SPOT POSITION INDICATION SIGNAL GENERATION

Abstract: A spot position indication signal (A) is generated by an address generator (4). A deflection current (Ih, Iv) for deflecting an electron beam spot in one direction in a raster scanned cathode ray tube (2) has a shape for obtaining an approximate linear scan in the one direction. The address generator (4) supplies a spot position indication signal (A) which is related to the spot position if: the spot position indication signal (A) is a linear function in time: A(t) = AO + dA.t, and two predetermined (wished) addresses (A1, A2) occur at two selected levels (I1, I2) of the deflection current to which belong two positions on the picture tube screen.

Description: Spot position indication signal generation. The invention relates to a method and a circuit for generating a spot position indication signal in a cathode ray tube display arrangement in which displays are realised by line by line scanning of a display screen of a cathode ray tube by an electron beam spot, the cathode ray tube display arrangement compπsing a deflection circuit for generating a deflection current to deflect the electron beam spot in a first direction, wherein the deflection current has a shape for obtaining an approximately linear scan on the display screen in the first direction duπng a scan period The invention also relates to a cathode ray display apparatus comprising such a circuit for generating a spot position indication signal Such a spot position indication signal may be used for generating position dependent waveforms for correcting deflection errors ot the cathode ray tube such as convergence errors or east-west distortions, or tor use as a dynamic tocusing waveform, or as a waveform influencing the brightness of a displayed picture to compensate tor brightness non-uniformity on the cathode ray tube It is known to obtain a line position indication signal which indicates the vertical spot position on a raster scanned display screen tor every line by using an analog to digital converter (further referred to as ADC) The ADC measures a value of the vertical deflection current at a moment a line occurs The vertical position of the spot and thus of a line on the display screen is determined by the value of the vertical deflection current in this line. So, the ADC supplies a hne position indication signal that is a measure ot the vertical position of the spot on the screen. If a repetition frequency, or an amplitude of the vertical deflection current changes, still the ADC supplies the actual vertical spot position at the moment a line occurs as the vertical spot position is still determined by the vertical deflection current For example, in convergence circuits using the output signal of the ADC for calculating convergence waveforms, the ADC must have a resolution of approximately 13 bits for display systems displaying about 600 lines. A striping would become visible on the screen if a lower resolution would be used This striping is a brightness modulation caused by different distances between adjacent lines due to inaccuracy in the position of the lines Such a high resolution ADC is expensive It is an object of the invention to provide a simple and cheap spot position indication signal. To this end a first aspect of the invention provides a method for generating a spot position indication signal as defined in claim 1 . A second aspect of the invention provides a circuit for generating a spot position indication signal as is defined in claim 6. A third aspect of the invention provides a cathode ray tube display apparatus comprising a circuit for generating a spot position indication signal as is defined in claim 7. Advantageous embodiments are defined in the dependent claims. The invention according to claim 1 offers a very simple and inexpensive method for generating a position indication signal representing the spot position on the screen of a cathode ray tube. It is important to know the actual position of the spot if a waveform has to be generated which should be a function of the position on the screen. This means that at a certain position on the screen a certain associated value of the waveform must occur. Such a position dependent waveform may be used to correct for position dependent geometry errors such as east-west, north-south or convergence distortions, or to correct for brightness non-uniformity across the screen, or for obtaining a position dependent scan velocity modulation. It is common practice to generate the position dependent waveform as a function of a variable depending on the time or a line number. The position dependent waveform which is generated in this way depends on the amplitude and the frequency of the horizontal or vertical deflection. For example, suppose, the cathode ray tube display arrangement displays a PAL-picture with 625 lines which has a vertical amplitude suitable to scan the screen along the whole height. A position dependent waveform depending on time is generated to obtain a certain correction, for example an east-west correction. This waveform has a suitable shape and extends during a vertical scan period over the whole vertical height of the screen. If then the amplitude of the vertical scan is decreased to scan only a part of the height of the screen, the generated position dependent waveform will have the same suitable shape extending during the same vertical scan period but now across only part of the height of the screen. So, the values of the position dependent waveform occur at a wrong position as now the same correction is performed on a smaller part of the screen. At different vertical frequencies (by example: 50Hz PAL, 60Hz NTSC, and 45Hz free running) a different total number of lines appears (as the line frequency does not change significantly) and thus a different position dependent waveform is generated as the lines at different vertical frequencies appear at different positions on the screen. Thus, the position address should be related to the vertical spot position at a moment a line in the picture occurs to obtain a position dependent waveform independent of the frequency and amplitude of the vertical deflection. A same reasoning holds for a position waveform depending on the horizontal position on the screen. The invention provides a position information signal (further referred to as address) generator based on the insight that the spot position on the screen is a linear function in time. The spot position is a linear function in time if the deflection current has a shape for obtaining a linear scan on the picture tube screen. Further, use is made of the fact that a certain deflection current corresponds to a certain position on the picture tube screen. As, in case of a linear scan, the spot position on the screen is a linear function of time, the address generator should generate an address which is a linear function in time. The address generator supplies an address which represents the spot position on the screen if the linear function in time representing the address is coupled to the linear function in time representing the spot position. Thus, the address generator supplies an address which is related to the spot position if two predetermined (desired) address values occur at two selected levels of the deflection current to which belong two positions on the picture tube screen. The actual value of an address at a certain moment depends on the choice of the predetermined address values which have to occur at the selected positions on the screen. Therefore, the invention determines in one scan period a first and a second moment in time at which a first and a second selected level of the deflection current is reached, respectively. The address which is generated as a linear function is related to the spot position if at the first and second moments in time, in a steady state, the address has the predetermined address values (position indication values), respectively. An embodiment of a method for generating a spot position indication signal (or address) according to the invention as claimed in claim 2 measures the two moments in time at which the two selected levels of the deflection current occur in a simple way. The count value occurring at the moment that a corresponding one of the selected levels is reached is stored. In an embodiment of the method according to the invention as claimed in claim 3 the address is written as a linear function in time which comprises an initial position indication value (further referred to as initial value) added to an incremental value multiplied with the time The initial value and the incremental value used in a certain scan period are determined from two linear equations The two linear equations are obtained by substituting in the linear function representing the address the two moments or the two mean values of these moments on which the deflection current reaches the two selected values in the preceding scan period, and the above mentioned predetermined address values at these moments, respectively In this way the address in the certain scan period is generated based on the initial value and the incremental value obtained in an easy way from the two linear equations. In an embodiment of the method according to the invention as claimed in claim 4, again, the address is written as a linear function in time which comprises the initial value added to the incremental value multiplied with the time. Now, the initial value and the incremental value used in a certain scan period are determined in a closed loop manner Therefore, the first and second values ot the address occurring at the first or second moment, are determined. These first and second addresses are compared with the predetermined address values, respectively The initial value and the incremental value tor use in the certain scan period are determined from the difference between these first and second addresses as occurring in a preceding scan period and the predetermined address values The determination ot the initial value and the incremental value can be performed in numerous ways depending on the requirements the method tor generating an address has to fulfil. An advantageous embodiment is described in the description of the Figures An embodiment ot the method according to the invention as claimed in claim 5 generates the address in a certain line by adding the incremental value to the address of the preceding line In this way the multiplication ot the incremental value with the time is replaced by a simple addition These and other aspects will be described and elucidated with reference to the accompanying drawings Figure 1 shows a cathode ray display apparatus with a position indication signal generator according to the invention, Figure 2 shows a waveform ot a deflection information being related to a deflection current, Figure 3 shows a waveform representing the position indication signal, Figure 4 shows two graphs representing two vertical deflection currents which have different amplitudes, Figure 5 shows a picture to elucidate the relation between the vertical deflection current, the vertical position on the screen and the position indication signal with respect to the two vertical deflection currents shown in Figure 4, Figure 6 shows two graphs representing two vertical deflection currents which have a scan period with a different duration, Figure 7 shows a picture to elucidate the relation between the vertical deflection current, the vertical position on the screen and the position indication signal with respect to the vertical deflection currents shown in Figure 6, Figure 8 shows an embodiment of a position indication signal generator according to the invention, Figure 9 shows an embodiment of a third calculation unit, and Figure 10 shows a diagram of the position indication signal to elucidate the operation of the embodiment of the position indication signal generator of Fig. 8 Figure 1 shows a cathode ray display apparatus with a position indication signal (or address) generator 4 according to the invention Figure 2 and 3 show waveforms elucidating the operation of the address generator 4. Figure 2 shows a waveform of a position information I being related to a deflection current lh; Iv The deflection current Ih,Iv may be the vertical Iv or horizontal lh deflection current. The address generator 4 has to supply an address A which is a linear function m time, as is shown in Figure 3 A(t) = AO + dA t wherein AO is an initial value and dA is an incremental value The address A is related to the spot position it two desired addresses A 1 ,A2 occur at two selected levels of the deflection current Ih;Iv This holds it the deflection current Ih;Iv has a shape for obtaining a substantial linear scan on the screen of the cathode ray tube 2. As, under this condition the spot position on the screen is a substantial linear function in time The address generator 4 generates the address A every scan period, the time t is relative within each scan period Two positions on the screen belong to two selected levels of the deflection current Ih;Iv. These two selected levels of the deflection current are represented by two levels 11 ,12 of the position information I, see Figure 2 These two levels 11 ,12 occur at two moments T1 ,T2, respectively The address A is generated to obtain the two wished addresses A 1 ,A2 at the two moments T1 ,T2, respectively This will be further elucidated below. An address generator 4 according to the invention as is shown in Figure 1, comprises a first and a second comparator 40,41 both receiving a position information I which is related to the deflection current and each receiving a reference level indicating the two levels 11 ,12 for supplying a first and second comparator signal Csl , Cs2, respectively, to a first and a second latch 42,43. The position information I may, tor example, be obtained (not shown) in a known manner from the deflection current lh; Iv via a current transformer, or a resistor through which the deflection current flows, or, if the deflection current Ih;Iv is generated by a power amplifier in a feedback loop, by a reference waveform to which the deflection current Ih;Iv is compared. Instead of the latches 42,43 any other storage circuit may be used. A counter 44 receives a reset signal R being active during a flyback period of the deflection current, and a clock signal Clk which is summed during the trace period to obtain a count value C which is supplied to a further input of both the first and second latch 42,43. A first calculation unit 45 receives an output value O l from the first latch 42, and an output value 02 from the second latch 43, to calculate the initial value AO and the incremental value dA The first calculation unit 45 receives a start information S indicating that new initial and incremental values A0,dA have to be calculated The start information may be related to the second moment T2 as after this second moment T2 all necessary information for the calculation is available The start information may also be the reset signal R. A second calculation unit 46 calculates the address A to supply the address A according to the expression: A(t) = AO + dA.t The second calculation unit 46 receives the reset signal R indicating that the new initial and incremental values A0,dA calculated by the first calculation unit 45 have to be used to calculate the address A in a next scan period (field or line). It is possible to combine the first and second calculation units 45,46 in one calculation unit If the address A represents a vertical position ot horizontal scanned lines the above expression may be written as: ι = hne A (line) = A0 + Σ dA ι = l wherein the address A is calculated to be the initial value A0 to which is added in every line the incremental value dA. In this case, the second calculation unit 46 only needs to perform an addition in every line instead ot multiplying the incremental value dA with the time t. The first and second comparator signals Csl ,Cs2 indicate two moments in time T1 ,T2 (Figure 2) at which two selected levels 11 ,12 of the position information I occur which correspond to the two selected levels of the deflection current Ih,Iv. A first count value Cl generated by the counter 44 is stored by the first latch 42 at the moment Tl that the first comparator signal Cs l indicates that the position information I has the first selected value I I . The second latch stores in the same way a second count value C2 at the moment that the second comparator 41 indicates that the position information I has the second selected value 12. The clock signal CLK needs to have a sufficient high repetition rate to be able to obtain sufficiently accurate count values Cl and C2. The first calculating unit 45 calculates the initial value AO and the incremental value dA from the first and second stored values C1 ,C2 knowing that these values represent the time moments T1 ,T2 at which the wished address values A l and A2 should occur, respectively The initial value AO and the incremental value dA are defined by the formulae. AO = (T1 .A2-T2.A 1 )/(T1 -T2) dA = (A 1 -A2)/(T1-T2) These formulae follow from a substitution of the measured moments T1 ,T2 and the wished address values A 1 ,A2 occurring at these moments in the linear function representing the address A. So, an address generator is realised for calculating with the second calculation unit 46 an address A from the initial value AO and the incremental value dA in such a way that, in a steady situation, at the moment the first comparator 40 detects a first selected level of the deflection current, the address A is equal to the first desired address value A l , and that at the moment the second comparator 41 detects a second selected level of the deflection current, the address A is equal to the second desired address value A2 The selected values of the deflection current Ih;Iv have to be selected such that in every display mode (by example a vertical compress mode to display pictures having a 16:9 aspect ration on a picture tube having a 4:3 aspect ratio) the deflection current Ih;Iv covers these values. Preferably, the selected values of the deflection current Ih;Iv are selected as far away as possible from each other to obtain a maximal accuracy. In case the position address A represents the vertical position on the screen, in the way described above, it is possible to obtain an address A which for every horizontal line represents the vertical spot position on the screen, independent on the deflection amplitude or frequency This will be further elucidated in the description of Figures 4, 5, 6 and 7. The address A in a certain line may be generated by adding the incremental value dA to the address A of a line preceding the certain line This addition can be performed with a suitably programmed computer or with the second calculating unit 46 adapted to be a hardware adder. Also the counter 44, the first and second latches 42,43 and the first calculating unit 45 may be replaced by a suitable programmed computer. An offset value has to be added to the initial value AO depending on the field if a picture is composed by interlaced fields. The address generator 4 according to the invention also supplies an address A which is independent on the deflection amplitude or frequency if the deflection current has a shape for obtaining only an approximate linear scan on the screen instead of a substantial linear scan. This is based on the insight that the absolute accuracy is not very important as a small difference between the actual spot position and the address A (a few lines if the address A is the vertical address) still produces an acceptable waveform. This difference causes a fixed error which will be compensated for by adjusting the shape of the waveform. In case the waveform generator is used to generate convergence waveforms, the differential accuracy of a vertical address from field to field is also not very critical, the difference should be below about one eight of a distance between two consecutive lines to obtain a good interlace. The differential accuracy (in case of a vertical address: from line to line) must be very high, otherwise striping occurs. As the address A is a linear function this only imposes a condition on the number of bits used to represent the address, this number must be high enough (about 14 bits in the vertical direction) to avoid striping. It is especially this high demand on differential accuracy that makes the known ADC expensive. Figure 4 shows two graphs representing two vertical deflection currents Ivl ,Iv2 which have different amplitudes during a scan period with a duration Ts. A first vertical deflection current Iv l starts at moment 0 with a start current value Is 1 , and ends at the moment Ts with an end current value lei . A second vertical deflection current Iv2 starts at moment 0 with a start current value Is2 < Is I , and ends at moment Ts with an end current value Ie2. As an example both vertical deflection currents Iv l ,Iv2 are S-corrected sawtooth waveforms which cause an approximately linear vertical scan on the cathode ray tube screen. The first and second vertical deflection current Iv l ,Iv2 reach a first predetermined value I I at moments T l and Tl ' , respectively. The first and second vertical deflection current Iv l ,Iv2 reach a second predetermined value 12 at moments T2 and T2\ respectively. Figure 5 shows a picture to elucidate the relation between the vertical deflection current Iv, the vertical position on the screen and the position indication A with respect to the vertical deflection currents Iv l ,Iv2 shown in Figure 4. The dashed line 3 represents a cathode ray tube screen with an aspect ratio of 4:3. The line positioned left from the screen 3 indicates values of the vertical deflection current Iv. At this line are indicated: the start current Isl and end current lei related to the first vertical deflection current Ivl as shown in Figure 4; the start current Is2 and end current Ie2 related to the second vertical deflection current Iv2; and the first and second predetermined vertical deflection current values 11 ,12. To a certain value of the vertical deflection current Iv corresponds in a one to one relation a certain vertical position on the screen 3. The first line Sc i indicates the scanned part of the screen 3 belonging to the first vertical deflection current Iv l . The first line Sci starts at the vertical position Psi corresponding to the start current Isl and ends at the vertical position Pel corresponding to the end current Iel . In this example the first vertical deflection current Iv l is chosen to obtain a vertical scan larger than the height of the screen 3. The second line Sc2 indicates the scanned part of the screen 3 belonging to the second vertical deflection current Iv2. The second line Sc2 starts at the vertical position Ps2 corresponding to the start current Is2 and ends at the vertical position Pe2 corresponding to the end current Ie2. In this example the second vertical deflection current Iv2 is chosen to obtain a vertical scan smaller than the height of the screen 3, for example for displaying a display information with an aspect ratio of 16:9. The predetermined vertical deflection current values 11 ,12 correspond to the vertical positions P l and P2, respectively. The references Tl , TT associated with the vertical position Pl denote the moments at which the first and the second vertical deflection current Ivl ,Iv2 reaches the first predetermined value I I , respectively (see also Figure 4). The references T2, T2' denote the moments at which the first and the second vertical deflection current Ivl ,Iv2 reaches the second predetermined value 12, respectively. The line positioned right from the screen 3 indicates values of the vertical spot position indication signal or the vertical address Av according to the invention in relation to the vertical position on the screen 3. The vertical address Av generated in response to the first vertical deflection current Iv l follows from the two equations: Av(Tl ) = AO + dA.Tl = A l Av(T2) = AO + dA.T2 = A2 Due to the first equation it is ensured that at moment Tl at which the vertical current Iv has the first predetermined value I I a vertical address Av is generated equal to a chosen value A 1 . Due to the second equation it is ensured that at moment T2 at which the vertical current Iv has the second predetermined value 12 a vertical address Av is generated equal to a chosen value A2. From this two equations the initial and incremental values AO, dA can be calculated, the calculated values substituted in the linear function representing the vertical address Av gives: Av(t) = (A 1.T2-A2.T1)/(T2-T1) + t (A2-A 1)/(T2-T1) wherein Av(Tl ) = A l and Av(T2) = A2. So, the vertical address Av(t) depends on a choice of the first and second address values A 1 ,A2. As the first vertical deflection current Iv l has a shape to obtain a linear vertical scan it is sufficient to lock the vertical address Av on two positions P1 .P2 to the screen. All other positions on the screen will then be locked to the vertical address Av as the vertical address Av is a linear function in tune. This means that the start value A0=0 corresponds to the vertical position Psi and that the end value As corresponds to the vertical position Pel In the following is elucidated that the vertical address Av generated as described above supplies the same address values at the same vertical positions if the amplitude of the vertical deflection current is changed The vertical address Av generated in response to the second vertical deflection current Iv2 follows from the two equations Av(Tl ') = AO + dA T T = A l Av(T2') = AO + dA T2' = A2 Due to the first equation it is ensured that at moment TT at which the vertical current Iv has the first predetermined value I I a vertical address Av is generated equal to a chosen value A l . Due to the second equation it is ensured that at moment T2' at which the vertical current Iv has the second predetermined value 12 a vertical address Av is generated equal to a chosen value A2 From these two equations the initial and incremental values AO, dA can be calculated, the calculated values substituted in the linear function representing the vertical address Av gives: Av(t) = (A l T2'-A2 Tl ')/(T2'-Tr) + t (A2-A 1 )/(T2'-T1 ') wherein Av(Tl ') = A l and Av(T2') = A2 Thus, the vertical address Av has the same first address value A l at the first vertical position Pl at which the vertical current has the first predetermined value I I And, the vertical address Av has the same second address value A2 at the second vertical position P2 at which the vertical current has the second predetermined value 12 As also the second vertical deflection current Iv2 has a shape to obtain a linear vertical scan it is sufficient to lock the vertical address Av on two positions P 1 ,P2 to the screen. All other positions on the screen will than be locked to the vertical address Av as the vertical address Av is a linear function in time. So, the vertical address generator 4 generates a vertical address Av which has the same values at same positions on the screen 3 although the amplitude of the vertical deflection current Iv has been changed. Figure 6 shows a first and a second vertical deflection current Ivl ,Iv2 which have a scan period with a different duration Ts l ,Ts2, respectively. The first and second deflection current Ivl ,Iv2 have the same amplitude. The first deflection current Ivl starts at moment 0 with a start current value Is, and ends at the moment Tsl with an end current value Ie. The second deflection current Iv2 starts at moment 0 with the same start current value Is, and ends at the moment Ts2 with the same end current value Ie. As an example both vertical deflection currents Iv 1 , Iv2 are S-corrected sawtooth waveforms which cause an approximate linear vertical scan on the cathode ray tube screen. The first and second vertical deflection current Iv l ,Iv2 reach a first predetermined value I I at moments Tl and TT , respectively. The first and second vertical deflection current Iv l ,Iv2 reach a second predetermined value 12 at moments T2 and T2\ respectively. Figure 7 shows a picture to elucidate the relation between the vertical deflection current Iv, the vertical position on the screen and the position indication A with respect to the vertical deflection currents Iv l . Iv2 shown in Figure 6. The dashed line 3 represents a cathode ray tube screen with an aspect ratio of 4:3. The line positioned left from the screen 3 indicates values of the vertical deflection current Iv. At this line are indicated: the start current Is, the end current Ie, and the first and second predetermined vertical deflection current values 11 ,12. With a certain value of the vertical deflection current Iv corresponds in a one to one relation a certain vertical position on the screen 3. The first line Sc i indicates the scanned part of the screen 3 belonging to the first vertical deflection current Ivl . The second line Sc2 indicates the scanned part of the screen 3 belonging to the second vertical deflection current Iv2. The first and second lines Sc i , SC2 both start at the vertical position Ps corresponding to the start current Is and both end at the vertical position Pe corresponding to the end current Ie. The vertical address Av generated in response to the first vertical deflection current Iv l follows from the two equations: Av(Tl ) = AO + dA.Tl = A l Av(T2) = AO + dA.T2 = A2 Again, the first and second predetermined vertical deflection current values 11 ,12 occurring at the moments T1 ,T2, respectively are locked to a first and a second predetermined vertical address value A 1 ,A2, respectively. All other vertical address values are locked to a vertical position, as the vertical address Av is a linear function of time and the shape of the first vertical deflection current Iv l causes a linear vertical scan. The vertical address Av generated in response to the second vertical deflection current Iv2 follows from the two equations: Av(Tl ') = AO + dA.TT = A l Av(T2') = AO + dA.T2' = A2 Now, the first and second predetermined vertical deflection current values 11 ,12 occurring at the moments T1 \T2\ respectively are locked to the first and a second predetermined vertical address value A 1 ,A2, respectively. Again, all other vertical address values Av are locked to a vertical position, as the vertical address Av is linear function of time and the shape of the second vertical deflection current Iv l causes a linear vertical scan. Figure 8 shows an embodiment of a position indication signal generator according to the invention. A same second calculation unit 46 as described in Fig. 1 is used. This second calculation unit 46 calculates the address A k (t) during a certain scan period designated by k. The address A k (t) is calculated as a linear function from the initial value A0 k .| and the incremental value dA k ., as determined by a third calculation unit 47 during the previous scan period k- 1 . The second calculation unit 46 receives the moments Tl , T2 at which the deflection current Ih;Iv reaches the first and second predetermined values I I , 12, respectively, during the certain scan period k. The moments Tl , T2 may also be mean values of moments at which in a number of scan periods preceding the certain scan period k the first and second predetermined values I I , 12 occur, respectively. The second calculation unit 46 supplies the values A T, A2' of the address A k as occurring at the moments Tl and T2. The embodiment of the position indication signal generator further comprises a first and a second comparing stage 48, 49. The first comparing stage 48 compares the address value A T with the desired address value A l to obtain a first difference value dA l k . The second comparing stage 49 compares the address value A2' with the desired address value A2 to obtain a second difference value dA2 k . The third calculation unit 47 calculates the initial value AO and the incremental value dA from the first and the second difference value dA l k , dA2 k . In a steady state situation, the first and the second difference value dA l k , dA2 k are zero, and the address A has the predetermined values A 1 ,A2 at the first and the second moments T1 ,T2 in time. At the moments T1 ,T2 the deflection current values 11 ,12 occur. To these deflection current values 11 ,12 correspond fixed positions on the display screen. The address generator supplies the addresses A 1 ,A2 at the moments T1 ,T2, and thus are these addresses A 1 ,A2 indicating the fixed positions on the display screen. In this closed loop system, it is possible to reach the steady state with the calculation performed by the third calculation unit 47 m many ways. A very effective embodiment is shown in Fig. 9. Figure 9 shows an embodiment of the third calculation unit 47. A first subtractor 470 subtracts the first difference signal dA l k from the second difference signal dA2 k , and has an output which is connected to a first input of a first multiplier 471 and to a first input of a second multiplier 475. The first multiplier 471 has a second input receiving the value T1 /(T2-T1 ) and an output for supplying an output value which is the multiplication of the values occurring at its first and its second input A second subtractor 472 subtracts the output value of the first multiplier 471 from the first difference signal dA l k to obtain a value VI . The value V I is supplied to a first time discrete integrator comprising an adder 473 and a delay or storage circuit 474 The delay or storage circuit 474 has an input which receives the addition of the value V I and the output value of the delay or storage circuit 474. The output value of the delay or storage circuit 474 is the initial value A0 k . The delay or storage circuit 474 delays or stores the input value for at least one scan period The second multiplier 475 has a second input receiving the value 1/(T2-T1 ) and an output for supplying an output value V2 which is the multiplication of the values occurring at its first and its second input. The value V2 is supplied to a second time discrete integrator comprising an adder 476 and a delay or storage circuit 477 The second time discreet integrator supplies the incremental value dA k and is arranged and operates in the same manner as the first time discreet integrator 474. Figure 10 shows a diagram ot the address A(t) to elucidate the operation of the embodiment of the position indication signal (address) generator of Fig. 8 which comprises a calculation unit 47 as shown in Fig. 9. It is assumed that the address generator is in a steady state situation up to and including a scan period k- 1 . During the scan period k- 1 , the address generator supplies an address A k ,(t) which is locked to the deflection current Iv;Ih as the address A k ,(t) has the desired value A l at moment T l at which the deflection current Ih.Iv has the predetermined value I I , and the address A k ,(t) has the desired value A2 at moment T2 at which the deflection current Ih;Iv has the predetermined value 12 Duπng the next scan period k it appears that the deflection current Ih;Iv changed in amplitude or duration. Despite this change of the deflection current Ih;Iv, the address A k (t) during the next scan period k is calculated by the second calculating unit 46 by using the initial value AO k , and the incremental value dΛ k , as calculated by the third calculating unit 47 during the scan period k-1, these values can be written as: A0 kl = (A2.T1-A1.T2)/(T1-T2) dA k l = (A1-A2)/(T1-T2) This gives rise to the address values Al and A2 at the moments Tl and T2. However, during this next scan period k, the changed deflection current Ih;Iv reaches the predetermined values II, 12 at the moments TT and T2', respectively. The second calculating unit 46 stores the address values occurring at these moments T1\T2' as. A k rττ) = AO M + dA M . TΓ = AΓ A„(T2') = A0 k , + dA k , . T2' = A2' The difference values dAl k ,dA2 k between the address values A1\A2' as occurring at the moments T1',T2' and the desired address values A1,A2 is determined by the first and second substractors 48, 49. The difference values dAl k ,dA2 k can be written as: dAl k = Al - AT = Al - A0 k l - dA k , . TT (1) dA2 k = A2 - A2' = A2 - A0 k , - dA k , . T2' (2) The third calculating unit 47 calculates from these difference values dAl k ,dA2 k an initial value A0 k and an incremental value dA k for use in the succeeding scan period k+1. The address generator would be locked to the deflection current Ih;Iv as desired, if during the scan period k+1 would be valid: A k+1 (TD = Al = A0 k + dA k . TT A k + ,(T2') = A2 = A0 k + dA k . T2' Because, in this case, again is valid that addresses A1,A2 correspond to the fixed positions on the screen which are determined by the predetermined values 11,12 of the deflection current Ih,Iv, respectively. From these two equations it follows that the initial value A0 k and the incremental value dA k should be calculated by the third calculating unit 47 to be: A0 k = (A2.Tr-A l .T2 , )/(Tr-T2') So, if this initial value A0 k and incremental value dA k are the outcome of the calculation performed by the third calculation unit 47 before the start of the scan period k+ 1 , then, the address A k + ,(t) calculated by the second calculation unit 46 during the scan period k+ 1 is locked to the position on the display screen. This has as a consequence that the address values occurring at the moments Tl ' and T2' equal the values A l and A2, respectively. So, the difference values dA l k + l ,dA2 k + , are zero during the scan period k+ 1 : dA l k + 1 = A l - A0 k - dA k . TT = 0 (3) dA2 k + l = A2 - A0 k - dA k . TT = 0 (4) Substitution of the address values A 1 ,A2 according to the equations (3) and (4) in the equations ( 1) and (2) gives: dA l k = A0 k - A0 k ., + (dA k - dA k .,) . TT (5) dA2 k = A0 k - A0 k ., + (dA k - dA k .,) . TT (6) From these two equations follow the initial value A0 k and the incremental value dA k written in terms of the difference values dA l k ,dA2 k : A0 k = A0 k ., + (dA l k .Tr- dA2 k .T2')/fTT-Tr) dA k = dA k ., + (dA l k - dA2J/(Tr-T2') Therefore, with the embodiment of the third calculating unit 47 as shown in Fig. 9, the address generator is locked to the changed deflection current Ih; Iv at two values 11 ,12 of the deflection current Ih;Iv in the scan period next to the scan period in which the deflection current changed. Because the deflection current has a shape to obtain an approximate linear scan on the display screen, the linear address generator is locked to the position on the display screen. The generation of an amplitude and frequency independent address A, as described above, can also be used in conjunction with so called transposed scanned cathode ray tubes whereby vertical scanned lines succeed in horizontal direction. Any reference signs in the claims cannot be construed as limiting the claim.

Claims: CLAIMS 1. In a cathode ray tube display arrangement m which displays are realized by line by line scanning of a display screen of a cathode ray tube (2) by an electron beam spot, the cathode ray tube display arrangement comprising a deflection circuit ( 1) for generating a repetitive deflection current (Ih;Iv) to deflect the electron beam spot in a first direction, wherein the deflection current (Ih;Iv) has a shape for obtaining an approximate iinear scan on the display screen in the first direction during a scan period, a method for generating a spot position indication signal (A), characterized in that the method comprises the steps of. generating ( 1 ), in a certain scan period, a position information (I) being related to the deflection current (Ih;Iv), measuring (40,42,44) a first moment in time (Tl ) at which the position information (I) has a first value (I I ), measuring (41 ,43,44) a second moment in time (T2) at which the position information (I) has a second value (12), calculating (45), in a later scan period, the spot position indication signal (A) as a linear function in time whereby, in a steady state situation, the spot position indication signal (A) has predetermined position indication values (A 1 ,A2) at corresponding first and the second moments in time (T1 ,T2) in this later scan period, respectively 2. A method as claimed in claim 1 , characterized in that the step of determining (40,42,43;41 ,43,44) the first and second moment in time (T1 ,T2) comprises the steps of: generating (44) a count value (C) indicative for a period of time, storing (42) a first count value (O l ) in response to detecting (40) that the position information (I) has the first value (I I ), the first count value (01 ) being representative for the first moment in time (Tl ), and storing (43) a second count value (O2) in response to detecting (41) that the position information (I) has the second value (12), the second count value (02) being representative for the second moment in time (T2) 3. A method as claimed in claim 1 , characterized in that the step of calculating (45) the spot position indication signal (A) in the certain scan period as a linear function in time comprises the step ot calculating A(t) - AO + dA t wherein AO is an initial position indication value and dA is an incremental value which are determined by AO = (T1 .A2-T2 A 1 )/(T1 -T2) dA = (A 1 -A2)/(T 1 -T2), wherein Tl is the first moment in time, T2 is the second moment in time, and A l and A2 are the predetermined position indication values (A 1 ,A2) at the first and second moments in time (T1 ,T2), respectively. 4. A method as claimed in claim 1 , characterized in that the method comprises the further steps ot determining a first value (A(T1 )) ot the spot position indication signal (A) at the first moment (T l ), determining a second value (A(T2)) ot the spot position indication signal (A) at the second moment (T2), and comparing the first and the second value (A(T1),A(T2)) with the predetermined position indication values (A 1 ,A2), respectively, to obtain difference values (dA l ,dA2), whereby the step ot generating (45) the spot position indication signal (A) in the later scan period as a linear function in time comprises the step ot calculating A(t) = AO + dA.t wherein AO is an initial position indication value and dA is an incremental value, the initial position indication value (AO) and the incremental value (dA) being determined based on the differences values to obtain, in a steady state situation, the spot position indication signal (A) having predetermined position indication values (A 1 ,A2) at the first and the second moments in time (T1 ,T2), respectively 5. A method as claimed in claim 3 or 4, characterized in that the spot position indication signal (A) is indicating a vertical position ot horizontally scanned lines, and wherein the step of calculating (45) the position indication signal (A) comprises the step of summing the incremental value (dA) to a value ot the spot position indication signal (A) representing the position of a line to obtain the spot position indication signal (A) of a succeeding line. 6. A circuit (4) tor generating a spot position indication signal (A), the circuit (4) receiving a deflection current (Ih,Iv) tor scanning an electron beam spot in a first direction along a display screen ot a cathode ray tube (2) line by line, wherein the deflection current (Ih;Iv) has a shape tor obtaining an approximately linear scan on the display screen in said first direction, characterized in that the circuit (4) comprises means tor generating ( 1 ), in a certain scan period, a position information (I) being related to the deflection current (Ih;Iv), means tor measuring (40,42,44) a first moment in time (Tl) at which the position information (I) has a first value (I I ), means for measuring (41 ,43,44) a second moment in time (T2) at which the position information (I) has a second value (12), means for calculating (45), in a later scan period, the spot position indication signal (A) as a linear function in time whereby, in a steady state situation, the spot position indication signal (A) has predetermined position indication values (A 1 ,A2) at corresponding first and the second moments in time (T1 ,T2) in this later scan period, respectively 7 A cathode ray tube display apparatus in which displays are realized by line by line scanning of a display screen of a cathode ray tube (2) by an electron beam spot, the cathode ray display apparatus comprising: a deflection circuit ( 1 ) receiving a horizontal or a vertical position signal (H,V) for generating a deflection current (Ih. lv) for deflecting the electron beam spot in a first direction, wherein the deflection current (Ih,Iv) has a shape tor obtaining an approximate linear scan on the display screen in said first direction, and a circuit (4) tor generating a spot position indication signal (A), characterized in that the circuit (4) for generating a spot position indication signal (A) comprises. means for generating ( 1 ), in a certain scan period, a position information (I) being related to the deflection current (111, Iv), means for measuring (40,42,44) a first moment in time (Tl ) at which the position information (I) has a first value (I I ), means for measuring (41 ,43,44) a second moment in time (T2) at which the position information (I) has a second value (12), means tor calculating (45), in a later scan period, the spot position indication signal (A) as a linear function in time whereby, in a steady state situation, the spot position indication signal (A) has predetermined position indication values (A 1 ,A2) at corresponding first and the second moments in time (T1 .T2) in this later scan period, respectively.

Inventors: VAN DALFSEN AGE JOCHEM, VAN DALFSEN, AGE, JOCHEM
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H04N1
IPC codes: G09G1, H04N3, H04N5, H04N17