Abstract:
An electronic viewfinder comprises a plurality of picture elements which form a matrix image display device. A drive circuit is connected to the display device to cause the electronic viewfinder to selectively display a portion of the image, corresponding to the video signal being received by the drive circuit, at an increased magnification. The drive circuit includes frequency divider circuits for providing shift clock pulse signals at frequencies corresponding to the available rates of magnification of the image portion. The drive circuit further includes delay circuits for delaying synchronizing signals to select the portion of the image which is to be displayed. The electronic viewfinder having this drive circuit, is capable of providing increased resolution for a portion of an image by enlarging the image, while allowing the electronic viewfinder to be formed by a relatively small number of picture elements, so that the electronic viewfinder may be miniaturized and manufactured at a reduced cost.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a viewfinder for a VTR (video tape recording) camera or the like which uses a dot matrix image display device, in which picture elements are arranged in matrix form. 
     The resolution, which determines the picture quality of a matrix image display device, depends on the number of picture elements which are arranged in rows and columns,(i.e. in matrix form), in the device. For example, in order to display a television image, a matrix of approximately 250 to 350 rows by 250 to 350 columns is required. However, there has been a tendency to reduce the number of picture elements in the display device of electronic viewfinders, in an effort to reduce manufacturing cost and to miniaturize the electronic viewfinder, thereby leading to a decrease in resolution. Despite this decrease in resolution, when a matrix image display device is applied to the electronic viewfinder of a VTR camera, the angle of view (i.e., the composition of the picture) can be determined. However, because of the low resolution, the focusing of the lens on an object; which is one of the essential functions of the electronic viewfinder, cannot be achieved readily. 
     SUMMARY OF THE INVENTION 
     It is a principal object of the present invention to provide an electronic viewfinder which overcomes the deficiencies of the prior art. 
     A further object of the present invention is to provide an electronic viewfinder, including a matrix image display device having a resolution which is lower than the inherent resolution of a display signal, which displays any desired portion of the image with increased magnification. 
     A still further object of the present invention is to provide an electronic viewfinder, including a matrix image display device having a small number of picture elements, which displays any desired portion of the image with increased magnification. 
     A still further object of the present invention is to provide an electronic viewfinder which increases the resolution of the image displayed by it&#39;s matrix image display device by enlarging the image, thereby allowing a VTR camera lens or the like to focus on an object. 
     The present invention is directed to an electronic viewfinder connected to receive a video composite signal. The electronic viewfinder includes first means for displaying an image and second means, connected to the first means and connected to receive the video composite signal, for providing a video signal to the first means. The second means includes a switch for providing first and second signals so as to cause the first means to provided a standard display or a magnified display. When the standard display is provided, the entire image is displayed so that the angle of view may be set. When the magnified display is provided, a portion of the image is displayed in an increased size so that focusing can be conducted by increasing the resolution of a portion of the image. 
     These together with other objects and advantages, which will become subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an equivalent circuit diagram of a matrix image display device used in an embodiment of the present invention; 
     FIG. 2 is an equivalent circuit diagram of a first example of the picture element P ij  of FIG. 1; 
     FIG. 3 is an equivalent circuit diagram of a second example of the picture element P ij  of FIG. 1; 
     FIG. 4 is an equivalent circuit diagram of a third example of the picture element P ij  of FIG. 1; 
     FIG. 5 is a block diagram of an embodiment of the drive control circuit, connected to the image display device of FIG. 1, for the electronic viewfinder of the present invention; 
     FIG. 6 is a schematic diagram illustrating the enlargement of a portion of an image in accordance with the present invention; and 
     FIG. 7 is a schematic diagram illustrating the enlargement of another portion of an image in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is an equivalent circuit diagram of a dot matrix image display device having picture elements arranged in a matrix of m rows and n columns. In FIG. 1, reference characters DX 1 , . . . DX i  . . . , and DX m  designate row scanning X shift registers; reference characters DY 1 , . . . DY j . . . , and DY n  designate column scanning Y shift registers; reference characters S 1 , . . . S i  . . . , and S m  designate X line selecting transistors; reference characters X 1 , . . . X i  . . . , and X m  designate X lines; Y 1 , . . . Y j  . . . , and Y n  designate Y lines; and reference characters P 11 , . . . P ij  . . . , and P mn  designate picture elements in matrix form. 
     Each picture element P ij  may be formed as shown in FIGS. 2, 3 or 4. FIGS. 2 and 3 include a selecting transistor 1, an image data storing capacitor 2 and a display element 3, such as a liquid crystal element, an EL (electroluminescent) element or a fluorescent element. In FIG. 3, the picture element further comprises a driving transistor 4. In FIG. 4, the picture element includes only the display element 3. In general, the picture elements of FIGS. 2 or 3 are so-called &#34;switch matrix type picture elements&#34; having an active switch, and the picture element of FIG. 4 is a so-called &#34;ordinary matrix type picture element&#34; having no active element. 
     The operation of the circuit of FIG. 1 will be described for the case of displaying a television image. When the Y 1  line is selected by a Y shift signal synchronous with a vertical synchronizing signal, and the X 1  line selecting transistor S 1  is rendered conductive (on) by an X shift signal synchronous with a horizontal synchronizing signal, the picture element P 11  is selected to emit light according to the amplitude of a video signal. When one X shift clock pulse is applied, the X 2  line selecting transistor S 2  is turned on, so that the picture element P 21  emits light in accordance with the level of the video signal at that time. Similarly, the remaining selecting transistors S 3  through S m  are scanned. Thus, the image data is written in the picture elements P 11  through P m1  connected to the Y 1  line. In the picture elements of FIGS. 2 or 3, the image data is stored in the capacitor 2, and therefore the emission of light is continued until the picture element is selected again in the next field. When one Y shift clock is applied and the X shift signal is supplied again, the picture element P 12  is selected. All the picture elements are scanned in this manner, so that one image field is written in the picture elements. 
     FIG. 5 is a block diagram of an embodiment of a drive control circuit, connected to the image display device of FIG. 1, for driving the image display device in accordance with the present invention. For convenience, the drive control circuit of FIG. 5 will be described for the case where an NTSC video signal is used to display an image without interlaced scanning. Furthermore, it will be assumed that in the image display device of FIG. 1, m (X lines)=96, and n (Y lines)=60 . 
     The drive circuit of FIG. 5 includes a synchronizing separator circuit 11 for obtaining a horizontal synchronizing signal, a vertical synchronizing signal and a video signal from an NTSC video composite signal. A Y delay circuit 12 delays the vertical synchronizing signal for a predetermined period of time, and an X delay circuit 13 delays the horizontal synchronizing signal for a predetermined period of time. The drive circuit further includes four 1/2 frequency dividers 14, 15, 16 and 17, and a high frequency oscillator 18 for oscillating in synchronization with the horizontal synchronizing signal. A pair of switches S 1  and S 2  are operated in tandem to select the shift clock pulse frequency, and the delay times (for controlling the shift signals) of the delay circuits 12 and 13 are set to predetermined values. When the switches S 1  and S 2  are operated and the delay times are set, a standard size image, an image portion which is enlarged by a magnification of ×2 (enlarged four times in area), or an image portion which is enlarged by a magnification of ×4 (enlarged sixteen times in area), can be obtained. An image display device 19, for example the display device of FIG. 1, is connected to the drive circuit. 
     An example of the relationship between magnification and clock frequency is set forth in Table 1 below. The portion of an image which is to be enlarged can be selected by varying the delay times. Table 1 lists the delay times for the case where a central portion of the image is enlarged (see FIG. 6). 
     
                       TABLE 1______________________________________    X shift          Y shift    clock   X delay  clock     Y delay    frequency            time     frequency time    (MHz)   (μs)  (KHz)     (ms)______________________________________Standard size      1.8       0        3.94    0×2 enlargement      3.6       13.3     7.88    4.17×4 enlargement      7.2       20.0     15.75   6.25______________________________________ 
    
     In order to display an image having the standard size, the X shift clock frequency is set to 1.8 MHz as illustrated in FIG. 5 and Table 1. As is well known in the display art, a horizontal period of 63.5 μs has an effective image data period of 53.3 μs. In displaying an image having the standard size, the video signal should be sampled equally for the number (96) of rows. Therefore, the X shift clock freqency fx is as follows: ##EQU1## On the other hand, the Y shift clock pulse should switch the selection of the sixty (60) Y lines with respect to 240 effective scanning lines (which are available when no interlaced scanning is carried out), so that the Y shift clock pulse selects one Y line per four effective scanning lines. Therefore, the Y shift clock frequency fY is as follows: ##EQU2## 
     As shown in FIG. 5, the oscillating signal (having a 7.2 MHz frequency) output by the high frequency oscillator 18 which oscillates in synchronization with the horizontal synchronizing signal, is frequency-divided by the 1/2 frequency dividers 16 and 17, to provide the X shift clock pulse having a frequency of 1.8 MHz. The Y shift clock pulse, with a frequency 3.94 KHz, can be obtained by directly frequency-dividing the 15.75 KHz horizontal synchronizing signal using the 1/2 frequency dividers 14 and 15. When an image having the standard size is to be displayed, the delay times of the Y delay circuit 12 and the X delay circuit 13 are set to zero. 
     When a portion of an image is to be displayed with a magnification of ×2 , both the X shift clock pulse and the Y shift clock pulse should have frequencies which are two times as high as the frequencies of the shift clock pulses used to display the standard size image. These doubled frequencies are obtained at the outputs of the 1/2 frequency dividers 14 and 16, respectively. In this case, if the X and Y shift signals are applied, without delay, based on the synchronizing signals, the upper left-hand portion of the image will be magnified. When it is required to enlarge the central portion A of the image (as illustrated in FIG. 6), the delay times of the X delay circuit 13 and the Y delay circuit 12 are determined as follows. The X delay circuit 13 should delay the horizontal synchronizing signal for one quarter (1/4) of the horizontal effective period (53.3 μs), so that the X delay time tdX is: 
     
         tdX=53.3×10.sup.-6 s×1/4≃13.3 μs (3) 
    
     Similarly, the Y delay circuit should delay the vertical synchronizing signal for one quarter (1/4) of one field time (16.67 ms), so that the Y delay time tdY is: 
     
         tdY=16.67×10.sup.-3 s×1/4≃4.17 ms (4) 
    
     The above determinations are also applicable to the case of displaying a portion of an image with a magnification of ×4, wherein the frequencies and delay times are as indicated in Table 1. When a ×4 magnification of an image is to be displayed, the switches S 1  and S 2  provide the horizontal synchronizing signal (15.75 KHz) and the output of the oscillator 18 (7.2 MHz) as the Y shift clock pulse signal and the X shift clock pulse signal, respectively. 
     FIG. 7 illustrates a portion of an image which is enlarged, the portion being selected by increasing the X and Y delay times. Further, any portion of the image can be selected for enlargement by changing the delay times of the Y delay circuit 12 and the X delay circuit 13. 
     In the above-described embodiment, the drive control circuit comprises digital delay circuits 12 and 13, and the digital frequency dividers 14, 15, 16 and 17. Therefore, the magnification of image portions is accurate in both the horizontal and vertical directions, so that the displayed images are stable. Further, any portion of the image can be enlarged merely by changing the delay times of the X and Y delay circuits 12 and 13. In addition, since the circuit for enlarging and displaying images has a relatively simple construction, it can be manufactured at low cost. 
     Although in the above-described embodiment two frequency divider stages are employed, the invention may alternatively include a single frequency divider stage or more than two frequency divider stages, so that the magnification can be set to a desired value or so that the number of possible magnifications can be increased. In the description of the invention set forth above with respect to FIG. 5, the matrix was described as having ninety rows and sixty columns; however, it should be understood that the technical concept of the invention is also applicable to image display devices having different numbers of rows and columns (e.g., a device having 192 rows and 120 columns). 
     As is apparent from the above description, the electronic viewfinder of the present invention employs a matrix display device with a small number of picture elements. Although it has a low resolution, it can set the angle of view (i.e., the composition of the picture), thereby contributing to a reduction in the manufacturing cost of the electronic viewfinder and to miniaturization of the same. However, the viewfinder of the invention can enlarge any portion of an image in the display. Therefore, with the electronic viewfinder of the present invention, the lens can be focused on the object and the resolution of a portion of the image can be increased. Thus, the electronic viewfinder of the present invention is most suitable for VTR cameras. 
     The many features and advantages of the invention are apparent from the detailed specification, and thus it is intended by the appended claims to cover all such features and advantages of the system which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described and, accordingly, all suitable modifications and equivalents may be restored to, falling within the scope of the invention.