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IEEE TRANSACTIONS ON COMPUTERS, JUNE 1970
solved by simply requiring that people avoid printing
them. This constraint apparently caused our printers
Ino real difficulty, although other reports [17] indicate
that people do not always make unbroken characters
even when they are required to do so.
Final conclusions should wait for experiments on several large data sets. It appears that algorithms T, U,
and V operating on binary vectors obtained from con-tours of characters quantized as in Fig. 3 yield recognition accuracies much better than those obtainable using
a partially trained optimum classifier, and that if sixpart area division--is used, reasonably good recognition
accuracies can be achieved, particularly if simple tests
are used to differentiate between commonly confused
characters and if contextual constraints are incorporated. These conclusions support those of Bakis et al.
[18 ] that curve-following features extract the significant
information from handprinting.
A Sonic Pen: A Digital Stylus System
REFERENCES
[1] J. M. Wozencraft and I. M. Jacobs, Principles of Communication Engineering. New York: Wiley, 1965, pp. 425-476.
[21 L. Kanal and B. Chandrasekaran, "On the dimensionality and
sample size in statistical pattern classification," 1968 Proc.
NEC, vol. 24, pp. 2-7.
[3] R. Alter, "Utilization of contextual constraints in automatic
speech recognition," IEEE Trans. Audio and Electroacoustics,
vol. AU-16, pp. 6-11, March 1968.
[4] R. 0. Duda and P. E. Hart, "Experiments in the recognition of
handprinted text: Part II-Context analysis," 1968 Fall Joint
Computer Conf., A FIPS Proc., vol. 33, pt. 2. Washington,
D. C.: Thompson, 1968, pp. 1139-1149.
[51 J. H. Munson, "The recognition of hand-printed text," in
Pattern Recognition, L. N. Kanal, Ed. Washington, D. C.:
Thompson, 1968, pp. 109-140.
[6] J. Raviv, "Decision making in Markov chains applied to the
problem of pattern recognition," IEEE Trans. Information
Theory, vol. IT-13, pp. 536-551, October 1967.
[7] J. K. Clemens, "Optical character recognition for reading
machine applications," Ph.D. dissertation, Dept. of Elec.
Engrg., M.I.T., Cambridge, Mass., August 1965.
"Optical character recognition for reading machine
[8]
applications," M.I.T. Electronics Research Lab., Cambridge,
Mass., Quart. Progress Rept. 79, pp. 219-227, October 1965.
[9] S. J. Mason and J. K. Clemens, "Character recognition in an
experimental reading machine for the blind," in Recognizing
Patterns, P. A. Kolers and M. Eden, Eds. Cambridge, Mass.:
M.I.T. Press, 1968, pp. 156-167.
[10] S. J. Mason, F. F. Lee, and D. E. Troxel, "Reading machine for
the blind," M.I.T. Electronics Research Lab., Cambridge, Mass.,
Quart. Progress Rept. 89, pp. 245-248, April 1968.
[11] A. L. Knoll, "Experiments with 'characteristic loci' for recognition of handprinted characters," IEEE Trans. Computers
(Short Notes), vol. C-18, pp. 366-372, April 1969.
[12] J. M. Wozencraft and I. M. Jacobs, Principles of Communication
Engineering. New York: Wiley, 1965, pp. 459-460.
[131 G. S. Sebestyen, Decision-Making Processes in Pattern Recognition. New York: Macmillan, 1962.
[14] I. J. Good, The Estimation of Probabilities, Research Monograph 30. Cambridge, Mass.: M.I.T. Press, 1965.
[15] W. H. Highleyman, "The design and analysis of pattern recognition experiments," Bell Sys. Tech. J., vol. 41, pp. 723-744,
March 1962.
[16] G. F. Hughes, "On the mean accuracy of statistical pattern
recognizers," IEEE Trans. Information Theory, vol. IT-14, pp.
55-63, January 1968.
[17] M. M. Chodrow, W. A. Bivona, and G. M. Walsh, "A study of
handprinted character recognition techniques," Rome Air
Development Center, Rome, N. Y., Tech. Rept. RADC-TR65-444, February 1966.
[18] R. Bakis, N. M. Herbst, and G. Nagy, "An experimental study
of machine recognition of hand-printed numerals," IEEE Trans.
Systems Science and Cybernetics, vol. SSC-4, pp. 119-132, July
,
1968.
A. E. BRENNER AND P. de BRUYNE
Abstract-An inexpensive computer graphical input device giving
Cartesian coordinates with better than 1-mm resolution is described.
The technique works equally well for two-or three-dimensional input
variants of the device.
Index Terms-Digital stylus, graphical input, sonic pen, three-dimensional input, two-dimensional input.
INTRODUCTION
Computer graphic displays often require interaction with
a human operator which cannot be conveniently handled by
a keyboard. The display program may then be written to
allow the use of a light pen as an operator control. Although
the hardware involved is very simple, the pen searching and
tracking technique which is required complicates the computer program and consumes appreciable computer time.
When time-sharing computer terminals are used, the available computer time is often insufficient for a live cycling
display, and use is made of a storage tube or local display
memory. Use of a light pen under these conditions becomes
impractical, and a RAND [1] tablet is a natural device to
perform such input functions in that it is an active device
generating a computer interrupt signal, either periodically
or on demand. The time required to store data following
each interrupt is very small; hence efficient use may be made
of the computer time. The sonic pen system [2] to be described performs functionally as a RAND tablet or the
Sylvania data tablet [3], although operationally it is a much
simpler device. It has the added advantages of being relatively inexpensive and having the capability of digitizing in
three dimensions. Finally, the scheme directly generalizes to
a three-dimensional graphical input device. In this sense,
although it is similar to the Lincoln Laboratory Lincoln
wand [4] in that it uses the propagation time of a sound
wave as a measure of distance, the two devices differ substantially.
SONIC PEN PRINCIPLE
The basic sonic pen digitizer shown in Fig. 1 consists of
an orthogonal pair of plane microphones positioned at two
edges of the digitized area and a pen or puck capable of
generating a short sonic pulse. In use the sonic pen generates
a spherical sonic wavefront by causing a small spark to
jump across a small gap in the tip of the pen. When the
wavefront first reaches the sensing microphone planes, an
output is obtained with a rise time of 1 to 2 xs. The distance
from the pen to each microphone is measured by counting
clock pulses during the transit time of the sound front from
the spark gap to the microphone. The limiting accuracy is
Manuscript received August 21, 1969; revised December 14, 1969.
This work was performed at Harvard University, Cambridge, Mass., and
was supported in part by AEC Contract AT(30-1)-2752.
A. E. Brenner is with the Department of Physics, Harvard University,
Cambridge, Mass.
P. de Bruyne is with R.C.A. Laboratories, Burfington, Mass.
547
SHORT NOTES
Fig. 1. A two-dimensional version of the sonic pen system.
Fig. 2. One possible embodiment of a sonic pen is shown. In this case a
ball point pen is used as the stylus and a hard copy sketch may simultaneously be made of the input data.
Fig. 3. A three-dimensional version of the sonic pen system.
TO COMPUTER
-300 v.
y STOP
x OVERFLOW
CLEAR ALL
Fig. 4. A block diagram of the overall electronic logic for the sonic
pen system. See the text for details.
obtain equal maximum x, y, and z dimensions; if only x
and y directions are used, shallow rectangular microphones
are convenient and improve signal output. The leading
surface of the wavefront will be parallel to a microphone
over a small area when it impinges and produces an electrical
impulse with a rise time of typically 1 ps. As sound travels
0.3 mm/ps in air, a resolution of about 0.2 mm is feasible at
distances to about 1 meter.
The two-dimensional tablet has a 30- by 30-cm plexiglass
HARDWARE DESCRIPTION
surface and the microphones, each with a 2.5- by 31-cm
Fig. 2 shows one embodiment of the tip of the pen, which sensitive area, are located for convenience at the left and
is typically pulsed at a rate of 60 times/second. If a z direc- top edges of the writing area. The frame can be placed on a
tion is used, the microphones are typically square planes to table or fixed in front of a large display CRT of a computer.
typically ± 0.2 mm, even at distances up to 1 meter, provided drafts can be kept to less than 0.3 meter per second and
temperature variations to + 1.0°C. Greater accuracy may
possibly be obtained using an automatic calibration technique utilizing a redundant microphone and by using an
enclosed temperature-stabilized air space. Experience to
date has given better than 1-mm accuracy without any of
these special procedures.
IEEE TRANSACTIONS ON COMPUTERS, JUNE 1970
548
Calibration is arranged so that in the latter case, the displayed points fall approximately under the tip of the pen.
Writing may be carried out on a stack of drawings, a book,
or even in space as no physical contact with any surface is
required. This device has been in use for over a year with a
display driven by a PDP-1 computer. Fig. 3 demonstrates a
three-dimensional arrangement. The top plane is used for
the z direction.
ELECTRONICS
The capacitor-type plane microphones consist of 25micron mylar aluminized on one side held against a perforated aluminum backing electrode by electrostatic forces.
They are biased with 600 volts and produce an output
voltage of typically 1 mV with 1 ,us rise time. A saturating
preamplifier of gain 10 000 drives a Schmidt trigger. The
output of each channel is connected to the main chassis
with coaxial cables. A block schematic diagram is shown
in Fig. 4.
At the time of the spark, three univibrators are triggered.
Two of these produced a read request of about 1 ms starting
1.5 ms after the spark. The third holds all flip-flops in the
reset state for about 20 /s until all electrical disturbance
from the spark has subsided. This includes overload recovery from the preamplifiers. When the reset state is
released, an oscillator of about 1 MHz is allowed to scale
into the x and y position scalers. When a preset count position is reached, another one shot is triggered causing a
brief 2-,s reset of the x and y scalers. This allows a digitalcontrollable offset to be obtained for the distances between
the microphones and the edge positions desired. The use of
the scalers for this purpose enables precise and stable positioning even for large distances.
After this second reset period, scaling in x and y continues until an output from the preamplifiers causes the
gating flip-flops to set. The x and y scalers are stopped individually when the sound arrives and hold the data of position until reset by a following spark. The data is read out to
the computer before this happens by the action of the univibrators as described earlier. If the position of the pen is
outside the designated field, then a computer interrupt
request is inhibited by overflow and underflow logic. Hence,
no false indications are possible and only writing in the
designated area is accepted. Besides the output from the x,
y, and possible z scalers, a function register is read out at
each interrupt request. The function switches enter control
information to the program allowing for flexible use of the
device.
It is thus seen that there has been provided a simple,
highly efficient arrangement for supplying graphic data to a
computer. While only a simple embodiment has been described, other variations will suggest themselves. For instance, specific nonlinear (such as logarithmic) operation
may be obtained by changing the frequency of the clock in
some programmed way.
ACKNOWLEDGMENT
Useful discussions during the initial conception of the
sonic pen idea were held with C. Zajde. Recent engineering
help was performed by A. Sanderson and M. Olmstead.
REFERENCES
[1] M. R. Davis and T. 0. Ellis, "The RAND tablet: A man-machine
graphical communication device," 1964 Fall Joint Computer Conf.,
AFIPS Proc., vol. 26. Baltimore, Md.: Spartan, 1964, pp. 325-331.
(Such a tablet is marketed under the trade name, Grafacon.)
[2] Patent pending. Details of the electronic circuitry are available from
the authors.
[3] J. F. Teixeira and R. P. Sallen, "The Sylvania data tablet: A new approach to graphic data input," 1968 Spring Joint Computer Conf.,
AFIPS Proc., vol. 32. Washington, D. C.: Thompson, 1968, pp.
315-321.
[4] L. G. Roberts, MIT Lincoln Lab. Rept., Lexington, Mass., June
1966.
On Information- Lossless Discrete-Time Systems
KEITH L. DOTY, MEMBER,
IEEE
Abstract-The concept of lossless state or system is generalized by
the definition of k-losslessness. If k>N(N-1)/2 for an N-state machine, k-lossless implies lossless. The series connection of a set of
discrete time systems {Ai}, where Ai is ki-lossless, results in a system
which is min{ki}-lossless. This result is a simple verification of the
intuitive notion that a noiseless communication channel is only as
lossless as the most lossy system in the channel. By means of inputoutput pair analysis, systems with the decomposition property (one
of the necessary but not sufficient conditions for linearity) are shown
to be information-lossless of finite order m, whenpne state has finite
order m. Further, every state must have exactly the same order. For
an N-state system with decomposition and zero-state linearity, information-lossless implies information-lossless of finite order m<N.
As a consequence, all information-lossless linear sequential machines
used as encoders allow decoding to begin after m received symbols,
provided that the encoder's starting state is known in advance.
Index Terms- Decomposition property, encoders, informationlossless machines, invertible, linear.
I. INTRODUCTION
A desirable property for a communication system is that
the message received be the message sent with as high a
degree of certainty as possible. Assuming no noise in the
encoder and decoder, this criterion requires that no information be lost because of the encoding or decoding process.
In this note we focus our attention on a specific class of
"information-lossless," noiseless, finite-state encoders
which meet such a requirement [1]. Where possible, statements are generalized to include discrete-time systems
having a countable number of states.
A system is information-lossless (IL) if, given the initial
state, observed response sequence, and the final state, the
excitation can always be determined or deciphered. If the
Manuscript received November 27, 1968; revised December 19, 1969.
Material for this paper was taken from a portion of the author's Ph.D.
dissertation, University of California, Berkeley, Calif.
The author is with the Department of Electrical Engineering, University of Florida, Gainesville, Fla. 32601.