Display control apparatus

Abstract: A display control apparatus permits the display by a single display control apparatus on a plurality of display devices which receive image data by an internal synchronization signal. The display control apparatus which outputs input image data to a plurality of display devices comprises a unit for selecting a longest one of the internal synchronization signals from the display devices and a unit for supplying the image data to the display devices in synchronism with the selected synchronization signal.

Description: The present invention concerns an image display system having a plurality of image display devices, an image data source and a display control apparatus for distributing image data from the image data source to each of the display devices. It also concerns a display control apparatus for use in such a system, as also system sub-combinations of the control apparatus and either the data source or the plurality of display devices. A CRT (cathode ray tube) has been used as a display device for a personal computer (hereinafter referred to as PC) and a work station (hereinafter referred to as WS). However, recently, a liquid crystal display device of a TN (twisted nematic) structure or an STN (super twisted nematic) structure has been used in a lap-top PC because of its superiority in light weight and thinness. Further, an FLC (ferroelectric liquid crystal) has many features of display element such as bistable memory characteristic and fast response. A display device which uses such a liquid crystal attains a high resolution by the memory characteristics of the element and it has been attracting a notice as a liquid display device capable of displaying in a large screen because of a simple electrode process of a matrix structure. In order to take synchronisation in supplying image data from a display control apparatus to the display device, an external synchronization method or an internal synchronization method is used depending on a characteristic of the display device. In a liquid crystal display which uses liquid crystal which responds to an effective value of an applied voltage, synchronization means for a CRT signal is provided in a display controller to share an interface of the CRT with the display controller to permit the display by the external synchronization method. A display device having a non-linear active device provided for each pixel to synchronize with the CRT signal has also been known although it accompanies difficulty in the manufacturing process. On the other hand, a liquid crystal display device which receives image data by the internal synchronization in order to use the property of a material to a maximum extent has been known. For example, in a ferroelectric liquid crystal, a displayable horizontal scan time varies with a temperature as shown in Fig. 3. Because of such a temperature characteristic of the material, in the display device which uses the ferroelectric liquid crystal, a display temperature of glass plates which hold the liquid crystal therebetween is detected and a period of reception of the image data is controlled by the internal synchronization. The liquid crystal display device has means for setting a horizontal display timing or a liquid crystal drive voltage by using an external temperature as a parameter. A horizontal display timing for the liquid crystal response speed is supplied to the display control apparatus as a reception synchronization signal of the image data. In demonstration or a reliability evaluation test of the display device, it may be required to simultaneously display on a plurality of display devices. In such a case, if the CRT display device which receives the image data by the external synchronization, the display is attained by arranging a distributor which causes the matching of the input impedance of the display device between the display device and the display control apparatus. In the liquid crystal display device which receives the image data by the external synchronization, the display is attained by arranging a distributor having a current buffer between the display device and the display control apparatus. This distribution method permits the activation of a plurality of display devices by using one host computer having the display device. However, in the liquid crystal display device which displays by the internal synchronization, the same number of host computers 10 to 15 as that of display devices 55 to 60 are used in the prior art system as shown in Fig. 7. In the demonstration or the reliability evaluation test of the display device, the environment parameters of the display device are stable in many cases but some variation of the environment parameters is unavoidable. Accordingly, it is difficult to drive a plurality of display devices by one host computer because of difference in the synchronization period of the display device which varied from environment parameter to environment parameter. Fig. 4 shows a prior art display system for displaying on one ferroelectric liquid crystal display device 20 by one host computer 30. In a flow of image data, a work station 40 first writes update image data to a video memory provided in a display control apparatus 32 upon request of draw primitive. Then, the display control apparatus 32 is supplied with a horizontal synchronization signal "a" shown in Fig. 5 from a display controller 21 built in the display device 20, and supplies image data d (a scan address e and image data f) to the display controller 21 at a timing shown in Fig. 5 in accordance with an image data transfer clock c. A horizontal scan period h of the horizontal synchronization signal a is determined by a temperature sensor 22 mounted on the glass plates which hold the liquid crystal element therebetween. The scan line address e and the image data f are discriminated by an address/data identification signal b. A difference between the image data and the horizontal scan period h is adjusted by a timing g. The image data f transferred to the display controller 21 is divided into odd and even pixel data which are supplied to segment drivers 23 and 24. Then, the segment drivers 23 and 24 apply appropriate voltages to segment electrodes of matrix structure electrodes of a liquid crystal panel 26. This voltage causes a potential difference from a voltage applied to a common electrode from a common driver 25 to drive the liquid crystal element. In this manner, the display by the display device 20 which displays by the internal synchronization is attained. Accordingly, for the simultaneous display by a plurality of display devices, a plurality of display control apparatus 32 for synchronously supplying image data and a plurality of work stations 40 are used for the horizontal synchronization signals 32 of different periods. Since the ferroelectric liquid crystal has the property of the response time to the temperature as shown in Fig. 3, a method has been known in which the horizontal display timing corresponding to the response time of the liquid crystal is supplied to the display control apparatus using the temperature as a parameter, as a synchronization signal of the image data transfer. In this display method, the synchronization period for the image data transfer from the display device to the display control apparatus is controlled in accordance with the environment temperature. In this case, if the supply period for one horizontal scan of image data supplied from the display control apparatus to the display device is longer than the period of the synchronization signal supplied from the display device, a loss of the image data occurs in the received data and a predetermined image is not displayed. On the other hand, if the supply period of one horizontal line scan of image data supplied from the display control apparatus to the display device is shorter than the period of the synchronization signal supplied from the display device, a drive voltage in a period other than the draw period on the liquid crystal may be set to a common voltage to improve contrast to some extent. Accordingly, the supply period of one horizontal line scan of image data supplied from the display control apparatus to the display device must not exceed the period of the synchronization signal supplied from the display device. It is an object of the present invention to attain the simultaneous display by a plurality of display devices operated by the internal synchronization, by one display control apparatus without trouble under a substantially stable environment. In accordance with one aspect of the present invention there is provided a display system in accordance with claim 1. An embodiment of the present invention aims to enable the simultaneous display by a plurality of display devices operated by the internal synchronization, by one display control apparatus without trouble under a substantially stable environment. In the accompanying drawings: Fig. 1 shows a block diagram of a configuration of a display system including a display control apparatus and a display device in accordance with one embodiment of the present invention; Fig. 2 shows a block diagram of a configuration of the display control apparatus of Fig. 1 and signals between the display control apparatus and the display device; Fig. 3 shows a graph of a temperature characteristic of a display device using a ferroelectric liquid crystal; Fig. 4 shows a block diagram of a configuration of a prior art display system and a configuration of the display device of Fig. 1; Fig. 5 shows a timing chart of signals in the apparatus and devices of Figs. 1 and 2; Fig. 6 shows a timing chart of drive waveforms to be supplied to a segment driver and a common driver by a display controller in the display devices of Figs. 1 and 2; and Fig. 7 shows a system configuration for displaying on a plurality of display devices by using a prior art display control apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is explained with reference to the drawings. Fig. 1 shows a block diagram of a configuration of a display system using a display control apparatus in accordance with one embodiment of the present invention. The system comprises a computer 40 such as a work station, a display control apparatus 50 and ferroelectric liquid crystal display devices 55 to 60. The display control apparatus 50 is provided on a mother board or an expansion bus of the computer 40. The ferroelectric liquid crystal display devices 55 to 60 are connected with the display control apparatus 50 by a plurality of wires. Fig. 2 shows a block diagram of a configuration of the display control apparatus 50. As shown in Fig. 2, the display control apparatus 50 comprises a set of video memories 51 for storing image data requested from the work station 40 from a client or a draw primitive, a circuit 53 for selecting a signal having a longest duration from horizontal synchronization signals supplied from a plurality of display devices 55 to 60 and a video timing circuit 52 for supplying image data d to the display devices 55 to 60 at a timing shown in Fig. 5 in synchronism with the longest duration signal 1 selected by the circuit 53. Numerals 71 and 72 denote conventional current buffer IC's for relaying currents of the image data and the control signal. The display devices 55 to 60 have the same construction as that of the display device 20 shown in Fig. 4. The image data supplied to the display devices 55 to 60 is applied to segment drivers 23 and 24 as drive waveforms shown in Fig. 6 by a display controller 21. C55 to C60 denote common drive waveforms in the display devices 55 to 60 and S55 to S60 denote segment drive waveforms (black) in the display devices 55 to 60. One horizontal synchronization period T is determined by the display controller 21 in accordance with temperature information from a temperature sensor 22 provided on a glass of a liquid crystal panel 26. In the above configuration, the display control apparatus 50 selects a longest duration signal from the horizontal synchronization signals S1 to S6 from the display devices 55 to 60 and supplies image data d to the display devices 55 to 60 in accordance therewith. In Fig. 6, the term T of the horizontal synchronization signal supplied from the display device 55 is shown as a longest one. As a result, adjustment periods T1 to T5 are created in the display devices 56 to 60. During the adjustment periods T1 to T5, a common voltage (VC) is supplied to the segment drivers 23 and 24 and the common driver 25. When the drive voltage is the common voltage (VC), the liquid crystal is not driven. The ferroelectric liquid crystal has a memory property in the behavior or molecules and hence the display is retained during the period of the application of the common voltage. The display control apparatus 50 normally supplies the image data of the display panel 26 to the respective display devices without the adjustment period as shown by the drive waveforms C55 and S55 of the display device 55. When the adjustment periods T1 to T5 are included in the drive waveforms, a contrast is somewhat enhanced. This is not disadvantageous in the demonstration of the display device. In the reliability evaluation test of the display device which is conducted under a substantially stable condition, the adjustment periods T1 to T5 are not included or in the order of one slot. Accordingly, by the use of the display control apparatus of the present embodiment, an installation space is saved, a cost is reduced and the display on the plurality of display devices which receive the image data by the internal synchronization is attained. In accordance with the present invention, since the image data is supplied to the respective display devices in synchronism with the longest one of the internal synchronization signals from the respective display devices, the supply period of one scan of image data supplied from the display control apparatus to the respective display devices is shorter than the period of the synchronization signal supplied from the display control apparatus. Accordingly, the image data can be simultaneously supplied to the plurality of display devices without loss of image data. In the present invention, the property of the liquid crystal and the margin in the drive condition are utilised to permit the simultaneous display on a plurality of display devices by one display control apparatus under the substantially stable environment. Thus, even when the display device is a type which receives the image data from the display control apparatus by internal synchronisation, the same image can be displayed on a plurality of display devices under a substantially stable environment with a simple construction and without a number of expensive host computers. Thus, cost is saved. Further, since the host computer can be installed in a limited space, space is saved. As described above, by using one host computer and a display control apparatus under a substantially stable operation environment, an image can be displayed without trouble on a plurality of display devices which receive image data by internal synchronisation.

Claims: A display system comprising: an image data source (40) to supply image data (d); a plurality of image display devices (55-60) each to display an image in response to said image data (d), each image display device having internal synchronisation means (21,22) to output a respective periodic horizontal scan synchronisation signal (a; S1-S6), the period of which is determined according to the displayable horizontal scanning time of the respective image display device; and a single display control apparatus (50), interposed between said image data source (40) and said plurality of image display devices (55-60), to transfer image data (d) received from said image data source (40) to each one of said plurality of image display devices (55-60), said display control apparatus (50) having: input buffer means (72) for relaying said image data and control signals; selection means (70) to select from the received scan synchronisation signals (S1-S6), a periodic horizontal scan synchronisation signal (1) of longest period thereamongst; timing means (52), responsive to the selected synchronisation signal (1), to control outputting of the image data (d) in synchronism with the selected synchronisation signal (1); and output buffer means (71) for relaying control signals and said image data. A display control apparatus (50), for use in the display system of claim 1 for transferring image data (d) received from an image data source (40), to each one of a plurality of image display devices, said control apparatus comprising: input buffer means (72) for relaying said image data and control signals; selection means (70) to select from the received periodic horizontal scan synchronisation signals (S1-S6), a periodic horizontal scan synchronisation signal (1) of longest period thereamongst; timing means (52), responsive to the selected synchronisation signal (1), to control outputting of the image data (d) in synchronism with the selected synchronisation signal (1); and output buffer means (71) for relaying control signals and said image data. An image data distribution apparatus, for use in the display system of claim 1, said apparatus consisting of an image data source (40) to supply image data (d); and the display control apparatus (50) of claim 2 arranged to receive image data (d) from said image data source (40). An image display apparatus, for use in the display system of claim 1, said apparatus consisting of: the display control apparatus (50) of claim 2; and a plurality of image display devices (55-60) each to display an image in response to image data (d) received from said display control apparatus (50), each image display device having internal synchronisation means (21,22) arranged to output, to said display control apparatus (50), a respective periodic horizontal scan synchronisation signal (a;Sl-S6), the period of which is determined according to the displayable horizontal scanning time of the respective image display device. A display system according to claim 1 or an image display apparatus according to claim 4, wherein each one of said plurality of image display devices (55-60) is a ferroelectric liquid crystal display device having display elements defined by respective segment and common electrodes each side of a ferroelectric liquid crystal display medium, each image display device (55-60) having internal synchronisation means (21), said synchronisation means (21,22) comprising a display controller (21) responsive to the image data (d), to control the timing of driving signal waveforms applied to the display elements by segment and common drivers (23-25) such that segment and common driving signals (S 55 ,C 55 ;...;S 60 ,C 60 ) are supplied to the drivers and a respective selected common electrode in each horizontal line scanning time (T) for the period of the respective periodic horizontal scan synchronisation signal (S1-S6), and a common voltage signal (V c ) is applied to the segment drivers and the respective selected common driver for a respective adjustment time (-,T1-T5) that is the difference in time between the period (T) of the selected synchronisation signal (l) and the period of the respective periodic horizontal scan synchronisation signal (S1-S6). A display system according to either of claims 1 or 5, or an image display apparatus according to either of claims 4 or 5 wherein: the displayable horizontal scanning time of each one of said plurality of image display devices is dependent on temperature; each one of said plurality of image display devices includes a temperature sensor (22); and said internal synchronisation means (21,22) is adapted to adjust the period of the respective periodic horizontal scan synchronisation signal (S1-S6) in response thereto.

Inventors: YUKI OSAMU, YUKI, OSAMU, Yuki, Osamu, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: G09G3
IPC codes: G02F1, G09G5, G09G3, G06F3