Abstract:
To facilitate the adjustment of the S correction in apparatus comprising cathode-ray tubes (televisions, monitors), the process consists in displaying visual marks on the screen, at the location of the picture points which remain stationary as the amplitude of the S correction which is applied to the sawtooth current flowing through the deflection coils of the cathode-ray tube is varied.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     The present invention relates to a process for correction of the deformation of a picture formed by a cathode-ray tube and a device implementing this process. It relates more particularly to the adjusting of the S correction in a television set. 
     2. Description of the Related Art: 
     Cathode-ray tubes are presently the most commonly used display elements in television sets. Represented diagrammatically in FIG. 1 a  is a television  6  comprising a conventional cathode-ray tube. The latter comprises an electron gun  1  which produces an electron beam  2  which is accelerated and focused onto a luminescent screen  3  to form a point of the picture. Deflection coils  4 , outside the tube, deflect the electron beam electromagnetically so as to scan the screen and form a complete picture. 
     Let us recall that a television picture is generally formed of two fields, even and odd, and that the electron beam scans the screen twice from top to bottom and from left to right in order to display a complete picture. 
     In what follows we shall be interested solely in the vertical deflection of the electron beam. 
     In FIG. 1 c  we have represented the current flowing through the coils  4  of the vertical deflector. The general shape of this signal is that of a sawtooth whose rising segments  8  correspond to the scanning of the screen by the electron beam from top to bottom (the active part of the picture), and whose segments  9  correspond to the return of the electron beam to the top of the screen (field blanking interval), the video signal sending a black level to the screen throughout the duration of the segment  9  (the inactive part of the picture). The duration of time between the start of a segment  8  and the end of a consecutive segment  9  is equal to the duration of a field. 
     Returning to FIG. 1 a , it may be noted that the distance between the electron gun  1  and the surface of the screen  3  is not constant: it increases as we travel from the centre of the screen towards the top or the bottom. Indeed, in recent cathode-ray tubes termed “flat screens” the radius of curvature of the screen  3  onto which the electron beam  2  is projected is much greater than the radius of curvature R between the centre of deflection C of the electron beam and its theoretical surface of projection  5 . 
     The consequences of this are that, when no correction is made, the picture which appears on the screen  3  is deformed, especially at the top and bottom of the picture. Thus, if a test pattern having equidistant horizontal lines is projected onto the screen, a picture such as that represented in FIG. 1 b  is obtained in which the gap between the lines is greater at the top and bottom of the picture than in the middle. 
     To correct this phenomenon, it is known practice to apply a current to the coils of the vertical deflector, not a perfect sawtooth current, as represented in FIG. 1 c , but one in which the sawtooth exhibits “S”-shaped portions  18 , as represented in FIG. 1 e . This correction is dubbed “S correction”. The image obtained when the correction is optimal is represented in FIG. 1 d.    
     This S correction must be set at the end of the television production line by altering the amplitude of the “S” shape of the portions  18  relative to the uncorrected shape of the sawtooth  8 ′. During this adjustment, three points  10 ,  11 ,  12  of the portion  18  of the curve are not modified by the changes of amplitude of the S correction, these points will be called the fixed points of the S correction hereinafter. 
     In order for the correction of the picture to be optimal, it is also necessary for the S correction to be centred relative to the screen. That is to say, at the instants at which the sawtooth current passes respectively through the points  10 ,  11  and  12 , the electron beam must scan the first line, the middle line and the last line of the screen respectively. 
     Moreover, in certain television receivers it is known practice to generate a few overscan lines at the top and bottom of the screen, these lines being used to perform tests. For example, these lines can be used to transmit colour information and to carry out auto-regulation of the colour temperature. 
     These overscan lines must not of course be displayed on the screen since they carry no information relating to the “useful” television picture transmitted. Hence, an adjustment must be made at the end of the television manufacturing line such that the electron beam does not scan the surface of the screen during these overscan lines. 
     Hence, at the end of a television receiver production line an operator must adjust a number of parameters: the position of the overscan lines, the position and amplitude of the S correction, etc. 
     In practice, he proceeds as follows: 
     firstly he dispatches a test pattern to the television, termed the convergence pattern and composed of a grid of equidistant lines, and he marks the zones of the picture which correspond to the fixed points of the S correction, that is to say he modifies the amplitude of the S correction so as to view on the screen those lines which remain still; 
     once the position of these lines has been marked, he must secondly position them at the top, middle and bottom of the screen respectively by adjusting the amplitude and the offset of the sawtooth current flowing through the coils of the deflector; the purpose of this is to centre the fixed points of the S correction with respect to the picture formed on the screen and this makes it possible furthermore to avoid displaying the overscan lines on the screen; 
     thirdly, he must readjust the amplitude of the S correction until the lines of the grid pattern appear equidistant on the screen. 
     SUMMARY OF THE INVENTION 
     This series of adjustments turns out to be fairly lengthy and an object of the invention is to simplify these operations in such a way as to reduce the time required to adjust each television set. 
     To this end, the subject of the invention is a process for S correction of the deformation of a picture formed by a cathode-ray tube, the amplitude of the S correction applied to the sawtooth current flowing through the deflection coils of the cathode-ray tube being variable. According to the invention, the process comprises the step of displaying visual marks on the screen at the location of the picture points which remain stationary as the amplitude of the S correction is varied. 
     By virtue of the invention, the first step of the above-described adjustment performed at the end of production is eliminated since the operator directly views the position of the zones of the picture corresponding to the fixed points of the S correction. Moreover, this marking is more accurate than in the prior art since it does not depend on the operators subjective perception of the position of the lines of the convergence pattern. 
     According to one aspect of the invention, the visual marks displayed are black lines. 
     It is thus sufficient for the operator to dispatch for example a white test pattern to the screen so as easily to mark the black lines corresponding to the fixed points of the S correction. 
     According to a particular embodiment, the step of displaying visual marks comprises: 
     the generation of pulses whose duration is at least equal to a video line at the instants at which the curve of the current flowing through the deflection coils passes through points, termed the “fixed points” of the S correction, which are not affected by the variation in amplitude of said S correction; and 
     the insertion of said pulses into the “Super Sand Castle” signal generated by the circuit controlling the scanning of the cathode-ray tube, said pulses indicating the position of those lines for which a black level is to be displayed on the screen. 
     Thus, the invention is implemented using a signal which already exists in the scan control circuit, the “Super Sand Castle” signal, which is generated, inter alia, so as to compel the video processor to deliver a black level to the electron guns of the cathode-ray tube during the line flybacks and field blanking intervals of the electron beam. 
     According to one aspect of the invention, the visual marks displayed are coloured lines. 
     This provides the operator with more attractive additional display capabilities. 
     According to another particular embodiment, the step of displaying visual marks comprises: 
     the generation of pulses whose duration is at least equal to a video line at the instants at which the curve of the current flowing through the deflection coils passes through points, termed the “fixed points” of the S correction, which are not affected by the variation in amplitude of said S correction; and 
     the insertion of said pulses into the text insertion signal generated by a character generator so as to control the displaying of characters at the locations of the picture corresponding to said fixed points of the S correction. 
     Thus, according to this embodiment, the invention is implemented using another pre-existing signal: the text insertion signal which is customarily used to insert text into a video signal. 
     It is another advantage of the present invention that it can be implemented very easily using circuits and signals which already exist in television sets. 
     According to a preferred aspect of the invention, the process is applied to the S correction in the vertical deflection circuit of the cathode-ray tube. 
     The invention also relates to a device for the S correction of the deformation of a picture formed by a cathode-ray tube, implementing the process described above, and comprising: 
     a sawtooth signal generator whose output is linked to the input of an S correction sinusoid generator, which also receives reference voltages as input and which outputs an S correction sinusoid which vanishes for input voltage values which are equal to the reference voltages, 
     means for adding together the sawtooth signal delivered by the generator and said S correction sinusoid, 
     means for comparing the sawtooth signal with each respectively of the reference voltages and for generating pulses at the instants at which the sawtooth signal passes respectively through each of the reference voltages; and 
     means for displaying, as a function of the signal received containing said pulses, visual marks on the screen, at the location of the picture points which are not affected by the variation in amplitude of said S correction. 
     The invention also relates to a television set with cathode-ray tube comprising an S correction device as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will emerge through the following description of two particular non-limiting embodiments of the invention given with reference to the appended figures in which: 
     FIG. 1 a , described above, diagrammatically represents a television set comprising a cathode-ray tube; 
     FIG. 1 b , described above, shows the appearance on the screen of a picture corresponding to a test pattern of equidistant lines when no correction is made; 
     FIG. 1 c , described above, represents the current flowing through the coils of the vertical deflector in the absence of any correction; 
     FIG. 1 d , described above, shows the appearance on the screen of the same picture as in FIG. 1 b  with an S correction; 
     FIG. 1 e , described above, represents the current flowing through the coils of the vertical deflector, with an S correction; 
     FIG. 2 diagrammatically represents a first part of a device implementing the invention; 
     FIGS. 3 a  and  3   b  diagrammatically represent two embodiments of a second part of a device implementing the invention; 
     FIG. 4 represents the transfer function of a circuit element of the device of FIG. 2; 
     FIG. 5 represents the profile of certain signals generated by the device of FIG. 2; 
     FIGS. 6 a  and  6   b  respectively represent the profile of a video signal and of a so-called “Super Sand Castle” signal generated by the device of FIG. 3 a.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 represents a part of a circuit for controlling the vertical scan, also termed the field scan, of a cathode-ray tube. The function of this circuit is to deflect the electron beam vertically in synchronism with the video signal received. 
     This circuit comprises a vertical sawtooth generator  20  controlled by the field synchronization signal arising from the video signal received by the television set. The vertical sawtooth generator is generally formed, as is known to the person skilled in the art, by placing in series a current source and a switch controlled by the vertical synchronisation cues (or field synchronization cues), a capacitor mounted in parallel with the switch making it possible to produce the sawtooth by charging up and discharging. 
     The signal present at the output of the sawtooth generator  20  is dispatched to the input V IN  of an S correction sinusoid generator  21 . This generator has a transfer function such as that represented in FIG.  4 . On its output V OUT  it generates a sinusoid  31  which vanishes for three voltage values V B , V M  and V H  received on its input V IN  and which exhibits a negative value between V B  and V M  and a positive value between V M  and V H . The reference voltages V B , V M  and V H  delivered to the sinusoid generator  21  are chosen by construction to be such that V B  is greater than the minimum voltage of the sawtooth  30  generated by the generator  20  (represented in FIG. 5) and that V H  is less than the maximum voltage of said sawtooth  30 , V M  corresponding approximately to the voltage in the middle of the sawtooth  30 . Their values are for example V B =3 volts, V M =4 volts and V H =5 volts. These voltage values correspond, as will be seen later, to the bottom position, to the middle position and to the top position of what was referred to above as the fixed points of the S correction. The amplitude of the sinusoid can be adjusted in a manner known per se. 
     An adder  22  sums the sawtooth  30  generated by the generator  20  and the sinusoid generated by the generator  21 . The signal resulting from this summation, represented by dashes  32  in FIG. 5, is a sawtooth whose segment of smallest slope exhibits the general shape of an “S”. 
     S This signal is transmitted to a variable-gain voltage/current amplifier  23  which delivers the current having an S correction to the coils  24  of the vertical deflector of the cathode-ray tube. By adjusting the gain of the amplifier  23  it is possible to adjust the size of the picture on the screen, expanding it to a greater or lesser degree in the vertical direction. There is also provision to adjust the offset of the current delivered to the coils of the vertical deflector so as to shift the picture on the screen vertically, in such a way as to centre it. 
     It may be observed that when the S correction sinusoid generated by the generator  21  is added to the sawtooth generated by the generator  20 , certain points on the resulting curve  32  are not modified relative to the initial sawtooth  30 . These points labelled  40 ,  41  and  42  in FIG. 5 are for this reason referred to as the fixed points of the S correction. 
     Indeed, in the picture formed on the screen of the cathode-ray tube, the zones of the picture which correspond to these fixed points of the sawtooth current flowing through the coils of the deflector are not modified when the operator adjusts the amplitude of the S correction (that is to say the amplitude of the sinusoid). 
     Moreover, it may also be noted that two portions  33 ,  34 , situated respectively beneath the bottom fixed point  40  and above the top fixed point  42  of the segment of smaller slope of the sawtooth  30 , are also not affected by adding the sinusoid. These portions correspond to the overscan lines which are not to be displayed on the screen. This is why the top and bottom fixed points of the S correction should correspond exactly to the first and to the last line displayed on the screen. 
     According to the invention, the zones of the picture corresponding to the fixed points of the “S” correction are marked on the screen by an appropriate display when the operator adjusts the S correction. 
     To do this, returning to FIG. 2, the sawtooth generated by the generator  20  is dispatched to the input of three comparators  25 ,  26 ,  27  which each receive the reference voltages V B , V M  and V H  respectively on their second input. These voltages are the same as those which are delivered to the sinusoid generator  21 . 
     Each comparator  25 ,  26 ,  27  outputs a voltage strobe exhibiting a rising edge at the instant at which the sawtooth passes respectively through the voltage values V B , V M  and V H . These instants correspond, as seen above, to the instants at which the sawtooth passes through the fixed points of the S correction. 
     The strobes delivered by the comparators  25 ,  26 ,  27  are dispatched to the input of a signal processing logic circuit  28  which outputs a signal  35  exhibiting pulses having a duration of one line with each rising edge of a strobe input. 
     This signal  35  is a periodic signal whose period is equal to the duration of one field of a video signal and which comprises, within each period, three pulses of duration equal to one line. This signal will be used to mark on the screen the zones of the picture corresponding to the fixed points of the S correction. 
     Furthermore, an input  29  is provided in the signal processing logic circuit  28  to dispatch a signal C/I for turning on or turning off the generation of the signal  35 . Depending on the value (0 or 1) of the signal C/I received on the input  29 , the signal  35  will or will not be generated. Indeed, when the television operates normally, the marks of the fixed points should not be displayed on the screen so as not to disturb the picture displayed. 
     According to a first embodiment, the pulses of duration equal to one line of the signal  35  will be added to a signal referred to as a “Super Sand Castle” on account of its crenellated profile. 
     The “Super Sand Castle” signal is a signal which is generated in a known manner in scan control circuits and which is used, inter alia, to inform the video processor of the moments of line flyback or field flyback of the electron beam, also referred to as the line blanking time and field blanking time, during which a black level is to be dispatched to the screen. 
     An example of a “Super Sand Castle” signal has been represented in FIG. 6 b . This signal is formed from a video signal represented in FIG. 6 a . The video signal comprises line synchronization pulses  14 ,  14 ′ at the start of each video line, these being followed by bursts  15 ,  15 ′. The role of the burst is to transmit, in the video signal, a phase reference and frequency reference for the chrominance sub-carrier of the composite video signal. The video signal next comprises a part  16  which corresponds to the active part of the picture which will be displayed on the screen. In the present example, this is a white line since the signal is at its maximum level. The period commencing at the start of the line synchronization pulse  14  and terminating at the start of the active part  16  of the line is called the line blanking interval. 
     During the period labelled T ST , a number of pulses regulate the field blanking period. This period corresponds to the time required for the electron beam to climb to the top of the screen once it has scanned the screen from top to bottom during a field. 
     During the line blanking and field blanking periods, the video should display a black level so that the trace of the electron beam is not visible. 
     To do this, it is known practice to generate line blanking and field blanking signals comprising strobes of duration equal to the line and field blanking times. It is furthermore known practice to generate burst gate pulses over the duration of the burst  15 ,  15 ′ so as to extract the chrominance-related information from the video signal. 
     By adding together the strobes of the line blanking, field blanking and burst gate signals, a signal with three levels is obtained: the “Super Sand Castle” signal. 
     FIG. 6 b  shows the profile of the “Super Sand Castle” signal obtained. It exhibits a porch  53  corresponding to the field blanking level, porches  54 , having a level greater than the field blanking level, corresponding to the line blanking level, and porches  55 , having a level greater than the line blanking level, corresponding to the burst gate. This is why this signal is sometimes also called a “Sand Castle” signal with three levels. The “Super Sand Castle” signal, which is transmitted to the video processor, is compared with a reference voltage whose level lies between the zero level and the field blanking level  53 . Depending on this comparison, the video processor displays a black level on the screen, whenever the “Super Sand Castle” signal passes through one of the porches  53 ,  54 ,  55  mentioned above which are greater than said reference voltage. By contrast, when the “Super Sand Castle” signal is at its zero level  56 , below said reference voltage, the video signal received is displayed on the screen. 
     According to the invention, the signal  35  (FIG. 2) containing the pulses indicating the position of the fixed points of the S correction is added to the “Super Sand Castle” signal. In FIG. 6 b  it can therefore be seen that a line blanking pulse  17  has been inserted into the “Super Sand Castle” signal instead of the level  19  which it should have had. This pulse  17  informs the video processor that it should not display the video line  16 ′ and that it should display a black level instead. 
     The operator, by dispatching a white test pattern to the screen of the television, that is to say a video signal making it possible to display solely white lines, will therefore be able accurately to view the position of the locations of the picture corresponding to the fixed points of the S correction since a black line will be displayed at each of these locations. 
     To adjust the positioning of the S correction, it will therefore be sufficient for the operator to modify the amplitude (by modifying the gain of the amplifier  23 ) and the offset of the current dispatched to the coils of the vertical deflector until the black lines corresponding to the locations of the fixed points of the S correction are positioned at the bottom, in the middle and at the top of the screen. 
     The adjustment achieved is thus faster and more accurate than in the prior art since the operator continuously views the lines portraying the position of the fixed points on the screen. 
     FIG. 3 a  illustrates a way of obtaining the modified “Super Sand Castle” signal described above. 
     On a first input, a logic OR gate  36  receives the signal  35  arising from the signal processing logic circuit  28  (FIG. 2) and comprising the pulses for positioning the fixed points of the S correction and, on a second input, it receives the output from a comparator  37 . 
     On a first of its inputs, the comparator  37  receives a signal HFLY (standing for “Horizontal Flyback”) containing parabolic line flyback pulses which are generated in a known manner by the horizontal scan control circuit and whose duration corresponds to the line flyback interval. The second input of the comparator  37  receives a reference voltage V ref  chosen to be close to the minimum value of the pulses of the signal HFLY so that the comparator  37  outputs a logic signal comprising strobes of duration equal to the line flyback interval. 
     The output of the logic OR gate  36  is linked to a first input of a “Super Sand Castle” signal generator  38  which also receives, on two other inputs, on the one hand the burst gate signal and on the other hand the field blanking signal (which were defined above). 
     The “Super Sand Castle” signal generator  38  delivers the modified “Super Sand Castle” signal to an output pin  43  of the scan control processor  45  (represented partially in FIG. 3 a ), which pin is linked to an input pin  44  of the video processor  46 . 
     FIG. 3 b  illustrates a second embodiment of the invention, in which the signal  35  containing the pulses indicating the position of the fixed points of the S correction is used to control the displaying of particular characters on the screen so as to mark the position of the zones of the picture corresponding to said fixed points. 
     The signal  35  is dispatched to one of the inputs of a microprocessor  51  which controls both the video processor  46  and a character generator  50 , which is also sometimes referred to as an OSD generator (standing for “On Screen Display”). 
     The video processor  46  comprises on the one hand three RGB video inputs  47  which receive the RGB signals emanating from the composite video signal decoder (for example a PAL or SECAM decoder) of a television set or emanating from a video recorder. During normal operation, these RGB video signals are transmitted to the RGB output  52  which controls the electron guns (one per colour) of the cathode-ray tube. 
     The video processor  46  additionally comprises three RGB Text inputs  49  which are linked to the corresponding outputs of the character generator  50  as well as a FBTXT (standing for “Fast Blanking Text” or “fast text insertion”) input  48  which receives a text insertion signal likewise generated by the character generator  50 , which makes it possible to transmit to the RGB output  52  of the video processor, the RGB Text signals received on its inputs  49  instead of the RGB video signals. 
     This makes it possible, as is known, to insert text generated by the character generator  50  into a video image. 
     According to the second embodiment of the invention, the microprocessor  51  instructs the character generator  50  to insert the pulses of the signal  35  into the text insertion signal which is dispatched to the FBTXT input of the video processor  46  so as to force the video processor to display, a coloured line for example, at the locations of the picture corresponding to the fixed points of the S correction. 
     The coloured lines are generated in a manner known per se by the character generator  50  but any other display capability can be envisaged (arrow, text, etc.). 
     Of course, the invention is not limited to the embodiments described above and encompasses all variants. In particular, the signal  35  can have pulses of longer duration than the duration of one line, for example it can comprise pulses with a duration of two or three lines. It can also comprise pulses with a duration of less than one line, in particular in the second embodiment. 
     The invention applies also to cathode-ray tubes in which scanning is continuous (such as those present in video monitors) rather than interlaced (with an even and odd field forming a picture).