Image display apparatus

An image display apparatus detects the position of a point on a screen opposed to a photosensor based on the average of the number of scanning lines displayed before detecting an electron beam in the first scanning direction by the photosensor, and based on the number of scanning lines displayed before detecting an electron beam in the second scanning direction by the photosensor. The point may further or alternately be detected using the average of a time required from the start of one horizontal line scanning operation to detection of the electron beam, by the photosensor, in the first scanning direction, and a time required from the start of another one horizontal line scanning operation to detection of the electron beam, by the photosensor, in the second scanning direction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image display apparatus in which a 
photosensor is opposed to a screen scanned with an electron beam, and the 
position of a point on the screen opposed to the photosensor can be 
correctly detected. 
2. Description of Related Art 
A variety of entertainment games using a screen of an image display 
apparatus such as a television set have recently been developed. In 
playing some of these entertainment games, a player aims a photosensor at 
a moving object displayed on the screen and pushes or triggers a switch of 
a game machine when he believes that the photosensor is exactly opposed to 
the moving object. When the moving object is actually opposed to the 
photosensor, the displayed moving object is erased or a broken and 
scattered image is displayed instead of the original moving object. 
FIG. 1 is a block diagram showing the structure of a television set in 
which the position of a point on a screen is to be detected by a 
photosensor and is used for such a game. A video signal SV inputted 
through a video signal input terminal 1 is inputted to a signal processing 
circuit 2 and a synchronous separation circuit 3. An output signal from 
the signal processing circuit 2 is inputted to a CRT driving circuit 6 via 
a switching circuit 5. An output signal from the CRT driving circuit 6 is 
supplied to a CRT 7. 
A vertical synchronization signal VSYC outputted by the synchronous 
separation circuit 3 is inputted to a character generator 4, a vertical 
output circuit 12 and a reset terminal 15R of a vertical counter 15. An 
output signal from the vertical output circuit 12 is supplied to a 
vertical deflection coil 8 fixed to the CRT 7. A horizontal 
synchronization signal HSYC outputted by the synchronous separation 
circuit 3 is inputted to the character generator 4, a horizontal driving 
circuit 10, a reset terminal 14R of a horizontal counter 14 and a clock 
terminal 15C of the vertical counter 15. 
An output signal from the horizontal driving circuit 10 is inputted to a 
horizontal output circuit 11. An output signal from the horizontal output 
circuit 11 is supplied to a horizontal deflection coil 9 fixed to the CRT 
7. A raster detection signal SA outputted by a photosensor 16 which is to 
be opposed to the screen of the CRT 7 is inputted to a gate circuit 18. A 
switching signal SS generated by a manually operated trigger switch 17 is 
inputted to a control terminal 18CNT of the gate circuit 18 and a 
processor 27. 
A signal outputted by the gate circuit 18 is inputted to a hold terminal 
14H of the horizontal counter 14, a hold terminal 15H of the vertical 
counter 15 and the processor 27. A clock signal outputted by an 
oscillation circuit 13 is inputted to a clock terminal 14C of the 
horizontal counter 14. The count value HCT of the horizontal counter 14 
and the count value VCT of the vertical counter 15 are supplied to the 
processor 27. A character generator control signal SCC outputted by the 
processor 27 is inputted to the character generator 4. A raster signal SR 
outputted by the character generator 4 is inputted to the CRT driving 
circuit 6 via the switching circuit 5. A character position data CD 
outputted by the character generator 4 is inputted to the processor 27. 
The operation of such a television set will be described below. FIG. 2 
shows the relationship between a point on the screen indicated by the 
photosensor and the scanning lines. 
When the photosensor 16 is opposed to a point on the screen C as shown in 
FIG. 2 and the trigger switch 17 is turned on, a switching signal SS 
generated by the trigger switch 17 is inputted to the processor 27 and the 
control terminal 18CNT of the gate circuit 18. Then, the processor 27 
starts detecting the position of the point on the screen C opposed to the 
photosensor 16. 
First, the processor 27 inputs a character generator control signal SCC to 
the character generator 4 so as to allow the character generator 4 to 
output the raster signal SR for one field of a white raster. Then, the 
character generator 4 outputs a raster signal SR for displaying one field 
of a white raster to the switching circuit 5. When a horizontal 
synchronization signal HSYC is outputted by the synchronous separation 
circuit 3, the horizontal output circuit 11 supplies an output signal to 
the horizontal deflection coil 9 in response to an output signal from the 
horizontal driving circuit 10. When a vertical synchronization signal VSYC 
is outputted by the synchronous separation circuit 3, the vertical output 
circuit 12 supplies an output signal to the vertical deflection coil 8. 
During the output of the raster signal SR, an alternative terminal of the 
switching circuit 5 connects to the character generator 4 so as to supply 
the raster signal SR to the CRT 7 via the CRT driving circuit 6. As a 
result, scanning is performed with an electron beam from the upper portion 
of the screen, thereby displaying one field of a white raster on the 
screen of the CRT 7. In the duration from the input of the switching 
signal SS to the control terminal 18CNT of the gate circuit 18 to the 
extinguish of the raster signal SR, the gate circuit 18 is on. Therefore, 
a raster detection signal SA outputted by the photosensor 16 can be 
inputted to the horizontal counter 14, the vertical counter 15 and the 
processor 27. 
Until the electron beam on the screen opposed to the photosensor 16 is 
detected by the photosensor 16, the horizontal counter 14 counts clock 
signals inputted by the oscillation circuit 13. The count value of the 
horizontal counter 14 is reset every time a horizontal synchronization 
signal HSYC is inputted to the reset terminal 14R of the horizontal 
counter 14. The horizontal synchronization signal HSYC is inputted to the 
clock terminal 15C of the vertical counter 15. The vertical counter 15 
counts horizontal synchronization signals HSYC, that is, the number of the 
scanning lines, until the electron beam at the point on the screen C 
opposed to the photosensor 16 is detected. 
When the electron beam at the point on the screen C opposed to the 
photosensor 16 is detected, the photosensor 16 outputs a raster detection 
signal SA. The raster detection signal SA is inputted to the hold 
terminals 14H of the horizontal counter 14 and the hold terminal 15H of 
the vertical counter 15 via the gate circuit 18. Thereby the count values 
HCT of the horizontal counter 14 and the count value VCT of the vertical 
counter 15 are held. By these held count values HCT and VCT, the number of 
the scanning lines between the uppermost scanning line L1 and the scanning 
line Ln when the raster detection signal SA is outputted, and a term tHF 
from the start of the scanning of the scanning line Ln to the output of 
the raster detection signal SA is obtained. 
Then, the processor 27 reads out the count values HCT of the horizontal 
counter 14 and the count value VCT of the vertical counter 15, and 
calculates the vertical and the horizontal direction positions based on 
the read count values HCT and VCT, thereby the position of the point on 
the screen C opposed to the photosensor 16 is detected. 
In the conventional television set having the aforementioned structure, the 
position of a point on the screen opposed to the photosensor 16 is 
detected based on the one-direction electron beam scanning conducted from 
the upper portion to the lower portion of the screen in order. Therefore, 
when the spot of the electron beam has a shape of, for example, a circle 
having a certain area as shown with a broken line in FIG. 2, the 
photosensor 16 detects the electron beam before the electron beam reaches 
the position on the screen C opposed to the photosensor 16. In other 
words, when the peripheral part of the circular spot coincides with the 
point on the screen C, the photosensor 16 detects a point on the screen A, 
which is the center of the circular spot, as if it were the point on the 
screen C. Therefore, the detection accuracy disadvantageously varies 
depending upon a variation of the spot shape of the electron beam and the 
focus characteristics and the fluctuation in the spot diameter of the 
electron beam owing to the difference in the size of the CRT. 
SUMMARY OF THE INVENTION 
The present invention has been developed to solve the above-mentioned 
problems. One of the objectives of the invention is to provide an image 
display apparatus in which an accuracy in the detection of a point on a 
screen opposed to a photosensor does not vary depending upon a variation 
of the spot shape and the focus characteristics of an electron beam as 
well as the fluctuation in the spot diameter of the electron beam due to 
the difference in the size of a CRT. 
The image display apparatus of the invention comprises scanning direction 
switching means for switching the direction of the electron beam scanning 
between a first scanning direction, in which scanning is conducted from 
the upper portion to the lower portion of the screen in order, and a 
second scanning direction, in which scanning is conducted from the lower 
portion to the upper portion of the screen in order; and means for 
calculating the average of a value indicating the position of a first 
point on a screen detected by the scanning in the first scanning direction 
and a value indicating the position of a second point detected by the 
scanning in the second scanning direction. Therefore, when the first 
scanning direction is selected, scanning is performed with an electron 
beam from the upper portion to the lower portion of the screen to obtain 
the value indicating the position of the first point at which the 
photosensor detects the electron beam is obtained. When the second 
scanning direction is selected, scanning is performed with an electron 
beam from the lower portion to the upper portion of the screen, to obtain 
the value indicating the position of the first point at which the 
photosensor detects the electron beam is obtained. Then, the average of 
the values indicating the first point and the second point is calculated 
to obtain a value indicating the position of the point opposed to the 
photosensor. 
In this manner, the detection accuracy does not vary depending upon a 
variation of the spot shape and the focus characteristics of the electron 
beam and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described referring to the accompanying 
drawings. 
FIG. 3 is a block diagram showing the structure of an image display 
apparatus according to the present invention. A video signal SV inputted 
through a video signal input terminal 1 is inputted to a signal processing 
circuit 2 and a synchronous separation circuit 3. An output signal from 
the signal processing circuit 2 is inputted to a CRT driving circuit 6 via 
a switching circuit 5. An output signal from the CRT driving circuit 6 is 
supplied to a CRT 7. 
A vertical synchronization signal VSYC outputted by the synchronous 
separation circuit 3 is inputted to a character generator 4, a vertical 
output circuit 12 and a reset terminal 15R of a vertical counter 15. An 
output signal from the vertical output circuit 12 is supplied to a 
vertical deflection coil 8 fixed to the CRT 7 via a switching contact 30a 
of a change-over switch 30. The vertical deflection coil 8 is grounded via 
a switching contact 30b. The change-over switch 30 is constituted such 
that the switching contact 30b is connected by switching to one terminal 
(the other terminal) of the vertical deflection coil 8, when the switching 
contact 30a is connected by switching to the other terminal (one terminal) 
of the vertical deflection coil 8. 
A horizontal synchronization signal HSYC outputted by the synchronous 
separation circuit 3 is inputted to the character generator 4, a 
horizontal driving circuit 10, a reset terminal 14R of a horizontal 
counter 14 and a clock terminal 15C of the vertical counter 15. An output 
signal from the horizontal driving circuit 10 is inputted to a horizontal 
output circuit 11. An output signal from the horizontal output circuit 11 
is supplied to a horizontal deflection coil 9 fixed to the CRT 7 via a 
switching contact 29a of a change-over switch 29. The horizontal 
deflection coil 9 is grounded via a switching contact 29b. The change-over 
switch 29 is constituted such that the switching contact 29b is connected 
by switching to one terminal (the other terminal) of the vertical 
deflection coil 9, when the switching contact 29a is connected by 
switching to the other terminal (one terminal) of the vertical deflection 
coil 9. Semiconductor switches are practically used as the change-over 
switches 29 and 30 to perform the above-mentioned switching operation. 
A raster detection signal SA outputted by a photosensor 16 opposed to the 
screen of the CRT 7 is inputted to a gate circuit 18. A switching signal 
SS generated by a trigger switch 17 is inputted to a control terminal 
18CNT of the gate circuit 18 and a processor 27. A signal outputted by the 
gate circuit 18 is inputted to a hold terminal 14H of the horizontal 
counter 14, a hold terminal 15H of the vertical counter 15 and the 
processor 27. 
A clock signal outputted by an oscillation circuit 13 is inputted to a 
clock terminal 14C of the horizontal counter 14. A count value HCT of the 
horizontal counter 14 and a count value VCT of the vertical counter 15 are 
inputted to the processor 27. A character generator control signal SCC 
outputted by the processor 27 is inputted to the character generator 4. A 
raster signal SR outputted by the character generator 4 is inputted to the 
CRT driving circuit 6 via the switching circuit 5. A character position 
data CD outputted by the character generator 4 is inputted to the 
processor 27. 
A scanning direction switching signal SCH outputted by the processor 27 is 
supplied to the change-over switches 29 and 30. The processor 27 is 
connected to, for example, a power source for signals (not shown) via a 
switch SW. A field term when the character generator 4 outputs a raster 
signal SR can be appropriately determined by turning on/off the switch SW, 
for example, depending upon the frequency of turning on/off the switch SW. 
The operation of the image display apparatus having the above-mentioned 
structure will now be described. FIG. 4 is a flow chart showing a control 
operation of the processor 27. FIGS. 5 and 6 are timing charts of each 
signal. FIG. 7 shows the relationship between the position of a point on 
the screen to be detected and scanning lines. 
First, the image display apparatus is actuated. A case where the position 
of a point on the screen C as shown in FIG. 7 is to be detected is herein 
described as an example. An operator aims the photosensor 16 at the point 
on the screen C of the CRT 7 and turns on the trigger switch 17 (step S1). 
By turning on the trigger switch 17, a switching signal SS is inputted to 
the processor 27. The processor 27 starts controlling the first scanning 
operation (hereinafter referred to as the "forward scanning") for 
detecting the position of the point on the screen C opposed to the 
photosensor 16 (step S2). The processor 27 supplies the character 
generator 4 with a character generator control signal SCC for allowing the 
character generator 4 to output a white raster of, for example, one field. 
In receiving the character generator control signal SCC, the character 
generator 4 outputs to the switching circuit 5 a raster signal SR for 
imaging one field of a white raster. During the output of the raster 
signal SR, the switching circuit 5 connects by switching to the character 
generator 4. Thereby, the raster signal SR is supplied to the CRT 7 via 
the CRT driving circuit 6. During the output of the raster signal SR from 
the time of supplying of the switching signal SS to the control terminal 
18CNT of the gate circuit 18, the gate circuit 18 is on. A horizontal 
synchronization signal HSYC outputted by the synchronous separation 
circuit 3 is inputted to the horizontal output circuit 11 via the 
horizontal driving circuit 10. A vertical synchronization signal VSYC is 
inputted to the vertical output circuit 12. 
An output signal relating to the horizontal synchronization signal HSYC 
from the horizontal output circuit 11 is supplied to the horizontal 
deflection coil 9 via the change-over switch 29. When the vertical 
synchronization signal VSYC is outputted by the synchronous separation 
circuit 3, an output signal from the vertical output circuit 12 is 
supplied to the vertical deflection coil 8 via the change-over switch 30. 
As a result, an electron beam is scanned on the screen of the CRT 7 from 
the upper portion to the lower portion of the screen in order, thereby 
displaying a white raster. 
The horizontal counter 14 starts counting clock signals outputted by the 
oscillation circuit 13 at the time of the input of a horizontal 
synchronization signal HSYC. Every time the horizontal counter 14 receives 
a horizontal synchronization signal HSYC, the count value is reset. The 
vertical counter 15 starts counting horizontal synchronization signals 
HSYC at the time of the input of a vertical synchronization signal VSYC. 
When a white raster is displayed up to the vicinity of the point on the 
screen C opposed to the photosensor 16, the photosensor 16 detects the 
electron beam, so that the photosensor 16 outputs a raster detection 
signal SA. The raster detection signal SA is sent to the gate circuit 18 
to reach the hold terminals 14H of the horizontal counter 14 and the hold 
terminal 15H of the vertical counter 15, the count values HCT of the 
horizontal counter 14 and the count value VCT of the vertical counter 15 
are held (step S3). 
Thus, as shown in FIG. 5, the obtained count value VCT of the vertical 
counter 15 corresponds to a term tVF from the rise of the vertical 
synchronization signal VSYC to the rise of the raster detection signal SA, 
and the obtained count value HCT of the horizontal counter 14 corresponds 
to a term tHF in one scanning operation from the start of scanning of a 
scanning line Ln corresponding to the nth horizontal synchronization 
signal HSYC to the rise of the raster detection signal SA. From these held 
count values HCT and VCT, the position of a first point on the screen A on 
the scanning line Ln as shown in FIG. 7 is determined. Then, the processor 
27 reads out the count values of the horizontal counter 14 and the count 
value VCT of the vertical counter 15 (step S4). 
The processor 27 then outputs a scanning direction switching signal SCH to 
invert the scanning direction (step S5). Thus, the processor 27 starts 
controlling the second scanning operation (hereinafter referred to as the 
"reverse scanning") (step S6). When the scanning direction switching 
signal SCH is supplied to the change-over switches 29 and 30, the 
switching contacts 29a and 30a change the connections as shown with broken 
lines in FIG. 7, resulting in inverting the directions of supplying an 
output signal from the horizontal output circuit 11 to the horizontal 
deflection coil 9 and the direction of supplying an output signal from the 
vertical output circuit 12 to the vertical deflection coil 8. 
The processor 27 also outputs a character generator control signal SCC to 
the character generator 4, thereby allowing the character generator 4 to 
output one field of a raster signal SR as mentioned in the description of 
the forward scanning. Then, scanning is performed on the screen with an 
electron beam successively from the lower portion to the upper portion of 
the screen in order, thereby displaying a white raster. The gate circuit 
18 is on during the output of the raster signal SR. The positions of the 
scanning lines in the white raster are the same as those in the forward 
scanning. Also as described above, the horizontal counter 14 starts 
counting clock signals outputted by the oscillation circuit 13 at the time 
of the input of a horizontal synchronization signal HSYC, and resets the 
count value HCT every time a horizontal synchronization signal HSYC is 
inputted. 
Also, the vertical counter 15 starts counting horizontal synchronization 
signals HSYC at the time of the input of a vertical synchronization signal 
VSYC. When, a raster is displayed up to the vicinity of the point on the 
screen C opposed to the photosensor 16, the photosensor 16 detects the 
electron beam, so that the photosensor 16 outputs a raster detection 
signal SA. The raster detection signal SA is sent to the gate circuit 18 
to reach the hold terminals 14H of the horizontal counter 14 and the hold 
terminal 15H of the vertical counter 15, the count values HCT of the 
horizontal counter 14 and the count value VCT of the vertical counter 15 
are held (step 7). As shown in FIG. 6, the obtained count value VCT of the 
vertical counter 15 corresponds to a term tVR from the rise of the 
vertical synchronization signal VSYC to the rise of the Paster detection 
signal SA. The obtained count value HCT of the horizontal counter 14 
corresponds to a term tHR in one scanning operation from the start of 
scanning of a scanning line Lm corresponding to the mth horizontal 
synchronization signal HSYC to the rise of the raster detection signal SA. 
From the held count values HCT and VCT, the position of a second point on 
the screen B on the scanning line Lm as shown in FIG. 7 is determined. 
Then, the processor 27 reads out the count values HCT of the horizontal 
counters 14 and the count value VCT of the vertical counter 15 (step S8). 
The processor 27 then performs calculations by using the following 
equations (step S9). Thereby a term tHA is obtained, required from the 
start of one scanning operation to reach the point on the screen C and the 
number of scanning lines 1 between the first scanning line L1 and the 
point on the screen C: 
##EQU1## 
In the above equations, tH indicates a horizontal scanning term; tHF 
indicates a term from the rise of a horizontal synchronization signal to 
the rise of a raster detection signal in the forward scanning; tHR 
indicates a term from the rise of a horizontal synchronization signal to 
the rise of a raster detection signal in the reverse scanning; N indicates 
the total number of the scanning lines between the upper portion to the 
lower portion on the screen; n indicates the number of scanning lines 
scanned before the output of the raster detection signal in the forward 
scanning; and m indicates the number of scanning lines scanned before the 
output of the raster detection signal in the reverse scanning. 
Then, the position of the point on the screen C is determined based on the 
calculated term tHA and the number of the scanning lines 1. Thus, the 
processor 27 thereby finishes controlling the one-field scanning (step 
10). In this manner, the operation for detecting the position of the point 
on the screen C is finished. 
As described above, the position of a point on the screen opposed to the 
photosensor is detected based on the average of the values indicating the 
position on the screen opposed to the photosensor when detecting the 
electron beam scanning in the forward direction (the first scanning 
direction), and the value indicating the position on the screen opposed to 
the photosensor when detecting the electron beam scanning in the reversed 
direction (the second scanning direction). When the electron beam has a 
spot in the shape of, for example, a circle as shown with a broken line in 
FIG. 7, the photosensor detects the electron beam before it reaches the 
point C in both the forward and the reverse scannings, so that the 
positions of the points on the screen A and B, which are equally spaced 
from the point C, are detected. Then, by calculating the average of the 
values indicating the positions of the points on the screen A and B as 
described above, the position of the point on the screen C opposed to the 
photosensor 16, which is located between the points A and B, can be 
correctly detected. 
Therefore, in this invention, the accuracy in detecting the position of a 
point on the screen does not vary depending upon the variation of the 
shape of the spot and the focus characteristics of an electron beam, or, 
the fluctuation in the spot diameter of the electron beam due to the 
difference in the size of the CRT. 
In the above-mentioned embodiment, the raster with one field term both in 
the forward and the reverse scannings is used. The accuracy in the 
detection, however, can be further improved when the rasters with a 
plurality of fields by adjusting the number of the fields by using the 
switch SW is used. 
Also in the embodiment, scanning is performed with an electron beam 
successively from the upper left to the lower right of the screen in the 
forward scanning and from the lower right to the upper left of the screen 
in the reverse scanning. Of course, the same effect can be obtained even 
when an electron beam is scanned from the upper right to the lower left in 
the forward scanning and from the lower left to the upper right in the 
reverse scanning. 
Further, in the embodiment, both of the average of the number of scanning 
lines in the forward scanning and that in the reverse scanning, and the 
average of the term required from the start of one scanning to the output 
of a raster detection signal in the forward scanning and that in the 
reverse scanning are used. However, it is possible to shorten the 
detection time by using either of the averages. 
As this invention may be embodied in several forms without departing from 
the spirit of essential characteristics thereof the present embodiment is 
therefore illustrative and not restrictive, since the scope of the 
invention is defined by the appended claims rather than by the description 
preceding them, and all changes that fall within metes and bounds of the 
claims, or equivalence of such metes and bounds thereof are therefore 
intended to be embraced by the claims.