TELEWRITING APPARATUS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP81-00120R000100020030-2
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 8
• Document Creation Date: December 20, 2016
• Document Release Date: December 4, 2003
• Sequence Number: 30
• Content Type: CONT

Uniteci Approved For Release 2007/09/21 : CIA-RDP81-0012OR000100020030-2
mates ratent umce Patented May ~ 14, 963
3,089,918
TELEWRITING APPARATUS
Robert E. Graham, Chatham Township, Morris County,
NJ., assignor to Bell Telephone Laboratories, Incor-
porated, New York, N.Y., a corporation of New York
Filed Sept. 9, 1960, Ser. No. 54,989
12 Claims. (Cl. 178-19)
information pertaining to graphic material and, more par-
ticularly, to apparatus for generating electrical signals
that are indicative of the position of a moving stylus upon
the face of a writing tablet. It has for an important
object the elimination of mechanical restraints that fetter
the free movement of the stylus.
Various types of telewriting devices for indicating the
"X" and "Y" co-ordinates of a moving stylus adjacent to
the plane formed by a writing tablet are well known.
Perhaps the most common of these involves the use of
2
the circular locus of the beam is centered at a point on
the face of the tube that lies opposite the center of the
photocell contained in the tip of the stylus, the output
from the photocell is essentially constant, i.e., it is a
steady state potential. Decentering of the beam, on the
other hand, produces a varying output potential from the
photocell of a frequency equal to the frequency of the
circular scan of the beam. The products of this signal
and each of the signals applied to the horizontal and
vertical deflection plates of the cathode ray tube to pro-
duce the circular rotation of the beam are related to the
degree of decentering in the horizontal and vertical de-
flection directions, respectively. Accordingly, the output
potential from the photocell is applied to a pair of multi-
pliers to produce two "error" signals. These error sig-
nals are separately integrated to produce a pair of sig-
nals which, when applied to the deflection plates of the
cathode ray tube, position the beam adjacent the tip of
the moving stylus and which are indicative of the posi-
tion of the stylus. In the present invention, the signals
from the integrators are added, respectively, to the sig-
nals that create the circular scan of the beam, and the
summed signals are applied to the deflection plates of
the cathode ray tube both to create the circular locus of
the beam and to center it on a point upon the face of
the tube opposite the tip of the stylus.
As an additional feature of the invention, an alterna-
tive scanning mode is instituted should the beam become
decentered beyond a certain predetermined amount. Dc-
centering may be detected by noting the magnitude of the
photocell output and generating an "out-of-contact" sig-
nal whenever the output falls below a predetermined
threshold. In the alternative mode, the electron beam is
made to scan the face of the cathode ray tube in normal
line sweep and frame sweep fashion until the beam again
commences to track the photocell. It may also be noted
that the above-mentioned "out-of-contact" signal is gen-
erated whenever the stylus is withdrawn from the face of
the tube. Thus, an indication is given of the formation
of discontinuous traces such as, for example, the forma-
tion of individual alphabetic characters or individual
words.
In another embodiment of the present invention, a
special cathode ray tube is employed as the writing tablet.
In this arrangement the cathode ray tube has a specially
constructed face comprising, for example, a resistive film
deposited on a thin dielectric layer. The stylus in this
case is composed entirely of a conductive material that is
grounded either directly, by the use of a flexible electrical
conductor, or indirectly utilizing the self-capacity of the
operator using the stylus. The electron beam is scanned
in a circular locus, as described before, and is also in-
tensity modulated. When the beam impinges upon a por-
tion of the resistive film that is opposite the conductive
stylus, the stylus shunts the majority of the varying beam
current to ground. By detecting the beam current in the
resistive film and producing a pair of signals proportional
to the product of the envelope of the current and the
potentials supplied to the deflection plates that create
the circular scan, the stylus may be tracked and its "X"
and "Y" co-ordinates upon the writing tablet noted in a
fashion similar to that described above.
Although the use of cathode ray tubes- as writing
pantographic arrangements in which the stylus is coupled 20
to, and thus restrained by, a series of mechanical linkages,
the relative movement of which effects a change, for ex-
ample, in the resistance of a variable resistor to which the
linkages are connected. Electrical signals indicative of
the position of the stylus are obtained by utilizing the 25
variable resistor in an appropriate electrical circuit. Gen-
erally, however, the use of such pantographic arrange-
ments in telewriting devices has been deemed undesirable,
since the mechanical linkages associated therewith im-
pede the completely free movement of the stylus and pre- 30
vent a person who is using the device from writing with
his normal and natural style and speed.
Telewriting devices are known, of course, in which
the above-mentioned pantographic apparatus is eliminated.
These generally utilize unrestrained conductive styli in 35
conjunction with resistive surfaces as the writing tablets.
Typically, the resistive surfaces have applied to them
electrical potentials that serve to create an electrical field
in the writing tablet. However, extreme nonlinearitics
are often developed within the field which tend to reduce
considerably the usab!e area of the writing surface.
Accordingly, another important object of the present
invention is the elimination in a telewriting device of a
resistive surface as the writing tablet and with it the elec-
trical potentials required to create an electric field.
These objects and others are realized in this invention
by departing from the usual type of telewriting device.
Specifically, the invention utilizes a conventional cathode
ray tube, whose face forms a writing tablet, and a special
stylus that contains 6 -photosensitive element, e.g., a photo- 60
cell,_ at its tip. As the stylus is moved in front of the
face of the cathode ray tube in forming cursive script,
electrical signals obtained from the photocell, via a flexi-
ble conductive lead, for example, are operated upon and
fe back- to the beam deflection plates of the tube, and
the electron beam in the tube is made to track the moving
stylus. Since the beam tracks the stylus, the feedback
signals are indicative of the "X" and "Y" co-ordinates of
the stylus upon the face of the tube, and thy maybe
transmitted to_.any remote location to indicate these co- 60
ordinates.
To elaborate further upon the principles of this inven-
tion, the tracking of the stylus by the electron beam is
accomplished by deflecting the beam in a circular scan 85
of high rotational frequency and small radius. When
tablets in telewriting devices is known, the present inven-
tion departs greatly from the principles of operation
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3,089,018
9
employed by such devices of the prior art. For example,
in a tctev+riting device disclosed in Patent 2,553,245
rr.rntcd to L. Fspcnschied on May 15, 1951, a source of
light contained in a stylus is required to impinge upon a
special photosensitive and electron emissive tube face to
create a beam of electrons. The beam created thereby is
passed through a set of plates and the potentials induced
therein are indicative of the position of the stylus. In
the present invention, however, an independently-formed
beam of electrons is employed, and by utilizing the special
tracking features outlined above, the beam is made to track
exactly the moving stylus. With the precise tracking,
potentials indicative of the exact position of the stylus are
obtained from the deflection plates of the cathode ray
tube.
The invention may be better comprehended by con-
sulting the following detailed description of illustrative
embodiments thereof taken in conjunction with the accom-
panying drawings in which:
FIG. 1 is a pictorial diagram, partially in block sche-
matic form, of an embodiment of this invention utilizing
a typical cathode ray tube as a writing tablet;
FIGS. 2 and 3 are graphical aids helpful in understand-
ing the operation of the device depicted in FIG. 1; and
FIG. 4 is a pictorial diagram, partially in block sche-
matic form, of an alternative embodiment of the invention
utilizing a specially constructed cathode ray tube as the
writing tablet.
Referring to FIG. 1, a typical cathode ray tube 10 is
illustrated having a beam producing electrode 11, an in-
tensity control grid 12, and sets of vertical deflection plates
13 and horizontal deflection plates 14. The signals respon-
sible for the deflection of the beam within the tube are
applied to the deflection plates 13 and 14 through the
conductors 53 and 54. The beam of electrons is made
to assume a circular scan of high rotational frequency and
small radius by applying, for example, a signal of cosine
wave form to the horizontal deflection plates and a signal
of sine wave form to the vertical deflection plates. The
apparatus for applying such signals to the deflection plates
will be described in more detail hereinafter.
A stylus 20 is supplied for forming cursive script on the
face of the tube. Conveniently, the tip of the stylus con-
tains a lens 22 which images upon a photocell 21, con-
tained within the stylus housing, the portion of the face
of the tube in front of the stylus. When the circular locus
of the beam upon the face of the tube is such that its
image is centered on the surface of the photocell, the
output from the photocell is essentially a steady state po-
tential. However, whets the image of the circular locus
of the beam is not centered on the photocell, a sinusoidally
varying potential is generated by the photocell of a funda-
mental frequency equal to the frequency of rotation of
the beam. A better understanding of the theory behind
the invention may be obtained at this point by consulting
FIG. 2.
In FIG. 2, the center of the photocell, indicated at point
Cp. may be considered to lie upon the face of the cathode
ray tube. The electron bean's counterclockwise circular
locus, of radius r, is skews with its center at CL, the latter
comprising the point of intersection of the two co-ordinates
that represent vertical deflection and horizontal deflection.
As pointed out above, the circular motion of the beam is
created by spQlyir.g potential of cosine wave form, e.g.,
a cos mt, to the boainostioe plates and a potential
of sine wave form, e.g., 41, sip wt, to the vertical deflection
plates.
In the figure. *c %saw is shown le a decentered position
ss compared wills tl$tp toi 1, the arvottat of decentering
being equal t that distance a bgWveen the points Cp and
Ct, The lion tKti neft of ltn s riuin,F, these two points
w0man ?Nagle b wiQh Ow horizontal deflection co-
Thqp..vos jr is the amount of decentering of
the bean Io it; hop. lgl dtpflecttion direction, while
s t.a r is the a01tFlj*t t4gWieg in the vertical deflec-
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4
tion direction. In order to reduce the distance s to zero, a
potential related to s cos c' must he applied to the horizon-
tal deflection plates of the cathode ray tube, while a poten-
tion related to v sin 9, must be applied to the vertical de-
flection plates. The present invention derives such poten-
tials from the photocell's output signal, ep, in the follow-
ing fashion.
Still referring to FIG. 2, the instantaneous photocell po-
tential, ep, is related to the distance between the position
of the beam at a given instant and the center of the photo-
cell at point Cp. The beam's angular velocity A is chosen
to be sufficiently great in relation to the rate at which the
stylus is moved across the face of the cathode ray tube
that the point Cp may be considered fixed during several
complete rotations of the beam around the point CL. It
may be seen that ep is greatest when the beam is in position
d. As the beam moves around to position e on its cir-
cular locus, the photocell potential gradually decreases to
a minimum value; as the beam moves from e back to d,
ep increases from its minimum value back to its maximum
value. Thus e, varies with time in an approximate sinus-
oidal fashion about some positive potential K, at a fre-
quency o equal to the frequency of rotation of the beam,
and with a maximum value at those times when the beam
is in position d. Thus, for example, if the beam com-
mences its counterclockwise circular locus at point c at
time t=0 and is at point d at time
w
the potential ep may be expressed in the following form:
ep=K+b cos (wt-'p)+p cos (2wt-2W)+q
cos (3wt-3')+ ... z cos (nwt-np) (1)
Only the term b cos (wt-4p), however, need be considered
in this analysis, since subsequent multiplication and in-
tegration operations upon the photocell signal cause the
contributions to the error signal from the constant term
K and the higher harmonics to be zero.
FIG. 3 depicts the variation in the value of b for vari-
ous amounts of decentering s. Positive values of s in-
dicate that, with regard to FIG. 2, the center of the
photocell lies to the right of the center of the circular
locus of the beam. Similarly, negative values of s in-
dicate that the center of the photocell lies to the left of
the center of the beam's locus. It may be noted, if the
response of the photocell is monotonically related to the
distance from the beam, when the center of the photocell
is circumscribed by the circular locus of the beam, i.e.,
when the absolute value of s is less than r, b varies mono-
tonically with s. When the center of the photocell is not
circumscribed by the locus of the beam, i.e., when the
absolute value of s is greater than r, the monotonic rela-
tionship between r and b no longer exists. However, the
algebraic signs of b and s are always the same whether or
not the center of the photocell is circumscribed by the
locus of the beam. Since b is related to s as shown in
FIG. 3, b cos p and b sin y, are related in the same fashion,
respectively, to s cos (p and s sin rp, the latter comprising
the components of decentering in the horizontal and ver-
tical deflection directions. Accordingly, the apparatus of
FIG. 1 resolves the potential ep into two components pro-
portional to b cos 9 and b sin rp, and these potentials are
utilized by virtue of their relationships to s cos w and s sin
rp, to center the locus of the beam on the photocell.
Referring once again to FIG. 1, the signal developed
by the photocell, the fundamental component of which is
given as b cos (wt-'p), is coupled to a tracking unit 30
by means of lead 23. Conveniently, lead 23 is a flexible
electrical conductor which neither fetters the free move-
ment of the stylus nor hampers the writing style of a per-
son using the device. An amplifier 31 may be utilized, if
required, to increase the magnitude of the photocell signal.
75 The signal from the amplifier 31 is applied to two mul-
CIA-RDP81-OO12OROOO1OOO2OO3O-2
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tiplicrs 37 and 38 and also to a peak rectifier 32. Two
signals from sources 36 and 39, a cos wt and a sin tut.
respectively, are also applied to the multipliers 37 and 38.
Since the fundamental component of the amplified sig-
nal from the photocell 21 may he designated as b cos
the output from multiplier 37, i.e.,
(a cos wt)(b cos [at-gyp])
may be expressed as:
Z, cos w I (cos 2wt)(2 cos p) [ (sin 2wt) (2 sin
Similarly, the output from multiplier 38, i.e.,
(a sin wt)(b cos [wt-w])
may be expressed as:
2 sin ,p+ (sin 2wt)(2 cos gyp) - (cos 2wt)(Z in `p
(3)
In the above multiplications, the constant term K and the
higher order harmonic components of the photocell sig-
nal have been neglected, since they produce product terms
that arc sine and cosine functions of time, and, such func-
tions are filtered out by subsequent integration operations.
As shown by Equations 2 and 3, each multiplication
produces a component of voltage that varies only with
V. An Equation 2, that component is ab/2 cos ip; in
Equation 3 it is ah/2 sin v. From the prior analysis, it
may be seen that these components of voltage are related
to the degree of decentering, in the horizontal and vertical
deflection directions, respectively, of the circular locus
of the beam from the center of the photocell. Accord-
ingly, to isolate these components, the signals from multi-
pliers 37 and 38 are applied to the summing integrators 44
and 45, respectively. The summing integrators depicted
are of conventional form, for example, as described in
Korn and Korn, Electronic Analog Computers, 143-45,
288-92 (lst ed., 1952). Each of the integrators provides
an output potential that is proportional to the integral
of the sum of the inputs applied thereto. For the present,
it will be assumed that the other inputs to the summing
integrators, i.e., the input from pulse gate 41 and the input
from the scale of n frequency divider 42, are each zero,
since they apply to an alternate scanning mode to be ex-
plained later. Summing integrators 44 and 45, are pro-
vided with a reset mechanism similar to the reset appa-
ratus as shown in the Korn and Korn reference, supra.
Conveniently, each may comprise a relay energized by a
signal from its reset lead, 46 or 47. Upon energization,
the relay connects the output of its associated summing
integrator to a common reference potential, e.g., ground.
Since, however, the resetting operations pertain to the
alternate scanning mode just referred to, no signals are
present on the reset leads, and accordingly, the only active
leads to integrators 44 and 45 are from multipliers 37 and
38. respectively.
Because the integrals of terms of the form sin nwt and
cos nwt are zero over a time long as compared with 27r/w,
the integrated signals from multipliers 37 and 38 repre-
sent the integrals of only the terms ab/2 cos 0 and ab/2
sin pp, respectively. Thus, the error potentials ab/2 cos V
and ab/2 sin W are integrated themselves to provide a
pair of signals which, when applied to the deflection
plates of he cathode ray tube, result in the circular locus
of the beam becoming centered on a point upon the face
of the tube opposite the tip of the stylus. In this respect
it should be noted that integration of the signals ab/2
cos v, and ab/2 sin W always produces deflection signals
of the proper polarity to reduce the error s. This may be
seen from the fact that, since the deflection voltages are
proportioned to the integrals of the terms ab/2 cos w and
ah/2 sin w, the terms themselves represent the rate of
change of the beam's deflection and, therefore, the veloc-
6
ity of the center of the rotating beam. From FIG. 3
it may be seen that the algebraic sign of b is always the
same as that of s. Thus, as the beam becomes centered,
i.e., as s approaches zero, the velocity of the center, CL,
5 of the circular beam locus is always in the proper direc-
tion independent of whether or not the beam circum-
scribes the stylus. If, however, the signals b cos 9 and b
sin p are derived by the use of low pass filters and ap-
plied directly, without integration, to the deflection plates
of the cathode ray tube, the rate of change of these terms
would represent the velocity of the center of the locus of
the beam. In this case it is apparent from FIG. 3 that,
as s approaches zero, the rate of change of b with respect
to time changes algebraic sign. The change in sign oc-
15 curs at s=r; and such a change in sign indicates a change
in the direction of the movement of the center of the
locus of the beam. Thus, without integration, operation
of the invention is readily achieved in the range of s
where b and s vary monotonically with each other, i.e.,
20 in the range where the circular locus of the beam cir-
cumscribes the center of the photocell.
Referring again to FIG. 1, the signals from integrators
44 and 45 are passed to the adders 34 and 35 where they
have added to them, respectively, the signals a cos wt
25 and a sin wt. These last-named signals are added at this
point to provide the sinusoidal signals that produce the
circular rotation of the beam. The signal from each of
the adders is thus composed of two components; first a
sinusoidal component that produces circular rotation of
30 the beam, and second a component that positions the
beam opposite the tip of the moving stylus. Accordingly,
these signals are applied directly to the deflection plates
13 and 14 of the cathode ray tube. by leads 53 and 54,
respectively.
35 Since the beam tracks the movements of the stylus,
the signals from the integrators 44 and 45 are indicative
of the "X" and "Y" co-ordinates of the stylus upon the
plane formed by the face of the cathode ray tube 10.
These signals are obtained from the leads 51 and 52 and
40 are applied to a utilization device 55 where they may be
utilized in any fashion, e.g., to effect a reproduction, at
a local monitor or at a remote point, of the graphic ma-
terial then being-"written" on the face of the tube 10.
invention contemplates an alternative scan-
45 ning mode that effects a "search scan" of the face of the
tube by the beam whenever the beam is not tracking the
stylus. Such a scan, which may be, for example, a
typical line by line sweep of the entire face of the tube
by the beam, continues until the beam once again tracks
50 the stylus. This scanning mode is useful to permit auto-
matic orientation of the beam near the stylus, so that
tracking may commence, for example, when the stylus
is returned to the face of the tube in a writing position
after it has been withdrawn therefrom. The alternative
55 scanning mode is effected in the following fashion.
The output from amplifier 31 is passed through a peak
rectifier 32 which develops a signal that is indicative of
the peak output potential produced by the photocell 21.
The signal from rectifier 32 is supplied to one input of a
comparator 33. the other input of which is energized by
a reference potential, for example, a fixed potential desig-
nated VR1t'. Whenever the signal from the peak recti-
fier falls below the reference potential, the output po-
tential of the comparator changes from, for example, 0
volts to some positive value. Thus, whenever the output
of the photocell falls below some predetermined magni-
tude, which indicates that the beam is neither tracking
the stylus nor in close proximity thereto, a signal is sup-
plied by comparator 33. The signal from the comparator
energizes pulse gate 41 which passes a series of pulses
that are formed, for example, from the output of the sine
wave generator 39 by the pulse former 40. Accordingly,
a series of positive pulses, for example, are applied to
one input of the summing integrator 44. These pulses
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arc added to the signal from multiplier 37, and the sum
is integrated. * However, since the photocell signal is ex-
tremely small at the time during which tracking is not
taking place, the output of multiplier 37 is virtually zero.
'I hus, the pulses from gate 41 are alone integrated to
produce an output potential from integrator 44 that in-
cteascs with time as a series of step voltages.
After each series of n pulses from the pulse gate 41,
a scale of n frequency divider 42 produces an output pulse
which serves to reset the summing integrator 44 and
which is applied to one input of the summing integrator
45. Summing integrator 45 integrates the series of pulses
from the scale of n frequency divider 42 in a fashion
similar to that of integrator 44. After each series of m
pulses from the frequency divider 42, the scale of m
frequency divider 43 produces a pulse which resets the
summing integrator 45. The net effect of the integrations
by the summing integrators 44 and 45 is to produce out-
puts that resemble sawtooth functions, although each
sawtooth is composed of a series of step voltages. The
output from the summing integrator 44 thus provides a
saes tooth output potential which builds up during every n
pulse from pulse gate 41 and which is applied through the
adder 34 to the horizontal deflection plates 14 of the
cathode ray tube. This in effect causes the beam within
the tube to sweep the face of the tube in normal line
sweep fashion. Similarly, the output from the summing
integrator 45, which also is of a sawtooth wave form
that builds up during every if times m pulses from pulse
gate 41, is applied through the adder 35 to the vertical
deflection plates 13 of the cathode ray tube. In this
fashion, the beam is made to sweep across the face of
the tube in typical frame sweep fashion in the vertical
direction.
Since the line sweep and frame sweep scanning raster
may commence at any point upon the face of the cathode
ray tube 10, the sawtooth waves should be of sufficient
magnitude to ensure that the beam scans an area of at least
twice that of the face of the tube. In this fashion, the
line sweep and frame sweep scanning raster effectively
encompasses the entire face of the tube, regardless of the
potentials previously supplied by the integrators.
Alternatively, the resetting of the integrators may be
effected by means of deflection voltage comparisons with
predetermined voltage values corresponding to maximum
required horizontal and vertical deflections, respectively.
By utilizing such comparisons and eliminating both the
scale of n divider 42 and the scale of nt divider 43, when
the horizontal deflection voltage reaches its maximum
value, the summing integrator 44 is reset and a pulse is
applied to the summing integrator 45. At maximum
vertical deflection, the vertical integrator is reset. In this
alternate arrangement the double-amplitude deflection
voltages are not needed.
During the search scan, the beam sweeps across the
entire face of the tube 10 in typical line sweep and frame
sweep fashion until the beam is in proximity to the tip
of the stylus 20. At this time, the amplified and peak
rectified potential from the photocell 21 is slightly greater
than the reference potential VRer. that is applied to com-
Nrator 33. The output potential of comparator 33 thus
ch..tngcs from the above-mentioned positive value to 0
"I". and the pulse gate 41 is no longer energized. Thus,
t*,e sumrning integrators 44 and 45 no longer have applied
*^rto the voltage pulses from gate 41 and frequency
42. respectively. However, since the beam in the
' ~`'vylc ray tube 10 is now adjacent the tip of the stylus
+re normal tracking mode takes over by virtue of
I" signals developed by multipliers 37 and 38,
}cam once again tracks the stylus. It should be
'`:t by adding the "error" signal from one of the
rtss 10 the associated pulse signal for the "search"
% P:s ing the sum to one integrator, the embodi-
?`e"' r~,,'u e1 can switch smoothly and permanently from
t~,e +lq s? fi...Y; r1
t hfdt to the tracking node. If a common
integrator was not utilized, such a transition most likely
could not take place, since it would be mere chance that
the integrated multiplier signal alone would be equal to
the search signal when the beam was opposite the stylus.
Thus, as the device switched hack from the searching
to the tracking mode, the beam would be deflected away
from the photocell, which would cause the searching mode
to start once again. The present arrangement overcomes
this problem.
The signal from the comparator 33 is also used to
generate an "out-of-contact" signal, as from the lead 50,
which is applied to the utilization device 55. In this
fashion, whenever the electron beam in the tube 10 as-
sumes a search raster and is sweeping the face of the
tube in line sweep and frame sweep fashion, a signal is
generated. This signal is present whenever the stylus
is either withdrawn from the face of the tube or when-
ever the beam has become decentered from the photocell
by a predetermined amount. Such an "out-of-contact"
signal may be utilized to indicate that the "X" and "Y"
signals from leads 51 and 52 are not indicative of informa-
tion pertaining to the tracing, upon the face of the cathode
ray tube, of an alphabetic character, a numeral, or other
graphic material.
In the embodiment illustrated in FIG. 1, no mechanical
restraints are placed upon the stylus. A conducting flex-
ible lead 23, which connects the photocell 21 and the
amplifier 31, is the only form of coupling that joins the
photocell wtih the rest of the circuitry. It may be appreci-
ated that this does not hinder or cramp a writing style to
any appreciable degree. However, alternative arrange-
ments may be employed to lessen even further the restraint
placed upon the movement of the stylus, such as, for
example, a radio transmitter contained within the stylus
to transmit a signal indicative of the photocell potential
to the amplifier 31 and thence on to the rest of the cir-
cuitry of FIG. 1.
Turning now to FIG. 4, there is depicted another em-
bodiment of the present invention. In this case, a special
cathode ray tube 10 is employed that is similar in all re-
spects to conventional cathode ray tubes except that it has
a specially constructed face 17. This face comprises an
outside dielectric layer 16 which conveniently may be of
glass. Deposited upon the inside of the layer is a thin
resistive film 15. An electrical connection is made by the
lead 19 to the thin resistive film at a point 18 on the inside
of the face of the cathode ray tube. Lead 19 is connected
to a demodulator 24, the output of which is connected to
a tracking unit 30, corresponding to the tracking unit 30
of FIG. 1.
A conductive stylus 20 is employed in this embodiment
in place of the stylus containing the photocell as depicted
in FIG. 1. The stylus 20 may be grounded directly, for
example, by the use of a flexible conductive lead con-
55 nected between it and ground, or it may be grounded indi-
rectly through the self-capacity of the operator writing
with the stylus.
The beam in the cathode ray tube, produced by the
electrode 11 that is connected to a suitable energizing po-
00 tential, not shown, is given a circular scan of high ro-
tational frequency and small radius, as in the embodiment
of FIG. 1. In addition, the beam is intensity modulated
by a radio frequency signal from the source 9 that is
applied to the beam intensity control grid 12. Intensity
,35 modulation of the beam is provided, since a varying sig-
nal is required to pass current from the resistive film 15
to the metal stylus 20, both of which may be considered
to comprise opposite plates of a capacitor. When the elec-
tron beam impinges upon the resistive film 15 at a point
70 opposite the tip of the metal stylus 20, a majority of the
varying beam current passes through the film to the stylus
and thence to ground. On the other hand, when the beam
impinges the resistive film at a point that is not opposite
the tip of the metal stylus, a majority of the varying beam
75 current flows within the resistive film to the demodulator
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24. Accordingly, by examining the current flowing in the
resistive film, one may obtain an indication of the flow of
current to the metal stylus.
It thus may be seen that when the locus of the circular
scan of the beam is opposite the center of the tip of the
metal stylus, the demodulator has applied to it a signal
that is small and whose envelope is non-varying. How-
ever, when decentering of the beam occurs. i.e., when the
beam is not spinning around an area directly opposite the
center of the tip of the metal stylus, the demodulator has
applied to its a signal of radio frequency containing a
varying or modulated envelope. The frequency of the en-
velope will be equal to the frequency of rotation of the
beam within the cathode ray tube. As in the embodiment
of FIG. 1, the products of this envelope and the potentials
applied to the plates of the cathode ray tube that cause
the circular scan are indicative of the magnitude and direc-
tion of decentering of the beam. Accordingly, the signal
from the resistive film is demodulated and applied to a
tracking unit 30 which is virtually the same tracking unit
as depicted in FIG. 1. The only differences in the units
are that. for the present embodiment, the inputs to multi-
pliers 37 and 38 from oscillators 36 and 39 are -a cos wt
and -a sin wt, respectively. Further, the comparator 33
is altered ;o energize the pulse gate 41 when the amplified
and peak rectified signal from demodulator 24 is greater
than VRef.. These changes are required since the funda-
mental component of the signal from the resistive film may
be expressed as -b cos whereas the fundamental
component of the signal from the photocell is
b cos (wt-0
and also because decentering of the beam results in an in-
crease in the signal from the resistive film, whereas decen-
tering causes a decrease in the photocell signal. With these
changes, the apparatus operates as that of FIG. 1, and the
beam tracks the movements of the metal stylus. Thus,
three signals are obtained, namely, an "out-of-contact"
signal from lead 50 indicating when the stylus is being
tracked, and "X" and "Y" signals from leads 51 and 52
indicating the co-ordinates of the stylus with respect to
the writing tablet.
For this embodiment, a special stylus containing a pho-
tocell is not required. Any metal stylus will suffice; an
ordinary metallic fountain pen or ballpoint pen may be
used.
Although specific embodiments of the present invention
have been depicted, it is obvious that numerous additions
and substitutions may be made without departing from the
spirit and scope of the invention. Accordingly, the inven-
tion should be deemed limited only insofar as it is re-
strictively defined in the following claims.
What is claimed is:
1. Te!ewriting apparatus comprising a cathode ray tube,
means for producing a beam of electrons within said tube,
means for deflecting said beam in response to input sig-
nals, means for supplying an oscillatory signal to said beam
deflection means for rotating said beam about a reference
axis, a photocell, means for detecting an output from said
photocell as it is moved in closed proximity to the face
of said cathode ray tube, means supplied with the output
of said photocell and said oscillatory signal for producing
a potential corresponding to the difference in phase there-
between and of a magnitude dependent on the distance
between said photocell and the point of impingement of
said beam on the face of said tube, means for integrating
said potential, means for applying said integrated po-
tential to said beam deflection means for deflecting said
beam so that said reference axis passes through said pho-
tocell, means for applying a signal to said beam deflec-
tion means to deflect said electron beam in a search raster
across the face of said tube whenever the magnitude of
said photocell output is less than a predetermined thresh-
old, and means for utilizing the aforementioned integrated
10
potential to indicate the "X" and "Y" co-ordinates of said
photocell.
2. Apparatus for transmitting electrical signals indica-
tive of the position of a photocell adjacent the face of a
cathode ray tube in which said tube includes means for
creating a beam of electrons and a first means for deflect-
ing said beam in a first direction and a second means
for deflecting said beam in a second direction; said appa-
ratus comprising means for applying a first signal to said
first deflection means and a second signal to said second
deflection means, said first and second signals exhibiting
a phase difference sufficient to impart to said beam a rota-
tional movement about a reference axis, a photocell, means
for detecting an output signal from said photocell, means
15 supplied with said photocell signal and said first signal for
producing a first potential proportional to the component
in said first direction of the distance between the point of
impingement of said beam upon said face and said photo-
cell, means supplied with said photocell signal and said
20 second signal for producing a second potential propor-
tional to the component in said second direction of the
distance between the point of impingement of said beam
upon said face and said photocell, means for producing an
out-of-contact signal whenever said output signal from said
25 photocell is less than a predetermined magnitude, means
responsive to said out-of-contact signal for producing a
first search signal, means responsive to said out-of-contact
signal for producing a second search signal, said first search
signal and said second search signal, when separately in-
30 tegrated and applied to said first and second deflection
means, respectively, imparting to said beam a movement
that effects a searching raster covering the entire face of
said tube, means for adding said first potential and said
first search signal to produce a first summed signal, means
for adding said second potential and said second search
signal to produce a second summed signal, means for in-
tegrating said first summed signal, means for integrating
said second summed signal, means for applying said inte-
grated first summed signal to said first deflection means,
means for applying said integrated second summed signal
to said second deflection means, and means for utilizing
said integrated first summed signal, said integrated see-
cnd summed signal and said out-of-contact signal to in-
dicate the movements of said photocell.
45 3. A telewriting device comprising a cathode ray tube,
means for producing a beam of electrons within said tube,
a first deflection means responsive to an input signal for
deflecting said beam in a first direction, a second deflection
means responsive to an input signal for deflecting said
50 beam in a second direction displaced 90 degrees from
said first direction, a stylus capable of being moved about
the face of said tube, a photosensitive element positiorcd
in said stylus, means for detecting an output signal from
said photosensitive element, means for multiplying said
55 output signal from said photosensitive element by a signal
of cosine wave form to produce a first product signal,
means for multiplying said output signal from'said photo-
sensitive element by a signal of sine wave form to pro-
duce a second product signal, means for producing an
60 out-of-contact signal whenever the magnitude of said out-
put signal from said photosensitive clement is less than
a predetermined value, means responsive to said out-of-
contact signal for producing a signal of a first pulse wave
form, means responsive to said out-of-contact signal for
i5 producing a signal of a second pulse wave form, means
for adding said first product signal and said first pulse
signal to produce a first summed signal, means for adding
said second product signal and said second pulse signal
to produce a second summed signal, first means for inte-
grating said first summed signal, second means for inte-
grating said second summed signal, means for resetting to
a first reference value the output of said first integrating
means after each series of n pulses from said first pulse
wave form means, means for resetting to a second refer-
ence value the output of said second integrating means
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after each series of pit pulses from said second pulse wave
fo; m means. means for adding said integrated first summed
'izaal and said signal of cosine wave form to produce a
first control signal, means for adding said integrated sec-
ond summed signal and said signal of sine wave form to
pr?duce a second control signal, means for applying said
first control signal to said first deflection means, means
for applying said second control signal to said second de-
flection means, and means for transmitting said integrated
first summed signal, said integrated second summed sig-
nal, and said out-of-contact signal to any location thereby
to provide signals indicative of the movement of said
stylus.
4. A device for the transmission of graphic informa-
tion comprising a cathode ray tube, the face of said
cathode ray tube comprising a conductive film placed upon
a layer of dielectric material, means for producing a beam
of electrons within said tube to impinge upon a portion
of said conductive film thereby to produce a flow of
current therein, means for modulating the intensity of
said beans in accordance with an applied signal to cause
intensity variations of said current, control means for de-
flecting said beam in response to an input signal, a con-
ductive stylus coupled to a source of reference potential
and capable of being freely manipulated adjacent said
dielectric layer to thereby produce an amplitude modula-
tion envelope on the intensity variations of said current,
demodulating means responsive to the flow of current in
said conductive film for developing a signal whose wave
form follows the envelope of said current flowing in said
conductive film, means responsive to the signals from
'said demodulating means and to said input signal for
supplying a control signal to said control means for posi-
tioning said beam to impinge the portion of said face of
said tube opposite said stylus, and means for utilizing
said control signal to indicate the movement of said con-
ductive stylus.
5. A device as recited in claim 4 further comprising
means responsive to said signal from said demodulating
means for supplying a varying potential to said control
means to cause said beam to be deflected in a search
raster across the face of said tube whenever the magnitude
of said demodulated signal is greater than a predetermined
threshold.
6. Telewriting apparatus comprising a cathode ray
tube, the face of said cathode ray tube comprising a thin
resistive fi!m and a layer of dielectric material, means
for producing a beam of electrons within said tube, said
beam producing a current flow in said resistive film upon
impingement therewith, means for modulating the in-
tensity of said beam in accordance with an applied sig-
nal thereby causing intensity variations in said current,
means for deflecting said beam in response to input sig-
nals, means for supplying an oscillatory signal to said
beam deflection means for oscillating said beam in a locus
about a reference axis, a source of reference potential, a
metal stylus capable of being moved freely about the
faze of said tube adjacent said dielectric layer and coupled
to aid source of reference potential, means for detecting
flow of current in said resistive film as said stylus is
rnuvcd in close proximity to said face of said tube, said
'0,, producing an amplitude modulation envelope on the
ccn.ety variations of said current when proximate to a
on said face which is closer to one part of the locus
said face of said tube whenever the magnitude of said de-
modulated signal is greater than a predetermined thresh-
old, and means for utilizing said integrated potential to
indicate the position of said stylus adjacent said face of
said tube.
7. Apparatus for transmitting electrical signals indica-
tive of the position of a conductive stylus adjacent the
face of a cathode ray tube in which said tube is char-
acterized by a face comprising a resistive film, said tube
being further characterized by means for creating a beam
of electrons, said beam producing a current flow in said
resistive film upon impingement therewith, means for
cyclically varying the intensity of said beam, a first means
for deflecting said beam in a first direction, and a second
15 means for deflecting said beam in a second direction; said
apparatus comprising means for applying a first signal to
said first deflection means and a second signal to said
second deflection means, said first and second signals ex-
hibiting a phase difference sufficient to impart to said
20 beam an oscillatory movement about a reference axis,
wave form developing means for developing a signal
whose wave form is the envelope of the current flowing
in said resistive film, means supplied with the signal from
said wave form developing means and said first signal
25 for producing a first potential proportional to the dis-
tance in said first direction between the point of impinge-
ment of said beam upon said face and said stylus, means
supplied with said signal from said wave form develop-
ing means and said second signal for producing a second
30 potential proportional to the distance in said second direc-
tion between the point of impingement of said beam upon
said face and said stylus, means for applying said first
potential to said first deflection means, means for apply-
ing said second potential to said second deflection means,
35 and means for utilizing said first potential and said second
potential to indicate at any location the movements of
said stylus.
8. Apparatus for transmitting electrical signals indica-
tive of the position of a conductive stylus adjacent the
40 face of a cathode ray tube in which said tube is char-
acterized by a face comprising a resistive film, said tube
being further characterized by means for creating a beam
of electrons, said beam producing a current flow in said
resistive film upon impingement therewith, means for
45 cyclically varying the intensity of said beam thereby pro-
ducing cyclical variations of said current, a first means
for deflecting said beam in a first direction, and a second
means for deflecting said beam in a second direction; said
apparatus comprising means for applying a first signal to
50 said first deflection means and a second signal to said
second deflection means, said first and second signals ex-
hibiting a phase difference sufficient to impart to said beam
an oscillatory locus about a reference axis, said stylus
producing an amplitude modulated envelope wave form
55 on the cyclical variations of said current whenever any
part of the locus of said beam on said resistive film is
closer than any other part of the locus to a point at which
said stylus is proximate to the resistive film, wave form
developing means for developing a signal whose wave
60 form is the envelope wave form of the current flowing
in said resistive film, means supplied with the signal from
said wave form developing means and said first signal
for producing a first potential proportional to the distance
in said first direction between the point of impingement
_ .....?, means and said second signal for producing a second
"Y signal for producing a potential corresponding potential proportional to the distance in said second direc-
? J;?tcrence in phase therebetween, means for intc- 70 tion between the point of impingement of said beam upon
{ ` '' "'tt Potential, means for applying said integrated said face and said stylus, means for integrating said first
potential, means for integrating said second
sa:d beam deflection means for deflecting said
r q, g potential,
"id reference axis passes through said stylus, means for applying said integrated first potential to said
tr M. tit sy +:
spying a signal to said beam deflection means first deflection means, means for applying said integrated
electron beam in a search raster across 75 second potential to said second deflection means, and
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means for utilizing said integrated first potential and said
integrated second potential to indicate the movements of
said stylus.
9. Apparatus for transmitting electrical signals indi-
cative of the position of a conductive stylus connected
to a source of reference potential as said stylus is moved
in close proximity to the face of a cathode ray tube, said
tube being characterized by a face comprising a resistive
film and said tube including means for creating a beam
of electrons, said beam producing a current flow in said
resistive film upon impingement therewith, means for
cyclically varying the intensity of said beam thereby pro-
ducing cyclical variations of said current, a first means
for deflecting said beam in a first direction, and a second
means for deflecting said beam in a second direction;
said apparatus comprising means for applying a first sig-
nal to said first deflection means and a second signal to
said second deflection means, said first and said second
signals exhibiting a phase difference sufficient to impart
to said beam an oscillatory locus about a reference axis,
said stylus producing an amplitude modulated envelope
wave form on the cyclical variations of said current when-
ever any part of the locus of said beam on said resistive
film is closer than any other part of the locus of said beam
to a point at which said stylus is proximate to the re-
sistive film, wave form developing means for developing
a signal whose wave form characterizes the envelope wave
form of the current flowing in said resistive film, means
supplied with the signal from said wave form developing
means and said first signal for producing a first potential
proportional to the distance in said first direction between
the' point of impingement of said beam upon said face
and said stylus, means supplied with said signal from said
wave form developing means and said second signal for
producing a second potential proportional to the distance
in said second direction between the point of impingement
of said beam upon said face and said stylus, means for
producing an out-of-contact signal whenever the magni-
tude of said signal from said wave form developing means
is greater than a predetermined magnitude, means respon-
sive to said out-of-contact signal for producing a first
search signal, means responsive to said contact signal for
producing a second search signal, said first search signal
and said second search signal imparting to said beam a
searching pattern which covers the entire face of said tube
when said signals are each integrated and applied to said
first deflection means and said second deflection means,
respectively, means for adding said first potential and said
first search signal to produce a first summed signal, means
for adding said second potential and said second search
signal to produce a second summed signal, means for in-
tegrating said first summed signal, means for integrating
said second summed signal, means for applying said inte-
grated first summed signal to said first deflection means,
means for applying said integrated second summed signal
to said second deflection means, and means for utilizing
said integrated first potential, said integrated second po-
tential, and said out-of-contact signal to indicate the
movements of said stylus.
10. A telewriting device comprising a cathode ray tube
having a face composed of a layer of dielectric material
upon which has been deposited upon one side thereof a
thin resistive film, means for producing a beam of elec-
trons within said tube, said beam producing a current
flow in said resistive film upon impingement therewith,
means for cyclically varying the intensity of said beam
thereby producing cyclical variations of said current, first
and second deflection means, means for applying a poten-
tial of cosine wave form to said first deflection means and
a potential of sine wave form to said second deflection
means for rotating said beam in a circle on said resistive
film, a source of ground potential, a grounded conductive
stylus capable of being moved freely about the face of
said tube. adjacent said dielectric layer, said stylus pro-
14
ducing an amplitude modulated envelope wave form on
the cyclical variations of said current when a part of said
circle is closer than another part of said circle to a point
at which said stylus is proximate to said dielectric ma-
terial, means for producing a signal proportional to the
amount of current in said resistive film, means for de-
tecting the envelope wave form of said proportional sig-
nal, means for multiplying said envelope wave form of
said signal by the potential of cosine wave form to pro-
duce a first product potential, means for multiplying said
envelope wave form of said signal by the potential of
sine wave form to produce a second product potential,
means for producing an out-of-contact signal whenever
the magnitude of said envelope wave form is greater than
15 a predetermined magnitude, means responsive to said out-
of-contact signal for producing a potential of a first pulse
wave form, means responsive to said out-of-contact signal
for producing a potential of a second pulse wave form,
means for adding said first product potential and said
20 first pulse potential to produce a first summed potential,
means for adding said second product potential and said
second pulse potential to produce a second summed po-
tential, first means for integrating said first summed po-
tential, second means for integrating said second summed
potential, means for resetting to a first reference value
the output of said first integrating means after each series
of n pulses from said first pulse wave form means, means
for resetting to a second reference value the output of said
second integrating means after each series of m" pulses
from said second pulse wave form means, means for
adding said integrated first summed potential and said
potential of cosine wave form to produce a first control
signal, means for adding said integrated second summed
potential and said potential of sine wave form to produce
a second control signal, means for applying said first
control signal to said first deflection means, means for
applying said second control signal to said second de-
flection means, and means for transmitting said integrated
first summed potential, said integrated second summed
potential, and said out-of-contact signal to any location
thereby to provide signals indicative of the movement
of said stylus.
11. Telewriting apparatus comprising a cathode ray
tube, means for producing a beams of electrons within
said tube, means for deflecting said beam in response to
input signals, means for supplying an oscillatory signal
to said beam deflection means for rotating said beam
about a reference axis, a photosensitive element, means
for detecting an output from said photosensitive element
as it is moved in close proximity to the face of said
cathode ray tube, means supplied with the output of said
photosensitive element and said oscilllatory signal for pro-
ducing an error potential corresponding to the difference
in phase therebetween, means for applying said error po-
tential to said beam deflection means for deflecting said
beam so that said reference axis passes through said
photosensitive element, means responsive to the output
of said photosensitive element for supplying a varying
potential to said deflection means to deflect said beam in
a search raster across the face of said tube whenever the
magnitude of said photosensitive element output is less
than a predetermined threshold, and means for utilizing
said error potential to indicate the "X" and "Y" co-
ordinates of said photosensitive element.
12. A device for the transmission of graphic informa-
tion comprising a cathode ray tube, means for producing
a beam of electrons within said tube, control means for
deflecting said beam in response to an input signal, means
for supplying an oscillatory signal to said beam deflection
means for rotating said beam about a reference axis,
stylus means positioned in a nonfixed fashion adjacent
the face of said tube, means responsive to the location
of said stylus means relative to the location of said beam
75 on the face of said tube for producing a control current,
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means supplied with said control current and said oscil-
latory signal for producing an error potential correspond-
ing to the difference in phase therebetween and of a mag-
nitude dependent on the distance between said stylus and
the point of impingement of said beam on the face of
said tube, means for supplying said error potential to
said control means to position said beam upon the portion
of said face of said tube opposite said stylus means,
means responsive to said control current for supplying
a varying potential to said deflection control means to
deflect said beam in a search raster across the face of said
16
tube whenever the location of said stylus means relative
to the location of said beam on the face of said tube is
greater than a predetermined distance, and means for
utilizing said error potential to provide information rele-
vant to the movement of said stylus means.
References Cited in the file of this patent
UNITED STATES PATENTS
2,455,532 Sunstein ---------------- Dec. 7, 1948
10 2,487,641 Denk ------------------ Nov. 8, 1949
2,849,707 White ----------------- Aug. 26, 1958
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