WO2005029841A1

WO2005029841A1 - Display apparatus

Info

Publication number: WO2005029841A1
Application number: PCT/IB2004/051696
Authority: WO
Inventors: Robert H. De Nie; Leendert A. D. Van Den Broeke
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-09-22
Filing date: 2004-09-07
Publication date: 2005-03-31

Abstract

The display apparatus comprises an output time-base corrector (OTC) that receives a digital linear saw-tooth (VR) as a vertical position indicator. A correction stage receives a multiplication factor (PMF) and/or DC-shift factor (PDC) for multiplying and/or DC-shifting the digital linear saw-tooth (VR) to obtain a corrected saw-tooth (CVR) that is supplied to a frame deflection circuit (DAC1, VDC). The DC-shift factor (PDC) is adjusted such that a midpoint (MP) of the corrected saw-tooth (CVR) coincides with a center (CE) of the display screen (SCR) in the direction of the frame scan. The multiplication factor (PMF) is adjusted to obtain a desired amplitude of the frame scan. Now, the corrected saw-tooth (CVR) provides a frame scan which is centered in the frame direction and which has the desired amplitude. Consequently, after this adjustment, the digital linear saw-tooth (VR) has become a position reference for the frame scan and thus is a one to one indication of the position in the frame scan direction on the screen (SCR).

Description

Display apparatus

FIELD OF THE INVENTION The invention relates to a display apparatus comprising an output time-base corrector, to a method of displaying an image, and to geometry correction circuit.

BACKGROUND OF THE INVENTION US-B 1-6,297,849 discloses an output time-base corrector which converts orthogonal sampled video into asynchronous sample values occurring at clock instants of a clock signal. The asynchronous sampled video is displayed on a screen of a display device. A discrete time oscillator of a time-discrete phase- locked loop supplies a time base signal. The time-discrete phase- locked loop determines a phase difference between the time base signal and reference instants indicating a timing of a line deflection of the display apparatus to obtain the time base signal being locked to the reference instants. The time base signal controls a sample rate converter such that the asynchronous video values that occur at the clock instants are interpolated from the orthogonal sampled video by the sample rate converter to obtain the video signal that is displayed on the correct line position on the display screen. All the circuits are clocked by clock signals originating from the same clock generator. Geometry pre-correction is required if the display device comprises a picture tube deflection coil arrangement which causes a non-constant scan rate of the electron beam across the screen of the picture tube, and if the commonly used corrections in the line deflection circuit for obtaining a constant scan rate have not been implemented. If geometry pre-correction is required, a waveform generator adapts the time base signal in accordance with a predetermined waveform. Consequently, geometry pre-correction is obtained by controlling the delay of the orthogonal sampled video signals with the sample rate converter such that it fits the distorted geometry on the screen. If a geometry correction is required in both the line scan direction and in the frame scan direction, a two-dimensional waveform generator is required. This two- dimensional waveform is a function of two variables: the position on the line and the position of the actual line in a raster of lines. Such a two dimensional waveform may be generated by a two-dimensional quadratic-spline waveform generator as disclosed in WO-A-97/41680. The position information fed to the two-dimensional quadratic-spline waveform generator is in US-B 1-6,297,849 the line-locked time base waveform instead of the address disclosed in WO-A-97/41680. Usually, the line scan is performed in the horizontal direction, and the frame scan is performed in the vertical direction, such that an image comprises horizontal lines of pixels that succeed each other in the vertical direction. To calculate the two- dimensional spline waveform, the screen is divided into horizontal segments each comprising 64 pixels and vertical segments comprising 64 lines. The output time-base corrector disclosed in US-B 1 -6,297,849 is able to deliver a high quality geometry correction for a given picture format, for example PAL. However, for another picture format, such as for example NTSC, to establish the same quality of the geometry of the displayed picture, it will be necessary to reprogram the alignment parameters determining the two-dimensional waveform. This is due to the fact that the two-dimensional waveform is generated based on the number of lines in a frame and that the numbers of lines per frame of these different picture formats differ. Moreover, if vertical expansion, compression or scrolling is applied by changing the vertical deflection current in time, the geometry corrections will not stay in place, which hampers the general applicability of this prior-art.

SUMMARY OF THE INVENTION It is an object of the invention to provide a display apparatus in which the output time base corrector provides a geometry correction that is less dependent on the number of lines in a frame. The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims. The display apparatus in accordance with the invention comprises an output time-base corrector which may be identical to the output time-base corrector disclosed in US- Bl -6,297,849. But now, instead of receiving as the frame position pointer the number of the actual line in the frame, a digital linear saw-tooth is generated. The digital linear saw-tooth is staircase shaped with a same height and duration of each of its steps. The digital linear saw- tooth need not actually by available as a waveform, it may consist of a sequence of numbers. A correction stage receives a multiplication factor and/or DC-shift factor for multiplying and/or DC-shifting the digital linear saw-tooth to obtain a corrected saw-tooth. A multiplier of the correction stage multiplies the value (level) of each step with the multiplication factor, the DC-shifting adds or subtracts a value to or from the value of each step dependent on the DC-shift factor. Thus, the multiplying causes a change of the amplitude and slope of digital linear saw-tooth, and the DC-shifting causes a DC-shift or offset of the digital linear sawtooth. A frame deflection circuit receives the corrected saw-tooth to supply a frame deflection current for obtaining a frame scan on the screen of the cathode ray tube. The DC- shift factor and/or multiplication factor are predetermined factors. The DC-shift factor is adjusted such that a midpoint of the corrected saw-tooth coincides with a center of the screen in the direction of the frame scan (usually the vertical direction). In fact, the DC-shift factor is adjusted such that a vertical deflection current is obtained which causes the electron beams of the cathode ray tube to hit the center of the screen in the direction of the frame scan. The multiplication factor is adjusted to obtain a desired amplitude of the frame scan with respect to a dimension of the screen in the direction of the frame scan. For example, if the frame scan is in the vertical direction of the screen, the desired amplitude may be adjusted such that the frame scan is somewhat larger than the height of the screen to obtain a predetermined amount of overscan. Now, the corrected saw-tooth provides a frame scan which is centered in the frame direction and which has the desired amplitude. Consequently, after this adjustment, the mid-point of the digital linear saw-tooth is known to correspond to the center of the screen in the frame scan direction. And, the extreme values of the digital linear saw-tooth are known to correspond to the edges (maximum distances with respect to the center of the screen) of the frame scan. Thus, due to the use of the digital linear saw-tooth as the basic waveform for determining the frame scan, this digital linear saw-tooth has become a position reference for the frame scan and thus is a one to one indication of the position in the frame scan direction on the screen. By using this digital linear saw-tooth as the position reference in frame scan direction for the output time-base corrector, the correction introduced by the time base corrector is referenced one to one to the position in the frame scan direction on the screen and is thus independent on the line number in the frame. WO-A-97/41680 describes the generation of a one-or two-dimensional spline waveform to correct for geometry errors of a deflection system. A vertical position information signal (also referred to as vertical address) generator is provided which generates a vertical digital saw-tooth of which the initial value and the increment are determined from two measured instants at which the vertical deflection current reaches two selected values. This is based on the insight that the spot position on the screen in the vertical direction is a linear function in time. This approach has the drawback that an accurate measurement of vertical deflection current is required to enable an accurate determination of the instants the two selected values are reached. Further, this approach requires a linear vertical scan which should be generated by a vertical deflection circuit. Thus, the vertical deflection circuit should generate an S-corrected saw-tooth without being able to use the vertical digital saw- tooth. FR-A-27570O0 discloses an analog circuit that generates a vertical analog saw-tooth that is used as a basic waveform for the vertical deflection, the east-west correction, and focus correction. From the basic vertical analog saw-tooth a S-correction signal is generated. Again a DC-shift and amplitude corrector is provided to be able to adjust the S-correction signal. The vertical deflection waveform is generated by adding the S- correction signal to the reference vertical analog saw-tooth, and again a DC-shift and amplitude adjustment circuit is required. This is a complex approach due to the many DC- shift and amplitude correctors. It is much more difficult to align the vertical deflection such that the center of the reference saw-tooth points to the center of the screen in the vertical direction, and such that the limit values of the reference saw-tooth point to the extreme deflection angles in the vertical direction. Further, the basic vertical analog saw-tooth is not multiplied around its mid- value. Consequently, if the amplitude of the basic vertical analog saw-tooth is adjusted, the DC-shift should be adjusted too. In an embodiment in accordance with the invention, if no geometry corrections in the frame scan are required (the horizontal geometry errors are compensated by shifting the position of the video with the output time base corrector), the corrected saw-tooth is directly used to drive the frame deflection circuit. If, however, a geometry correction in the frame scan is required, the display apparatus further comprises a frame waveform generator as defined in claim 3. This frame waveform generator receives the digital linear saw-tooth as a frame position reference and supplies a frame correction waveform to the correction circuit during a normal operating mode. In the normal operating mode, the display apparatus displays input video, and thus a corrected frame scan is required to enable a distortion free picture in the frame direction. For example, usually, the frame scan has at least to be S- corrected according to a well-known S-correction. In the adjustment phase the multiplication factor and the DC-shift factor are adjusted such that the desired amplitude of the frame scan is obtained and such that the midpoint of the corrected saw-tooth coincides with the center of the display screen in the frame direction. During the adjustment phase, the digital linear saw-tooth is directly fed to the correction circuit. This may be obtained by by-passing the frame waveform generator, or by controlling the waveform generator to realize a one to one relation between its input and output signal. Thus, the correction circuit is used during the adjustment phase to calibrate the amplitude and mid-point position of the corrected saw-tooth, and during the normal operating mode to correct the amplitude and shift of the frame correction waveform supplied by the frame waveform generator which receives the digital linear waveform as the position reference in the frame direction. Consequently, the frame correction caused by the frame correction waveform is determined from the digital linear waveform that has a one to one relation with the position on the screen in the frame direction and thus is also independent on the number of lines in a frame. It has to be noted that FR-A-2757000 does not disclose that in an adjustment mode, the analog vertical reference waveform is directly used to feed the vertical deflection stage. The S-correction is always in the path between the analog vertical reference waveform and the vertical deflection stage. Preferably, the adjustment is performed during the production of the display apparatus. The adjustment may also be performed by a service technician. In an embodiment as defined in claim 4, the frame saw-tooth generator comprises a digital integrator that generates the digital linear saw-tooth. The digital integrator receives a start value, a step size, and a clock rate. At each instant a clock or enable signal with the clock rate is received, a step of step size is made, starting at the start value at a start instant, until the process is stopped at a stop instant. The start instant and the stop instant are determined by a start-stop signal that is related to the frame synchronization signal of the input video signal. Usually, a well-known frame divider which receives the frame synchronization signal and the line synchronization signal counts down at the line rate to generate the start-stop signal. This approach has the advantage that the interlacing of the frames is optimal because the start-stop signal is directly related to the frame synchronization. In an embodiment as defined in claim 5, the display apparatus further comprises a digital signal processor that receives a mode signal indicating a field rate of the video signal to be displayed. This frame rate depends on the format or standard of the input video signal. For example, the frame rate of a PAL signal is 50Hz, while the frame rate of an NTSC signal is 60 Hz. If frame doubling is used, these frame rates double to 100 Hz and 120 Hz, respectively. Now, for each different frame rate, different fixed values are selected for the step size and/or the clock rate such that the digital linear saw-tooth has a fixed slope that is different for different frame rates. This has the advantage that the amplitude of the digital linear saw-tooth is kept constant at different frame rates. Because the digital linear saw-tooth has (after the adjustment) a one to one relation with the position on the screen in the frame direction, and it is used as the position reference for both the frame deflection and the output time base corrector, both the frame deflection and the geometry correction caused by the output time base corrector will automatically be correct at the different frame rates. If a VCR, or a converter is used which doubles the frame rate, the duration of successive frames may vary. These duration variations can be coped with because the slope of the digital linear saw-tooth is kept constant and not its amplitude, and the interlacing of successive fields will stay optimal. In an embodiment as defined in claim 6, the display apparatus further comprises a digital signal processor that receives a mode signal indicating a field rate of the video signal to be displayed. This frame rate depends on the format of the input video signal. For example, the frame rate of a VGA signal supplied by a computer may vary between 60Hz and 120Hz (or even within a broader range). Now, for each different frame rate, different fixed values are selected for the step size and/or the clock rate such that the digital linear saw-tooth has a same fixed amplitude for different frame rates. Because the digital linear saw-tooth has (after the adjustment) a one to- one relation with the position on the screen in the frame direction, and it is used as the position reference for both the frame deflection and the output time base corrector, both the frame deflection and the geometry correction caused by the output time base corrector will automatically be correct to obtain a frame scan with a constant amplitude at the different frame rates. In the embodiment as defined in claim 7, the digital signal processor receives an input signal indicating compression or expansion and/or a DC-shift of the video in the frame direction. The video in the frame scan direction might be expanded, for example, in reaction on a user command, if a video input signal with a 3:4 aspect ratio or a letter box signal has to be displayed on a display screen with a 16:9 aspect ratio. The DC-shift may be controlled by the user to shift the expanded 3:4 picture to make a sub-title visible. The compression might be relevant if a 16:9 picture is displayed on a display screen with a 3:4 aspect ratio. In the embodiment as defined in claim 8, the display apparatus further comprises an east- west correction waveform generator that supplies an east- west correction waveform to the well-known east-west correction circuit that usually is a diode modulator of the line deflection. Because the east-west correction waveform generator receives the digital linear saw-tooth as the position reference, also the east-west correction will be independent on the number of lines of the vertical scan, and on the different frame rates of the input video signal. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 shows a block diagram of a display apparatus, Fig. 2 shows a block diagram elucidating the generation of the digital linear saw-tooth and the frame deflection drive waveforms, Figs. 3A and 3B show the digital linear saw-tooth, the corrected saw-tooth and the frame position on the screen, Figs. 4A and 4B show waveforms to elucidate the generation of a frame correction waveform with the frame waveform generator based on the digital linear saw-tooth as the frame position indicator, Figs. 5A and 5B show the digital linear saw-tooth if expansion, compression, or a DC-shift is required, Fig. 6 shows the frame correction waveforms obtained if expansion or compression is required, Fig. 7 shows waveforms to elucidate the generation of an east- west correction waveform with the east-west correction waveform generator based on the digital linear sawtooth as the frame position indicator, and Fig. 8 shows the east- west correction waveforms if expansion or compression is required.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The same references in different Figures refer to the same items which function in the same manner. In the description of the Figures, it is assumed, by way of example, that the frame scan occurs in the vertical direction and that the line scan occurs in the horizontal direction to obtain horizontal lines (lines extending in the horizontal direction) that succeed each other in the vertical direction. The frame scan is therefore referred to as vertical scan and the line scan is referred to as horizontal scan. If a transposed scan is generated, the line scan occurs in the vertical direction and the frame scan occurs in the horizontal direction to obtain vertical lines (lines extending in the vertical direction) that succeed each other in the horizontal direction. The invention is also applicable to such a transposed scanned display.

Fig. 1 shows a block diagram of a display apparatus. The dashed box OTC depicts a simplified form of the prior-art output time base corrector disclosed in US-B1 - 6,297,849. The output time base corrector comprises a memory MEM, a sample rate converter SRC, a digital to analog converter DAC, a horizontal reference time base generator HTB, a horizontal deflection drive circuit HD, and a two-dimensional waveform generator WGl. The memory MEM stores the input video signal, which in this example comprises the component input signals red, green and blue Ri, Gi, Bi, to obtain stored component signals Rs, Gs, Bs, respectively. The scan rate converter SCR receives the stored component signals Rs, Gs, Bs to supply the output signals Ro, Go, Bo, respectively. The digital to analog converter DAC converts the digital output signals Ro, Go, Bo to analog output signal R, G, B, respectively. The analog output signals R, G, B are supplied to a video drive circuit VDC which generates the video drive signal VDS suitable to modulate the intensity of the electron beams of the picture tube CRT. Usually, the video drive signal VDS comprises red, green and blue drive signals that are supplied to corresponding drive electrodes of the picture tube CRT. The horizontal reference time base generator HTB receives the horizontal synchronization signal H to generate a horizontal reference Href. The horizontal deflection drive circuit FED receives this horizontal reference Href and a horizontal flyback pulse Hfb from the horizontal deflection circuit HDC to supply a horizontal drive pulse Hdr to the horizontal deflection circuit HDC. The horizontal deflection drive circuit HD further supplies a horizontal position indication FB to the two-dimensional waveform generator WGl . The horizontal deflection circuit HDC has further an input to receive an east- west correction waveform EW and supplies the anode voltage Va to the cathode ray tube CRT. The horizontal deflection circuit HDC is coupled to a horizontal deflection unit (not shown) of the CRT to supply a horizontal deflection current IH to a horizontal deflection coil of the horizontal deflection unit to deflect the electron beams in the horizontal direction. Alternatively, the output signal IH may be a voltage driving deflection plates of the horizontal deflection unit. The two-dimensional waveform generator WGl further receives the horizontal position indication FB, a vertical position indication OS which is either LN (in the prior art) or VR (in accordance with the invention) and horizontal geometry parameters (which may depend on the vertical position) HGP to supply a control signal CS to the sample rate converter SRC. A vertical divider VD receives the vertical synchronization signal V and the horizontal synchronization signal H to generate a vertical linear saw-tooth LN which starts which a start value determined by the vertical synchronization signal V and which, starting from the start value increments or decrements with a fixed value at each horizontal line. Thus, this vertical linear saw-tooth LN indicates the position of the lines in the frame. The vertical divider VD further supplies a start-stop signal STST which is related to the vertical synchronization signal V. This vertical linear saw-tooth LN is used in the prior art to indicate the line positions to the output base converter OTC. The frame saw-tooth generator FSG receives the start-stop signal STST and generates a digital linear saw-tooth VR that is also referred to as the vertical reference sawtooth or the vertical reference. This digital linear saw-tooth VR has a one to one relation with the vertical position on the screen SRC. This will be elucidated in detail with respect to Fig. 2. The frame saw-tooth generator FSG further receives information ECS on the amount of expansion, compression and or shift desired. The vertical deflection waveform generator WG2 receives the vertical reference VR and the vertical geometry parameters VGP to supply a vertical waveform CVR to a digital to analog converter DACl. The digital to analog converter DACl provides differential drive voltages V+ and V- to the vertical output stage VDC. The vertical output stage VDC supplies an output signal IV to a vertical deflection unit (not shown) of the picture tube CRT to deflect the electron beams in the vertical direction. The output signal IV may be a vertical deflection current if the vertical deflection unit comprises a vertical deflection coil, or the output signal IV may be a vertical deflection voltage if the vertical deflection unit comprises vertical deflection electrodes. The operation of the vertical deflection waveform generator WG2 is elucidated in detail with respect to Fig. 2. For the understanding of the invention it is relevant that the vertical deflection waveform generator WG2 comprises a waveform generator (DI in Fig. 2) and a correction circuit (MUL and SHI in Fig. 2). The east- west waveform generator WG3 receives the vertical reference VR and east- west geometry parameters EWGP to supply an east- west waveform EWW to the digital to analog converter DAC2. The digital to analog converter DAC2 provides the east- west correction waveform EW to the horizontal deflection circuit HDC. The vertical deflection waveform generator WG2 and the east- west waveform generator WG3 may optionally receive information EHT on the anode voltage Va supplied to the picture tube CRT by the horizontal deflection circuit HDC. Usually the anode voltage Va is generated by using a line output transformer of which the primary is used as a choke for the line deflection. Alternatively, it is possible to use a separate high voltage generator to obtain the anode voltage Va. This information EHT is used to correct for the influence the value of the anode voltage has on the sensitivity of both the horizontal and vertical deflection. The operation of the output time base corrector OTC is described in detail in US-B 1-6,297,849 which is incorporated herein by reference. In short, the control signal CS determines a time shift of the input samples Rs, Gs, Bs to obtain time shifted output samples Ro, Go, Bo which occur at the correct position on a screen SRC of a picture tube CRT. In the prior art US-B 1-6,297,849, the two-dimensional waveform generator WGl receives its vertical reference LN from a line count mechanism or vertical divider VD via the dashed arrow. The geometry correction caused by the output time base corrector OTC will depend on the number of lines in a frame and on the position of the lines in the frame. In the present invention one and the same vertical reference saw-tooth VR is used, both for the output time base corrector OTC and the vertical deflection waveform generator WG2, if present. The vertical reference VR is a linear, digitally generated, saw-tooth of which only the slope and dc-level may vary. The, preferably DSP (Digital Signal Processor) based, one-dimensional waveform generator DI takes care of all necessary vertical geometry corrections, using the vertical reference saw-tooth VR as a position-on-screen reference. The, preferably DSP- based, two-dimensional waveform generator WGl also uses the vertical reference saw-tooth VR as the vertical position-on-screen reference. Consequently, the vertical geometry corrections will become only position- dependent, and thus independent on the number of lines in a frame, or on an actual position of the lines in a frame due to a desired compression, expansion of a DC-shift of the picture. The vertical reference saw-tooth VR indicates the vertical position on the screen because the correction circuit MCUL, SHI is adjusted during an adjustment mode such that the vertical reference saw-tooth VR multiplied by a multiplication factor (PMF in Fig. 2) and DC-shifted by a shift factor (PDC in Fig. 2) to obtain a corrected saw-tooth CVR which causes the desired vertical scan on the screen SCR of the picture tube CRT. In the desired scan, the mid-point of the corrected saw-tooth CVR coincides with the center of the screen in the vertical direction, and the amplitude of the corrected saw-tooth CVR causes the desired extreme positions of the scanned area in the vertical direction. Thus, by adjusting the vertical reference saw-tooth VR as described it is known that the mid-point of the vertical reference saw-tooth VR corresponds to the center of the screen SCR in the vertical direction, and it is known that the maximum and minimum values of the vertical reference saw-tooth VR correspond to the extreme positions. Consequently, the vertical reference saw-tooth VR is a pointer to the vertical position on the screen SCR. Fig. 2 shows a block diagram elucidating the generation of the digital linear saw-tooth and the frame deflection drive waveforms. A digital signal processor DSP comprises a processor core PRC, and an output register OR, a parameter register file PRF, a program ROM PRO, a program RAM PRA, and a data register file DRF which all are connected to the processor core PRC via an address bus Adr and a data bus Dat. The parameter register file PRF and the program RAM PRA are also connected to an on-chip I/O bus, consisting of an address bus A and a bi-directional data bus D. A central controlling processor unit (not shown) may be used to write the geometry parameters into the parameter register file PRF and to write the program of the digital signal processor DSP into the RAM PRA. The processor core PRC fetches the parameters from the parameter register file PRF, exchanges data with the data register file DRF and can fetch instructions from the program RAM PRA and from the program ROM PRO in a known manner to perform the program stored and to supply the results to the output register OR. In the output register OR are stored the values of the shift factor PDC, the multiplication factor PMF, and the geometry coefficients CO, C 1, C'2 which determine the frame correction waveform VW supplied by the one-dimensional waveform generator DI, and the start value stv, the step value ste and the clock rate rat to the frame saw-tooth generator FSG. The processor core PRC further receives the vertical reference saw-tooth VR, the horizontal reference Href and the information EHT on the anode voltage Va. The frame saw-tooth generator FSG may comprise a single integrator that produces each field the digital linear saw-tooth VR. The digital linear saw-tooth or vertical reference saw-tooth VR is determined by the three parameters: the start- value stv, the step- size ste and the clock-rate rat. At each instant determined by the clock-rate rat a step with the step-size ste is made, starting at the start instant with the start- value stv until the process is stopped by the start/stop signal STST from the vertical divider VD (see Fig. 1). This stop instant coincides with the occurrence of the vertical synchronization pulse V. The values of the parameters stv, ste, rat are calculated by the DSP and depend on the field-rate of the picture to be displayed, as well as on the settings for vertical expansion/compression and vertical scroll. Normally, the frame saw-tooth generator FSG operates in fixed-slope mode, wherein one fixed slope is generated for each field-rate standard, such as 50Hz, 60Hz, 75Hz, 100Hz or 120Hz. Alternatively, a second operation mode is available which is referred to as the fixed amplitude mode. In this mode, suitable for e.g. VGA signal formats, the amplitude of the saw-tooth is kept constant, independent on the field-rate to be displayed. For this amplitude mode the measuring signal mea that indicates the amplitude of the vertical reference saw-tooth VR has to be supplied to the processor core PRC. The vertical deflection waveform generator WG2 comprises the one- dimensional waveform generator DI, the multiplier MUL and the shifter SHI. To start with, the operation of the circuit of Fig. 2 is elucidated during the adjustment phase that usually is performed during factory or service alignment. In this mode, the one-dimensional waveform generator DI of the vertical deflection waveform generator WG2 is by-passed such that the frame correction waveform VW is the vertical reference sawtooth VR. Or the one-dimensional waveform generator DI is controlled by the coefficients CO, C'l, C2 to supply the vertical reference saw-tooth VR as the vertical correction waveform VW. The multiplier MUL receives the frame correction waveform VW which is the vertical reference saw-tooth VR to supply a multiplied frame correction waveform MVW to the shift circuit SHI. The multiplier MUL multiplies the correction waveform VW around the mid- value of the frame correction waveform VW to assure that this mid-point remains at precisely half the screen height. During the adjustment phase, the amplitude of the vertical correction waveform VW is adjusted until the vertical scan covers the desired height of the screen SCR of the picture tube CRT. Usually, the vertical scan is made larger than the height of the screen SCR to obtain some over-scan. It might be required to generate markers in the video signal which should occur at a predetermined distance on the screen SCR. The multiplier MUL does not change the mid-point value of the vertical reference saw-tooth VR. In fact, the multiplier MUL changes the slope of the vertical reference saw-tooth while keeping the mid-point fixed. The multiplication factor PMF required to obtain the desired height of the vertical scan is stored in the parameter register file PRF. The multiplier MUL is followed by the shift stage SHI, essentially an adder. The shift stage performs a DC-shift on the multiplied frame correction waveform MVW to obtain the corrected saw-tooth CVR which is the multiplied frame correction waveform MVW DC- shifted by an amount indicated by the shift factor PDC. During the adjustment phase, the shift factor PDC is varied until the mid point of the corrected saw-tooth CVR coincides with the center of the screen SCR in the vertical direction. In fact, the picture on the screen SCR is shifted in vertical direction over a distance required such that the vertical scan is substantially symmetrical around the center of the screen SCR. The shift factor PDC required to obtain the desired vertical shift is stored in the parameter register file PRF. It might be required to generate a marker in the video signal which indicates the mid point of the corrected saw-tooth CVR, for example as a white horizontal line, on the screen SCR. The marker may also be a blanking exactly at the mid-point of the saw-tooth. When the multiplication factor PMF and the shift factor PDC found in this manner during the adjustment phase are used during normal operating mode wherein the one- dimensional waveform generator DI provides a correction required to obtain a more linear vertical scan it is known that the mid-point of the vertical reference saw-tooth VR corresponds to the centre of the screen SCR in the vertical direction and that the maximum and minimum values of the vertical reference saw-tooth VR correspond to the upper and lower edges of the vertical scan area on the screen SCR. Thus, the vertical reference sawtooth VR is a one-to-one pointer to the vertical position on the screen SCR. In the now following the normal operating mode is elucidated. The one- dimensional waveform generator DI of the vertical deflection waveform generator WG2 may be a double integrator as disclosed in Fig. 8 of the prior-art US-B 1-6,297,849. This double integrator may generate a spline waveform as disclosed in detail in WO97/41680. The spline waveform is determined by start values STV1 and STV2 and the coefficients CO, Cl, C2 of this Figure 8. The complete spline waveform is calculated by dividing the vertical direction in segments. In the embodiment in accordance with the invention the digital signal processor DSP supplies the coefficients CO, C l, C'2. The same calculations have to be performed as explained with respect to Fig. 8 of US-B 1-6,297,849. The double integrator DI of the one-dimensional waveform generator WG2 receives its determining parameters from the output register OR, also. The coefficient values CO, C 1 and C'2 do not need any further calculation; they can be loaded directly into the integrators. The moments for loading are determined by the vertical reference saw-tooth VR. The digital signal processor DSP uses the same vertical reference saw-tooth VR and has to take care that any new coefficient value CO, C 1, C2 is written into the output register OR before the moment the value is required by the integrators. The values of the coefficients CO, C 1 and C'2 determine the frame correction waveform VW at the output of the double integrator DI and thus the final waveform of the vertical drive signal V+ and V-. The digital signal processor DSP uses the vertical reference saw-tooth VR and horizontal reference Href not only as vertical and horizontal segment references, but also as timing references. Each vertical retrace period, a few line times are used to calculate intermediate values from the stored parameters in the parameter register file PRF. These intermediate values are written into the data register file DRF. Since the calculation of the coefficients CO, Cl, C2 for the integrators only requires these intermediate values, no extra clock cycles are wasted during the visible lines, while any change of parameters during the visible scan period will not be noticeable. Instead of a single DSP it is also possible to use a separate digital signal processor to provide the waveform generator WGl, and another digital signal processor to provide the other functionality. This last digital signal processor then calculates the coefficients. For example, the horizontal segments are selected 128 pixels (clock cycles) long, while the vertical segments are 1/8 of the maximum amplitude of the vertical reference saw-tooth VR. This is accomplished by taking the 3 most significant bits of the vertical reference saw-tooth VR for determining the segments. The remaining bits determine the delta within a vertical segment, necessary to calculate the exact start-values for the integrators. The multiplier MTUL receives the frame correction waveform VW to supply a multiplied frame correction waveform MVW to the shift circuit SHI. The multiplier MUL multiplies the correction waveform VW around the mid- value of the frame correction waveform VW to assure that this mid-point remains at precisely half the screen height. The multiplier stage MUL is used both for gain correction of the overall gain between this stage and the final vertical deflection coil current IV and preferably also for the EHT correction to prevent vertical breathing of the picture as function of the CRT beam current which causes variations of the anode voltage Va. The multiplier MUL is followed by a shift stage SHI, essentially an adder, to compensate for any offset in the circuitry between this stage SHI and the final vertical deflection current IV. The processor core PRC calculates the multiplication factor PMF from factory settings determined during the adjustment phase, and the anode voltage information EHT. For example, in a practical implementation the multiplier stage MUL is able to adjust the amplitude of the vertical deflection current IV between -25% and +25% of the nominal value. For variations of the anode voltage Va an additional -6% to +6% is available. With the shift control signal PDC a vertical shift of -6% to +6% can be achieved. In addition, Moire effects can be reduced considerably by adding a small extra shift between 0 and half a line to the vertical shift value defined at the moment of factory or service alignment. This extra shift changes in successive frames. The shift stage SHI which supplies the corrected saw-tooth CVR is driving two, preferably on-chip, digital to analog converters DAC+ and DAC-, which yield together a complementary driving signal V+, V- for the vertical power amplifier VDC to supply the vertical deflection current IV to drive the vertical deflection coil. The two digital to analog converters DAC+ and DAC- are also indicated as DACl. The processor core PRC combines the stored factory adjusted factors and the correction for the variation of the anode voltage Va and the moire compensation to calculate the actual multiplication factor PDC and the shift factor PMF. It has to be noted that both the vertical deflection current IV and the vertical position indication OS for the output time base corrector OTC use the same vertical position indicator which is the vertical reference saw-tooth VR. Thus, all geometry corrections, either in the vertical deflection current IV or caused by shifting the video with the output time base corrector OTC are generated with respect to the actual position on the screen SCR and thus are independent on the number of lines in a frame or line number of a particular line in the frame. Consequently, all geometry corrections occur on the required position independent on the frame rate of the video signal or on the DC-shift, expansion or compression desired. The user desired compress, expand and/or shift factors provided by the user settings influence the start value stv, the step size ste and the rate rat. These user settings may be required if the aspect ratio of the picture to be displayed and the aspect ratio of the display screen SCR are different. Instead or on top of the user settings, automatically generated settings might be used. The automatically generated settings may be determined based on detecting the characteristics of the input signal.

Figs. 3A and 3B show the digital linear saw-tooth, the corrected saw-tooth and the frame position on the screen. Fig. 3 A shows the digital linear saw-tooth VR and the start- stop signal STST both as function of time t. The vertical axis of the digital linear saw-tooth VR indicates the levels L of the digital linear saw-tooth VR. Fig. 3B shows the corrected saw-tooth CVR and the start-stop signal STST both as function of time t. The vertical axis of the corrected saw-tooth CVR indicates the levels L of the corrected saw-tooth CVR. Fig. 3B further indicates the link between the levels of the corrected saw-tooth CVR and the position on the screen SCR. The digital linear saw-tooth or vertical reference saw-tooth VR (also referred to as the reference saw-tooth VR) of Fig. 3A starts at a level A2 determined by the start value stv at the instant t2 the start-stop signal STST indicates that the reference saw-tooth VR should start. At each clock instant t2 to t9 the value of the reference saw-tooth VR decreases with a fixed step ste. The clock instants tl to t20 are equidistant. Successive clock instants ti follow each other with the clock rate rat. Thus the period of time between two successive PHNL031173 p_CT/l _B2oo4/051696 16 clock instants ti is 1/rat. At the instant t6, the reference saw-tooth VR crosses its mid-value Ml. At the instant t9 the minimum value Al of the reference saw-tooth VR is reached. At the instant tlO, the start-stop signal STST indicates that the end instant is reached and the value of the reference saw-tooth VR is reset to its start value stv. The corrected saw-tooth CVR of Fig. 3B is obtained by multiplying and shifting the reference saw-tooth VR in the correction circuit, which comprises the multiplier MUL and the shifter SHI. During the adjustment phase, the multiplication factor PMF and the shift factor PDC are determined such that the mid-point MP of the corrected saw-tooth CVR corresponds to the center CE of the screen in the frame direction which in this example is the vertical direction, and that the amplitude DAM of the corrected saw-tooth CVR causes the indicated scan height SCH which covers the screen SCR at least over its complete height SH. Due to the shift, the mid-point of the corrected saw-tooth CVR occurs at the value MP which may differ from the mid-point value Ml of the reference saw-tooth VR. However, because of the adjustment with the multiplication factor PMF and the shift factor PDC it is known that at the mid-value of the reference saw-tooth VR indicates the center CE of the screen SCR. In the same manner, due to the adjustment of the multiplication factor PMF and the shift factor PDC, it is known that the maximum value A2 of the reference saw-tooth VR became the maximum value CA2 of the corrected waveform CVR and thus indicates the highest vertical scan position MVS. Again, in the same manner, due to the adjustment of the multiplication factor PMF and the shift factor PDC, it is known that the minimum value Al of the reference saw-tooth VR becomes the minimum value CA1 of the corrected waveform CVR and thus indicates the lowest vertical scan position LVS. Thus, every level of the reference saw-tooth VR is a one-to-one pointer to the vertical position on the screen SCR. For clarity only, Figs. 3 show a reference saw-tooth VR and a corrected reference saw-tooth CVR with a few levels only. In a practical realization, these saw-tooths VR and CVR should have a sufficient high number of levels to allow indicating the position of the vertical position with sufficient accuracy. For example, the reference saw-tooth VR may be a 17-bit word.

Figs. 4A and 4B shows waveforms to elucidate the generation of a frame correction waveform with the frame waveform generator based on the digital linear saw-tooth as the frame position indicator. The digital linear saw-tooth VR is shown as a straight line, because for a very high number of steps the steps are too small to make them visible in the scale used for Fig. 4A. Once the vertical reference saw-tooth VR is adjusted to the height SH and mid point CE of the screen SCR by means of the amplitude and shift controls, this saw-tooth VR forms a perfect vertical place pointer. The linear saw-tooth VR starts with the start value A2, which has the value 2ⁿ-l if the word defining the saw-tooth VR is n bits wide. The linear saw-tooth VR starts at the start instant tsta which is indicated by the start-stop signal STST shown in Fig. 4B. The mid-point value Ml has the value 2"^*1 and is reached at the instant tm, and the minimum value Al is zero and is reached at the stop instant tsto, which is indicated by the start-stop signal STST. Although this saw-tooth VR is linear, this does not mean that, if this saw-tooth VR is used as the waveform determining the vertical deflection current IV, the corresponding positions on the screen SCR are uniformly distributed across the height SH of the screen SCR. This is due to the vertical linearity errors in the vertical deflection coil picture tube CRT combination if a linear saw-tooth shaped vertical deflection current IV is supplied. To correct the vertical linearity errors a well known S-corrected vertical deflection current IV is required. To obtain the S-corrected vertical deflection current IV, the double integrator DI of the waveform generator WG2 should supply a correction waveform VW which is S-corrected as is shown in Fig. 4A. Such a waveform can be generated with the one- dimensional spline waveform generator disclosed in US-B 1-6,297,849. By way of example, this waveform VW is defined by 10 parameters pi to plO, which are positioned at saw-tooth values of the reference saw-tooth VR which are exactly in the middle of the vertical segments segO to seg7. The parameters pi to pi 0 are programmed to the value of the desired curve VW at these positions. The double integrator DI will construct the complete curve through these points indicated with pi to plO via the quadratic spline method described in the US-B1 - 6,297,849.

Figs. 5 A and 5B show the digital linear saw-tooth if expansion, compression, or a DC-shift is required. Fig. 5A shows at the left hand the digital linear saw-tooth VR as function of time t. If neither compression nor expansion is required, the digital linear saw- tooth VR1 is generated which is the digital linear saw-tooth VR used during the adjustment phase. If an expand mode is required, the digital linear saw-tooth VRE is generated, and if a compress mode is required, the digital linear saw-tooth VRC is generated. Fig. 5A shows at the right hand the display of a circle on the display screen SCR. It is assumed that the circle in the input video is displayed correctly as the circle Cl if the digital linear saw-tooth VR1 is used. In the compress mode, this correct circle in the input video is displayed as the compressed circle CC. In the expand mode this correct circle in the input video is displayed as the expanded circle CC. In a practical implementation, the compress mode will be used if the circle of the input video is displayed as an expanded circle if the digital linear saw-tooth VRl is used. The expand mode will be used if the circle in the input video is displayed as a compressed circle if the digital linear saw-tooth VRl is used. If the slope of the reference saw-tooth VR is changed with the half value Ml as the rotation point as shown in the left hand picture of Fig. 5A, the result is an expansion or compression of the displayed picture as shown in the right hand picture of Fig. 5 A. This is caused by the fact that the deflection current IV goes faster or slower from top to bottom, while the points of the circle remain at the same positions in time. The values of the sawtooth VR, however, still point to the same positions on the screen. Hence, the corresponding corrections remain at their original position as required. It is thus not required to adapt the correction coefficients. If the slope of the saw-tooth VR is made so steep that the start point and/or end point would lie above the maximum value A2 of 2n-l or below the minimum value Al of 0, the saw-tooth is clipped at these extreme values Al or A2 and the video output signals R, G, B are automatically blanked with a vertical blanking signal to prevent electrons hitting areas which are too far away from the screen SCR. If the digital linear saw-tooth VRE is used as the vertical reference saw-tooth, the video signal is blanked until the instant tbl and from the instant tb2 onwards. Thus in fact, the video signal is blanked as long the vertical reference saw-tooth has the maximum value A2 or the minimum value Al . Fig. 5B shows at the left hand the digital linear saw-tooth VR as function of time t. If no vertical shift is required, the digital linear saw-tooth VRl is generated which is the digital linear saw-tooth VR used during the adjustment phase. If a vertical shift upwards is required, the digital linear saw-tooth VRS1 is generated, and if a vertical shift downwards is required, the digital linear saw-tooth VRS2 is generated. Fig. 5B shows at the right hand the display of a circle on the display screen SCR. It is assumed that the circle in the input video is displayed correctly as the circle Cl which is centered around the centre of the screen SCR if the digital linear saw-tooth VRl is used. In the shift upwards mode, this circle in the input video is displayed as the upwards shifted circle CU. In the shift downwards mode this circle in the input video is displayed as the downwards shifted circle CD. In a practical implementation, the shift mode may be used to make a subtitle visible which is displayed at the bottom of a video signal which is expanded. If the reference saw-tooth VR is shifted as shown in the left hand picture of Fig. 5B, the result is a shift of the displayed picture as shown in the right hand picture of Fig. 5B. This is caused by the fact that the deflection current IV gets a DC-offset, while the points of the circle remain at the same positions in time. The values of the saw-tooth VR, however, still point to the same positions on the screen. Hence, the corresponding corrections remain at their original position, as required. It is thus not required to adapt the correction coefficients. If the saw-tooth VR is shifted so far that the start point or end point would lie above the maximum value A2 of 2n-l or below the minimum value Al of 0, the saw-tooth is clipped at these extreme values Al or A2 and the video output signals R, G, B are automatically blanked. If the digital linear saw-tooth VRSl is used as the vertical reference saw-tooth, the video signal is blanked until the instant tbl. If the digital linear saw-tooth VRS2 is used as the reference saw-tooth, the video signal is blanked from the instant tb2 onwards. Thus, again, the video signal is blanked as long the vertical reference saw-tooth has the maximum value A2 or the minimum value Al. Of course, any combination of expand/compress and scroll is possible, without the need to re-align the geometry correction coefficients.

Fig. 6 shows the frame correction waveforms obtained if expansion or compression is required. Fig. 6 shows the reference saw-tooth VR and the frame correction waveform VW as function of time t for three situations. The frame correction waveform VW is generated by the double integrator DI, which is a spline waveform generator which receives as vertical position indication the reference saw-tooth VR. The spline waveform generated depends on the coefficients CO, CT, C2 and this vertical position indication VR. If neither compression nor expansion is required, the reference saw-tooth is VRl . If this reference saw-tooth VRl is supplied to the spline waveform generator as the vertical position indication, with the selected coefficients CO, Cl, C2 the S-corrected sawtooth VW1 is generated. This curve VW1 is representative for the vertical deflection current IV. Usually the coefficients CO, C 1, C'2 are determined to obtain a linear vertical scan which has the height adjusted during the adjustment phase. If vertical expansion is required, the reference saw-tooth VRE is used as the vertical position indication of the spline waveform generator, and the S-corrected waveform VWE will be generated. Now, with the same coefficients CO, C 1, C2, at a same value A3 of the reference saw-tooths VRE and VRl, the same value Bl of the S-corrected waveforms VWE, VW1 will be obtained. But, these same values occur at different instants: at the instant te for the expansion mode, and at the instant tn for the normal mode. Or said in different words, the same correction is made at the same position on the screen SCR, but depending on the amount of expansion at a earlier instant te than the instant tn. Thus, although the picture is expanded (the same value Bl is reached earlier in time) the same correction still occurs at the same vertical position on the screen SCR. In the same manner, if compression is required, the reference saw-tooth VRC is used as the vertical position indication to obtain the S-corrected waveform VWC. Again, with the same coefficients CO, C 1, C'2, at a same value A3 of the reference saw-tooths VRC and VRl, the same value Bl of the S-corrected waveforms VWC, VW1 will be obtained. Because this value Bl of the S-corrected waveform VWC occurs later in time the picture is compressed. A same reasoning is true for a vertical shift: the same correction is obtained at the same vertical position at a different instant. Thus the correction is always position dependent although the displayed picture is vertically shifted.

Fig. 7 shows waveforms to elucidate the generation of an east- west correction waveform with the east-west correction waveform generator based on the digital linear sawtooth as the frame position indicator. The digital linear saw-tooth VR is shown as a straight line, because for a very high number of steps the steps are too small to make them visible on the scale used for Fig. 7. Once the vertical reference saw-tooth VR is adjusted to the height SH and mid point CE of the screen SCR by means of the amplitude and shift controls, this saw-tooth VR forms a perfect vertical place pointer. The linear saw-tooth VR starts at the start instant tsta, which is indicated by the start-stop signal STST with the start value A2 which has the value 2ⁿ-l if the word defining the saw-tooth VR is n bits wide. The mid-point value Ml has the value 2"^"1 and is reached at the instant tm, and the minimum value Al is zero and is reached at the stop instant tsto, which is also indicated by the start-stop signal STST. To correct the east-west errors a well known parabolic correction signal EW is required which usually is fed to the diode modulator of the horizontal deflection circuit HDC to obtain a horizontal deflection current IH which has an amplitude dependent on the vertical position on the screen SCR. To obtain the parabolic correction signal EW, the waveform generator WG3, which preferably is a one-dimensional spline waveform generator as disclosed in US-B1 -6,297,849, receives coefficients EWGP and the reference saw-tooth VR as the vertical position indicator to obtain the parabolic waveform EWW as is shown in Fig. 7. By way of example, this PHNL031173 _{pcτ/| B20}04/051696 21 waveform EWW is defined by 10 parameters pi 1 to p20, which are positioned at saw-tooth values of the reference saw-tooth VR which are exactly in the middle of the vertical segments segl 1 to seglδ. The parameters plO to p20 are programmed to obtain the value of the desired curve EWW at these positions. The one-dimensional spline wave generator WG3 will construct the complete curve through these points indicated with plO to p20 via the quadratic spline method described in the US-B 1-6,297,849.

Fig. 8 shows the east-west correction waveforms if expansion or compression is required. Fig. 8 shows the reference saw-tooth VR and the east-west correction waveform EWW as function of time t for three situations. The east-west correction waveform EWW is generated by the spline waveform generator WG3 that receives as vertical position indication the reference saw-tooth VR. The spline waveform generated depends on the coefficients EWGP and this vertical position indication VR. If neither compression nor expansion is required, the reference saw-tooth is VRl. If this reference saw-tooth VRl is supplied to the spline waveform generator WG3 as the vertical position indication, with the selected coefficients EWGP the parabola EWW1 is generated. If vertical expansion is required, the reference saw-tooth VRE is used as the vertical position indication of the spline waveform generator WG3, and the east- west waveform EWWE will be generated. Now, with the same coefficients EWGP, at a same value A4 of the reference saw-tooths VRE and VRl, the same value B2 of the east-west waveforms EWWE, EWW1 will be obtained. But, these same values occur at different instants. Said differently, the same correction is made at the same position on the screen SCR, but depending on the amount of expansion at a earlier instant for the waveform EWWE than for the waveform EWW1. Thus, although the picture is expanded the same correction still occurs at the same vertical position on the screen SCR. In the same manner, if compression is required, the adapted reference sawtooth VRC is used as the vertical position indication, which automatically causes the correct east-west waveform EWWC. Again, with the same coefficients EWGP, at a same value A4 of the reference saw-tooths VRC and VRl, the same value B2 of the east- west waveforms EWWC, EWW1 will be obtained. Because this value B2 of the east-west waveform EWWC occurs later in time the picture is compressed. A same reasoning is true for a vertical shift: the same correction is obtained at the same vertical position at a different instant. Thus the correction is always position dependent although the displayed picture is vertically shifted. It is further possible to correct the east- west correction with information EHT on the anode voltage Va to decrease the influence of variations of the anode voltage Va on the width of the picture displayed and on the shape of the parabola.

To conclude, the same reference saw-tooth VR is supplied to the spline waveform generator DI of the vertical waveform generator WG2, to the east- west spline waveform generator WG3, and to the output time base corrector OTC. Thus, the geometry correction generated by the spline waveform generators WGl (of the output time base corrector OTC, to generate the correction signal CS controlling the sample rate converter SRC), DI (of the vertical waveform generator WG2, to generate the desired waveform of the vertical deflection current IV) and WG3 (which generates the east- west waveform EWW) all are coupled to the vertical position indicator VR. Thus, all geometry corrections are generated by using the same reference saw-tooth VR which is a true one-to-one vertical position indicator. Consequently, all correction waveforms generated are independent on the number of lines in a frame or the actual position of a line in a frame. As long as the same correction waveform is required, the same coefficients can be used.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Geometry correction circuit comprising a frame saw-tooth generator (FSG) for generating a digital linear saw-tooth (VR), a correction stage (MUL, SHI) for receiving a multiplication factor (PMF) and or DC-shift factor (PDC) for multiplying and/or DC-shifting values of the digital linear saw-tooth (VR) to obtain a corrected saw-tooth (CVR), and an output time-base corrector (OTC) for receiving an input video signal (Ri, Gi, Bi) and a time-base signal (OS) comprising the digital linear saw-tooth (VR) to supply an output video signal (Ro, Go, Bo) having a position on a screen (SCR) being shifted with respect to the input video signal (Ri, Gi, Bi) under control of the time-base signal (OS), wherein the DC-shift factor (PDC) and/or multiplication factor (PMF) are predetermined to obtain, respectively (i) a midpoint (MP) of the corrected saw-tooth (CVR) coinciding with a center (CE) of the screen (SCR) in the direction of the frame scan, and/or (ii) a desired amplitude (DAM) of the corrected saw-tooth (CVR) with respect to a dimension (DI) of the screen (SCR) in the direction of the frame scan.

2. A display apparatus comprising: a cathode ray tube (CRT), a geometry correction circuit as claimed in claim 1, and a frame deflection circuit (DACl, VDC) for receiving the corrected saw-tooth

(CVR) to supply a frame deflection current (IV) to obtain a frame scan on a screen (SCR) of the cathode ray tube (CRT).

3. A display apparatus as claimed in claim 2, wherein the display apparatus further comprises a frame waveform generator (DI) for receiving the digital linear saw-tooth (VR) as a frame position pointer to supply a frame correction waveform (VW) to the correction stage (MUL, SHI) during normal operation wherein a corrected frame scan is required, and to supply the digital linear saw-tooth (VR) to the correction stage (MUL, SHI) during an adjustment phase for determining the predetermined multiplication factor (PMF) and/or DC-shift factor (PDC).

4. A display apparatus as claimed in claim 2, wherein the frame saw-tooth generator (FSG) comprises a digital integrator (FSG) for generating the digital linear sawtooth (VR) based on a start value (stv), a step size (ste) and a clock rate (rat), values of the digital linear saw-tooth (VR) being obtained by either adding or subtracting the step size (ste) at instants (tl , ... , tl 0) occurring at the clock rate (rat), starting from the start value (stv) at a start instant (t2) until an end instant (tlO) is reached, and wherein the display apparatus comprises a vertical synchronizing circuit (VD) for supplying a start-stop signal (STST) being related to a frame synchronization pulse (V) of the input video signal (Ri, Gi, Bi) and indicating the start instant (t2) and the end instant (tlO).

5. A display apparatus as claimed in claim 2, wherein the display apparatus comprises means (DSP) for receiving a mode signal (MS) indicating a field rate to be displayed, to select, for each different field rate, different fixed values for the step size (ste) and/or the clock rate (rat) to obtain a fixed slope of the digital linear saw-tooth (VR).

6. A display apparatus as claimed in claim 5, wherein the display apparatus comprises means (DSP) for receiving a mode signal (MS) indicating a field rate to be displayed, to select, for each different field rate, values of the step size (ste) and/or clock rate (rat) to obtain a substantially fixed amplitude of the digital linear saw-tooth (VR).

7. A display apparatus as claimed in claim 5, wherein the display apparatus comprises means (DSP) for receiving a mode signal (MS) indicating an amount of compression or expansion and/or DC-shift in the frame direction, to select values of the step size (ste), the start value (stv), and the clock rate (rat) for each different amount of the compression or the expansion and/or the DC-shift to obtain a corresponding amount of the compression or the expansion and/or the DC-shift of the digital linear saw-tooth (VR).

8. A display apparatus as claimed in claim 2, wherein the display apparatus further comprises an east- west correction waveform generator (WG3) for receiving the digital linear saw-tooth (VR) as a frame position pointer to supply an east-west correction waveform (EWW) to an east-west correction circuit (DAC2, EWC).

9. A method of displaying an image on a CRT, the method comprising generating (FSG) a digital linear saw-tooth (VR), receiving a multiplication factor (PMF) and or DC-shift factor (PDC) for multiplying (MUL) and/or DC-shifting (SHI) values of the digital linear saw-tooth (VR) to obtain a corrected saw-tooth (CVR), frame deflecting (DACl, VDC) receiving the corrected saw-tooth (CVR) to supply a frame deflection current (IV) to obtain a frame scan on a screen (SCR) of the cathode ray tube (CRT), the DC-shift factor (PDC) and/or multiplication factor (PMF) being predetermined to obtain, respectively (i) a midpoint (MP) of the corrected saw-tooth (CVR) coinciding with a center (CE) of the screen (SCR) in the direction of the frame scan, and/or (ii) a desired amplitude (DAM) of the corrected saw-tooth (CVR) with respect to a dimension (DI) of the screen (SCR) in the direction of the frame scan, and output time-base correcting (OTC) receiving an input video signal (Ri, Gi, Bi) and a time-base signal (OS) comprising the digital linear saw-tooth (VR) to supply an output video signal (Ro, Go, Bo) having a position on the screen (SCR) being shifted with respect to the input video signal (Ri, Gi, Bi) under control of the time-base signal (OS).