Graphic Display System

Abstract

Graphic display system for graphically indicating the responses from a number of students to a question wherein the sampling cycle for displaying the answers to each question is based on the time that a certain percentage of the students take to answer that question.

Description

BACKGROUND OF THE INVENTION

The present invention relates to the graphic display system and more in particular, the present invention relates to the graphic display adopted for indicating dynamic phenomenon.

The so called teaching machine for giving questions to a large number of students and for finding the total of the answers from the students, is generally known to those skilled in the art. In the conventional teaching machine, the periodically changing answering phenomena of students to a certain question, are recorded in penrecorder in most cases. The above mentioned penrecorder system is adopted for carrying out the analysis and analytical research of the answering phenomena thereafter, but it is not adopted for instantaneously observing the periodically changing answering phenomena.

In order to overcome the above mentioned drawback of the conventional penrecorder system, those skilled in the art can easily think of the employment of graphic display for graphically indicating various kinds of phenomena electro-optically, in place of penrecorder. However, in accordance with the conventional display devices, the following problems are brought about because the sampling cycle, i.e., the time corresponding to the unit scale width in the direction of time axis on the indication surface. For example, when a certain question is given to students, the time required for the answers of the students to the question, is greatly varied by the ease or difficulty of the question or the abilities of the students. Therefore, when all the answering phenomena ranging from the case where the answering time is short, to the case where the answering time is long, are to be displayed on the same display indication surface, it is necessary to extend the time axis on the indication surface, and the size of the display device is excessively increased. On the contrary, when the time axis on the display surface is made short, it is impossible to sufficiently indicate the answers in the case of a question that takes a long time to answer.

The object of this invention is to provide a graphic display system which overcomes the above mentioned problems of the conventional devices.

Another object of the present invention is to provide a graphic display system capable of automatically changing the sampling cycle in accordance with the change of the speed with which the questions are answered.

In accordance with an embodiment of the present invention, when a certain question is given to a number of students, the time rate of the answers of the students to the question can be graphically indicated on the indication surface by luminescent diodes, discharge tubes on a CRT. First, the time (.tau.) required from the presentation of a question to the response of, for instance, 5 percent of the students, is measured. Next, the repetitive cycle of the sampling pulse for displaying the answers is determined on the basis of the time (.tau.), and thereafter, the student answers are displayed on the indication surface. At the same time, the scale in the direction of time axis on the indication surface is determined on the basis of said time (.tau.).

The details of the present invention are explained in accordance with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the rate at which the students provide answers to a typical question;

FIG. 2 is a block diagram showing a sampling pulse generating circuit as the main portion of the graphic display device of the present invention;

FIG. 3 is a diagram showing the circuit of a display panel utilizing a number of discharge tubes;

FIG. 4 is a diagram showing the circuit of a display panel utilizing a plural number of luminescent diodes;

FIGS. 5A and 5B, when fitted together as shown in FIG. 5, are a block diagram showing an embodiment of this invention utilizing the luminescent diode display panel;

FIGS. 6A and 6B, when fitted together as shown in FIG. 6, are a block diagram showing another embodiment of this invention utilizing a CRT.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the diagram showing how the rate of response from the students to a typical question. In FIG. 1, the abscissa shows time, and the ordinate indicates answer-rate.

What is meant by "answer-rate" in this specification is defined by the following formula;

FORMULA

Answer-rate = Number of students who have answered/Total number of students .times. 100 (1)

From FIG. 1, it is seen that the answer rate of the students is extremely low till a certain period of time has passed from the time when the question is given to the students (the time is shown by .tau.) and when the time .tau. has passed, it is a logarithmical curve. And, for example, when the 3.tau. time has passed, the answer rate arrives at certain level. The time .tau. is greatly varied by the ease or difficulty of the question, or by the ability of students. In the same manner, the gradient of the curve after having passed the time .tau. is closely related to the time .tau.. In other words, when the time .tau. is large, the gradient of the curve becomes "dull," and when the time .tau. is small, the gradient becomes "sharp."

Therefore, in accordance with the present invention, the time till the answer rate becomes such as 5 percent is measured, this time is defined as the time .tau., and from the time .tau., the time, i.e., the sampling cycle of the display curve is determined. In other words, the unit time (t) showing one unit of the time axis of the display is changed in correspondence to the time (.tau.). Therefore, in accordance with the present invention, it becomes possible to accurately and conveniently indicate the student answers on the same display surface even when the answer time varies greatly from question to question the answer time.

In the following paragraphs, the method for generating the sampling pulse, which is the essence of the present invention, is explained in accordance with FIG. 2.

In FIG. 2, A-Counter 1 is a binary counter for counting a first pulse train P.sub.1, and B-Counter 2 is also a binary counter for counting a second pulse train P.sub.2. A start signal P.sub.s changes to a high level at the time when a question is given to the students. On the other hand, a stop signal P.sub.t changes to a high level when the rate of the answer to the question reaches 5 percent. The stop signal P.sub.t may be provided, for example, by a counter 2a which receives as an input answer responses over a line l and provides and output when its count reaches a predetermined number corresponding to 5 percent of all answerers. Namely, the time ranging from the time when the start signal P.sub.s has become high, to the time when the stop signal P.sub.t becomes high, corresponds to the time (.tau.). When the start signal P.sub.s becomes high a flipflop F.sub.1 takes its set state, and the set output of flipflop F.sub.1 is supplied to an AND Gate G.sub.1. Therefore, when the flipflop F.sub.1 is set, the first pulse train P.sub.1 passes through the AND Gate G.sub.1 and is supplied to A-Counter 1, and is counted. Thereafter, the student answers start and are suitably counted, and when the answer-rate reaches 5 percent, the flipflop F.sub.1 is reset by the stop signal P.sub.t. Thereby the counting operation of A-Counter 1 stops. In this case, a value corresponding to the time (.tau.) till the answer rate has reached 5 percent is set in A-Counter 1.

The reset output of flipflop F.sub.1 is supplied to the AND Gate G'.sub.2, and therefore when the flipflop F.sub.1 is reset, the second pulse train P.sub.2 passes through the AND Gate G'.sub.2 and is supplied to B-Counter 2 to be counted.

When the count in B-Counter 2 agrees with the contents of A-Counter 1, a coincidence output is emitted from a coincidence circuit 3, to drive a mono-multi-vibrator 4. P.sub.c is the output pulse from the mono-multi-vibrator 4. B-Counter 2 is cleared by this pulse P.sub.c, and thereafter, when pulse P.sub.c is eliminated, B-Counter 2 starts counting again, and the content thereof is compared with the content of A-Counter 1. Thereafter, the above mentioned operation is repeated, and the pulse P.sub.c having the predetermined repetitive cycle is emitted from the mono-multi-vibrator 4.

Suppose that the repetitive cycles (the periods) of the first pulse train P.sub.1 and the second pulse train P.sub.2 are respectively set to be T.sub.1 and T.sub.2, and that the two repetitive cycles are interrelated by the below given formula;

Formula:

t.sub.1 = n.sup.. T.sub.2 (2)

when the counting of A-Counter 1 is set to be n.sub.1, the time (.tau.) is obtained by the following formula;

Formula:

.tau. = n.sub.1.sup.. n.sup.. T.sub.1 (3)

when the formula (2) is substituted in the formula (3), the following formula can be obtained;

Formula:

.tau. = n.sub.1.sup.. n.sup.. T.sub.2 (4)

on the other hand, when the counting of B-Counter 2 is supposed to be n.sub.2, the cycle T.sub.c of the pulse P.sub.c can be obtained by the following formula;

Formula:

t.sub.c = n.sub.2.sup.. T.sub.2 (5)

therefore, from the formulae (4) and (5), the below given formula (6) can be obtained;

Formula:

.tau. = n.sub.1 /n.sub.2.sup.. n.sup.. T.sub.c (6)

When the contents of the two counters 1 and 2 coincide, n.sub.1 becomes equal to n.sub.2, and therefore, the formula (6) becomes as follows;

Formula:

.tau. = n.sup.. T.sub.c (7)

From the above given formula (7), when it is presumed that one graduation of the display surface time axis corresponds to the pulse P.sub.c, the time .tau. is always represented by the (n) number of graduations. In other words, the repetitive cycle (i.e. the period) of the pulse P.sub.c is changed in correspondence to the way the time (.tau.) changes. In the present invention, the pulse P.sub.c is utilized as the sampling pulse. The cycle T.sub.1 represents the fineness of the divisions of the unit time t, and is the factor for determining the precision of the cycle T.sub.c, and therefore, it is preferable that T.sub.1 is smaller. On the other hand, the integer n stands for the number of the graduations dividing the time (.tau.), and determines the fineness of the reproduced graph, and therefore it is better when the value of n is greater. When the graduations are determined as mentioned above, the counting (n) of A-Counter 1 is indicated on the position of graduations corresponding to the time (.tau.) on the time axis of the indication surface, and the values of 2n or 3n are indicated on the graduations corresponding to 2.tau. and 3.tau..

FIG. 3 is a diagram showing an embodiment of the graphic display panel in which indicating discharge tubes are used.

As shown in FIG. 3, X drive lines X.sub.1 -X.sub.n are extended from the stages of a counter 6, which is an X drive circuit and Y drive lines Y.sub.1 -Y.sub.m are extended through transistors TR.sub.yl -TR.sub.ym, which are an Y drive circuit 7. A decoder 5 is connected with the Y drive circuit 7. Indicating lamps L.sub.ll -L.sub.mn are disposed at the points of intersection between the X and Y drive lines and the voltages V.sub.1 and V.sub.2 are applied to the terminals of the indicating lamps L.sub.ll -L.sub.mn through resistors.

The answer indicating data arrives on the input lines l. The number of answers generally increases as time elapses and converges toward a certain value after a predetermined time. A low-level signal appears on the Y drive line of the stage which corresponds to the decode output of the decoder 5. In response to the timming pulses P.sub.s illustrated in FIG. 2 a time axis counter in the x-drive circuit 6 steps so that low-level signals appear on the X drive lines sequentially. As a result the indicating lamps L at the points of intersection of the X and Y drive lines with the low-level signals are turned on sequentially. Once the indicating lamps are turned on, they may remains turned on even after the drive signals on the X and Y drive lines have returned to the high level. The above described graphic display device is of a conventional type, for example disclosed in U.S. Pat. application Ser. No. 256,373, filed May 24, 1972 by Takashi Inoue. It is seen that the segment connecting the adjacent turned-on indicating lamps L indicates the input data on the line l, for example, the rate of change in response ratio with respect to time.

FIG. 4 is a diagram showing a graphic display panel in which luminescent diodes are used. In the embodiment of FIG. 4, an X driver circuit 8 is composed of transistors TR.sub.x1, TR.sub.x2, . . . , TR.sub.xn, X driver lines X.sub.1, X.sub.2, . . . , X.sub.n are connected to the respective collectors of the transistors TR.sub.xl .about. TR.sub.xn, and in the same manner Y driver lines Y.sub.1, Y.sub.2, . . . , Y.sub.m are connected to the respective collectors of transistors TR.sub.yl .about. TR.sub.ym which form a Y driver circuit 9. Luminescent diodes D.sub.l .sub.l .about. D.sub.m .sub.n are respectively connected to the respective crossing points on the X driver lines and Y driver lines X.sub.l .about. X.sub.n, Y.sub.l .about. Y.sub.m. The respective emitters of the transistors TR.sub.yl .about. TR.sub.ym composing the Y driver circuit 9 are connected to a common power source +V, and current controlling resistances R are connected to the Y driver lines Y.sub.l .about. Y.sub.m.

For example, when X driver pulse P.sub.xl and Y driver pulse P.sub.yl have arrived, the transistor TR.sub.xl of X driver circuit 8 and the transistor TR.sub.yl of Y driver circuit 9 are put on, and X driver lines X.sub.l and Y driver lines Y.sub.l are selected. As a result, current is passed through the luminescent diode D.sub.l .sub.l, and said diode luminesces. Next, when the driver pulse P.sub.x2 and P.sub.x1 have arrived, X driver circuit L.sub.x2 and L.sub.y1 are selected, and the diode D.sub.1 .sub.2 luminesces. Thus, in the the same manner, the desired diodes luminesce in turn. Luminescent diodes do not remain on after the energizing current is removed. Therefore, for statically indicating a desired curve on the display panel, it is necessary to repeat the above-stated action at a pre-set cycle in a conventional manner.

FIG. 5, which comprises 5A and 5B, is a block-diagram of an embodiment of the invention in which produced when the display panel FIG. 4 is used.

In the block diagram of FIG. J, block 10 is a sampling pulse generating circuit as is explained in accordance with FIG. 2, and block 11 is the display unit in which the luminescent diodes are used, as is explained in accordance with FIG. 4. The indication surface 12 of the display unit 11, is composed of 400 luminescent diodes (20 .times. 20) arranged in the form of matrix. For the sake of convenience, the explanation here is given on the presumption that the abscissa (X-axis) X.sub.1 .about. X.sub.20, and the ordinate (Y-axis) Y.sub.1 .about. Y.sub.20 on the indication surface 12, are both designated by 5 bits binary code. 15 is a recirculating type shift register of 100 bit structure storing binary code signals of the ordinate (Y-axis) corresponding to each of the respective X.sub.1, X.sub.2, . . . , X.sub.20 signals in the direction of the abscissa (X-axis) on the indication surface 12.

Suppose that the Y-axis codes Y.sub.j (j = 1, 2, . . . , 20) corresponding to position X.sub.1 are stored in the bit positions 99 to 95 of the register 15, and that Y-axis codes Y'.sub.j corresponding to X.sub.2 position are stored on the following bit positions 94 to 90 of the register 15, and in the same manner Y-axis codes corresponding to X.sub.20 position are stored in bit positions 4 to 0. Every time a clock pulse P.sub.l is given on the line l.sub.1, the shift register 15 is read out serially by being shifted to the right by one bit, and then the bit that has been shifted out is transferred to the register 16 through the line l.sub.2. At the same time, the read out bit is circulated through the line l.sub.3, AND Gate G.sub.3, OR Gate G.sub.4, and is re-stored into the register 15. The register 16 is 5 bit shift register having the memory capacity sufficient for storing one Y-axis code. Therefore, when the first 5 pulses are given to the line l.sub.1, an Y-axis code Y.sub.j corresponding to X.sub.1 position is stored in the register 16.

The counter 19 counts the clock pulse P.sub.l on the line l.sub.1, and emits a 1 signal per 5 counts. The above mentioned 1 signal is supplied to the AND Gates G.sub.5 -1 .about. G.sub.5 -5, through l.sub.4. As a result, the gates G.sub.5 -1 .about. G.sub.5 -5 are energized, and the content of the register 16 are parallelly read out. As is apparent from the above given description, when the first 5 clock pulses P.sub.l have arrived at the line l.sub.1, the Y-axis code stored in the register 16 corresponding to the X.sub.1 position is parallelly read out, and the is transferred to the buffer register 17 through the gates G.sub.5 -1 .about. G.sub.5 -5. Thereafter, the content of the buffer register 17 is decoded by Y decoder 18, and one Y-drive signal P.sub.yj corresponding to the above mentioned Y-axis code Y.sub.j is emitted.

The 1 signal emitted from the Counter 19 is supplied to Counter 20 through the line l.sub.5. The counter 20 is 5 bit binary counter, and is a stepped forward by the 1 signal emitted from the Counter 19. In other words, every time the shift register 15 is shifted to the right by 5 bits, the Counter 20 is stepped by one count. As is apparent from the above description, the Counter 20 designates the X positions corresponding to the respective Y-axis codes stored the register 16. The content of the counter 20 is decoded by an X-decoder 21.

When the first 5 clock pulses have arrived at the line l.sub.1 , the counter 20 counts 1 and an X-drive signal P.sub.x1 for selecting X.sub.l position is emitted from the X-decoder 21. The X and Y driver circuits 8, 9 are driven by said Y-drive signal P.sub.yj and said X-drive signal P.sub.xl, and the luminescent diode on the coordinates (X.sub.l, Y.sub.j) luminesces. The operation of the display unit 11 was explained before in accordance with FIG. 3, and it is omitted here. Y-axis codes (Y'.sub.j, Y".sub.j, . . .) are stored in the shift register 16 every time 5 clock pulses P.sub.l arrive, and the X positions (X.sub.2, X.sub.3, . . .) corresponding to the respective Y-axis codes, are designated by Counter 20. The contents of the shift register 16 and Counter 20 are decoded by the decoders 18 and 21, respectively and a result, the luminescent diodes on the coordinates (X.sub.2, Y'.sub.j), (X.sub.3, Y".sub.j) . . . are selected and luminesce. Thus, when a luminescent diode on the final position along the X-axis, i.e., on X.sub.20, the above mentioned operation is repeated. The content of the shift register 15 is visually indicated on the indication surface 12 in the form of curve.

In this case, the indication surface 12 shows the student response to a question. The code signals designated the answer-rate are transmitted, after the 5 percent point, into the answer-rate register 13 through the input line l.sub.6. The answer-rate register 13 is 5 bit register as in the case of the shift register 16. The 1 signal from the Counter 19 is supplied into the AND Gates G.sub.1 -1 .about. G.sub.1 -5 through the line l.sub.7 and therefore, the content of the answer-rate register 13 is parallelly sent into the shift register 14 every time the five clock pulses P.sub.l have arrived, and thereafter, the content of the shift register 14 is serially read out on the line l.sub.8 by the clock pulse P.sub.l but Gate G.sub.2 is in its off state, and therefore it is not sent to the register 15.

In this case, the 5 percent answer-rate corresponds to the graduation Y.sub.1 of Y-axis, and the 10 percent answer-rate correspondes to the graduation Y.sub.2.

In the same manner 100 percent correct answer rate corresponds to Y.sub.20 axis. As explained in FIG. 2, when the time from the presentation of a question to the time when 5 percent answer rate is obtained, is set to be (.tau.), the repetitive cycle T.sub.c of the timing pulse P.sub.c emitted from the timing pulse generating circuit 10 is represented by the formula given below;

Formula:

t.sub.c = .tau./n

In the above given formula n is presumed to be 5. And, the repetitive cycle of clock pulse P.sub.l is remarkably quick when compared with that of the timing pulse P.sub.c.

The timing pulse P.sub.c emitted from the timing pulse generating circuit 10 is counted by Counter 22. The Counter 22 is a 5 bit binary counter as in the case of the Counter 20. The comparator circuit 23 compares the countings of Counter 20 and Counter 22, and when the two countings coincide, 1 a signal is emitted to the line l.sub.9. AND Gate G.sub.2 is put on ON state by 1 signal on the line l.sub.9, and AND Gate G.sub.3 becomes in the OFF state. Here, N is an inverter circuit. Therefore, the answer-rate data read out on the line l.sub.8 are written into the shift register 15 through the AND Gate G.sub.2 and OR Gate G.sub.4. Thus, the above mentioned operation is repeated every time 1 signal is emitted onto the line l.sub.9 from the comparator circuit 23, and the answer-rate data are written into the shift register 15. The content of the Counter 20 indicates the respective positions X.sub.1, X.sub.2, . . . on X-axis, and therefore, the data to be written into the shift register 15 correspond to the respective X positions. The positions X.sub.1, X.sub.5, X.sub.15, and X.sub.20 in the direction of X-axis correspond to 2.tau., 3.tau., and 4.tau.. Thus, the time graduations in X-axis are changed in correspondence to the change of time .tau..

It should be understood that the system shown in FIG. 5 can be applied to a display panel provided with discharge tubes.

FIG. 6, consisting of FIGS. 6A and 6B, is a diagram showing another embodiment in which a CRT is used. In FIG. 6, block 10 is a sampling pulse generating circuit, and 100 is an indicating CRT. In this embodiment the indication surface of the CRT 100 is divided into a total of 20 .times. 20 = 400 dots. A circulation type shift register 104 stores 400 bits, and the respective bit positions correspond to the dot positions on the CRT 100. The content of the shift register 104 is supplied to the amplifier 105 by right hand serial shifting on the clock pulse P.sub.l, and at the same time, the content of the shift register 104 is rewritten through AND Gate G.sub.2 and an OR Gate G.sub.4. An amplifier 105 gives a predetermined brightness signal to the CRT 100 when a signal is given thereto from the register 104.

A Y-Counter 106 is a 5-bit binary counter for counting the dot positions in Y-axis (the perpendicular direction) while being supplied with clock signal P.sub.l. The content of said Y-Counter 106 is converted into the corresponding analogue voltage with D-A converter 107. The analogue voltage of the converter 107 is amplified with the amplifier 108, and it is supplied to CRT 100 as the perpendicular deflection signal. In the same manner, X-Counter 109 is a 5 bit binary counter stepped forward by one every time the Y-Counter 106 makes 20 counts. As is apparent from the above description, X-Counter 109 determines the respective graduation positions of the electron beam on the X-axis. The content of the X-Counter 109 is converted into the corresponding voltage analogue by D-A converter 110, and then amplified with the amplifier 111, and is supplied as the horizontal deflection signal to the CRT 100. The above mentioned structure is the same as the structure of the conventional CRT display device, and therefore the explanations of the details of the conventional CRT display device are omitted here.

The indication data are supplied into the register 101 from the input line l.sub.10, and are decoded by the decoder 102. The output line of the decoder 102 is composed of 20 lines in correspondence to the number of the dot positions in the direction of Y-axis on the indication surface, and the 1 signal from the Counter 106 is supplied also to AND Gates G.sub.1 -1 .about. G.sub.1 -20. Therefore, every time 20 clock pulses P.sub.l have arrived, the output of the decoder 102 is transferred parallelly to the shift register 103 through the Gates G.sub.1 -1 .about. G.sub.1 -20. The content of the shift register 103 is serially read out on the line l.sub.ll by being right hand shifted under the control of the clock pulse P.sub.l. The AND Gate G.sub.3 is in the OFF state normally, and therefore the data on the line l.sub.11 are not written in the shift register 104.

In the same manner as in FIG. 5, the timing signal emitted from the block 10 is counted by Counter 112. The comparator circuit 113 compares the counting of Counters 109 and 112, and when the countings of the two counters are the same, a 1 signal is emitted onto the line l.sub.12. Thereby, AND Gate G.sub.2 is turned into OFF state, and AND Gate G.sub.3 is turned into ON state, and the 20 bit data on the line l.sub.ll are written into the shift register 104 through the gates G.sub.3 and G.sub.4. As can be easily understood, the position of the data in the direction of X-axis written in said shift register 104, is designated by the counting of X-Counter 109.

The embodiments shown in the attached diagrams are mere examples, and various kinds of embodiments other than those given in the attached diagrams, can be thought of.

Claims

What is claimed is:

1. A graphic display system for indicating the dynamic answer response of a set of answerers to a question comprising:

means for detecting the time lag between the posing of a question and the response thereto by a defined subset of said answerers;

means for providing a timing signal having a cycle which is a defined function of said time lag;

means for detecting the total number of responses to the question at each of a plurality of sample times occurring at a frequency which is a defined function of the cycle of said timing signal; and

means for displaying the detected total number of responses for each of said sample times on uniformly spaced consecutive portions of an indicating surface to provide a representation of a curve showing the dynamic answer response of the answerers to the question, wherein the spacing between said consecutive portions is a function of said time lag.

2. A graphic display system as in claim 1 wherein the displaying means comprise a two-dimensional indicating surface, an X-drive and a Y-drive each indicating a position along one of said dimensions, means for providing a display point at the intersection of the Y-axis indicated by the Y-drive and the X-axis indicated by the X-drive, and wherein the means for detecting the total number of responses at sample times occurring as a defined function of said timing signal comprise a current response register for storing a count of the total number of answer responses that have occurred, a timing signals counter for storing the current count of the timing signals that have occurred, means for transferring a decoded representation of the contents of the current responses counter to the Y-drive of the display means, and means for transferring a decoded representation of the contents of the timing signals counter to the X-drive of the display means, whereby the display means displays a decoded representation of the total number of answer responses at each occurrence of a timing signal, at an X-position defined by the current number of timing signals.

3. A graphic display system as in claim 2 wherein the display means include a matrix of luminescent diodes.

4. A graphic display system as in claim 1 wherein the display means comprise a cathode ray tube and wherein the means for detecting the total number of answer responses at sample times occurring as a defined function of said timing signals comprise a register storing the total current count of answer responses, a counter storing the total number of timing signals that have occurred, means for driving a first axis of the cathode ray tube as a function of the contents of said register, and means for driving a second axis of said cathode ray tube as a function of the contents of said counter.

5. A graphic display system as in claim 1 wherein the display means comprise a matrix of gas discharge tubes.

6. A graphic display system as in claim 1 wherein the display means comprise a matrix of luminescent diodes.

7. A graphic display system as in claim 1 wherein the display means comprise a cathode ray tube.