Method for centering and dimensioning an image on a cathode-ray tube

A method for centering and dimensioning an image on a cathode ray tube receiving display signals supplied by a display calculator includes measuring durations of vertical black edges of the image, and modifying adjustment values for a horizontal centering of the image to obtain equal lateral vertical edges. The method further includes measuring the durations of the vertical black edges of the image to calculate adjustment values for a horizontal dimension of the image to cause the vertical black edges to disappear. Durations of the horizontal black edges of the image are measured to calculate adjustment values of a vertical dimension of the image, and to calculate adjustment values of a vertical centering of the image to cause the horizontal black edges to disappear for centering the image vertically. The adjustment values obtained are recorded in a memory of the display calculator.

The present invention relates to cathode ray tubes used for displaying images on television (TV) sets and personal computers (PC). More particularly, it relates to a method of automatically centering and dimensioning a displayed image.

A cathode ray tube for displaying images on a screen formed by its faceplate generally comprises electronic circuits which control the scanning of the screen by an electron beam so as to activate or not the luminescence of screen pixels and thus produce the desired image.

These electronic scanning circuits are driven via a display controller through electrical signals which can have different sources, such as computer signals, signals from laser disks or game consoles. Because of their diversity, a same setting for the size and position of the image cannot be suitable for all sources and can result in a bad centering of the image on screen and a distorted image. These defects can moreover exist for a cathode ray tube from the factory in the case where the settings are not properly adjusted.

This can account for the presence of black stripes on the vertical or horizontal edges of the image, a horizontal or vertical shift of the image, or an image distortion in the horizontal or vertical direction.

In the state of the art, these faults are corrected manually by the user through control buttons which bring up adjustment menus and sub-menus. Such an operating procedure is neither fast nor simple, notably owing to the fact that there are only few control buttons, which requires the user to employ a same button for several different functions.

This is all the more inconvenient as these adjustments must be made as a function of the screen's operating mode, for example to pass from one video mode to another or from a classical 640×480 pixel screen to a higher definition 1280×1024 pixel screen.

Accordingly, when changing from one video mode to another, the display calculator analyzes the new horizontal and vertical synchronization signals, calculates their frequencies and carries out the necessary adjustments to display a new image in the new mode. However, the image obtained is never perfectly adapted to the screen size, and consequently suffers from faults as regards centering and dimensioning or size mentioned above.

Therefore, in the state of the art, the user must activate the control buttons provided for that purpose until the desired image is obtained, and the adjustments made are written into a memory of the display calculator, not only for the current session, but also for later sessions with the same display mode.

However, despite this memory storage of the settings, the user often needs to readjust the latter during a subsequent use of the same display mode, and all the more so as the settings of the mode entered in memory are not always adapted to all software which use that mode.

An object of the present invention is thus to implement a process for automatically centering and dimensioning an image on the screen of a cathode ray tube.

The invention relates to a method of centering and dimensioning an image on a cathode ray tube whose display signals are supplied by a display calculator, the method being characterized in that it comprises the following steps:

(a) measuring the durations of the vertical black edges of the image and modifying step by step the adjustment (HPOS) for horizontal centering to obtain equal lateral vertical edges;

(b) measuring the durations of the vertical black edges of the image to calculate the adjustment (HSIZE) of the horizontal dimension of the image so as to cause the vertical black edges to disappear;

(c) measuring the durations of the horizontal black edges of the image to calculate the adjustment of the vertical dimension of the image (VSIZE′) and the adjustment of the vertical centering of the image (VPOS′) so as to cause the horizontal black edges to disappear and to center the image vertically, and

(d) recording the adjustment values obtained (HPOS, HSIZE, VSIZE′ and VPOS′) in a memory of the display calculator.

Steps (a), (b) and (c) can be performed in any order because they are independent of each other, but it is advisable to perform step (a) before step (b), given that the precision for the calculation of the setting (HSIZE) for the horizontal dimension of the image depends on the perfect horizontal centering of the image.

Step (d) can come into play after each step (a), (b) or (c) to record the value of the setting obtained by the step having just been finished.

The method is implemented only if the image is sufficiently stable, this being detected by checking that the positions of the vertical and horizontal edges have fluctuations below a certain threshold. This stability is checked before each step (a), (b) or (c)

FIG. 1-Ashows the faceplate10of cathode ray tube12, on the screen of which appears an image14whose vertical edges16and horizontal edges18are black (i.e. dark), so indicating that the image14is not centered at the center of the screen and that it only occupies a part of the screen.

As indicated in the introductory portion above, the adjustments for centering and dimensioning the image are at present made by the user through buttons20which bring down menus and sub-menus on the screen to guide the user in the adjustments.

These control buttons20are active for the adjustments via a display calculator which supplies the values of horizontal and vertical scanning signals. This display calculator is capable of receiving the video signals and analyzing them to output these scanning signals.

In accordance with the invention, a control button22(FIG. 1-B) is added to implement the inventive process and obtain in a few seconds the correctly centered and dimensioned image ofFIG. 1-B.

The process of the invention is based on the measurement of the length, in units of time, of vertical and horizontal black edges, these measurements then serving for carrying out algorithmic operations and calculations leading to a modification of the image centering and its dimensions.

FIG. 2shows the image14and the corresponding horizontal scanning signal30as a function of time t for a line of the image, i.e. the current IHflowing in the horizontal deflection coil (yoke). The figure also shows the horizontal synchronization pulses32and34(HFBACK) which determine the start and end points of a horizontal scanning signal, the start of horizontal scanning corresponding to the falling edge of pulse32and the end corresponding to the rising edge of pulse34. The duration of the scanning return (flyback) is given by the duration of pulse32or34.

When the image exhibits vertical black edge portions, this comes from the fact that signals of the Red, Green and Blue components at the start and end of horizontal scanning are all below a certain level. The measure of the time duration T1HAVbetween the falling edge and the start of the left of the image indicates the extent of the left vertical black edge portion while a measure of T2HAVbetween the end of the right of the image and the rising edge indicates the extent of the right vertical black edge portion.

It then follows that if T1HAV=T2HAV, then the image is centered horizontally, whereas it is not centered if T1HAVis different from T2HAV.

The process in accordance with the invention obtains horizontal centering of the image by:measuring T1HAVand T2HAVin a repetitive manner,comparing T1HAVand T2HAVat each time,displacing the image by one unit towards:the right if T1HAV<T2HAV,the left if T1HAV>T2HAVuntil is obtained the equality T1HAV=T2HAV.

The measurement of T1HAVand T2HAVis performed by the display calculator using a device provided to that effect and known per se.

TIHAVand T2HAVdo not allow to obtain the horizontal dimensioning of the image for making the vertical black edge portions disappear, since the time interval between two horizontal synchronization pulses32and34is fixed, irrespective of the horizontal width of the image. The process of the invention produces this horizontal dimensioning by modifying the amplitude of the curve30in accordance a formula, as shall be described below.

FIG. 3shows the image14and the corresponding vertical scanning signal40as a function of time t for a complete image, i.e. the voltage Vvof the vertical deflection sawtooth signal for line-by-line vertical screen scanning. The figure also shows the vertical synchronization signals42and44(VFBACK) which determine the start and end points of a vertical scanning signal, the duration of the pulse determining the duration of the return period for the vertical scanning signal.

As in the case of horizontal line scanning, the time periods T1VAVand T2VAVrespectively define the extents of the top black edge portion and the bottom black edge portion of the image. However, these time periods cannot serve to center the image vertically because the time interval between the top and bottom edges of the image and the corresponding pulses42and44remain constant irrespective of the vertical position of the image.

Likewise, the time periods T1VAVand T2VAVcannot serve directly for vertically dimensioning the image because the time period of the vertical synchronisation pulses42,44remains the same irrespective of the image height. The measurement of T1VAVand T2VAVis carried out by the display calculator using the above-mentioned measuring device for measuring T1HAVand T2HAV.

The process according to the invention provides the vertical centering and the vertical dimensioning by modifying the amplitude of the curve40in accordance with a formula as shall be described hereafter.

The diagram ofFIG. 4illustrates the main steps of the invention, which comprises the steps of:

(a) measuring T1HAVand T2HAVto calculate the adjustment HPOS to perform in order to obtain the horizontal centering of the image (box50),

(b) measuring T1HAVand T2HAVto calculate the adjustment to perform HSIZE in order to obtain the horizontal dimensioning of the image (box52),

(c) measuring T1VAVand T2VAVto calculate the adjustment to perform VPOS and VSIZE in order to obtain at the same time vertical centering and the vertical dimensioning of the image (box54), and

(d) recording the values HPOS, HSIZE, VPOS′ and VSIZE′ in a memory (box56) of the display calculator.

If an error arises during one or another of steps50,52and54, notably in the case of image instability, the starting values are restored in the memory (box58). These errors can arise from an image which is unstable, which is shifting, which is too small to be adjusted, or for any other reason.

Note that steps (a), (b) and (c) can be performed in any order, but it appears logical to start with the simplest, which is the horizontal centring step, owing to the fact that it stems directly from the measurement of T1HAVand T2HAV. Moreover, step (b) yields more precise results if it follows from step (a).

The diagram ofFIGS. 5-Aand5-B shows in detail the operations to be performed during step (a) for horizontal centring. However, the first operations60,62,64,66,68and70are repeated, wholly or in part, at the start of each step (a), (b) or (c) to check that the image is stable within the established limits. These first operations comprise the steps of:pressing on button22(arrow60) to trigger off the operations,performing a first series of measurements to obtain a first set of pairs of values T1HAV1and T2HAV1, T1VAV1and T2VAV1(box62),performing a second series of measurements to obtain a second set of pairs of values T1HAV2and T2HAV2, T1VAV2and T2VAV2(box64),subtracting the second set of pairs of values from the values from the first set (box66) to obtain difference values DIFF in terms of absolute values,comparing the difference values DIFF with a threshold TMUDIFF (lozenge68),stopping the operations if DIFF>TMUDIFF, for the image is then considered to be unstable or shifting, or passing onto the next operation (lozenge70) in the opposite case.

Note that the series of measurements T1AVand T2AVwhich concern the horizontal deflection are preferably only performed just before each horizontal adjustment (a) or (b) to determine the horizontal stability of the image.

Likewise, the series of measurements T1VAVand T2VAV, which concern the vertical deflection, are only performed just before the vertical adjustments, preferably for centering and dimensioning to determine the vertical image stability,comparing T1HAVand/or T2HAV(lozenge70) with a maximum value MAX and stopping the operations if it is reached, for the image is then considered to be too small and hence not exploitable, or that the video signal is bad (lozenge70); in the case of a negative comparison, passing on to the next operation, the first concerning the horizontal centering proper, which comprises the steps of:checking whether the negative comparison arrives for the first time or not (lozenge72), andin the case of a positive check, passing on to the next operation comprising the steps of:comparing T1HAVwith T2HAV(lozenge74), andstopping the horizontal centering operations in the case of an inequality, for the image is already horizontally centered, and passing on to step (b),the image must be displaced to the right if T1HAV<T2HAV, such an event being memorized by a flag at the 0 state,the image must be displaced to the left if T1HAV>T2HAV, such an event being memorized by the flag, but in this case at the 1 state,in the case of a negative check, or in the case where the image must be displaced, passing on to the next operation.

The value 0 or 1 for the flag indicates the direction in which the image is to be displaced, the displacement being effected in a stepwise manner by incrementing or decrementing the centering adjustment value HPOS.

The following operations involve comparing T1HAVwith T2HAVand modifying the centering adjustment value HPOS in the direction indicated by the value of the flag until detection of the equality T1HAV=T2HAV. These operations are presented in the diagram ofFIG. 5-B.

The first operation (box80) consists in checking whether the flag is at logic 1, indicating that the image is off-centered in the right direction and must be brought back to the left.if the check is positive, the following operation consists in checking whether T1HAV>T2HAV(lozenge82), and there are three possible responses:(i) T1HAV=T2HAV, in which case the image is centered and the horizontal centering operations are stopped to pass on to step (b),(ii) T1HAV>T2HAV, in which case the image is off-centered in the right direction and must be displaced to the left by decrementing the adjustment value HPOS by one unit (box86); moreover, a loop counter90is incremented by one unit;(iii) T1HAV<T2HAV, in which case the image which was off-centered in the right direction since the start of the operations is now off-centered towards the left, which means that the centering value HPOS has been exceeded by one unit. This overshoot is corrected by incrementing the horizontal adjustment value HPOS by one unit (box94). With this incrementation, the value of HPOS corresponds to the center position, and the horizontal centering operations are stopped to pass on to step (b).

If the flag is not at logic 1, i.e. the image is off-centered to the left and must be brought back to the right, the following operation (box84) consists in checking whether T1HAV<T2HAV, and there are three possible solutions as in the previous case:(i) T1HAV=T2HAV, in which case the image is centered and the horizontal centering operations are stopped to pass on to step (b),(ii) T1HAV<T2HAV, in which case the image is off-centered in the left direction and must be displaced to the right by incrementing the adjustment value HPOS by one unit (box88); moreover, a loop counter90is incremented by one unit;(iii) T1HAV>T2HAV, in which case the image which was off-centered in the left direction since the start of the operations is now off-centered towards the right, which means that the centering value HPOS has been exceeded by one unit. This overshoot is corrected by decrementing the horizontal adjustment value HPOS by one unit (box96). With this incrementation, the value of HPOS corresponds to the center position and the horizontal centering operations are stopped to pass on to step (b).

If the loop counter90is incremented, this means that the centering value HPOS has not yet been obtained and that it is necessary start again all the operations described above (new loop) starting from step62consisting of measuring new values of T1HAVand T2HAVsubsequent to the new value of HPOS.

However, this new loop is performed only if the number of loops has not exceeded a certain threshold BMAX. The operation consists in:comparing (lozenge92) the contents of the loop counter90with BMAX,stopping the operations if the centering has not been achieved after a set number of shifts BMAX,or starting a new loop if BMAX is not attained.

To set the horizontal dimension of the image such that it takes up the entire width of the screen, i.e. without vertical black edges, it is necessary to change the amplitude setting for the current flowing in the horizontal deflection coil, such an adjustment being represented by a value HSIZE which can vary e.g. between 0 and 255. It is this value HSIZE for obtaining a maximum image width which is calculated by the method according to the invention, this value being dependent on many parameters, and notably T1HAVand T2HAV.

The formula which enables to calculate HSIZE is:

In which formula:AVoptiis the optimum amplitude of the current in the horizontal deflection coil to obtain an image of optimum width; this amplitude is measured for a type of cathode ray tube and in a reference video mode,HSIZEMAXis the maximum value of HSIZE, e.g. 255 as indicated above,HAMPMAXis the maximum variation of the current in the horizontal deflection coil to obtain a maximum horizontal deflection; this value varies as a function of the horizontal scanning frequency f(fH) as described below;HAMPMINis the minimum variation of the current in the horizontal deflection coil to obtain a minimum horizontal deflection; this value varies as a function of the horizontal scanning frequency f(fH) as described below;T′ is the total duration of a horizontal line, i.e. the duration of the period T of the horizontal synchronization signal, from which are subtracted the duration of the flyback pulse TFLYBACK, in general three microseconds, and a duration of safety margins, e.g. 0.6 microseconds, and
−Td′=T−(T1HAV+T2HAV+TFLYBACK)

i.e. the duration of the image on screen between these black edges.

This formula is established by supposing that the current varies linearly, which is not the case, so that to take into account the fact that the curve is S shaped, the coefficient applied AVoptimust be replaced by
(1.8T′−Td′)/2.8Td′,

which coefficient can change depending on the type of cathode ray tube and its control device.

The values for HAMPMINand HAMPMAXare determined by means of, curves as a function of the horizontal scanning frequency f(H), this being effected for frequency ranges.

For instance, curve100ofFIG. 6shows the function of HAMPMIN=f(fH) for a range of frequencies from 34 kHz to 41 kHz for the case of a given cathode ray tube. The abscissa x is graduated in kHz while the ordinate is graduated in HAMP×10 mA. There is thus obtained a straight line whose equation is:
Y=−5.22x+1265.1=ax+b.

This equation is different for another range of frequencies.

Coefficients a and b determined for each range of frequencies are recorded in a memory so that they can be read in view of calculating HAMPMINaccording to the horizontal scanning frequency.

HAMPMAXis obtained in the same manner as for HAMPMIN.

As a result, if there are eight frequency ranges, there shall be sixteen pairs of coefficients (a,b) which define the sixteen variation curves, eight for HAMPMINand eight for HAMPMAX.

To achieve vertical centering and vertical dimensioning, the method according to the invention consists in measuring the values T1VAVand T2VAVfor the image which appears on the screen, and then first calculating VSIZE to obtain the vertical dimensioning and subsequently VPOS′ to obtain the vertical centering according to the following formulae:
VSIZE′=0.5[(3VSIZEMAX+2VSIZE)]·[(Td×T′)/(TD′×T)]−1.5(VSIZEMAX) and
VPOS′=VPOS+(A−B)
with
A=[(2.25+1.5.(VSIZE/VSIZEMAX)]×[(0.5−T1/T).(VPOSMAX/0.6)]
and
B=[(2.25+1.5.(VSIZE′/VSIZEMAX)]×[0.5−T1′/T′).(VPOSMAX/0.6)]

To define the parameters of these formulae, reference shall be made toFIG. 7, which shows the sawtooth for vertical scanning40, but inversed with respect to that ofFIG. 3. The abscissa shows the duration and the ordinate shows the voltage VOUT. On this sawtooth is placed the image112to be vertically centered and dimensioned and a reference image which is appropriately vertically centered and dimensioned.

InFIG. 7, T1, T2correspond in time periods respectively to the start and end of the reference image114, while T1′ and T2′ correspond in time periods respectively to the start and end of the image112to be centered and dimensioned. The following relations are then established:

The duration Td of the reference image is given by Td=T2−T1, and the duration Td′ of the image to be centered is given by Td′=T2′−T1′.

Also, T1=T1VAVand T2=T−T2VAV, T being the total duration of a sawtooth. Similarly, T1′=T1′=VAVand T2′=T′−T2′VAV.

The reference image114is obtained by a manual adjustment in a reference video mode on a given type of cathode ray tube and the values T1VAVand T2VAVare measured and entered into a memory to be used for the automatic adjustments on that type of cathode ray tube. The same applies for the value VSIZE, which corresponds to that reference image, while VZIZEMAX is the maximum adjustment value, for example 256.

These elements allow to calculate the value VSIZE′ according to the above formula, i.e. the adjustment value that will allow to obtain an image which is appropriately vertically dimensioned.

By knowing VSIZE′, it is possible to calculate VPOS′ according to the above formula, which also uses the value VPOSMAX, which is the maximum adjustment for the vertical centering.

The invention has been described in its application to the adjustment of a cathode ray tube by the user of a computer or a television set in which the cathode ray tube forms the display screen. The invention also applies to the implementation of the process for adjusting the horizontal and vertical deflection coils at the end of a cathode ray tube production line.

Indeed, at the end of a cathode ray tube production line, the image generated to test for the correct operation of the cathode ray tube exhibits faults which an operator corrects in various ways. One of the faults concerns a bad alignment between the image and screen centers and, to correct it, the operator first performs image centering and dimensioning adjustments using the buttons20(FIG. 1-A) and then the adjustments in the electronic and magnetic circuits (deflection coils) to displace the image center and make it coincide with the center of the screen. In this sequence of operations, the method of the invention can be implemented to obtain the centering HPOS and HPOS′, and possibly the dimensioning HSIZE and VSIZE′.

For this adjustment, the operations to be performed would then be as follows:display a calibrated image, for example a white image with a perfectly centered cross,launch the process of the invention wholly or in part,modify the electronic and magnetic settings for the screen to bring the cross to the center of the screen.