A pattern generator

NUCLEARINSTRUMENTS

AND METHODS

40(1966)

A PATTERN

121-124;ONORTH-HOLLAND

PUBLISHING co.

GENERATOR

J. KONTOS Nuclear Research Centre “Democritos”, Aghia Paraskevi Attikis, Athens, Greece Received 9 August 1965 A pattern generating system is described capable of generating trains of pulses that can be associated with patterns consisting of line segments. The output of this system is useful in testing

pattern analysis systems in connection with photograhs from high energy physics experiments. A transistorized version of the generator was built and tested.

1. Introduction The system described in this paper is capable of generating trains of pulses that can be associated with patterns consisting of line segments. This is the kind of pattern that one has to analyze for the recognition and measurement of the tracks of nuclear particles inside bubble and spark chambers as recorded on photographic film. Considerable effort is being put recently on automatic recognition and measurement of photographic plates from high energy physics experiments. In similarity with other fields of artificial intelligence, there exist two main methods of attack here too. These are the software and the hardware method respectively. The software method uses the programming of a general-purpose digital computer while the hardware method uses special-purpose analogue, digital or hybrid machines. When testing a particular software system of pattern analysis it is quite simple to generate artificial test-patterns as a part of the computer program. On the other hand it seems that patterngenerating systems are required in order to be able to

test easily special purpose machines that themselves have no test-pattern generating facilities. The pattern generator described here produces a repetitive pulse train with a long period corresponding to the time of scanning of a whole picture. Within this long period, there are a number of shorter periods, each corresponding to the time of a single line scanning. During these shorter periods, a reference pulse and one or more signal pulses occur in accordance with the pattern selected. Photographs of typical patterns are given in fig. l-5. The patterns of figs. 1-3 may be obtained when photographing a single charged particle entering a chamber and either passing straight through

Fig. 1. Particle passing straight through.

Fig. 5. Two particles passing straight through.

122

J. KONTOS I

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Fig. 7. Reconstruction of a straight line.

or producing an event. Fig. 4 corresponds to the case of a neutral particle producing a charged pair. And finally fig. 5 corresponds to the case of two non-interacting particles passing straight through the chamber. The description of the pattern generator begins in the next section where the basic unit used is described. This unit belongs to the class of systems called “neuroids” elsewhere’). 2. The basic nit The basic unit used in building the pattern generator

Fig. 8. Inputs and outputs of the basic unit without feedback.

has a very simple structure and its block diagram is shown in fig. 6. This system is well known as a pulse modulator, but it can also be considered as a very simple member of the class of systems called “neuroids” elsewhere’). When this system is used as a modulator one is mainly interested in its response to slow-varying

Fig. 9. Inputs and outputs of the basic unit with feedback.

A PATTERN

Fig. 10. Reconstruction

of a broken line.

while in the present context its response to pulse inputs is relevant. The use of this system as a pattern generator unit can be understood by considering first the system without feedback (a = 0). In that case a sign-inverting integrator cascaded by the Schmitt trigger has to be considered only. Let the input to the integrator be the sum of the two waveforms shown in fig. 8a and 8b respectively. By appropriate choice of parameters the output of the integrator and the Schmitt trigger will be as shown in fig. 8c and 8d respectively. Considering now the falling edges of this output of the Schmitt trigger as defining the positions of dark spots and the waveform of fig. 8a as corresponding to the scanning signal, one can reconstruct a picture of the form shown in fig. 7. This picture may be taken to correspond to the case of a photograph containing a single straight particle track. inputs

123

GENERATOR

In this simple illustrative case there are only five scans per picture but the extension for more scans is quite straightforward. The case considered above, i.e. the basic unit without feedback, has the disadvantage that the pulses produced vary in width within one picture period. The feedback can be used to correct this. By choosing the appropriate value for a and the other parameters involved, an output with constant width pulses can be obtained. Using a single basic unit, one can obtain patterns similar to fig. 3 by appropriate choice of the input waveforms. As an example consider the case where the input to the basic unit is the sum of the two waveforms shown in fig. 9a and 9b respectively. The outputs of the integrator and the Schmitt trigger will then be of the form shown in fig. 9c and 9d respectively. A picture of the form shown in fig. 10 can then be reconstructed using the waveforms of fig. 9. In this case there are eight scans per picture but the extension to more scans is again quite straightforward.The quantitative analysis of the behaviour of the system can be accomplished by the methods given in ‘). More complicated patterns can be obtained by the combination of a number of basic units as described in the next section. 3. The system The block diagram of the whole pattern generator C M

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F/n

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Fig. 11. The block diagram of the whole pattern generator.

124

J.

KONTOS

system is shown in fig. 11. It may consist of k basic units N,, . . , Nk. The inputs to the basic units come from adders S. The waveform representing the line scanning signal is produced by the monostable M. The waveforms defining the picture frequency and shaping the lines of the pattern are produced by a frequency divider f/n. By f is denoted the frequency of the line scanning and by n the number of lines contained in a picture. The whole system is driven by a clock, C. The output of the system is obtained from an OR-gate G with inputs the outputs of all the basic units and the monostable M. The form of this output is controlled by the potentiometers Pi1, P12,. . ., Pkl, PkZ and the voltages V, , . . . V,. These controls are brought out on the front panel of the device. In addition to the main output of the system there is a display output for convenient monitoring, of the pattern being generated. This display output is formed by adding to the main output the output of a digital to

analogue converter (D/A) which is driven by the frequency divider. The photographs shown in figs. l-5 were taken using this display output. It is the main output, however, that will be used as an input to any pattern analyzer that is to be checked. A transistorized system based on the above structure was built that requires about 10 transistors for each basic unit and about 30 transistors for the rest of the system. The line frequency of this system was set at 5 kc/s and the number of lines making up a single picture may be set at 8, 16, 32 or 64. This pattern generator has already proved an invaluable aid to our work in progress on pattern analysis. Thanks are due to G. Fragakis for his collaboration. Reference 1) J. Kontos,

Neuroid Studies, 4th Intern. netics, Namur (Oct. 1964).

Congr. on Cyber-