Liquid crystal display device with display holding device

A liquid crystal display device is provided for receiving a video signal and displaying a visual image. The display device employs an active matrix liquid crystal display member having a plurality of display elements, and a plurality of switching elements, respectively. An image signal included in the video signal is applied to the display elements in accordance with a horizontal synchronous signal. The signal corresponding to the synchronous signal included in the video signal to be applied to the switching elements is inhibited for a predetermined period of time so that the image is kept displayed by the display elements for a certain period of time without the need to supply another video signal.

BACKGROUND OF THE INVENTION 
The present invention relates to an active matrix liquid crystal display 
(LCD), and more particularly to a video signal recording/regenerating 
device, such as an electronic still camera and the like, employing a 
liquid crystal display as a monitor. 
Recently, liquid crystal displays are being used as monitors in video 
signal recording/regenerating devices, making for a compact and relatively 
small power consumption device. Among the various types of liquid crystal 
displays, active matrix type displays having display elements and 
switching elements are becoming more and more prevalent, because displays 
employing other methods such as a direct multiplexing method are hardly 
capable of providing high-density display. In the active matrix type 
display, display elements are selectively driven by selectively turning ON 
corresponding switching elements. 
FIG. 1 is a circuit diagram illustrating an example of a conventional thin 
film transistor (TFT) active matrix LCD. 
As shown in FIG. 1, a gate drive circuit 1 supplies switching pulses to a 
predetermined number of gate buses 2, whereas a source drive circuit 3 
applies video signals to a predetermined number of corresponding source 
buses 4. 
At each position of the matrix a thin film transistor (TFT) 5 functions as 
a switching element. A capacitor 6 accumulates the signal connected to 
each drain of the TFT's 5, and a display element 7, having transparent 
electrodes and liquid crystal layers respectively, are disposed at each 
position. 
The operation will now be described with reference to a timing chart of 
FIG. 2. 
The gate drive circuit 1 supplies switching pulses to the corresponding 
gate buses 2 being synchronized with horizontal synchronous signals 
included in the video signals. 
Suppose a switching pulse (FIG. 2(a)) is supplied to a gate bus 2 of the 
i-th line with the predetermined timing synchronized with the horizontal 
synchronous signal, and a switching pulse (FIG. 2(b)) is supplied to the 
gate bus 2 of the (i+1)-th line as with the timing of the horizontal 
synchronous signal approximately one field later in the case of interlaced 
scanning on one field basis. Note that in the case of line sequential 
scanning, the interval between adjacent switching pulses of the ith and 
the (i+1)th gate buses respectively equals one horizontal scanning period. 
The TFT's 5 connected to the gate bus 2 to which the switching pulse has 
been supplied are thus turned ON. 
The source drive circuit 3 divides the received video signal into a 
multiplicity signals corresponding to a number of horizontal scanning 
lines these signals are synchronized with the horizontal synchronous 
signal included in the video signal. These video signals are accumulated 
in each of the capacitors 6 via the source and drain of the each TFT 5 and 
delivered to each display element 7 for display. 
The signals accumulated in the capacitors 6 are held until a switching 
pulse is again supplied, after a lapse of time corresponding to the period 
of one frame. It should be noted that if the capacity of the liquid 
crystal and the resistance ratio R.sub.off /R.sub.on (wherein R.sub.off is 
the internal resistance of the TFT 5 when it is in an OFF condition and 
R.sub.on is the internal resistance when it is in an ON condition) of the 
TFT 5 are sufficiently larger, the capacitor 6 can be omitted. 
When the switching pulse is supplied to the gate bus 2 of the i-th line, 
the video signal (FIG. 2(c)) applied to the source bus 4 of the j-th 
column, is sampled and held in capacitor 6 as shown in FIG. 2(d). 
If the potential shown in FIG. 2(e) has been supplied to the reference 
terminal of the capacitor 6, the voltage shown in FIG. 2(f) is applied to 
the display element 7. 
It should be noted that since the charges accumulated by the capacitors 6 
are in accordance with the preceding input image signal (FIG. 2(f)), the 
succeeding image signal is applied to each display elements 7 in reversed 
polarity. That is, the image signal is applied to each source bus 4 
through the source drive circuit 3 as an alternating current. An interval 
between the image signals applied to each of display elements 7 
corresponds to the period for one frame in an interlaced scanning method; 
or one field in a line sequential scanning method. 
In this way, a video signal corresponding to each pixel is supplied so that 
a whole image is displayed. 
In the case of an electronic still camera or the like, for instance, such a 
liquid crystal display can be used to confirm an image to be photographed 
to view a recording of a video signal of the image on a magnetic disk. 
Moreover, the liquid crystal display can be used to monitor the image by 
regenerating the video signal from the magnetic disk. 
As set forth above, when a video signal is supplied to a conventional 
liquid crystal display, the display functions to show an image for only a 
period of time that corresponds to one frame. This permits the display to 
indicate multiple frames, thus simulating animation (or motion) of an 
image. Accordingly, in order for a conventional liquid crystal display 
device to display a still image, a frame member (or field memory) has to 
be used to store a video signal equivalent to one frame (or field) and 
then apply the video signal to the liquid crystal display for display by 
repeatedly reading the stored signals or repeatedly regenerating the 
signals recorded on the magnetic disk. 
As a result, expensive memory is required to regenerate a still image and 
the disadvantage is that the generation of the still image tends to become 
costly. 
To obtain the still image by regenerating the video signal from the 
magnetic disk is also disadvantageous in that a battery, a magnetic head 
or disk, or the like is quickly consumed. 
SUMMARY OF THE INVENTION 
To overcome the above disadvantages, it is therefore an object of the 
present invention to provide a low cost, simple construction liquid 
crystal display that is capable of displaying a still image. 
Another object of the present invention is to provide a video signal 
processing device that is capable of confirming a recording image 
immediately after it has been recorded, and monitoring the recorded image 
by regenerating the recorded video signal while minimizing the power 
consumption of the device. 
According to one aspect of the invention, there is provided a liquid 
crystal display device that receives a video signal and displays a visual 
image, comprising: 
an active matrix liquid crystal display member having a plurality of 
display elements with a plurality of switching elements, respectively; 
means for supplying drive signals to the display elements through 
respective switching elements based upon the video signal; 
control means for keeping the switching elements in an OFF condition for a 
predetermined period of time so that the supply of the drive signal to the 
display elements is interrupted, whereby a still image is displayed for 
the predetermined period of time without applying another video signal to 
the display member. 
According to another aspect of the invention, there is provided a video 
signal processing device, comprising: 
means for photographing an object and converting the image of the object 
into a video signal having an image signal, a horizontal synchronous 
signal, and a vertical synchronous signal; 
an active matrix liquid crystal display member having a plurality of 
display elements with a plurality of switching elements, respectively; 
means for supplying drive signals to the display elements through the 
respective switching elements based upon video signal; 
control means for keeping the switching elements in an OFF condition for a 
predetermined period of time so that the supply of the drive signal to the 
display elements is interrupted, whereby the visual still image is 
displayed for the predetermined period of time without applying another 
video signal to the display member.

DESCRIPTION OF AN EMBODIMENT 
FIG. 3 is a block diagram illustrating the configuration of a liquid 
crystal display embodying the present invention. 
In FIG. 3, a gate driver 11 supplies switching pulses to a predetermined 
number of gate buses 12, while a source driver 13 supplies video signals 
to a predetermined number of source buses 14. A TFT active matrix LCD 15 
is driven by the gate driver 11 and the source driver 13. 
The aforementioned arrangement of the TFT active matrix liquid crystal 
display 15 is similar to what is shown in FIG. 1. 
An LCD circuit 19 operates as follows. 
Upon receiving the video signal, a source drive signal generating circuit 
16 generates a source drive signal and supplies the signal to the source 
driver 13. Upon receiving the video signal, a gate drive signal generating 
circuit 17 generates a gate drive signal. A still image control circuit 18 
causes the gate drive signal to be intermittently supplied from the gate 
drive signal generating circuit 17 to the gate driver 11. 
A system control circuit 20 controls the operation of the LCD circuit 19, 
which will be described with reference to a timing chart shown in FIGS. 
4(a) -4(f). 
Upon receiving the video signal, the source drive signal generating circuit 
16 generates the source drive signal and applies it to the source driver 
13. The source driver 13 applies the source drive signal (including an 
image signal) via the corresponding source bus 14 to the source of the TFT 
5 of the TFT active matrix LCD 15. 
On the other hand, the gate drive signal generating circuit 17 generates 
the gate drive signal synchronously with the horizontal synchronous signal 
included in the video signal. The gate drive signal is applied via the 
still image control circuit 18 to the gate driver 11. The gate driver 11 
supplies the switching pulse to each gate bus 12 in accordance with the 
gate drive signal. The switching pulse is supplied to the gate of the TFT 
5 to turn ON and OFF the TFT 5. 
In this way, an image corresponding to the video signal is displayed on the 
TFT active matrix LCD 15. 
When a still image control signal is applied by the system control circuit 
20, the still image control circuit 18 stops supplying the gate drive 
signal from the gate drive signal generating circuit 17 to the gate driver 
11. 
As shown in FIGS. 4(a) and 4(b), the switching pulses supplied to the gate 
buses 12 of the i-th and (i+2)-th lines (i-th and (i+1)-th lines in the 
case of line sequential scanning) are inhibited from being supplied after 
the generation of the still image control signal. 
When the video signal, shown in FIG. 4(c), has been applied to the source 
bus 14 of the j-th row, the voltage sampled and held in the TFT 5 and 
capacitor 6 that has been electrically connected to the source bus 14 of 
the j-th row is kept held thereafter, as shown in FIG. 4(d). 
While the signal shown in FIG. 4(e) is being supplied to a reference 
terminal of the capacitor 6, the signal shown in FIG. 4(f) is applied to 
the corresponding display element 7. As no switching pulse is supplied to 
the gate bus 12, the voltage is kept constant. A still image is thereby 
displayed on the TFT active matrix LCD 15. 
The supply of the switching pulse from the gate drive signal generating 
circuit 17 to the gate drive 11 is inhibited for a certain period of time 
(e.g., several seconds) which is sufficient to confirm the still image. 
Actually, the charge upon capacitor 6 is spontaneously discharged via the 
OFF resistance of the TFT 5 with the holding of the image on the liquid 
crystal displaying element 7. Experiments show that a still image becomes 
obtainable for approximately 10 seconds by interrupting the supply of the 
switching pulse. 
FIG. 5 is a block diagram illustrating an arrangement of an electric still 
camera employing a liquid crystal display device embodying the present 
invention. The elements similar to those in FIG. 1 are given the same 
reference characters as in FIG. 1. 
As shown in FIG. 5, when an object (not shown) is photographed, a 
photographic lens 31 causes light deriving from the object to be incident 
on a CCD 32. A CCD driver 33 electrically drives the CCD 32. A process 
circuit 34 processes the output of the CCD 32 and supplies the processed 
video signal to an LCD circuit 19 and a recording/regenerating circuit 35, 
based upon a process control signal. In this case, a switching circuit 40 
is switched so that the process circuit 34 and the LCD circuit 19 are 
electrically connected upon a selection signal from the system control 
circuit 20. 
A magnetic head 36 records the video signal on a recording medium such as a 
magnetic disk 37, that's rotated by a spindle motor 38. An operating 
switch 39, such as a release switch is operated when an object is 
photographed. 
On the other hand, when the video signal is regenerated from the magnetic 
disk 37, the system control circuit 20 outputs a selection signal to the 
switching circuit 40 so that the output of the recording/regenerating 
circuit 35 is supplied to the LCD circuit 19. In this case, the magnetic 
head 36 regenerates the video signal from the magnetic disk 37, and the 
recording/regenerating circuit 35 transmits the regenerated video signal 
to the LCD circuit 19 in accordance with the recording/regenerating signal 
from the system control circuit 20. 
The operation of the circuit in FIG. 5 will now be described with reference 
to FIGS. 6(a)-6(f). 
While the photographic lens 31 causes an image of the photographing object 
to be formed on the CCD 32, the CCD 32 is electrically driven by the CCD 
driver 33. The system control circuit 20 receives a CCD drive signal from 
the CCD driver 33 and applies a photoelectric conversion signal to the 
process circuit 34. The process circuit 34 processes the received signal 
and converts it into a video signal having a certain format, such as an 
NTSC (National Television System Committee) format. The video signal is 
applied to the LCD circuit 19 and displayed, as stated above. 
When the operating switch 39 is operated at a certain time while the image 
is being monitored by the LCD circuit 19, the system control circuit 20 
supplies a recording/regenerating control signal to the 
recording/regenerating circuit 35. At this time, the 
recording/regenerating circuit 35 generates a recording pulse, shown in 
FIG. 6(a) for a period corresponding to one field, frequency modulates 
(FM) the video signal supplied from the process circuit 35 during that 
period and outputs the FM signal onto the magnetic head 36. The video 
signal corresponding to one field (one frame) is accordingly recorded on 
one track of the magnetic disk 37, which is rotated by the spindle motor 
38 at a speed of 3,600 rpm. 
In the case of regenerating the video signal from the magnetic disk 37, the 
recording/regenerating circuit 35 transmits the regenerated video signal 
to the LCD circuit 19. 
Further, the system control circuit 20 supplies the still image control 
signal, shown in FIG. 6(b), the LCD circuit 19 synchronously with the 
recording/regenerating control signal. 
In the LCD circuit 19, the generation of the switching pulse is 
interrupted, i.e., the supply of the gate drive pulse is interrupted (in 
other words, the TFT 5 switching elements are kept in the OFF condition, 
as shown in FIG. 6(c), while the still image control signal is being 
supplied, as shown in FIG. 6(b). The video signal supplied to the source 
bus 14 of the j-th row, as shown in FIG. 6(d), is sampled and held in the 
field immediately after the operating switch 39 is operated, as shown in 
FIG. 6(e). FIG. 6(f) shows the potential of the capacitor reference 
electrode. 
This holding condition is continued until the still image control signal is 
released. As a result, the photographed image is displayed on the LCD 
circuit 19 as a still image during that time, whereby the user is able to 
confirm the image thus photographed without any additional operations. 
Note that, from the point in time when the still image control signal has 
become OFF till the next switching pulse is supplied to the gate buses 12, 
no video signal is sampled and capacitors 6 keep holding the accumulated 
charge (which is discharged spontaneously). 
The present invention is applicable to not only interlaced scanning, as 
above, but also to line sequential scanning. 
As set forth above, the supply of the switching pulse to the active matrix 
LCD is inhibited for a predetermined time, and a still image is obtainable 
using a simple construction that is low in cost and does not require using 
a field memory, frame memory or the like. 
As the supply of the switching pulse is inhibited for a predetermined 
period of time during which the recording operation is performed to have a 
still image displayed on the liquid crystal display, the image that is 
actually recorded can be monitored without an additional operation. 
Moreover, an advantages is obtained from the present invention if it is 
applied to a portable video signal recording/regenerating device that uses 
batteries, because not only is wear of the magnetic disk, head or the like 
reduced, but also the electrical power consumption of the device is 
minimized.