Method and device of alignment of a video image with an edge of a display screen

The aligning a video image with an edge of a display screen, the video image being displayed by a scanning of lines of the display screen by at least one electron beam modulated by a modulation signal, including, for each scanned line of the screen, storing successive digital video data in a storage device; detecting the end of the scanning of the line preceding the scanned line; initiating a delay upon detection of the end of the scanning of the line preceding the scanned line; starting the reading, from the storage device, of the successive digital video data stored at the end of the delay; and providing the modulation signal based on the read successive digital video data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and a device for aligning a video image with an edge of a display screen.

2. Discussion of the Related Art

Generally, a video image is displayed on a display screen of a display terminal by exciting phosphors arranged on the screen by means of one or several electron beams, emitted by electron guns. In the case of a color screen, a cathode-ray tube with three electron beams, each exciting a type of phosphor respectively emitting a red, green, or blue light, is generally used. The electron beams are modulated in intensity by modulation signals representative of the image to be displayed on the screen.

Generally, the electron beams are focused at one point of the screen and are deviated together to scan screen lines. The electron beams scan the screen lines from the right to the left of the screen, returning to the left of the screen after the scanning of each line. The screen scanning is performed from the upper horizontal edge to the lower horizontal edge.

The electron beam deviations are obtained by two deflection coils, one horizontal deflection coil which controls the scanning of each screen line, and one vertical deflection coil which controls the deviations of the electron beams in the vertical direction.

FIG. 1shows a conventional example of a timing diagram of current I supplying the horizontal deflection coil. For clarity, the timing diagram is not shown to scale. Curve10, representative of current I, comprises linear ascending ramps11linked by descending portions12. Each ascending ramp11corresponds to the scanning, by the electron beams, of a line from the left to the right of the screen. Each descending portion12corresponds to the return of the electron beams to the left of the screen.

FIG. 2shows timing diagrams of signals implemented to form ramps11of current I and to display an image on the screen.

From the image signal received by the display terminal, is extracted a square signal HSYNC, represented by curve15, from which are generated the different clock signals synchronizing the elements of the display terminal. Blocks of video signals VIDEO, shown as blocks16A,16B of curve16, each block16A,16B comprising the video data necessary for the display of an image line, are also extracted from the image. Each video block16A,16B is transmitted to a modulation system to generate the signals for modulating the intensity of the electron beams.

A rising edge of signal HSYNCindicates the beginning of the display of a line. Duration ΔTVIDEO separating one rising edge of signal HSYNCfrom the beginning of the transmission of a video block16A,16B is constant.

The stopping of an ascending ramp11is controlled by the rising edge of a signal HOUT, represented by curve17. Signal HOUTis provided by a device for controlling the display terminal based on signal HSYNC. Signal HOUTis, for example, a square pulse signal, each rising edge of which, separated by a duration ΔTHOUTfrom the preceding rising edge of signal HSYNC, corresponds to the end of an ascending ramp.

The display frequency of the lines on a screen is generally high, for example, on the order of some thirty kilohertz, whereby the implementation of ascending ramps11of current I supplying the horizontal deflection coil must be performed directly at the level of the power stage of the horizontal deflection coil by discrete components. The control device transmits signal HOUTto the power stage of the horizontal deflection coil to control the end of each ascending ramp11. The power stage transmits in return to the control device a signal HFLY, represented by curve18, each pulse of which indicates a descending part12of the supply current of the horizontal deflection coil, that is, the return of the electron beams to the left of the screen.

Response times tSof the power stage components may be variable. There may thus appear a variable duration between the reception of signal HOUTby the power stage and the end of the associated ascending ramp11. Now, it is necessary to ensure a constant duration ΔTVID between signal HFLYand the beginning of the transmission of the next video data block16A,16B so that the pixel display on a screen line starts at a substantially constant distance from the screen edge.

FIG. 3shows in the form of a block diagram a conventional control device20implementing a regulation of duration ΔTVID between signal HFLYand the beginning of the transmission of the next video data block16A,16B.

Control device20receives as inputs signal HSYNCand signal HFLY. Signal HSYNCis transmitted to a counter22which, after a variable delay ΔTHOUT, transmits signal HSYNCto a calculation unit (SYNT.)24which provides signal HOUT. Control device20also comprises a comparator26, which determines duration ΔTHFLYseparating the reception of a rising edge of signal HSYNCand of the next signal HFLY, compares the measured duration with a reference duration ΔTREF, and accordingly provides an error signal to a correction unit28. Correction unit28transmits to counter22a new duration ΔTHOUTto be used to correct the position of the next pulse HOUTto obtain a better position of the next signal HFLY.

The correction performed on duration ΔTHOUTwill affect the next signal HOUTtransmitted after signal HSYNC. Thereby, the ascending ramp11which immediately follows the signal HFLYfrom which the correction has been determined, is not corrected, since it originates from an already-transmitted signal HOUT. The video block immediately following signal HSYNCmay thus not be correctly synchronized with ascending ramp11. The line of the video image associated with the video block may be displayed in a shifted manner with respect to the screen edge while the next line of the video image will be properly displayed.

SUMMARY OF THE INVENTION

The present invention aims at providing a device and a method for displaying lines of a video image enabling immediate synchronization of the screen scanning and of the displaying of an image.

For this purpose, the invention provides a method for aligning a video image with an edge of a display screen, the video image being displayed by a scanning of lines of the display screen by at least one electron beam modulated by a modulation signal, comprising the steps of, for each scanned line of the screen, storing successive digital video data in a storage means; detecting the end of the scanning of the line preceding the scanned line; initiating a delay upon detection of the end of the scanning of the line preceding the scanned line; starting the reading, from the storage means, of the successive digital video data stored at the end of the delay; and providing the modulation signal based on the read successive digital video data.

According to another object of the present invention, the determined delay is identical for all the scanned lines.

According to another object of the present invention, the electron beam is deviated by a horizontal deflection coil receiving, for the scanning of each line, a linear current ramp.

According to another object of the present invention, each of the digital video data corresponds to an intensity of a color component of a pixel to be displayed on screen.

The present invention also provides a device for aligning a video image with an edge of a display screen, the video image being displayed by a scanning of lines of the display screen by at least one electron beam emitted by an electron gun and modulated by a modulation signal, comprising a means for storing successive digital video data; a counter receiving a signal representative of the end of the scanning of a line and transmitting said representative signal after a determined delay; a means for providing a read start control signal based on the representative signal transmitted by the counter; a means for reading from the means for storing the successive digital video data upon reception of the read start control signal; and a means for providing the modulation signal based on the read successive digital video data.

According to another object of the present invention, the storage means is of first-in-first-out type.

According to another object of the present invention, the device further comprises a means for receiving an image signal transmitted from the outside of the device; a means for providing a synchronization signal based on the image signal; and a means for providing a control signal of the beginning of the scanning of a screen line based on the synchronization signal.

According to another object of the present invention, the synchronization signal is a substantially periodic square pulse signal, the control signal being provided upon each rising edge of the synchronization signal.

DETAILED DESCRIPTION

In the following description, reference numerals identical to those previously used will be used to designate signals performing identical functions.

FIG. 4shows an example of implementation of a control device30according to the present invention equipping a display terminal. As previously described, control device30receives signals HSYNCand HFLYand outputs signal HOUTwhich controls the end of the ascending ramps of the supply current of the horizontal deflection coil of the display terminal.

Control device30also receives digital pixel data PIX_DATA_R, PIX_DATA_G, PIX_DATA_B, which are determined from the image signal received by the display terminal. As an example, each of the digital pixel data PIX_DATA_R, PIX_DATA_G, PIX_DATA_B is representative of an intensity value of a red, green, or blue component of a pixel of the image to be displayed.

Device30according to the present invention comprises three memories32,34,36of FIFO-type. The digital pixel data PIX_DATA_R, PIX_DATA_G, PIX_DATA_B are respectively and successively stored in each memory32,34,36.

Device30according to the present invention comprises a counter38which receives as an input signal HFLY, or a signal resulting from a pre-processing of signal HFLY, indicating the end of an ascending ramp of the supply current of the horizontal deflection coil. Counter38transmits signal HFLYto a calculation unit (SYNT.)40after a fixed delay ΔTVID′ has elapsed. Calculation unit40determines from signal HFLYa read start control signal READ, which is, for example, a square signal.

At the rising edge of signal READ, each memory32,34,36successively transmits to a digital-to-analog converter (DAC)42,44,46pixel data PIX_DATA_R, PIX_DATA_G, PIX_DATA_B. Duration ΔTVID′ then represents the duration separating signal HFLYfrom the beginning of the transmission of pixel data PIX_DATA_R, PIX_DATA_G, PIX_DATA_B to converters42,44,46. Converters42,44,46convert the read digital pixel data PIX_DATA_R, PIX_DATA_G, PIX_DATA_B into analog signals PIX_R, PIX_G, PIX_B. Analog signals PIX_R, PIX_G, PIX_B are then transmitted to the modulation system of the display terminal which determines, based on analog signals PIX_R, PIX_G, PIX_B the signals of modulation of electron beam intensity.

Control device30also comprises a counter48which receives signal HSYNC. Counter48transmits, after a fixed delay ΔHOUT′ has elapsed, signal HSYNCto a calculation unit50which provides signal HOUT.

FIG. 5shows timing diagrams of the signals ofFIG. 2with device30ofFIG. 4. Curve55represents signal HSYNC, curve56represents video blocks56A,56B of a signal VIDEO′ transmitted to the electron beam modulation system, that is, any one of signals PIX_R, PIX_G, or PIX_B, curve57represents signal HOUTand curve58represents signal HFLY.

InFIG. 5, the rising edge of signal HOUTis simultaneous with the rising edge of signal HSYNC. This corresponds to a delay ΔTHOUT′ set to zero. Counter48can, in this case, be eliminated.

Control device30according to the present invention enables controlling the time of the beginning of the transmission of video blocks to the modulation system. Such a time is modified according to signal HFLYso that the duration between signal HFLYand the beginning of the transmission of the video data blocks remains constant. It is thus ensured that for each line, the image line is displayed at a fixed distance from the left side of the screen.

The present invention has many advantages.

First, as compared to a conventional control device, the device according to the present invention performs a particularly simple open-loop regulation.

Second, the present invention enables synchronizing in real time the transmission of video data to the modulation system with the scanning of a line. Even if an ascending ramp of the supply current of the horizontal deflection coil is offset in time and/or has a duration longer or shorter than a reference duration, the pixels of the next image line will be correctly displayed. Indeed, the video blocks, corresponding to the pixels to be displayed, are only transmitted to the modulation system after a fixed duration following the reception of signal HFLYindicating the end of the preceding ramp, and thus independently from the ramp duration.

Third, the present invention enables getting rid of the constraints linked to variations tSbetween signal HOUTand signal HFLY. Indeed, it is the time of the beginning of the video block transmission which is directly controlled, and no longer the time when control signal HOUTis transmitted.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the present invention may apply to display terminals of computer monitor type, automatic teller machine screen, etc.