NATIONAL BUREAU OF STANDARDS REPORT

3804

A Study of Print Reading Systems Leading to a

Proposed Reader for Typewritten Material

By

Herbert D. Cook

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

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

NSS PROJECT NBS REPORT

0107-20-5515 November 26, 1954 3804

A Study of Print Reading Systems Leading to a

Proposed Reader for Typewritten Material

By

Herbert D. Cook

Electronic Instrumentation Section

Electricity and Electronics Division

Developed through

Data Processing Systems Division

for

Department of the Navy

Bureau of Supplies and Accounts

BuSandA 56782

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

rh«p<ibiicstion.r» Approved for public release by the

unieeipermiMien n Director of the National Institute of „d„j,,wuu.r.<

25,d,c. Such par Standards and Technology (NIST) port ha. been » Pecmcally prepared f* 1 on October 9, 2015 .port for it« own ute.

] INTRODUCTION

The purpose of this study is to investigate the principles

of reading print in order to establish a suitable design for a reader for typewritten material. This will form the basis for a proposed project to develope a prototype model of such a reader.

As a background for the study a number of existing print

reading systems were analyzed with respect to methods of operation

and also practical considerations such as speed, accuracy and suitablility for the application. Analysis of these systems is not

presented in this report because of the difficulty of obtaining

complete details of the many developments now under way in this

field. It is believed, however, that a sufficiently wide cross section of the field has been examined to formulate reliable conclusions with respect to the suitability of the methods. In this

report a brief analysis of the basic principles of print reading

is given and also a description of the design of a proposed reading

system.

The proposed reader is expected to read at the rat e of at

least 10 characters per second.

1.1 Characteristics of Typewritten Material

The special problems encountered in reading typewritten material

arise from the nature of the typewriter itself. It is essentially

a portable device, constructed with a minimum of precision, for

writing what is sometimes little more than barely legible print.

Most typewritten material is readable with processes of which the

eye is capable, but which would be difficult to build into a machine

reader. The eye sees characters largely by their outlines, taking

less account of the total black area of the printed material. A

machine reader must rely on both the area and the shape, and this

leads to the special difficulties.

First, the shape of the characters is not uniform because of

uneven distribution of ink transfer in the printing process. This

is due in part to the texture of the ribbon which is usually a woven fabric, giving a superimposed pattern of threads on the character and in the space between and surrounding the type face.

Also the roughness of the paper surface prevents an even deposit of ink over the surface. As a result, the characters are generally imperfect especially at the edges, although any part of
the character may have missing portions. This difficulty must be

taken care of by the reader so that each character can be identified

in spite of defects, unless the defects cause it to be changed so

as to be indistinguishable from some other character. Some improvement in quality may be obtained by the use of a paper tape ribbon

that is now commercially available.

Also, the position and spacing of the characters is not uniform.

Often the characters are so close as to leave no space between them.

This rules out any system that utilizes this space in the register

or recognition processes.

The shape and size of the characters are not uniform between

different typewriters due to the large number of makes of typewriters

and the variety of type faces available. Unless a program of standardization of type face is undertaken, which appears impractical

at present, a reader for material gathered from many sources must

be capable of reading all the various fonts. No reader has yet

been built that will do this without a change in the reader for

each font being read. If a large amount of machine reading is con- templated, the proposal for font standardization as set forth by

Rabinow^ deserves study.

1,2 Register Problem

Other problems also arise that are basic to all print reading

systems. These are the problems of register and speed. Obtaining

register is the most difficult practical problem in any system.

By register is meant the relative geometrical positions of two

images or shapes. This is without reference to a fixed coordinate

system but only with reference to the relationship between the images

themselves. Two identical images are said to be in register when

all points of one coincide spatiall}'- with the corresponding points

of the other.

In a recognition system the comparison is between the image

presented to the system and a reproduction of the image stored in

the system. In this case register does not require that the images

be superimposed actually, but only in a relative manner depending

on the manner in which the image is stored. It is convenient to

think of register by superposition as taking place by an inverse

process of projection of a stored image back on the image being

read.

1 Report on standardization of the 5x7 font, Diamond Ordnance

and Fuse Laboratories Report No, TR-39

2
The material is generally read one character at a time, each

of which is placed in turn in the field of view of the optical pickup head of the reader. Accurate register of each character is re- quired in order to achieve recognition. The accuracy required is

too high for mechanical positioning because of the inaccurate manner

in which the material is written on the paper.

In order to locate the characters properly, several steps are necessary. First the paper must be inserted in approximate position

and orientation to put the appropriate line of print in view of the

reader. Even if this is done accurately, there is generally sufficient variation along the line of print to require an additional

process of searching for each character. The accuracy of register

that must be attained is high in order to make it possible to distinguish among the 40 or more characters that may appear. It is

estimated that the tolerance from exact register for ordinary print

to be distinguishable by machine cannot be greater than about one thirtieth of the overall width of the \iddest character. Better

reading accuracy would be obtained in any system with a tolerance

of less than this value.

In the various reading systems now being developed a number

of different methods are used to attain register. Some require

that there be a clear space surrounding each character, and utilize

this in the searching process. Mien it is impractical to require

that such a space exist, as in typewritten material, these systems

cannot be used. In this case a system must use other methods to

achieve register. This may take the form of relying on the recognition process as an indicator for register while searching blindly

in the area where the character may be found.

When recognition is used in register sensing, a complete test

is made of many possible positions throughout the area in which

each character may exist. This is usually done in a geometric

pattern of tries with several in the vertical direction, which is

already positioned closely, and with a continuously moving pattern

in the horizontal direction, in order to cover all possible positions. Since register must be obtained within a distance of not

more than one thirtieth of the character spacing, a large number

of tries are necessary for each character. There are ways of re- ducing the number of tries such as taking advantage of the kmwledge gained from the position of one character to help find the

next one, and some increase in speed may be obtained by such methods.

1.3 Speed Problem

The large number of individual operations required for comparing

3
all of the stored characters with the field of view throughout the area where the characters are likely to be found is an indication

of the large amount of information processed during the reading

operation®

Consider a system in which the field of view is sampled at about

1000 points 9 as in a 32 line raster and 32 point resolution along

each line® Without knowledge of the horizontal register , and with

the vertical register known within a tolerance of one fourth the

character height ? there is required a total of 8x32 or 256 individual

rasters to find each character® It is assumed that 8 complete

rasters are made at each horizontal position? each consecutive

raster being moved one line vertically® Since there are 1000 points

to sample in each field? this amounts to 256?000 points per character® If 50 characters are in the vocabulary and all must be tried

in sequence;, there are 12?800?000 points to be sampled per recognition® This must be multiplied by the number of characters per

second to find the number of point samples per second®

This rate is too high to be achieved with any single existing

electronic device alone® In order to achieve a practical reading

speed j as many operations as possible must be done simultaneously?

or in parallel® Every practical existing system has some portion

operating in a parallel manner® An example of this is the comparison

of the scanned input with all of the stored characters simultaneously®

Other methods for Increasing the reading speed utilize some means

for sensing misregister in order to reduce the searching required

for recognition®

1®4 Basic Operations

Although the methods used in reading print differ in many respects? there are certain necessary operations basic to all systems®

First? there must be conversion of the image on the paper to the

medium used in the device? usually an electrical signal to be

handled by electronic means® Next? there must be stored in the device in some manner the complete set of characters to be recognized®

Then there must be a comparison between the stored character information and that derived from the printed image® Beyond this, there

is a selection process to indicate which of the characters is recognized? and a coding of the selection that is appropriate to the

computer or other device into which the reader operates® These operations are not immediately apparent in every system because of

the many combinations in which they may appear? but are essential

to the operation of any system®

It may be seen that since a variety of methods can accomplish
each basic operation, if used in their possible combinations, a very large number of different reading machines might be built all

of which would operate successfully. This is apparent from the

large number of memory systems that have been used in computers

that could serve in a reader. A practical reader must, however, use components that are practical in themselves and that work veil in

combination.

1.5 System Classification

The various systems for reading print can be classified roughly

by the manner in which the image is analyzed. Recognition is always

by comparison with stored information that includes all of the

possible characters, and in order to make the comparison, the image

as viewed by the optical reading head must be converted to the same type of information that is available in the storage. The classification suggested here is in terms of the dimensions of the converted

image.

The three categories will be called (A) Optical matching, (B)

Scan line analysis, (C) Image point analysis. In "optical matching"

(A) the complete image in two-dimensional form is compared directly

with two-dimensional images stored in the reader. In (B) the image

is scanned with a continuously moving spot, forming a line pattern

over the image field and producing a time varying signal. The signal

is compared with a similar signal produced by the storage mechanism.

In (C) the image is analyzed point by point over the entire field

according to a specific pattern, either in sequence or in combination.

The values obtained from examining each point are then processed in

a digital manner to determine the nature of the image. Given the

values from a sufficiently large number of points, a digital computer

is capable of recognizing the characters that appear in the field

of view of the reader.

A system using the "optical matching" principle (A) requires

some means for making the image of the field of view coincide with

that of each of the characters in the vocabulary. Since thi-s is an optical-mechanical process, the speed of such a system is generally

limited.

With the scan line analysis (B), less information is derived

from the image than for (A) but the information does maintain con- tinuous dimensional characteristics in time that allow accurate comparisons. Further, the derived information is compatible with established continuous signal techniques in electronics.

In image point analysis (C), some accuracy is lost in the loss

5
of the interpolation possibilities that exist in continuous signals*

As a result more image points are required to reproduce an image

with the same accuracy* This means in turn that more sampling of the

field is required* The techniques of analysis are those common to

digital computers*

A number of readers may be briefly mentioned as examples of the

combination of principles*

An early reader for the blind (RCA Laboratories) uses a method

of scanning and makes a comparison with the character storage with

respect to the number of intersections the scan makes in passing

over different parts of the character. Only a small amount of information about the character is utilized*

More information is stored in an early reader by Sheppard in

which the character being read is compared with stored parts of

characters by direct optical means* Combinations of parts determine

the identity of the character*

A later reader by Sheppard scans the character by image points

as in (C) above* The necessary computing is done at the same time

that the scanning progresses*

The reader built by Rabinow is in the category of (A) with a direct optical comparison between read and stored images* Rabinow 1 s chief contribution is in the comparison process in which the two

superimposed images are scanned with a raster* searching over the

field sequentially for unmatched image points*

2 The reader demonstrator built by Greenough is a more rapid

model of Rabinow9 s system with electronic scanning and control circuitry* Intermittent mechanical motion of the sequential vocabulary storage also is a factor in the increased speed*

Although all of the readers are capable of reading they require

much improvement in speed and accuracy to be practical* It is believed that this improvement can be achieved with methods that are available in the present state of electronics* One of the important

factors is that there be a large amount of information stored in

the vocabulary storage of the reader,, and that it be available very

quickly on demand* Also important is that a suitable method be

used for attaining accurate register as has already been mentioned.

It is believed that the above considerations point to method (B) of

system classification as most suitable for a practical reader of

typewritten material,, largely because of the high degree of dimensional accuracy that can be obtained with a minimum of information

processing.

2 MBS Report No. 3634 "Technical Details of Print Reader Demonstrator" * M* L* Greenough and C* C. Gordon

6
2 THE PROPOSED SYSTEM

On the basis of the previous considerations and on experimental

work done while this study was in progress a reading system was designed incorporating methods that are presently achievable in electronic practice o It is of a type characterized by comparison of

electrical signals derived from scanning as described in the classification of (B) in the previous section,, The comparisons are made

against all characters in parallel as is required for a practical

reading speed* The process is entirely electronic except for motion

of the paper through the reader* The block diagram of the system

is shown in Figure 1*

A description of the proposed reader in terms of its functional

operation follows.

2.1

Paper-moving Equipment

This is the only mechanical part of the reader* It consists

of a carriage , yet to be designed, for holding the document in

position beneath the optical pickup head. A continuous relative

motion between the paper and the pickup head moves the reading

point horizontally along the line of print. The line to be read is

chosen by the operator in accordance with the proposed application,

as some choice is necessary as to which material on the document is

of value. Automatic machinery can be designed for paper insertion

but is complicated unless similar automatic feed were used in printing.

For typewritten material it is doubtful whether the value of such

equipment would outweigh its complexity.

2,2

Light Source

The field of view of the reader is illuminated by a light

source consisting of a projection type lamp and condensing lenses

to focus the light on the required paper. The paper, illuminated

at about 20,000 foot candles as required, can be viewed by the operator through an appropriate dark glass filter.

2.3

Optical Pickup Head

The illuminated field is seen by an optical system consisting

of a lens and an “image dissector" converter tube. The image

7
dissector converts the optical image to electrical signals in ac- cordance with an applied scan pattern. This tube was chosen for

its ability to operate with an open light source rather than in

darkness, and also, since it is a non-storage type of converter,

its ability to operate with the irregular scan patterns that are necessary in the reading process. The image dissector is of a type

used in commercial closed-circuit 2,4 television.

Scanning of Pickup

The image is scanned in a raster similar to television scanning, covering the field in a linear grid pattern. This produces

a tine-varying signal that is compared with similar signals generated is* the vocabulary storage. A 32«>line raster is considered

suitable for recognition of the typewritten characters.

2,5

Storage of the Character Vocabulary Images

The information required for recognition of the forty or more characters in the vocabulary is stored in the form of masks of all

of the images. These correspond in shape to the original type

face and are changeable for different varieties of type. Each

mask is scanned by an individual scanner consisting of a small

cathode ray tube for a light source and a phototube pickup. These

generate signals in the same manner as the pickup head.

2,6

Scanning of the Storage Masks

The scan pattern of the mask scanners is identical to that of

the optical pickup head and thus the generated signals are of the same type. The signal derived from scanning any mask is identical

to that from the pickup only when the corresponding character

appears in register before the reading head.

2.7

Unitizing

The signals generated from both sources (pickup and storage)

vary in amplitude depending on the density of ink, darkness of

paper and for other reasons. Since only the shape of the character is of interest, signals corresponding to white and black

are more desirable. The variable signals are therefore given

8
values of either one or zero by a circuit that makes a decision as to whether the instantaneous value of the signal is above or below

a prescribed level* This makes all signals uniform in order that

comparison may be made only on the basis of shape and area. 2.8

Mismatch Indicating

The unitized signals from the pickup are compared for identity

with the signals from all of the storage scanners simultaneously.

For each stored character a comparison circuit indicates for each

part of the scanned field the disagreement with the image dissector

signal. The disagreement or mismatch is zero only for the case of

a character in register, compared with the corresponding mask scanner.

All other comparison circuits indicate some mismatch as does also

the correct character when register is not proper.

2.9

Integrating

An integrator for each mask adds up all the integrated mismatch

every time the field is scanned for a comparison. A low value is an indication of correspondence and a small amount of mismatch.

2.10

Comparison of Integrated Values

Comparing the values indicated by the integrators associated

with each mask, if any one is sufficiently small, it is assumed

that a character has been recognized.

2.11

Selection of Character

The integrator giving a value of mismatch less than the threshold value is that designating the character being read. If more

than one indicate less than the required mininmm value, the one indicating least is taken as the designating integrator.

2.12

Encoding for Computer Input

Wien a character is identified a code signal is generated that

is appropriate for the input to the computer.

9
2*13 Output to the Computer

The coded output signal goes either directly to a computer or

to some intermediate medium such as magnetic tape or teletype tape.

2 6 14 Checking

With even the most perfect reader there is a possibility of

error due to imperfect print or defects in the paper surface. Accordingly, it is necessary to include some ability to make checks

on the accuracy of reading. There are several forms this may take, some of which may reject doubtful reading and some of which may

ask for re-reading the material, taking cognizance of significant

portions of the characters in view of the information gained on

first reading. Other methods anticipate the possible characters

being read and sensitize only those in order to make a more certain

identification. As an example it is possible to sensitize only

the numeric characters when no alphabetic characters are expected.

This would be done automatically in a programmed sequence of operat3.ons

«

Other rejections of material may come after reading because

of logical inconsistencies. It is possible to arrange the material

when written to assist in such checks by such means as putting a known number of characters in each line etc.

With respect to the over-all accuracy of the reader, it can be

assumed that if material is rejected for inconsistency, whether

due to faulty printing or inability of the machine to read, it is

not necessarily to be considered a machine error.

3 EXPERIMENTAL WORK

In order to check the validity of the foregoing proposal,

the various portions of the proposed reader were tried experimentally. A complete scanning system was constructed including

an image dissector tube and its associated deflection amplifiers.

A mask scanner with a single character was operated in conjunction

with this in order to develope the comparison circuits. This effort resulted in a number of system improvements that are important

in the solution of the reading problem.

10
3.1

Automatic Threshold Bias Control

In typewritten material there is encountered a wide variation

in the density of the printing ink and the reflectivity of the paper

surface. In scanning such material the signals vary as a result of

these differences. Unless special provision is made, the effect is

to change the apparent size and shape of the characterso This effect

is of such a magnitude occasionally as to prevent the recognition

circuits from operating,,

A solution of this problem is found in the use of a circuit

that automatically controls the threshold bias with respect to which

the signals are unitized. This threshold value should properly be

midway between the values of signal corresponding to the ink density

and that of the paper surface. The circuit as developed makes the

adjustment automatically whenever a character appears in the field

of view of the reader. The effect of using the circuit is equivalent

to improving the optical resolution of the image and also the resolution as affected by the bandwidth of the system and persistence effects of the cathode ray scanners.

3.2

Two-Image Mask

This is a means for deriving two separate images from each mask

scanner of the vocabulary storage. The two images are used for the

operations of recognition and register control. Register control

is best achieved by using a complete character image that is identical in shape to the type face, as accurate positional information

can be derived with its use. The other image is used in the recognition process. It is thinner and is designed to fit entirely within the boundaries of the complete character image. Recognition by

the use of this second image is tolerant of slight amounts of residual misregister and tilt of the character, and this results in

greater accuracy.

The two images are produced by scanning both the normal black

mask and a semi-transparent mask superimposed on it. When this

combination is scanned in the normal manner, the information of

both appears in the signal. These are separated by establishing

two separate threshold bias levels at which two Schmitt trigger

circuits operate, each of which then responds to one of the images.

3.3

Image Register Sensing

The number of operations required for searching to obtain

register can be substantially reduced if some knowledge can be

11
gained as to where the searching might best be done. Various devices

external to the reading head such as photocells servoed to the

paper carriage or to the position controls of the reading scan can

be used, but some searching is generalljr still required to achieve

accurate register. Other methods using information obtained internally by the use of the normal reading scanner with non-uniform

scan patterns have been used, but also are limited in the accuracy

with which they ultimately achieve register, especially if there

is no clear space horizontally between characters,

A method of sensing the direction of misregister has been devised in which information is derived from the whole area scanned

during the reading process. From the information derived from com- paring the stored image and the pickup image, a positive or negative

voltage is obtained from the comparison circuits depending on the

direction of misregister. This is done for both the vertical and

horizontal directions. When the direction voltages are applied to

the positioning controls of the scanning tubes, a complete servo

loop is formed that moves the images into register. It is not

necessary to allow a definite space between characters as the information is derived from the same scan field as the information

used in the recognition process, and this can be made to cover

only the area occupied by the character.

This registering scheme and the other circuits described were built into the experimental system, and were found to operate as expected. In addition to testing these circuits, the construction

of the experimental equipment was of value in the solution of

other problems incidental to the development of a prototype model

of reader.

4 SUMMARY

The considerations upon which the design of the proposed

reader are based were derived from analysis of the problem of

reading print, and information with respect to known existing

readers and their development. The operation of these readers

has demonstrated that a successful reader can be built 9 although

there are problems associated with their speed and accuracy. One

of the greatest problems has been that of register of the material

with respect to the reader, and this can be considered the main

one in the way of increasing reading speed.. It is believed that

the system chosen here has a high probability of success in solving

these problems when fully developed.

12 'OMAf-dC.- 58, / 73
BLOCK

DIAGRAM

OF

PROPOSED

READER
TICAL

HORIZONTAL

CAN

SCAN

DETAIL

OF

VOCABULARY

STORAGE

UNIT