# Patent

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

## Abstract

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.

## Background
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)



Other characteristics and advantages of the present invention shall become apparent from reading the following description of a preferred embodiment, in relation with appended drawings in which:
FIG. 1-A shows an image on a screen of a cathode ray tube which is not centered and exhibits a black surround, and FIG. 1-B shows the same image after implementation of the process in accordance with the invention;
FIG. 2 is a diagram showing the relations between the horizontal position of the image on screen and the horizontal scanning signal for an image line;
FIG. 3 is a diagram analogous to that of FIG. 2, but showing the relations between the vertical position of the image on screen and the vertical scanning signal for a complete image;
FIG. 4 is a diagram showing the main steps of the process in accordance with the invention;
FIGS. 5-A and 5-B show the steps in the horizontal image centering algorithm in accordance with the process of the invention, this algorithm being preceded by an algorithm for checking the image stability;
FIG. 6 is a curve showing the variation of HAMPMIN as a function of the horizontal scanning frequency for a given range of frequencies; and
FIG. 7 is a diagram showing the adjustment for vertical centering and vertical dimensioning.



FIG. 1-A shows the faceplate 10 of cathode ray tube 12, on the screen of which appears an image 14 whose vertical edges 16 and horizontal edges 18 are black (i.e. dark), so indicating that the image 14 is 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 buttons 20 which bring down menus and sub-menus on the screen to guide the user in the adjustments.
These control buttons 20 are 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 button 22 (FIG. 1-B) is added to implement the inventive process and obtain in a few seconds the correctly centered and dimensioned image of FIG. 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. 2 shows the image 14 and the corresponding horizontal scanning signal 30 as a function of time t for a line of the image, i.e. the current IH flowing in the horizontal deflection coil (yoke). The figure also shows the horizontal synchronization pulses 32 and 34 (HFBACK) which determine the start and end points of a horizontal scanning signal, the start of horizontal scanning corresponding to the falling edge of pulse 32 and the end corresponding to the rising edge of pulse 34. The duration of the scanning return (flyback) is given by the duration of pulse 32 or 34.
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 T1HAV between 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 T2HAV between 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 T1HAV is different from T2HAV.
The process in accordance with the invention obtains horizontal centering of the image by:

    
    
        measuring T1HAV and T2HAV in a repetitive manner,
        comparing T1HAV and T2HAV at each time,
        displacing the image by one unit towards:
        
            the right if T1HAV<T2HAV,
            the left if T1HAV>T2HAV until is obtained the equality T1HAV=T2HAV.
        
        
    
    


The measurement of T1HAV and T2HAV is performed by the display calculator using a device provided to that effect and known per se.
TIHAV and T2HAV do 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 pulses 32 and 34 is fixed, irrespective of the horizontal width of the image. The process of the invention produces this horizontal dimensioning by modifying the amplitude of the curve 30 in accordance a formula, as shall be described below.
FIG. 3 shows the image 14 and the corresponding vertical scanning signal 40 as a function of time t for a complete image, i.e. the voltage Vv of the vertical deflection sawtooth signal for line-by-line vertical screen scanning. The figure also shows the vertical synchronization signals 42 and 44 (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 T1VAV and T2VAV respectively 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 pulses 42 and 44 remain constant irrespective of the vertical position of the image.
Likewise, the time periods T1VAV and T2VAV cannot serve directly for vertically dimensioning the image because the time period of the vertical synchronisation pulses 42, 44 remains the same irrespective of the image height. The measurement of T1VAV and T2VAV is carried out by the display calculator using the above-mentioned measuring device for measuring T1HAV and T2HAV.
The process according to the invention provides the vertical centering and the vertical dimensioning by modifying the amplitude of the curve 40 in accordance with a formula as shall be described hereafter.
The diagram of FIG. 4 illustrates the main steps of the invention, which comprises the steps of:
(a) measuring T1HAV and T2HAV to calculate the adjustment HPOS to perform in order to obtain the horizontal centering of the image (box 50),
(b) measuring T1HAV and T2HAV to calculate the adjustment to perform HSIZE in order to obtain the horizontal dimensioning of the image (box 52),
(c) measuring T1VAV and T2VAV to 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 (box 54), and
(d) recording the values HPOS, HSIZE, VPOS′ and VSIZE′ in a memory (box 56) of the display calculator.
If an error arises during one or another of steps 50, 52 and 54, notably in the case of image instability, the starting values are restored in the memory (box 58). 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 T1HAV and T2HAV. Moreover, step (b) yields more precise results if it follows from step (a).
The diagram of FIGS. 5-A and 5-B shows in detail the operations to be performed during step (a) for horizontal centring. However, the first operations 60, 62, 64, 66, 68 and 70 are 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 button 22 (arrow 60) to trigger off the operations,
        performing a first series of measurements to obtain a first set of pairs of values T1HAV1 and T2HAV1, T1VAV1 and T2VAV1 (box 62),
        performing a second series of measurements to obtain a second set of pairs of values T1HAV2 and T2HAV2, T1VAV2 and T2VAV2 (box 64),
        subtracting the second set of pairs of values from the values from the first set (box 66) to obtain difference values DIFF in terms of absolute values,
        comparing the difference values DIFF with a threshold TMUDIFF (lozenge 68),
        stopping the operations if DIFF>TMUDIFF, for the image is then considered to be unstable or shifting, or passing onto the next operation (lozenge 70) in the opposite case.
    
    


Note that the series of measurements T1AV and T2AV which 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 T1VAV and 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 T1HAV and/or T2HAV (lozenge 70) 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 (lozenge 70); 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 (lozenge 72), and
            in the case of a positive check, passing on to the next operation comprising the steps of:
        
        
        comparing T1HAV with T2HAV (lozenge 74), and
        stopping 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 T1HAV with T2HAV and 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 of FIG. 5-B.
The first operation (box 80) 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 (lozenge 82), 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 (box 86); moreover, a loop counter 90 is 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 (box 94). 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 (box 84) 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 (box 88); moreover, a loop counter 90 is 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 (box 96). 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 counter 90 is 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 step 62 consisting of measuring new values of T1HAV and T2HAV subsequent 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 (lozenge 92) the contents of the loop counter 90 with 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 T1HAV and T2HAV.
The formula which enables to calculate HSIZE is:




  HSIZE
  =
  
    
      
        
          
            A
            Vopti
          
          ⁡
          
            (
            
              
                T
                ′
              
              /
              
                Td
                
                  
                    
                  
                  ⁢
                  ′
                
              
            
            )
          
        
        -
        
          H
          AMPMIN
        
      
      
        
          H
          AMPMAX
        
        -
        
          H
          AMPMIN
        
      
    
    ×
    
      HSIZE
      MAX
    
  




In which formula:

    
    
        AVopti is 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,
        HSIZEMAX is the maximum value of HSIZE, e.g. 255 as indicated above,
        HAMPMAX is 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;
        HAMPMIN is 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 AVopti must 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 HAMPMIN and HAMPMAX are determined by means of, curves as a function of the horizontal scanning frequency f(H), this being effected for frequency ranges.
For instance, curve 100 of FIG. 6 shows 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 HAMPMIN according to the horizontal scanning frequency.
HAMPMAX is 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 HAMPMIN and eight for HAMPMAX.
To achieve vertical centering and vertical dimensioning, the method according to the invention consists in measuring the values T1VAV and T2VAV for 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 to FIG. 7, which shows the sawtooth for vertical scanning 40, but inversed with respect to that of FIG. 3. The abscissa shows the duration and the ordinate shows the voltage VOUT. On this sawtooth is placed the image 112 to be vertically centered and dimensioned and a reference image which is appropriately vertically centered and dimensioned.
In FIG. 7, T1, T2 correspond in time periods respectively to the start and end of the reference image 114, while T1′ and T2′ correspond in time periods respectively to the start and end of the image 112 to 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=T1VAV and T2=T−T2VAV, T being the total duration of a sawtooth. Similarly, T1′=T1′=VAV and T2′=T′−T2′VAV.
The reference image 114 is obtained by a manual adjustment in a reference video mode on a given type of cathode ray tube and the values T1VAV and T2VAV are 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 buttons 20 (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.

## Claims

1. A display comprising
a cathode ray tube for displaying an image thereon; and
a calculation circuit connected to said cathode ray tube for centering and dimensioning the image being displayed by
(a) measuring durations of vertical border edges of the image and iteratively modifying adjustment values for a horizontal centering of the image to obtain equal lateral vertical edges,
(b) measuring the durations of the vertical border edges of the image to calculate adjustment values for a horizontal dimension of the image to cause the vertical border edges to disappear, and
(c) measuring durations of horizontal border edges of the image 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 border edges to disappear and to center the image vertically;

said calculation circuit further checking stability of the image at a start of at least one of the steps (a), (b) and (c), by

(a0) measuring a pair of durations of at least one of the vertical and horizontal border edges of the image,
(a1) subtracting two successive measurements to determine a variation in the durations,
(a2) cornparing the variation with a first threshold, and stopping if the first threshold is exceeded or continuing if the first threshold is not exceeded, and
(a3) comparing the measurements with a second threshold, and stopping if the second threshold is exceeded or continuing with at least one of steps (a), (b) and (c) if the second threshold is not exceeded.

2. A display according to claim 1, further comprising a display calculator connected to said cathode ray tube for providing display signals thereto.

3. A display according to claim 2, wherein said display calculator comprises a memory for recording the adjustment values obtained.

4. A display according to claim 1, wherein the durations of the vertical border edges of the image are indicated by variables T1HAv and T2HAV; and wherein said calculation circuit further comprises performing:
(a4) checking that the measurements of the durations T1HAV and T2HAV are a first measurement since a start of steps (a), (b) or (C), and if a response of the checking is negative, then passing on to a sub-step (a6), and if the response of the checking is positive, then passing on to a substep (a5); and
(a5) comparing the measurements for T1HAV and T2HAV.

5. A display according to claim 4, wherein step (a4) further comprises:
setting a flag to a first logic value if T1HAV <T2HAV to indicate that the image is to be displaced towards the right;
setting the flag to a second logic value if T1HAV<T21HAV to indicate that the image is to be displaced towards the left; and
stopping the horizontal centering if T1HAV=T2HAV, since the image is centered horizontally, and passing on to step (b).

6. A display according to claim 5, wherein said calculation circuit further comprises performing:
(a6) checking whether the flag is at the second logic value, and if the checking is a positive response, then passing on to a next sub-step (a7), and if the checking is a negative response, then passing on to a sub-step (a8);
(a7) checking whether T1HAV<T2HAV, and if T1HAV=T2HAV since the image is horizontally centered, then stopping, and if the checking is a positive response, then passing on to a sub-step (a9), and if the checking is a negative response, then passing on to a sub-step (a10); and
(a8) checking whether T1HAV<T2HAV, and if T1HAV=T2HAV, since the image is horizontally centered, then passing on to step (b), and if the checking is a positive response, then passing on to a sub-step (a11), and if the checking is a negative response, then passing on to a sub-step (a12).

7. A display according to claim 6, wherein said calculation circuit further comprises performing:
(a9) decrementing by one unit the adjustment value for centering the image to displace the image by one incremental step to the left, and passing on to a sub-step (a13);
(a10) incrementing by one unit the adjustment value for centering the image to displace the image towards the right, since the image had been displaced by one incremental step too far towards the left, and stopping the horizontal centering since the image is centered horizontally and passing on to step (b);
(a11) incrementing by one unit the adjustment value for centering the image to displace the image by one incremental step to the right, and passing on to sub-step (a13);
(a12) decrementing by one unit the adjustment value for centering the image to displace the image towards the left, since the image had been displaced by one incremental step too far towards the right, and stopping the horizontal centering since the image is centered horizontally and passing on to step (b);
(a13) incrementing a loop counter by one unit each time the sub-step (a9) or (a11) has been performed, then passing on to a following sub-step (a14); and
(a14) checking whether content of the loop counter has attained a third threshold, and if the checking is a positive answer, then stopping on account of an impossibility of adjustment, and if the checking is a negative answer, then restarting the loop at step (a2).

8. A display according to claim 1, wherein the durations of the vertical border edges of the image are indicated by variables THAV and T2HAV; and wherein calculating the adjustment of the horizontal dimension of the image in step (b) is based upon the formula:




  HSIZE
  =
  
    
      
        
          
            A
            Vopti
          
          ⁡
          
            (
            
              
                T
                ′
              
              /
              
                Td
                
                  
                    
                  
                  ⁢
                  ′
                
              
            
            )
          
        
        -
        
          H
          AMPMIN
        
      
      
        
          H
          AMPMAX
        
        -
        
          H
          AMPMIN
        
      
    
    ×
    
      HSIZE
      MAX
    
  




wherein:
Avopti is an optimum amplitude of a current in a horizontal deflection coil of the cathode ray tube to obtain an image of optimum width, with the amplitude being measured for a type of cathode ray tube,
HSIZEMAX is a maximum value of the adjustment of the horizontal dimension of the image,
HAMPMAX is a maximum variation of a current in the horizontal deflection coil to obtain a maximum horizontal deflection, this value varies as a function of a horizontal scanning frequency;
HAMPMIN is a 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;
T′ is a total duration of a period T of a horizontal synchronization signal, from which are subtracted a duration of a flyback pulse TFLYBACK and a duration of safety margins, and
Td′=T−(T1HAV+T2HAV+TFLYBACK), which is a duration of the image on the screen between the border edges.

9. A display according to claim 8, wherein the coefficient (T′/Td′) in the formula is replaced by 1.8T′−Td′/2.8Td′ to take into account a form of a curve for the current in the horizontal deflection coil.

10. A display according to claim 1, wherein the durations of the horizontal border edges of the image are indicated by variables T1VAV and T2VAV; and wherein calculating the adjustment of the vertical dimension of the image in step (c) is based upon the formula:


VSIZE′=0.5[(3VSIZEMAX+2VSIZE)].[(Td×T′)/(Td′×T)]1.5 (VSIZEMAX);


and wherein calculating the adjustment of the vertical centering of the image in step (c) is based upon the formula:


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)],


wherein:
Td is a duration of a reference image such that:


Td=T2−T1 with T1=T1VAV and T2−T−T2VAV,


T is a total duration of a vertical scanning sawtooth,
Td′ is a duration of the image to dimension and to center such that:


Td′=T2′−T1′ with T1′=T1′VAV and T2′=T′−T2′VAV,


VSIZE is an adjustment value for the vertical dimension of the reference image,
VSIZEMAX is a maximum adjustment value for the vertical dimension of the image,
VPOS is an adjustment for the vertical centering of the reference image, and
VPOSMAX is a maximum adjustment value for the vertical centering.

11. A display comprising
a cathode ray tube for displaying an image thereon; and
a calculation circuit connected to said cathode ray tube for horizontal centering and horizontal dimensioning the image being displayed on said screen by
(a) measuring durations of vertical border edges of the image and iteratively modifying adjustment values for a horizontal centering of the image to obtain equal lateral vertical edges, and
(b) measuring the durations of the vertical border edges of the image to calculate adjustment values for a horizontal dimension of the image to cause the vertical border edges to disappear;

said calculation circuit further checking for stability of the image by

(a0) measuring a pair of durations of at least one of the vertical and horizontal border edges of the image,
(a1) subtracting two successive measurements to determine a variation in the durations,
(a2) comparing the variation with a first threshold, and stopping if the first threshold is exceeded or continuing if the first threshold is not exceeded, and
(a3) comparing the measurements with a second threshold, and stopping if the second threshold is exceeded or continuing with at least one of steps (a) and (b) if the second threshold is not exceeded.

12. A display according to claim 11, further comprising a display calculator connected to said cathode ray tube for providing display signals thereto.

13. A display according to claim 11, wherein the durations of the vertical border edges of the image are indicated by variables T1HAV and T2HAV; and wherein said calculation circuit further comprises performing:
(a4) checking that the measurements of the durations T1HAV and T2HAV are a first measurement since a start of steps (a) or (b), and if a response of the checking is negative, then passing on to a sub-step (a6), and if the response of the checking is positive, then passing on to a sub-step (a5); and
(a5) comparing the measurements for T1HAV and T2HAV.

14. A display according to claim 13, wherein step (a4) further comprises:
setting a flag to a first logic value if T1HAV<T2HAV to indicate that the image is to be displaced towards the right;
setting the flag to a second logic value if T1HAV<T2HAV to indicate that the image is to be displaced towards the left; and
stopping the horizontal centering if T1HAV=T2HAV, since the image is centered horizontally, and passing on to step (b).

15. A display according to claim 14, wherein said calculation circuit further comprises performing:
(a6) checking whether the flag is at the second logic value, and if the checking is a positive response, then passing on to a next sub-step (a7), and if the checking is a negative response, then passing on to a substep (a8);
(a7) checking whether T1HAV<T2HAV, and if T1HAV=T2HAV since the image is horizontally centered, then stopping, and if the checking is a positive response, then passing on to a sub-step (a9), and if the checking is a negative response, then passing on to a sub-step (a10); and
(a8) checking whether T1HAV<T2HAV, and if T1HAV=T2HAV, since the image is horizontally centered, then passing on to step (b), and if the checking is a positive response, then passing on to a sub-step (a11), and if the checking is a negative response, then passing on to a sub-step (a12).

16. A display according to claim 15, wherein said calculation circuit further comprises performing:
(a9) decrementing by one unit the adjustment value for centering the image to displace the image by one incremental step to the left, and passing on to a sub-step (a13);
(a10) incrementing by one unit the adjustment value for centering the image to displace the image towards the right, since the image had been displaced by one incremental step too far towards the left, and stopping the horizontal centering since the image is centered horizontally and passion to step (b);
(a11) incrementing by one unit the adjustment value for centering the image to displace the image by one incremental step to the right, and passing on to sub-step (a13);
(a12) decrementing by one unit the adjustment value for centering the image to displace the image towards the left, since the image had been displaced by one incremental step too far towards the right, and stopping the horizontal centering since the image is centered horizontally and passing on to step Cb);
(a13) incrementing a ioop counter by one unit each time the sub-step (a9) or (a11) has been performed, then passing on to a following sub-step (a14); and
(a14) checking whether content of the loop counter has attained a third threshold, and if the checking is a positive answer, then stopping on account of an impossibility of adjustment, and if the checking is a negative answer, then restarting the loop at step (a2).

17. A display according to claim 11, wherein the durations of the vertical border edges of the image are indicated by variables T1HAV and T2HAV; and wherein calculating the adjustment of the horizontal dimension of the image in step (b) is based upon the formula:




  HSIZE
  =
  
    
      
        
          
            A
            Vopti
          
          ⁡
          
            (
            
              
                T
                ′
              
              /
              
                Td
                ′
              
            
            )
          
        
        -
        
          H
          AMPMIN
        
      
      
        
          H
          AMPMAX
        
        -
        
          H
          AMPMIN
        
      
    
    ×
    
      HSIZE
      MAX
    
  




wherein:
Avopti is an optimum amplitude of a current in a horizontal deflection coil of the cathode ray tube to obtain an image of optimum width, with the amplitude being measured for a type of cathode ray tube,
HSIZEMAX is a maximum value of the adjustment of the horizontal dimension of the image,
HAMPMAX is a maximum variation of a current in the horizontal deflection coil to obtain a maximum horizontal deflection, this value varies as a function of a horizontal scanning frequency;
HAMPMIN is a 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;
T′ is a total duration of a period T of a horizontal synchronization signal, from which are subtracted a duration of a flyback pulse TFLYBACK and a duration of safety margins, and
Td′=T (T1HAV+T2HAV+TFLYBACK), which is a duration of the image on the screen between the border edges.

18. A display according to claim 17, wherein the coefficient (T′/Td′) in the formula is replaced by 1.8T′−Td′/2.8Td′ to take into account a form of a curve for the current in the horizontal deflection coil.

19. A display comprising
a cathode ray tube for displaying an image thereon; and
a calculation circuit connected to said cathode ray tube for centering and dimensioning the image being displayed by
(a) measuring durations of vertical border edges of the image and iteratively modifying adjustment values for a horizontal centering of the image to obtain equal lateral vertical edges,
(b) measuring the durations of the vertical border edges of the image to calculate adjustment values for a horizontal dimension of the image to cause the vertical border edges to disappear, and
(c) measuring durations of horizontal border edges of the image 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 border edges to disappear and to center the image vertically;

said calculation circuit indicates durations of the vertical border edges of the image by variables T1HAV and T2HAV; and calculates the adjustment of the horizontal dimension of the image in step (b) is based upon the formula:





  HSIZE
  =
  
    
      
        
          
            A
            Vopti
          
          ⁡
          
            (
            
              
                T
                ′
              
              /
              
                Td
                ′
              
            
            )
          
        
        -
        
          H
          AMPMIN
        
      
      
        
          H
          AMPMAX
        
        -
        
          H
          AMPMIN
        
      
    
    ×
    
      HSIZE
      MAX
    
  




wherein:
Avopti is an optimum amplitude of a current in a horizontal deflection coil of the cathode ray tube to obtain an image of optimum width, with the amplitude being measured for a type of cathode ray tube,
HSIZEMAX is a maximum value of the adjustment of the horizontal dimension of the image,
HAMPMAX is a maximum variation of a current in the horizontal deflection coil to obtain a maximum horizontal deflection, this value varies as a function of a horizontal scanning frequency,
HAMPMIN is a 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,
T′ is a total duration of a period T of a horizontal synchronization signal, from which are subtracted a duration of a flyback pulse TFLYBACK and a duration of safety margins, and
Td′=T (T1HAV+T2HAV+TFLYBACK), which is a duration of the image on the screen between the border edges.

20. A display according to claim 19, further comprising a display calculator connected to said cathode ray tube for providing display signals thereto, and wherein said display calculator comprises a memory for recording the adjustment values obtained.

21. A display according to claim 19, wherein said calculation circuit further checks for stability of the image by
(a0) measuring a pair of durations of at least one of the vertical and horizontal border edges of the image;
(a1) subtracting two successive measurements to determine a variation in the durations;
(a2) comparing the variation with a first threshold, and stopping if the first threshold is exceeded or continuing if the first threshold is not exceeded; and
(a3) comparing the measurements with a second threshold, and stopping if the second threshold is exceeded or continuing with at least one of steps (a) and (b) if the second threshold is not exceeded.

22. A display according to claim 21, wherein the durations of the vertical border edges of the image are indicated by variables T1HAV and T2HAV; and wherein said calculation circuit further comprises performing:
(a4) checking that the measurements of the durations T1HAV and T2HAV are a first measurement since a start of steps (a), (b) or (c), and if a response of the checking is negative, then passing on to a sub-step (a6), and if the response of the checking is positive, then passing on to a sub-step (a5); and
(a5) comparing the measurements for T1HAV and T2HAV.

23. A display according to claim 22, wherein step (a4) further comprises:
setting a flag to a first logic value if T1HAV<T2HAV to indicate that the image is to be displaced towards the right;
setting the flag to a second logic value if T1HAV<T2HAV to indicate that the image is to be displaced towards the left; and
stopping the horizontal centering if T1HAV=T2HAV, since the image is centered horizontally, and passing on to step (b).

24. A display according to claim 23, wherein said calculation circuit further comprises performing:
(a6) checking whether the flag is at the second logic value, and if the checking is a positive response, then passing on to a next sub-step (a7), and if the checking is a negative response, then passing on to a sub-step (a8);
(a7) checking whether T1HAV<T2HAV, and if T1HAV=T2HAV since the image is horizontally centered, then stopping, and if the checking is a positive response, then passing on to a sub-step (a9), and if the checking is a negative response, then passing on to a sub-step (a10); and
(a8) checking whether T1HAV<T2HAV, and if T1HAV=T2HAV, since the image is horizontally centered, then passing on to step (b), and if the checking is a positive response, then passing on to a sub-step (a11), and if the checking is a negative response, then passing on to a sub-step (a12).

25. A display according to claim 24, wherein said calculation circuit further comprises performing:
(a9) decrementing by one unit the adjustment value for centering the image to displace the image by one incremental step to the left, and passing on to a sub-step (a13);
(a10) incrementing by one unit the adjustment value for centering the image to displace the image towards the right, since the image had been displaced by one incremental step too far towards the left, and stopping the horizontal centering since the image is centered horizontally and passing on to step (b);
(a11) incrementing by one unit the adjustment value for centering the image to displace the image by one incremental step to the right, and passing on to sub-step (a13);
(a12) decrementing by one unit the adjustment value for centering the image to displace the image towards the left, since the image had been displaced by one incremental step too far towards the right, and stopping the horizontal centering since the image is centered horizontally and passing on to step (b);
(a13) incrementing a loop counter by one unit each time the substep (a9) or (a11) has been performed, then passing on to a following sub-step (a14); and
(a14) checking whether content of the loop counter has attained a third threshold, and if the checking is a positive answer, then stopping on account of an impossibility of adjustment, and if the checking is a negative answer, then restarting the loop at step (a2).

26. A display according to claim 19, wherein the coefficient (T′/Td′) in the formula is replaced by 1.8T′−Td′/2.8Td′ to take into account a form of a curve for the current in the horizontal deflection coil.