the states of the various portions of the system

at the beginning of the operating cycle. See block diagram in

Figure 1.

1) Unknown letter in place

2) Vocabulary masks in motion

3) Stepping Switch indicating previous character

3
li) Vertical Scan Oscillator on continuously

5 ) Horizontal Scan off, at left edge of mask area

2 .lib Operation

When the Reset Switch is actuated, the Stepping Switch is rapidly

advanced to its home position. In this position the Start Alphabet

Pulse Generator is made operable. Nothing else happens until the

Vocabulary Belt passes through the beginning of the alphabet.

In the course of its motion, the Vocabulary Mask Belt comes to

the blank or clear frame. At the end of the blank frame which comes

at the beginning of the first letter in the alphabet, a clear area

pulse is generated in the Start Alphabet Pulse Circuit. This is fed

to the Master Flip-Flop, turning it on. Relative timing of these

actions is indicated in Figure 2.

At every frame change in the Vocabulary Belt, a synchronizing

pulse is generated which occurs after the new mask is in position,

phased to come at the left hand position of the Deflector Vane.

This pulse starts the ^rame Flip-Flop.

The Horizontal Scan Generator is gated on by the output of the

Frame Flip-Flop, producing a series of staircase waves of about 5> milliseconds period. Figure I4 shows significant relative timing

during the Field period. The discharge at the peak of each wave

is called the End of Field pulse. This is counted in the Field

Counter, resulting in a pulse to stop the Frame F-F after the sixth

pulse. Thus a series of six horizontal scans is produced. As

can be seen from Figure 3, showing timing during the Frame period,

these six scans or fields occur while the Deflector Vane is moving

gradually from left to right extreme horizontal positions. In

effect there are "tries" at a match at six positions of the unknown

letter with respect to each mask.

Turning off the Frame F-F at the end of the six fields results

in a pulse for advancing the Stepping Switch. This pulse must pass

through the Electronic Switch however, which is actuated by the

Master F-F. Thus the Stepping Switch can only be advanced while

the Master F-F is on. The Stepping Switch therefore advances from

the Home position at the end of each group of six fields, or at

the same time the film is being changed to a new mask. Due to the

relative phasing of Master and Frame F-F gates, one extra advancing

pulse is generated, so that the first letter of the alphabet can be separate from the Home position.

To sum up the scanning process so far, at each change of trial

masks, a series of six complete scanning rasters is made. These

rasters or fields are generated while the Deflector Vane shifts

h
the unknown letter with respect to the mask. Since the total

amount of horizontal travel is small* the amount during one field is assumed negligible. At the end of this period the

Stepping Switch advances to the next position. Approximately

simultaneously* the Vocabulary Belt changes to the next mask

in alphabet* whereupon the cycle begins again.

During each field the optical signals are analyzed for

agreement between mask and letter. First they are passed

through a threshold Clipper to decide in effect whether the

instantaneous photoelectric current is indicative of black

ink or white paper appearing through the transparent areas of the

mask under trial* The photoelectric current is very small

when the scanning beam is over the opaque areas of the mask.

Thus the only opportunity for signals of any magnitude is when

the beam is over the transparent portions. The signals from

the Clipper emerge as either positive (for black) or negative

(for white) . The threshold for transition is adjustable depending upon the heaviness of the typewritten character.

The function of the Error Integrator* which is next in

line* is to sum up all areas where white paper is visible

through the mask* Since the scanning velocity is constant*

the actual summation is carried out as the time integral of

the white signal from the Clipper. At the end of each field

the integrator is sampled to determine whether the integral

is above or below the chosen threshold level. If at the end

of the field the integral is above the threshold* there is no interference with the overall cycle. The Integrator is reset

immediately after the sampling.

However* if the integral is below the threshold* as is the

case when the mask matches the letter* a recognition pulse appears

at the output of the Coincidence circuit. This is amplified*

shaped* and fed to the Master F-F-to turn it off® This in turn*

disables the Electronic Switch* preventing the Stepping Switch

from advacing any farther. Finally* as the output indication

in the Demonstrator model* an appropriately labelled lamp is

energized through the Stepping Switch contacts.

To take care of the case of non-recognition of an unknown

character* the last mask on the belt is entirely opaque; thus

recognition is bound to occur on this one if it has not on the

other masks. This provides a convenient means for indicating

non-recognition of the standard characters.

After recognition on one of the masks* only the Stepping

Switch is stopped. There is no interference with the mask changing* scanning* integrating and other functions. Since the Master
F-F can only be reset on the Home position of the Stepping Switch,

the Master F-F remains off until the Reset Switch is actuated.

3. THEORY CF OPERATION - DETAILED

3*1 Optical System

3.1a Illuminator

The Illuminator is a standard Eastman Kodak 2X2 slide projector

with a l£0-watt bulb, arranged with a supplementary lens of 20

diopters located in front of the regular projection lens. The light

is focussed to an area of about .2 X .3 inches on the paper. Mounting of the projector is arranged for an angle of illumination of

about 35° from the paper* The estimated intensity of the light Is about 100,000 to £00,000 foot candles* Uniformity is probably good

to about 10% over the scanned area.

3.1b Uhkown Letter

In the Demonstrator the characters with recognition to be

shown are typed on a roll of white bond paper taped to the typewriter platen. The platen is then turned up until the characters

are at the top. Either single or double spacing may be used

between letters.

The typewriter itself is a Remington with Elite type face.

The dimensions of the letters themselves are about 0.10 inch

high by 0.10 inch wide for the letter M. Spacing of characters

horizontally is 12 to the inch. Best results with regard to the

kind of ribbon seem to be with the heavily-inked variety, or with

a paper carbon ribbon.

3.1c Pick-up Lens

This is a movie-camera type lens made by Bell and Howell

labelled Lumax f/l.9, focal length 1 inch. It is normally used at

f/2.8 for better contrast than is provided at full open.

3. Id Deflector Vane

The Horizontal Deflector Vane is a plate of l/8 inch thick

lucite arranged to oscillate about an axis which is equivalent to

the vertical direction on the letter masks® The vane is driven

synchronously through a cam on the same shaft which changes the

frames on the film. As the vane is tilted, light rays passing

through it emerge with a positional shift which is a nearly linear

6
function of tilt. It is moved through a range of about + 15

degrees, providing a total shift equal to about V~>% of the

width of the letter M, or about l|r line widths on the mask.

The Vertical Deflector Vane is a hand-operated means for

lining up the letters vertically with the masks.

3.1e Masks

These are photographic (negative) pictures of hand-drawn

master figures representing the essential elements of each

letter. In the drafting process, the typewritten characters

were enlarged on the viewing screen of a microcard reader to

about 2 ijh inches high. The width of the lines was made about

3 A of the line width in the character when typed with medium

inked ribbon. Normal line width in the mask is about 10 mils

referred to the typed character, or about 10% of the width of

the letter M« Examples of magnified typed characters and the

resultant drawn figure are shown in Figure 5°

As photographed on the mask, the dimensions of the letter

M are 0.18 inch high by 0.16 inch wide®

The order of the letters on the Vocabulary Belt is listed

in Table I. This order is dictated principally by the ambiguities

such as 0 and Q, where it is necessary to place the Q mask first

in order to screen out recognition of the Q as an apparent 0.

The Vocabulary Belt was made by photographing the drawn letters

sequentially on a motion picture camera arranged for frame-byframe exposure. The film itself is Recordak Micro-File panchromatic

film, 16 mm. width, and perforated on one side. Virtually the only

important photographic characteristic is that the unexposed areas be clear after processing. Ordinary 16 mm. negative film has a gray base which absorbs at least 5

% of the transmitted light in

the unexposed areas, making it unsuitable.

3. If Projection Lens

This is the regular projection lens for the projector assembly.

The focal length is 2 inches; the aperture, f/l.6. In the optical

arrangement as set up, this lens focusses an image on the dissector

tube which is about one inch high. Thus the overall magnification

from typed character to image dissector tube is 10 times. Illumination intensity at the screen is about 2.5 foot candles with white

paper and clear mask.

3.1g Image Dissector

This type of pick-up tube was chosen because signals may be

extracted on an aperiodic basis. That is, a continuous signal may

be obtained if the scanning spot is held motionless. Since there

is no storage, as is the case for iconoscopes, orthicons and

7
vidicons, there is no necessity for scanning and discharging the entire

screen area. Thus the scan may be limited to any desired area.

At the photosensitive screen of the dissector tube, the boundary

of the mask frame is 1.75> inches high and 1.25> inches wide. The ref- erence orientation here is with respect to the projected letters as viewed on the screen.

Orientation and placement of the dissector tube is important

in this application. There is an effective rotation of the pattern

of about 30° as it is scanned, brought about by the combination of

magnetic focussing field and electrostatic accelerating f ield.

Furthermore, this particular style of dissector tube, having its

screen internal rather than on the front surface, has the electron

multiplier elements blocking the light path from a centrally located

lens. To avoid a shadow with the present optical system, the dissector tube is tilled so that the light misses the multiplier structure.

Proper orientation of the tube is furnished by observing a television type presentation wherein a CRO is arranged for identical

sweeps, and the CRT grid blanked by optical signals from the

dissector. Centering and rotation are easily checked under these

conditions. A typical pattern is shown in Figure 6,

From this presentation and a knowledge of size of the projected

image, the present sweep amplitudes have been measured as l.U inches

vertically and 1.0 inch horizontally at the photo-sens itve area. These

sweeps are derived from the vertical and horizontal deflection amplifiers, which feed currents through the respective deflection coils

of the yoke. While the frequencies are not those for which the yoke

was designed, the coils are used in the manufacturer ! s original

intention as regards high and low frequencies. The low frequency

coil contains a ferromagnetic structure, and is the relatively thin

coil on the yoke. In ordinary television this would of course be

the vertical deflection coil.

Scanning current amplitudes are l.U ampere peak-to-peak triangular wave for vertical deflection, and 0.6 ampere peak-to-peak staircase wave for the horizontal deflection. Other detailed information

on the tube is given in Table 2.

The scanning spot is approximately square, because of the shape

of the aperture in the collector structure. As measured to the half

signal or $0% response points, the spot width is about 80 mils wide

at the tube, or 8 mils referred to the typewritten characters. This

size was chosen to provide large signals and at the same time minimizes

the effect of typing imperfections. It will be noted that the spot

width is somewhat less than the mask opening width, which is about

10 mils. The horizontal rate of travel per vertical sweep is U mils.

8
Thus the horizontal travel per sweep is about one-half the spot width. There are about 20 vertical passes in the width of the

letter M. Vertically the measured bandwidth of 60 kilocycles

provides a resolution of at least 1|0 lines in the height of the

letters

Control of the magnitude of the optical signals is provided

by means of the potentiometer on the image dissector power supply.

This control varies the potential applied to one of t he intermediate

dynodes, changing its emission efficiency. Maximum gain is secured

in the center of its rotation.

3.1h Synchronizing Pulse Generator

This assembly produces a pulse of an amplitude of 0.3 volts

and a duration of 1% of one rotation of the film drive motor

shaft. The synchronizing pulse originates in the lamp-bulb and

photocell assembly arranged around a slotted disc on the film

motor shaft. The shaft rotates once for each frame on the

Vocabulary Belt. Phasing is such that this pulse occurs at the

same time as the film claw retracts at each frame change.

3.2 Electronic Circuitry

Details of the operation of the various portions of the circuitry are given here. Because the circuit is complex, separate

diagrams are given for the Gating, Scanning and Recognition circuits

,

as well as for the deflection amplifiers. Reference is suggested

first however to the detailed block diagram Figure ?. Waveforms

of significance are included on the diagrams and in the notebook

accompanying the equipment.

3.2a Gating and Indicator Circuits - Figure 8a

1) Master Flip-Flop

This is a conventional bistable circuit, pulsed negatively

at the two grids whenever transfer is desired. The output of the

Master F-F is a gate which is either zero or approximately -50 volts.

The latter condition is considered the "on" state of the circuit.

2) Start Alphabet Pulse Generator

The function of this circuit is to generate a pulse whenever the Vocabulary Belt passes through the blank frame coming immediately before the alphabet itself. The circuit begins with a lowpass filter to attenuate the normal scanning signals in proportion

to the relatively large one caused by the blank frame. Then a controllable bias is inserted through the 25>0 K potentiometer to

9
allow adjustment of the threshold. This is followed by a Schmitt

trigger of conventional design which flops over whenever the

threshold is crossed. Effectively at this point the signals from

the blank frame are "squared up". When differentiated, negative

and then positive pulses are generated. The pulse inverter V30B

is normally biased beyond cut-off at -20 volts, so that only the

positive input pulses are amplified. The negative pulse appearing

at the plate of V30B is then fed to the Master F-F to turn it on.

An additional condition imposed upon the circuit is that the

Master F-F is to be started only when the Stepping Switch is in

the Home position, but must not be operable while the Reset Button

is pushed. The circuitry for accomplishing this is obvious from

the diagram; the basic arrangement is to hold the grid of V30B

highly negative whenever the circuit is to be disabled. As part

of the arrangement to provide a separate position on the Stepping

Switch for the first letter in the alphabet, it is necessary to

turn on the fester F-F before the cessation of optical signals.

Otherwise the pulse would not come until the "A" mask is nearly

in position, leaving too short a pulse for the Stepping Switch.

The early start of the Stepping S'witch is brought about by coupling

the Frame F-F gate to the Schmitt input so that the Schmitt is

returned to the "off" state at the- End-oft^rame time if it has been

previously turned on. The last condition can only be satisfied

when the clear frame is in position. Thus the negative pulse

at the plate of V30B appears at the time of the End-of-Frame pulse

when the clear frame mask is in place.

Therefore the output of this circuit is a negative pulse

occurring at the end of the scanning over the clear area of the

blank frame. Timewise it comes at the End of Frame pulse, coinciding approximately with the beginning of the film travel which

brings the "A" mask into position.

3) Electronic Switch

This circuit is provided to gate the actuating pulses to

the Stepping Switch in accordance with the state of the Master F-F.

The actuating pulses are really gates v/hich last during the "off"

time of the Frame F-F. The input is derived from one plate of

the Frame F-F, the one which is positive while the scan is "off".

The f irst actuating pulse comes when the Master F-F is turned on,

which coincides with the End of Frame pulse on the clear frame

mask. This arrangement provides the initial advance signal which

allows a separate Stepping Switch position for the first letter in

the alphabet, as distinct from the Home position.

h) Stepping Switch Actuator

8 _ The pulses or gates which are allowed to pass by the Electronic

Switch are fed to a power amplifier for eventual actuation of a

relay. This relay in turn controls the Stepping Switch. Motion of

10
the Stepping Switch occurs when its coil is de-energized , thus

the Switch advances to the next light slightly after each synchronizing pulse. The normal advancing action of the Stepping

Switch is therefore externally controlled by the relay in the°

plate of V31.

5) Stepping Switch

The Stepping Switch is of the 20 position telephone

type. Alternate rotor contacts are removed,, providing through

the use of alternate bays of fixed contacts , a I4O position

selector switch. The first position is defined as the Home

position. Next to this is the commencement of the alphabet.

For rapid self-advance to the Home position when the

Reset Switch is actuated, voltage is applied to the Switch

coil through its Interruptor contacts. Stopping at the Home

position is assured, through an additional switch which disables

the rapid-advance circuit when the Home position is reached.

6 ) Indicator Lamps

These are 6-watt , 115 volt bulbs run on approximately

half-voltage to avoid over voltage on the Stepping Switch.

In the Demonstrator the alphabet as laid out on the Indicator

Panel is the same as on the Vocabulary Belt.

3«2b Scanning Circuits - Figure 8b

l) Frame Flip-Flop

This flip-flop is turned "on” by the synchronizing pulse

coming after each new mask is in place, and is shut off by the

output pulse from the Field Counter at the completion of six

fields. It is thus the gate generator for controlling the

scanning program.

The "off" state of the Frame Flip-Flop is with the right

hand (V26B) tube conducting. In this condition the output gate

to the Horizontal Scanning Generator is most positive (zero),

disabling the horizontal scan through V27A. In the other condition

the gate is at -50 volts.

The synchronizing pulse originates in the lamp-bulb and

photocell assembly on the film drive motor shaft. A negative

pulse of about .35 volts appears at the anode of the photocell.

This pulse is amplified through the two stages of V32, after

which it is applied to the Frame F-F to turn it "on".

11
The output of the Frame F-F is a gate to permit horizontal scan. This gate is zero in the off position and about -50 volts in the

"on" position.

2) Vertical Scan Generator

This circuit begins with a free-running multivibrator

oscillating continuously at about 3000 cps. In order to generate

the desired trigangular wave for vertical scan, the oscillator is

fed to a clipper Vop for squaring, then to an integrating circuit.

The various time constants including the two low-pass networks at

the input of cathode follower V~[)|A and the 2 pbi output capacitor,

are all tailored to match the characteristics of the StrombergCarlson amplifier. With another amplifier, for example one like

that used for horizontal deflection, these integrating circuit

values would require change.

The output of the Vertical Scan Generator is a triangular

wave of about 3000 cps and 25 volts peak to peak amplitude.

3) Horizontal Scan Generator

The horizontal scan is designed to advance the sweep to a new horizontal position each time the vertical scan completes one up or down trace. Therefore the waveform is a staircase wave of

about 30 steps. At the conclusion of the 30 steps, the wave is

discharged and is repeated. Six rasters or fields are generated,

after which the horizontal scan returns to its quiescent value of

+8 volts a

The 3000-cycle square wave Vertical Oscillator charges

the 2000 mmf capacitor through diodes Vq£ each time the square

wave multi-vibrator flops over. The amount of charge is governed

by the voltage change applied, and by the ratio of small capacitors

(Ij.7 and 68 mmf) to the summing capacitor (2000 mf). The accumulation of voltage on the staircase summing capacitor is fed to the

cathode follower V-jhp. To enhance linearity, a bootstrap arrangement from the cathode follower is employed.

Retrace or discharge of the horizontal scan voltage is

provided by the thyratron Vt£. As the output voltage rises, the

tap on the voltage divider UJOK, 50 K pot, 510 K) reaches the

conducting range of the thyratron, bringing about discharge of

the staircase summing capacitor 0 After deionization of the thyratron, the charging process resumes.

In order to gate off the horizontal scan voltage, a gate

tube V27A is made conducting during the "off" time of the Frame F-F,

thus the horizontal scan can only occur when the Frame F-F is "on".

12
The major output of the Horizontal Scan Generator

is a series of 6 staircase waves of 30 steps each. After the

6 waves have occurred the voltage remains at the bottom or quiescent level. Amplitude is approximately 60 volts peak to

peak. Each step in the staircase is about 167 microseconds in

duration, thus each whole staircase has a period of 5 milliseconds.

The other output of this circuit is the End of Field

pulse generated by the discharge of the summing capacitor. As

its name implies, this pulse coincides Tilth the end of each

complete field of scan over the letter area.

li) Field Counter

This circuit begins with a lf>0 microsecond delay multivibrator to generate a pulse for discharging the Error Integrator

after it has been read out. The delayed End of Field pulses are then counted in a staircase counter which is adjusted to provide

an output pulse at the sixth input pulse.

The Field Counter is essentially similar to that in the

Horizontal Scan Generator. Control of the count is by means of

the £00 k potentiometer in the cathode follower ou"tPu’t circuit.

The output of the Counter is a series of 6 negative

pulses delayed l£0 microseconds from the input pulses, plus one counter output pulse for six input pulses.

3) Vertical Deflection Amplifier

This is a commercial St romb erg-CarIs on type Model AP

25 audio amplifier. It is rated at I|. 5 watts output. Normally

the gain control is set to provide about l.U amperes peak to

peak output current of triangular waveform through the yoke.

6) Horizontal Deflection Amplifier

This amplifier is of the current-summing type with

a high degree of negative feedback. It is arranged for constantcurrent or high impedance output.

The amplifier is in two nearly-identical portions, one active and the other inactive. The function of the inactive side

is to serve as a stable voltage regardless of whether current is

drawn from it or fed into it. The active amplifier feeds the

reference point through the Horizontal Deflection Yoke and a resistance of 100 ohms. The feedback for the active side is taken

from the midtap between yoke and resistance. Thus the aroltage across

13
the 100-ohm resistance is made to follow the input signal to the amplifier, hence the output current is proportional to the voltage from

the Horizontal Scan Generator.

Centering of the raster horizontally over the masks is

accomplished by feeding a controllable bias current into the input

terminals.

The output of the Horizontal Deflection Amplifier is a staircase wave of current of 0.6 amperes peak to peak. After the 6

fields for each mask, the current returns to its quiescent value which

holds the scanning beam off to the left of the mask.

3* 2c Recognition Circuits - Figure 8c

l) Clipper

This is a conventional Schmitt trigger circuit, having two

output states which depend upon whether the input is above or below

a chosen threshold value. At the input the signal has added to it,

by means of current through the 10 K resistor, a controllable bias

voltage. With the divider chosen, the effective bias change, for a change in the setting of the 100 K pot, is one-tenth the measured

change at the arm of the potentiometer. Thus a voltmeter at this

point furnishes a convenient reference for bias changes. The Schmitt

circuit is designed for low hysteresis, so that the threshold is substantially the same for increasing (negative-going) or decreasing

photoelectric signals.

'

' The relation between actual threshold level and the measured

voltage at the potentiometer am is shown in Figure 9. From the

relation, any desired threshold may be chosen, as for example 95

volts at the test point provides a threshold level of about -15

volts

.

A discussion of the factors involved in setting the

threshold is given in Section k°

The output of the Clipper is either a signal representing

black (Ov.) or white (-bOv.) at the grid of V-^g.

2) Error Integrator

The chief action of this circuit is to accumulate charge

on the 320 micro-microfarad integrator capacitor whenever the Clipper

signal represents "white” * Then at the end of the field the accumulated voltage is fedto the Coincidence circuit. Finally the integrator

capacitor is discharged by the Delayed End of Read pulse.
Charging of the integrator capacitor is accomplished

through the series combination of 330 K and diode V^c^, whenever

Vig is cut off. When VqQ is conducting there can be no accumulation

of charge. Effectively the final voltage on the capacitor is proportional to the total area that white paper is seen through the

letter mask. An output cathode follower Flap is added to permit

current to be drawn from the integrator voltage

,

The integrator charging function is gated by Vpyg to permit the acquisition of charge only while the horizontal scan is on. A discharge tube V22B resets the capacitor voltage when its grid is

pulsed positively by the inverted (through V22A ) Delayed End of Field

pulse.

3) Coincidence

This circuit receives the final integrator voltage, using

it to enable or suppress the passage of the End of Field pulse. If

the integrator output is below a chosen threshold, as set by the

100 K potentiometer, the End of Field pulse appears at the plate of as a large positive spike of short time constant. If the integrator output is too large (positive), is conducting at the time

the End of Field pulse occurs. Under this condition, no appreciable

positive spike can appear at the common plates of Vo an<^ ^21B* The

actual transition between pulse and no-pulse is not an abrupt one;

there is a narrow band of proportionality for a small range of

integrator voltage.

It) Recognition Pulse Shaper

In order to trigger the thyratron V0 g reliably, the sharp

spike at the plates of V2 q is lengthened by tfte rectifier time constant

circuit of Vgg. The lengthened pulse is then fed to the normally cutoff thyratron. The thyratron fires when it receives a pulse from the

Coincidence circuit, creating a rapid, negative Recognition pulse at

its plate. This is the pulse which shuts off the Master Flip-Flop

upon recognition of a character.

As part of the Demonstrator, firing of the thyratron is

rendered audible on a small loudspeaker through which the tube

discharges

5>) Meter Circuit

This circuit is added to furnish an indication of optical

signal magnitudes. In general the optical gain seems to disply some instability, necessitating occasional adjustment of the image dissector gain control or the Illuminator lamp voltage.

15
The circuit is essentially a negative peak rectifier

with a very long time constant to retain the peak reading.

Primarily the meter circuit derives its reading from the peak

signal voltage during the clear frame. The meter indication

is characterized by a pulse whenever the clear frame passes,

followed by a slow decay until the next pulse. In operation

the magnitude of the optical signals should be adjusted until

the meter reads 100 at the beginning of the slow decay.

The circuit itself is a negative peak rectifier

V23A followed by a cathode follower V2313, The zero control

is set with the Illuminator off but with the ambient light

at its prevailing valve. In general dim ambient light is

preferable, since there will be less variation with movement

of the operator.

h. CONCLUSIONS

Threshold and Error Level Adjustments

There are two important controllable variants involved in the

discrimination between wanted and unwanted characters. These are

l), the setting of the level of transition for black to white

signals, and 2), the threshold for maximum area of disagreement

allowing recognition. These two functions are somewhat interrelated, hovrever an optimum balance between the two will result

in the greatest degree of discrimination. This means proper

recognition over the greatest range of practical input conditions.

An examination of actual typewritten characters shows two

general f orms of degradation which are always present in varying degrees. These are 1) the inked portions are effectively

gray rather than black, and 2) the inked areas differ in geometry

from the idealized shapes drawn for the masks. The former produces

an obvious increase in the reflectance of the inked areas, lessening the range between it and the reflectance of white paper. It

is in this range that the Clipper Threshold must be placed. The

latter condition however has been found to be the more bothersome.

In this category, are> included characters in which the lines are too thin as from light typing, characters in which the lines are

so wide as to spread into the areas of unwanted masks from heavy

typing, and finally characters with defects in the type face

itself. Factors in this category are the ones which principally

influence the setting of Error Threshold.

All of these conditions make recognition more difficult. If one of these is very pronounced, for example, if the lines are gray rather than black, the permissible range of other factors is

in general reduced. To provide as large a range of these

factors as possible is the object of optimizing the controllable

factors of clipper level and error level.

16
Measurements of the effective reflectance of the inked areas

of typewritten characters vary from 30 to 50 percent of that of

white paper alone.

These are figures derived when the letter is stationary with

respect to the mask. However in actual operation, the letter is

moving slowly horizontally because of the motion of the Deflector

Vane. During the time of one field, for example, the total horizontal travel is about 2% of the width of the letter M or about 20^

of the width of the lines of the letter. This means that if a letter and mask match at the left side at the instant of scan, they

will not coincide exactly at the right side by the time the scan reaches the right side. Instead some white will be visible, raising the effective reflectance of this part of the letter. The net

effect of this motion is therefore to narrow the range between the

reflectances of inked areas and white paper.

This effect could be compensated by shrinking the horizontal

dimension of the letters as the master patterns are dram. In the

Demonstrator however, the compensation was not included. Undoubtedly

the measured range of Clipper Threshold could be extended if the

horizontal distortion had been introduced.

Experience with the present equipment shows an optimum setting

for Clipper Threshold at about 75p equivalent reflectance, for an ink reflectance of about percent. Thus the clipping level

is approximately half way between black and white signals.

These reflectance figures may be converted to electrical voltages

with the assignment of a standard reference level for signals from

white paper. It has been arbitrarily set at 30 volts peak-to-peak

for signals during the scan over the clear mask. When scanning over

a letter mask, these signals are slightly reduced, because of bandwidth

limitations, to about 25 volts average peak-to-peak.

The optical signals are least (most positive) when the scan is

over the opaque areas of these masks. These signals begin a +U volts,

going to ““21 volts on white paper, and to -26 volts on the clear frame.

Over°the Inked portions 'on the paper, the signals average perhaps 10

volts from the origin, or about -6 volts from ground. Thus the Clipper

Threshold extremes can be from -6 to -21 volts. Under typical type

conditions, correct recognition has been observed for the threshold

set between -13 to -17 volts. Mid-range is therefore about -15 volts.

The relation between the Clipper Threshold bias and the voltage

read at the am of the potentiometer is plotted in Figure 9. The

optimum point appears to be at about +95 volts, giving the clipper level

mentioned above.

17
Conditions surrounding the threshold of error level are that

it must provide for discrimination of unwanted characters without

setting unreasonably high standards for the quality of the matching letter. With the particular type face (Elite) used in the

Demonstrator, the optimum level appears to be about 0.75$ of the

maximum letter area (product and height and width of letter M)

At this level recognition of all alphabetic and numeric characters

has been demonstrated. Presumably there is a range of tolerance

for this value, however the time available did not permit a detailed study of this item*

18
REFERENCES

1. Fournier D’Albe, "On a Type-Reading Optophone", Proc. of the

Royal Society, 90 , 191U.

2. V. K. Zworykin and L. E. Flory, "Reading Aid for the Blind"

Electronics,; August 19U6.

3. Zwozykin, Flory and Pike, "Letter Reading Machine",

Electronics, June 19h9.

I;, D. H. Shepard, "Apparatus for Reading", U. S. Patent No.

2,663,758, December 22, 1953.

5. C, Hillyer, "Method of an Apparatus for Comparing Information" U. S. Patent No. 2,679,636, May 25, 195U*

19
TABLE I

Order of Alphabet on Vocabulary Belt

A K M N V XTZB8SHR

PEDFUJQOGC LTT123

k 5 6 7 9 Opaque Frame Clear Frame

20
TABLE II

IMAGE DISSECTOR DATA

Type D-I4.O-SE Dissector

Mfgr. Farnsworth Radio and Television

Yoke Data

Vertical deflection winding

Horizontal deflection winding

Focus winding

Rotation

1.3 mh., 8 ohms

2.7 mh., 2 ohms

1,27 h, 2I4.O ohms

70 degrees approx.

Signal Data

Peak signal current 1^0 fiA approx.

Circuit Diagram - Fig. 8d

ILLUMINATOR

IMAGE DISSECTOR

UNKNI

ILLUMINATOR

STEPPING SWITCH

THRESHOLD ADJ. FIGURE S BLOCK DIAGRAM OF PRINT READER DEMONSTRATOR

IRISCOGN1TION)

RECOGNITION

PULSE
•• -
3WVMJ awns

swvad inowo

3WVM3 XNV18

3WVH3 3HOVdO

1130

A«vinevooA

NOIXISQd

M01IMS 9Nldd31S

SECONDS

APPROX.

—

FRAME

PERIOD

50

MILLISECONDS,

APPROX.

HORIZONTAL SCAN 4o

ecUJ> z<u

Vi

£CoH

°sc o:<5UJ

h*Z o 3

UJ H- U. «t • u.

uj 9 3 Q 3 Z

ft. !aJ

OO

csUJ

£L

IO

y

ao

JK

U1>

h~<

_!UJ

eeo

Sfc

MILLISECONDS,

APPROX.

PHOTO

OF MAGNIFIED

TYPEWRITTEN

CHARACTER

PHOTO

OF MASTER

CHARACTER

AS DRAWN

FOR MASK

FIG* 5 MAGNIFIED TYPEWRITTEN

LETTER AND ASSOCIATED

<£*£>

MASTER CHARACTER

FIG. 6 EXAMPLE OF

TELEVISION - TYPE

PRESENTATION FOR

CHECKING ORIENTATION

OF IMAGE DISSECTOR



riourxL (

STEPPIUO SWITCH DETAIL BLOCK DIAGRAM

PRINT RLADt R Dt'i, ) I S

R A

(

HOME POSITION

HOMING GATE TO ALLOW START OF MASTER FLIP-PLOP

ONLI WHEN STEPPING SWITCH IS IN HOME POSITION a:

RESET SWITCH

STEPPING

SWITCH ACTUATOR

OPTICAL SIGNALS

FROM IMAGE DISSECTOR

AND 6C4 OPTICAL SIGNALS

START ALPHABET

PULSE GENERATOR

(SCHMITT TRIGGQO START IT

MASTffi FLIP-FLOP START-FIRST HASH IN VOCABULARY (END OF BLANK MASK) STOP-RECOGNITION OF LETTER

L _29A* B

BL I ELECTRONIC SWITCH

TUBE

T.

4rRECOGNITION PULSE

25 V. P-P TRIANGULAR WAVE

es TizCy

VERTICAL

SCAN. OSC. 3IC MULTIVIBRATOR

Ulf INTEGRATOR

(CLIPPER) - TTOTICAL

DEFLECTION

AMPLIFIER

TOTIOAL

DXFLIOTION COIL • l.S AMPS. P-P

9 OHKS

FRAME FLIP-FLOP START-NEW MASK

IN PLACE

STOP- 6 FIELDS | COMPLETED

26A,B >

—*|ATsen|<- 3DTir

HORIZONTAL

SCAN

GENERATOR A LDOKARlTT’

TUBE

BOOTSTRAP,,

lATH. POLL.)

I STAffiC *

; ^ HORIZONTAL SCAN STAIRCASE 2Q0cp«

HORIZONTAL

DEFLECTION

AMPLIFIER

BOOTSTRAP

:-sarw rwsy—

FRAME DIS- FIELD DIS- CHARGE 1 CHARGE

TOBE V17A I I”6*

WHITE PAPER mN- RECOGNITION) \ LETTER (RECOGNITION) aril —*

OPTICAL SIGNALS

(ANALOG)

HCWZONTAL

DEFLECTJOB COIL

0.45 AMPS. P-P , 1.0 05X3 i

<JSbs>

j clippm

- ^ . _ INTEGRATOR GATS

. 'W

ERROR INTEGRATOR

(CHARGE DIODE, OATH.

COINCIDraCB

(RECOGNITION) RECOGNITION PULSE

SHAPER (STRETCHESTSTRATUON

(AUDIO)

i (SCHMITT TRIGGER)

T

17 V

18 /

FOLLOWER OUTPUT

V19A,B /

PULSE AMP. T91a /

7

8B

f

28

ir.

OPTICAL SIGNALS

(BLACK OR WHITE)

• PULSE AT BCD OF

FIELD IF ERROR TOTAL IS BELOW

THRESHOLD

FROM FHAMF F-F

optical

S/G-MALS

Z2.DK •WWVj

.OP MF

&ATE

FROM FfiPME F-F •

• I

li

—

t

START ALPHABET PULSE GENERATOR MASTER FLIP-FLOP

SCHMITT TRIGGER CLIPPER PULSE INVERTER

OPTICAL

S/G-HALS

FROM FRAME F-F

ZZOK S.IM

FZRO

FRAME GATE

FRON1 FRAME F-F

ELECTRONIC SW.

SWITCH TUBE

V30/I

'/Z.LSN7 ZOOK —AAWVV

STEPPING- SWITCH ACTUATOR

HOME POS.

RESET SN

"nu m\"N

STEPPING- SWITCH

INDICATOR LAMPS

( TOTAL - 30

FIG-ORE 2 a. GATING & INDICATOR CIRCUI TS

NOTE: ALL CAPACITORS *1MF UNLESS MAPPED

<NB^>

FRAW

—H- SLO

1 1

SO

N6T£

FRAME FLIP-FLOP

a

FIGURE 26 0)

SCANNING CIRCUITS

NOTE-. AU CAPACITORS MMT UNLESS MARKED

-«e

\JERT. SCAN osc. - 3000 cps MULTI VIBRATOR

V/3 LSN7

L eZDO integrator

Clipper output-c.F. +zod

'/i LSN7

1 1 FZCO a CHARGE _

HORIZ. SCAN.

!DK

Q
.
ACTIVE AMPL!FIB

V-7K ww

INPUT

INACTIVE AMPL/F/I

MTES

1-3M

+300 t300

-100

-ZOO

RATES : ALL TR/ODES - CS/V7

ALL PENTODES - LAOS

CAPfiC/ TOPS M/Af i/NLESS marked

+300

-ZOO

ZZK

+30D

HORIZONTAL DEFLECTION TONE

+ 300

REGULATED TO

APPRO*.. /DO V.

SIV.IL AS IN iHfiPt TO MUIT MLPUTUPC - 40 * f-P BAS* -HZlrdC.

FIGURE 8b I'3) HORIZONTAL DEFLECT/ON CIRCUITS

INPUT

OPTICAL SIGNALS

TEi

DELA'/BO END

OF FIELD PULSE

INTEGRATED

OUTPUT

NOTE :

ERROR INTEGRATOR

+zoo

COINCIDENCE RECOGNITION PULSE SHAPER

INTEGFATED

OUTPUT

-ZOO

: +ZOO ZOO

PULSE STPE TCPEP

CPAPO-E DIODE

Vsb '/zLstn

-zoo

END OF

FIELD PULSE

NOTE'. All capacitors n\me unless mapped FIGURE. Sc

recognition circuits

CONNECTIONS

TO

/MAGE

DISSECTOR

TUBE

METER

AMPLIFIER
V

'
SJLTOA 'QTOHS3VJH1 a3ddll0 3A1JL01JJ3

FIG.

9

CLIPPER

THRESHOLD

LEVEL

VS.

BIAS

SUPPLY

VOLTAGE

J

-j u

FIG.

10

PHOTOGRAPH

OF

PRINT

READER

DEMONSTRATOR
.
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

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