Abstract:
A method for obtaining line synchronization information items from a video signal is proposed. The inventive method is based on convolving the relevant part of an analogue video line signal with a pattern function. The result of the convolution operation is further processed to determine the time instants of the occurrence of the horizontal sync signals. The time instants are subsequently filtered to generate horizontal pulses. A video line memory allows to utilize subsequent horizontal sync signals for calculating the horizontal sync pulse of a current video line. The invention also relates to an apparatus for carrying out the method.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a method for obtaining line synchronization information items from a video signal. According to a second aspect the invention is related to an apparatus for generating a line synchronisation pulse for a video line signal.  
         BACKGROUND  
         [0002]    In today&#39;s television receivers and computer TV cards it is necessary to separate synchronisation signals (or in short: sync signals) to achieve a proper representation of the video image on the display, e.g. a cathode ray tube.  
           [0003]    Even though digital system solutions will become increasingly predominant in future television technology, analogue source signals will still exist for many years in the future. Examples include the terrestrial reception of video signals, which is still widespread to date, and the analogue recording methods, e.g. according to the VHS standard in the case of video recorders. Such analogue signal sources represent critical signal sources for digital systems, and their signal processing requires special measures. The situation is the same for future multimedia terminals as long as they are equipped with an analogue video input. Due to the existence of large quantities of analogue video material, e.g. VHS tape libraries, it is unlikely that the use of analogue video signals could disappear in the near future.  
           [0004]    Conventionally, a sync slicer is used for sync separation. The sync slicer is combined with a PLL (Phase Locked Loop) for “smoothing” the extracted sync information. The PLL technology is mature but has well known limitations due to two conflicting requirements: On the one hand the PLL must have a low-pass characteristic to suppress disturbances in the sync signal detection caused by noise. On the other hand, tape recorders, in particular camcorders, output the video signal on a variable time base due to mechanical tolerances. The time base variations also appear as disturbances of the sync signal to the PLL. However, this kind of disturbance has to be passed through without attenuation because any alterations of the time base would create horizontal instabilities of the displayed video picture. In other words: The PLL has to suppress noise on the one hand and has to pass time-base variations on the other hand. Fortunately, the two effects are distinguishable by their frequency. Time-base variations are a low frequency effect above 1 kHz. As the PLL is always a second order loop, corner frequencies and stability are selected in every new designed system for best compromise.  
           [0005]    Television receivers with digital signal processing (e.g. in the case of the 100 Hz technology) have been operating, as a rule, with clock systems, which are synchronized with the respective input signal. Since the input signal is the analogue CVBS signal, either the horizontal sync pulse (line-locked clock) or, alternatively, the colour subcarriers or colour synchronizing pulses (burst) (colour subcarrier-locked clock) are frequently used as reference point for the synchronization. The sync separation in the video lines has usually been carried out to date by means of analogue methods using so-called sync separator stages and a PLL filter stage connected downstream. In television receivers with digital signal processing, a PLL filter stage, which is a digital realization of the known analogue sync signal processing is commonly used. The filter stage is then a digital PLL (Phase-Locked Loop). Examples of such digital PLL circuits are the circuits SAA 7111 from Philips, HMP 8112 from Harris and Digit 3000 from Micronas. The principal problem with such digital PLL circuits is that the known instabilities in the picture occur when the input signal present is an analogue video signal picked off from an analogue video recorder, which is currently operating in the search mode (fast forward or reverse run). Many users of analogue video recorders are sufficiently acquainted with such instabilities. Specifically, disturbing horizontal stripes appear in the picture when the video recorder is operating in the search mode. These disturbing stripes originate from the fact that in the search mode, the video heads no longer run on a single slanted track but rather sweep across two or more slanted tracks, depending on the search speed. During the transition from one slanted track to the next, abrupt sudden phase changes arise with regard to the occurrence of the sync pulses of the video lines. These sudden phase changes are actually governed by the geometry in magnetic tape recording in accordance with the slanted track method. The sudden phase changes are therefore determined by the system and, in addition, virtually unavoidable.  
           [0006]    Irregular occurrence of line sync pulses also arises, however, in the case of video signals generated by camcorders. In this case, the instabilities that occur are, as a rule, more severe than in the case of a normal video recorder, because the regulation of the head-drum speed is subject to greater fluctuations on account of the larger component tolerances.  
           [0007]    EP-A 0 266 147 discloses a digital PLL circuit for a television receiver. In the case of this digital PLL circuit, in order to avoid-the abovementioned problem in the search operating mode in video recorders, a switching unit is provided which drastically shortens the time constant of the phase-locked loop in the event of identification of a sudden phase change caused by the head changeover at the end of a slanted track, with the result that the region of instability in the picture is reduced in size. The disadvantage of this solution is that the reduction of the time constant of the phase-locked loop provided by this solution means that noise components in the video signal are able to be suppressed less well and disturbing lines still remain visible, even though to a lesser extent than when the time constant is larger.  
           [0008]    In EP-A 0 899 945 a method is for obtaining line synchronization information is described. According to the known method a video line is convoluted with an idealized horizontal synchronization pulse. The result of the convolutions processed in an open loop system replacing the PLL. The open loop system is realized by a linear regression to extrapolate the best guess of a current sync pulse using past sync pulses.  
           [0009]    Convolving, or convolution, is a well-known term meaning the integral of one function multiplied by another function, which is shifted in time, see for example in “New IEEE Standard Dictionary of Electrical and Electronics Terms”, 1993.  
           [0010]    Using the known systems as a starting point it is desirable to have a method providing an even better performance with regard to horizontal synchronization of the video lines.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention suggests a method for obtaining line synchronization information from a video line signal. According to the inventive method the relevant part of the video line signal is analysed to determine time instants defining the temporal position of the line synchronization pulses. A predetermined number of video lines is stored in a line delay. Using this as a data base a filtered time instant for a video line preceding the currently received video line by the number of video lines stored in the line delay.  
           [0012]    In an embodiment of the invention the calculation is an interpolation of the time instants determined for video lines preceding and following the currently displayed video line. Specifically, in another embodiment the interpolation is a linear interpolation.  
           [0013]    In order to define the time instants of the synchronization pulses precisely it is suggested to convolute the entire or relevant part of the video line with a pattern function. An idealized synchronization pulse may be used as the pattern function.  
           [0014]    According to the second aspect of the invention an apparatus for generating line synchronization pulses from a video line signal is suggested. The inventive apparatus comprises means for analysing the entire or the relevant part of the video line to determine time instants defining the temporal position of the line synchronization pulses. The apparatus further includes a line delay for storing a number of video lines. Finally, means are provided for calculating a filtered time instant for a video line preceding the currently received video line by the number of video lines stored in the line delay.  
           [0015]    In another embodiment the apparatus comprises means for convolving the entire or the relevant part of the video line with a pattern function.  
           [0016]    In yet another embodiment the apparatus is provided with an FIR filter having a set of predetermined filter constants. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]    Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. In the figures it is shown:  
         [0018]    [0018]FIG. 1 simulated results for the reciprocal of the current horizontal frequency of line sync pulses in a video signal, which has been picked off from a camcorder;  
         [0019]    [0019]FIG. 2 a top level block diagram for baseband sync and video processing;  
         [0020]    [0020]FIG. 3 a diagrammatic illustration of a convolution operation of two square-wave pulses;  
         [0021]    [0021]FIG. 4 the structure of an MTA filter (moving time average) for carrying out the convolution operation;  
         [0022]    [0022]FIG. 5 a diagrammatic illustration for the calculation of the zero of the first derivative of the result function of the convolution operation according to the invention;  
         [0023]    [0023]FIG. 6 a graphical illustration of the timing of the video signal, the horizontal sync signal and the system clock;  
         [0024]    [0024]FIG. 7 the time positions of the horizontal sync pulses, a schematic structure of the FIR filter and a graphical representation of t he used filter coefficients;  
         [0025]    [0025]FIG. 8 the circuit section handling the time management of the synchronisation signal processing;  
         [0026]    [0026]FIG. 9 the timing of the counters incorporated in the time management circuit section;  
         [0027]    [0027]FIG. 10 the time positions of the detected H sync  pulses, the output pulses of the H sync  detector and the output pulses of the horizontal filter;  
         [0028]    [0028]FIG. 11 a comparison of the error transfer function and transfer function of different types of H sync  processing means;  
         [0029]    [0029]FIG. 12 the filter coefficients of different types of filters, and  
         [0030]    [0030]FIG. 13 a video picture exemplifying the improvement of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Simulated values for the position of the line sync pulses in a video signal are plotted in FIG. 1. The number of the respective line is plotted in the direction of the abscissa in FIG. 1. The reciprocal of the current horizontal frequency 1/f H  for the line sync pulses is plotted in the direction of the ordinate of FIG. 1. The letter “n” generally designates the line number of each video line. The plotting of the reciprocal of the current horizontal frequency means that the time-domain positions of the respective line sync pulses can be compared with one another. The values corresponding to the actual measured values for the individual positions of the line sync pulses are in each case specified by the broken vertical lines. The end of each broken vertical line then specifies the line sync pulse position established. If the video signal present were ideal, then all of the broken. lines would have the same length. The illustration is made for a video signal generated by a camcorder. Fluctuations in the line sync pulse positions about an average value are clearly discernible in a first region of FIG. 1. A steady increase in the line sync pulse positions can be discerned in a second part of FIG. 1. This behaviour can be explained by the regulation of the speed of rotation of the head-drum, this regulation being slow to start. Component tolerances and noise may be responsible for the variations of the line sync pulses about an average value. The crosses in FIG. 1 specify the line sync pulse positions corrected by the PLL circuits, which are usually used in the television receivers. However, the regulating behaviour of this PLL circuit results in the outputting of a phase error with regard to the line sync pulse positions as long as the vertical frequency in the video signal steadily changes. This is clearly discernible in the second part of FIG. 1 from the difference between the crosses and the broken lines. Since the sign of this phase error may be randomly distributed in the individual sub pictures and, in addition, the value of the phase error likewise does not always remain constant, the phase deviation D is manifested as a visible disturbance in the video picture displayed. The disturbance corresponds to a horizontal jitter effect in large parts of the picture. The picture gives the impression that somebody is shaking it in the horizontal direction. In order to eliminate the phase error with frequency changes occurring in the video signal, it is customary in the case of an analogue PLL control loop to increase the bandwidth of the PLL circuit. If this measure is taken, then the line sync pulse positions, which are indicated by circles in FIG. 1 are produced. However, this solution has the disadvantage that the noise suppression of the PLL circuit is impaired, as a result of which momentary picture disturbances in the form of a jitter behaviour are still discernible. Vertical lines no longer appear straight but rather distorted.  
         [0032]    [0032]FIG. 2 shows a top level diagram of the sync and video processing of a TV incorporating the present invention.  
         [0033]    The proposed architecture is based on a free running system clock, e.g. a crystal oscillator. Hence, the system clock has no correlation to the video sync or colour subcarrier frequencies. The free running system clock is particularly advantageous for processor-based systems adapted to receive video signals of various types and properties. However, colour burst-locked clocked systems are also possible to realize with the architecture described in the following if this is desirable for certain embodiments.  
         [0034]    On the left hand side of FIG. 2 the inputs for analogue base band video signals are shown. The inputs  1   a  and  1   b  are used for S video signals having separate chrominance C and luminance Y inputs. The input  2  is provided for receiving composite video signals. The analogue video signals are converted into corresponding digital signals by A/D converters  3   a  and  3   b , respectively. The digital output signals are then stored in FIFO (first-in-first-out) line delays  4   a  and  4   b , respectively, having a size of e.g. eight video lines. The FIFO line delays are necessary for filtering time data used in horizontal sync processing, as it will be described further below.  
         [0035]    In the signal path processing the composite video signal a comb filter is provided to separate the luminance Y and the chrominance C signals. Subsequently, the signals are fed into a chrominance decoder to generate the luminance signal Y and the chrominance signal C.  
         [0036]    A switch  8  is controlled to select the luminance and chrominance signals received from line delay  4   a  or from chroma decoder  7  to be further processed in a pixel interpolation filter  9 , which will be described in more detail below.  
         [0037]    The output of filter  9  is buffered in a line buffer  11  and passed on to a colour dematrix  12  for generating digital R, G, B signals. The digital R, G, B signals are converted into analogue signals, amplified by associated drivers in block  13  and supplied to a cathode ray tube  14  for display.  
         [0038]    In case a display device different than a CRT, e.g. an LCD, TFT or plasma display is used, the signal processing may be different than shown in the present embodiment. However, these differences do not depart from the scope of the present invention.  
         [0039]    For synchronization signal processing a digital luminance signal Y or a digital composite video signal is selectively used depending on which analogue inputs are active. The signal for sync signal processing is selected by switch  16  controlled by the same control signal as the switch  8 . The vertical sync processing in block  17  is conventional and is therefore not described in more detail. The information regarding which field of a video frame is currently being processed is also obtained in this unit. The vertical sync signal V sync  is delayed in a line delay  18  to re-establish the timing to the video signals delayed in the line delays  4   a  and  4   b , respectively.  
         [0040]    The V sync  signal is input to a microprocessor  19 , which is among other functions operative to execute the synchronization logic.  
         [0041]    In a H sync  processing unit  21  the H sync  signals are detected and processed by the inventive method, which will be described in more detail below. The horizontal sync information is supplied to the μP  19 . The μP  19  uses the horizontal and vertical sync information to generate output signals for a deflection driver  22  being connected to a deflection apparatus not shown in FIG. 2.  
         [0042]    The deflection apparatus is associated with the CRT  14  and effects the scanning of the electron beams inside the CRT in a conventional way.  
         [0043]    In case the display device is not a CRT the deflection driver  22  is replaced by an appropriate device effecting the representation of the video signal line by line on the screen of the respective display device.  
         [0044]    The H sync  processing unit  21  comprises a H sync  detector  23  to determine the timing of H sync  signals contained in the received video signal. The H sync  signals are filtered in a H-filter  24  and the resulting signals H o  and φ o  are provided to the μP 19  and to the pixel interpolation filter  9 . The operation of the H sync  detector  23 , the H-filter  24  and the V sync  processor  17  is controlled by an event control  25 , which will be described in more detail in connection with FIG. 8.  
         [0045]    In contrast to the otherwise widespread edge detection for the falling edge of a line sync pulse in the CVBS signal, the present H sync  detector  23  operates according to the correlation principle. In this case, the CVBS signal is convolved with an ideal line sync pulse and then the minimum is sought. This principle is illustrated in FIG. 3, where it is possible to discern in principle that the convolution operation of two square-wave pulses generates a triangle function as result function. This function then has a minimum or maximum, which specifies the position of the line sync pulse. The CVBS signal for a video line is designated by the reference symbol f in (k). The reference symbol S ideal  (k) designates an ideal line sync pulse. The result function of the convolution operation is designated by the reference symbol φ SV (k). The reference symbol k s  specifies the position of the minimum of the result function. The convolution operation is carried out in the H sync  detector  23  for example in such a way that the CVBS signal present in one of the line stores  4   a ,  4   b  for a video line is digitally convolved with a corresponding ideal line sync pulse. The execution may alternatively be configured in such a way that instead of the CVBS signal for the entire video line being convolved with the ideal line sync pulse, only the relevant part for the line sync pulse is convolved with the idealized line sync pulse.  
         [0046]    This detection method behaves extremely robustly even for example in the case of terrestrial signals with multipath propagation which are subjected to a great deal of interference. If a constant correlation length l s  is used for the convolution operation, the MTA (Moving Time Average) filter known from the prior art is suitable, for example for the circuitry realization of the convolution operation, the structure of which filter is illustrated in FIG. 4, where the line sync pulse is represented in an idealized manner with a square-wave pulse response.  
         [0047]    In order to determine the maximum or minimum in the result function of the convolution operation, the zero of the first derivative of the result function is calculated. This computation operation is illustrated in more detail in FIG. 5, where the reference symbol φ SV   (k)  designates the first derivative of the result function, the variable k stands for the respective sample of the derivative function, k s  specifies the position of the zero of the derivative and k 0  specifies the last sample with a negative sign in the transition region of the first derivative of the result function. For an exact determination of the zero, a linear regression is carried out in the transition region of the derivative function. The zero is then calculated in a simple manner using the regression line established. The point of intersection of the regression line with the zero axis is designated by the reference symbol N s  in FIG. 5. The regression length l amounts to nine samples in the example illustrated. In this way, the minimum of the first derivative is calculated with subpixel resolution. The subpixel resolution is necessary since the subsequent vertical filtering cannot effectively eliminate pixel quantization. The subpixel resolution is also necessary because, e.g. given a sampling rate of 18 MHz for the A/D conversion in the A/D conversion unit  20  and a display having a width of 56 cm, the visibility limit for picture details is approximately 0.17 pixel. Investigations with various input signals have shown that a linear regression yields an optimum result for the calculation of the subpixel resolution. For the calculation of the centre of the line sync pulse, which corresponds to the minimum of the result function of the convolution operation, approximately 10 samples are sufficient for the region around the zero of the derivative function. The computation rule that approximately 10 samples are sufficient was established at a sampling rate of 18 MHz using video signals having a constant horizontal frequency for various signal-to-noise ratios in the case of terrestrial reception. In this case, the standard deviation for a signal having a signal-to-noise ratio of 15 dB was 0.93 pixel. In the case of a low-noise signal having a signal-to-noise ratio of 35 dB, the standard deviation is 0.07 pixel.  
         [0048]    If video signals originating from video recorders are processed, the horizontal frequency can deviate by up to 4% in the trick mode in video recorders, which also proportionally affects the length of the line sync pulse. This is then manifested in a deterioration in the identifiability of the edge in the course of the derivative function of the result function of the convolution operation. However, it has been shown that deviations up to this degree have no relevant influence on the detection accuracy in the course of the zero determination. For the calculation of the centre k s  of the line sync pulse by means of linear regression, it is possible to use an arithmetic unit instead of a more complex microprocessor, since approximately 32 μs remain for this calculation, which corresponds to half of one video line in the PAL system. Moreover, the equidistant samples enable a distinct simplification of the calculation. The formula for the calculation of the centre k s  of the line sync pulse with subpixel accuracy then reads as follows:  
         k   s     =       k   0     -       k   l              ∑     i   =   1     l            ϕ   SV          (     i   -     l   2       )             ∑     i   =   1     l          (       (     i   -     l   2       )            ϕ   SV          (     i   -     l   2       )         )                                   
 
         [0049]    In this case, k l  is a constant, which can be calculated as a function of the regression length l. All the other symbols are known from the description regarding FIGS. 5 and 3.  
         [0050]    Following the time base described in connection with FIG. 6 each video line duration T H  can be calculated by the following equation: 
           TH ( n )=Δ H   I ( n )+φ I ( n )−φ I ( n− 1). 
         [0051]    For an undistorted PAL standard input signal the line duration is T H =64 μs. With a constant system clock frequency of f clk =18 MHz the number of clock cycles between two H sync  pulses would be constant at ΔH I =1152. The phase difference between the H sync  pulses and the beginning of a clock cycle would also be a constant value for all video lines.  
         [0052]    However, in practice the video line duration is not constant but variable. Noise and low frequent distortions are super imposing the input video signal causing a time shift of the detected temporal position of the H sync  pulse.  
         [0053]    As a consequence the detected video line duration T H  changes line by line causing horizontal jitter. To avoid this kind of jitter it is necessary to filter the detected temporal positions of the H sync  pulses.  
         [0054]    For each time instant t n  the H sync  detection block  23  outputs two values, H I  and φ I . H I  describes the time difference between two H sync  pulses measured in integer cycles of the internal system clock. φ I  represents a fraction of one cycle allowing to determine the temporal position of a specific H sync  pulse with subpixel resolution.  
         [0055]    In FIG. 6 the details of the timing of the different signals are shown. FIG. 6 a  displays an analogue composite video signal CVBS with incorporated H sync  pulses, which are indicated by “H”. FIG. 6 b  illustrates the. temporal position ti of the H sync  pulses by vertical lines  25 . The abscissa in FIG. 6 b  is divided into units of the nominal duration T H  of a video line. The temporal position ti falls into a particular cycle of the system clock. The relevant cycles define the integer number H I . A comparison between FIGS. 6 b  and  6   c  illustrates that the nominal beginning of a video line as shown in FIG. 6 b  coincides with the beginning of a cycle of the system clock as illustrated in FIG. 6 c . However, FIG. 6 b  also shows that the temporal positions ti of the detected H sync  pulses do not coincide with a cycle of the system clock. The time shift also discernible in FIG. 6 b  as a deviation from the nominal beginning of a video line is quantified by φ I . φ I  defines the fraction of a system clock cycle the respective H sync  pulse lags behind the clock cycle.  
         [0056]    Several examples of the time shift are illustrated in FIG. 6 d  with reference to FIG. 6 c.    
         [0057]    The time instants t i  determined according to the described method are subsequently filtered in a horizontal filter  24 . The horizontal filter  24  is a FIR filter having symmetrical filter coefficients.  
         [0058]    At a given time the FIR filter  24  takes  1  line durations ΔT n , n=0, . . . , 1 into account to calculate the filter output T m  as it is illustrated in FIG. 7 a . One line duration ΔT n  is the time difference between two H sync  pulses n and n+1, defined by the time instants t n  and t n+1 .  
         [0059]    The FIR filter then calculates the filter output signal according to the equation  
           T   m     =       ∑     n   =   0       -   l          Cn       ,     l   ·   tn                           
 
         [0060]    where C n,1  are the filter coefficients, (l+1) is the filter length, and t n  the time instants of the H sync  pulses. In FIG. 7 b  the FIR filter  24  is shown in more detail. The time instants are stored in delay stages  26   -1  . . .  26   o . The contents of the delay stages is multiplied with the associated filter coefficient c n,m  in multipliers  27   -1  . . .  27   o . The individual products are summed up in an adder  28  to yield the filter output T m . The index m of the calculated filter output T m  is related to the number of video lines that can be stored in the line delay  4   a  or  4   b . In the present embodiment the line delay has a size of eight video lines and therefore m=7. In other embodiments having a different size of line delay m may take a different value.  
         [0061]    It is important to note that this filter design does not contain any feedback of a previous result to a subsequent result. The output of the horizontal filter depends exclusively on the input signals, i.e. the detected time instants t i . The H-filter  24  is an open loop filter.  
         [0062]    The horizontal filter outputs the filtered temporal position of the H sync  pulse in terms of values H o  and φ o  indicating the length of the video line in terms of a number of system clock cycles and a phase shift relative to the system clock, respectively. The time correction of the video signal is done by the pixel interpolation filter block  9 , which is arranged to shift each video line in time with subpixel resolution corresponding to the value of φ o , wherein 0≦φ o ≦1. The output signal H o  of the H filter defines the start of each new video line with the accuracy of one system clock cycle.  
         [0063]    In the previous paragraphs it has been shown that the H I  pulse in connection with the additional phase information φ I  is used as input for the H-filter  24 . The summed up time information of each H sync  pulse (ΔH I +φ I ) is stored for a defined number of lines, which equals the FIR filter length.  
         [0064]    In the present embodiment of the inventive apparatus the time measurement is implemented by means of counters, requiring only a few basic hardware components. A practical realization is shown in FIG. 8.  
         [0065]    [0065]FIG. 8 shows in more detail the circuit section being effective for the time management. The time management circuit section, which is referenced as a whole with the reference number  31 , is structured into several blocks comprising a counter block  32 , a comparator block  33 , a phase memory  34 , the FIR filter  24  and an event control  25 . The details of the FIR filter  24  have already been described in connection with FIG. 7 b.    
         [0066]    The counter block  32  includes two counters  36   a ,  36   b  to measure the time difference between the occurrences of two subsequent H I  signals in units of clock cycles. The H I  signal is also supplied to the event control  25  outputting a counter reset signal. During normal operation the counter reset signal corresponds the H I  signal.  
         [0067]    A counter control unit  37  controls a switch  38  to connect the counter reset signal emitted by the μP  19  with either counter  36   a  or counter  36   b  for resetting it. At the same time the counter control unit  37  operates two further switches  39   a  and  39   b  to connect the output of the respective counter, which is not reset to one input of a comparator  41 . The second input of the comparator  41  is provided with the predicted value calculated in the H-filter  24 . The calculation is based on past and future H I  values. If the two input values of the comparator  41  are equal it outputs the H o  pulse.  
         [0068]    [0068]FIG. 9 shows the timing of the H I  signal, the H 0  pulse and the contents of the counters  36   a ,  36   b . One counter counts each H I  pulse and the other counts the number of clock cycles for counter memory storage (circle) and output comparison (cross) and vice versa. Or in other words: The selected counter for output comparison counts the number of clock cycles from the last input pulse H I  until the output pulse is generated by the prediction value (cross in FIG. 9). At the same time the selected filter is reset. Fractions of a clock cycle are not important for the counter time scheme. Fractional parts of a clock cycle are stored in the phase memory  34 .  
         [0069]    Some lines of delay/memory are needed for the new horizontal filter technique based on an interpolation principle. In this example, a 8 line delay block is used to delay the video signal by 8 video lines. This is needed, because the signal delay allows the H filter block to obtain timing information from past video lines (still stored in line memory), before the video signal is output to the pixel interpolation filter. After passing the pixel interpolation filter, the video lines are orthogonalized with the processed timing information of the H filter and further time corrections are not possible. Using this line delay, the filter behavior can be improved as shown in the next chapter.  
         [0070]    In FIG. 10 the timing of different types of signals is shown in an overview. From top to bottom the processing of the horizontal sync signals is visualized beginning with the detected H sync  signals in FIG. 10 a . On the abscissa of all diagrams in FIG. 10 the time grows from left to right. FIGS. 10 a  to  10   c  correspond to the illustration in FIGS. 6 b  to  6   d  describing the output of the H sync  detector  23  (FIG. 2). The output signals H I  and φ I  are provided to H-filter  24  to generate the filtered signals H o  and φ o  (FIGS. 10 d ,  10   e ) supplied to the μP  19  and the pixel interpolation filter  9 , respectively.  
         [0071]    Finally, the filtered H pulse (FIG. 10 f ) is supplied to the deflection driver  22  (FIG. 2). Apart from the timing, FIG. 10 also illustrates the effect of the time delay. Using a line delay corresponds to case b) in FIG. 10 b , where the estimation time point is approximately seven lines in the past, referring to the actual detected H sync  pulse. This configuration is adapted to the present embodiment having eight lines of video memory. Of course, the size of the memory and the parameters of the line delay may be different in other embodiments of the invention. The line delay following the vertical signal processing block is needed to compensate the line delay of the video streams (Y/C or composite video) and uses the same delay time as the video stream line delay. The realization of the line delay for the vertical signal needs much less hardware; it is a binary signal (V-Imp, Field).  
         [0072]    The last building blocks “Line-Memory Dual-Port” and “Synchronization-Logic” are used to synchronize the output signals for different display techniques.  
         [0073]    The present embodiment of the invention uses a free-running output clock frequency and outputs a fixed number of pixels per line. Depending on the variation of horizontal and vertical input frequency, the number of lines per each output field can vary. Field synchronization is possible by changing the number of output lines per field. Therefore, this mode is ideally suited to drive CRTs for the following reasons.  
         [0074]    i) Tube displays are horizontally stable, when the horizontal frequency of the driver stage is essentially constant (crystal locked).  
         [0075]    ii) DC-coupled vertical CRT driver stages are able to adapt to changing line numbers per field without any visible degradation.  
         [0076]    iii) This mode is also compatible with future multimedia-systems, where the usage of a line-locked clock system might be not acceptable.  
         [0077]    A constant horizontal (H sync ) frequency can be obtained by the proposed sync algorithm, when some lines of buffer memory are used to compensate the different line durations T H  between the buffer input and output during one field (T H ≠const., T H,out =const.). This corresponds to a variation of the number of pixels per line at the buffer input and a constant number of pixels per line at the buffer output.  
         [0078]    The vertical coupling is field synchronous. This means, that the vertical frequency of the output adapts at the end of each field to the vertical timing of the input to obtain correct field synchronization (T V =T V,out ≠const.). The maximum needed line buffer size is defined by the maximum difference between the input and output memory addresses during one field. Hence, for a maximum averaged H-frequency variation of ±0.5% including phase-skips at the writing and constant output H-frequency at the reading, the line buffer memory size must at least be capable to store three video lines. The implementation of the vertical synchronization with changing vertical frequency in CRT-TVs does not present difficulties for a person skilled in the art. This type of synchronization can also be used for other display technologies like LCD or plasma displays.  
         [0079]    It is noted that the use of a video line memory according to the invention for the purpose to take into account subsequent horizontal sync pulses is not limited to the use of FIR filters. The same concept can be applied with linear regression as filtering function. Filtering by linear regression with different regression lengths is disclosed in EP-A 0 899 945. In combination with the video line memory the known filter concept allows “to look into the future” to yield improved horizontal filtering. The results achieved by the different techniques will be discussed further below.  
         [0080]    The advantage of the new approach can be seen best in the “frequency domain”. FIG. 11 shows a comparison between the conventional PLL, a system based on a linear regression filter technique without video line memory and with four lines of video memory and finally the actual new approach based on the FIR filter technology. FIG. 11 a  shows the time-base variation tracking and FIG. 9 b  the noise suppression. To get a relevant measure of the tracking quality of the sync separation, FIG. 11 a  shows the error transfer function |1-H e (f)|, where H e (f) is the transfer function of the FIR filter. The error at typical distortion frequencies at approximately 30 Hz must be in the order of −60 dB to avoid visible jitter. FIG. 11 b  details the improvement of sync-noise suppression compared to PLL and also compared to the previous patent application. Alternatively, when approximating the Linear Regression by a FIR filter the filter quality is in between the symmetrical FIR filter and the linear regression without memory. This is a suitable solution for one to approximately five line delays or line memory resulting in asymmetrical FIR coefficients.  
         [0081]    In FIG. 12 a diagram shows the asymmetrical and symmetrical impulse responses corresponding to filter coefficients of the various filter types described above. The + symbols indicate the coefficients for linear regression without memory; the x symbols indicate the coefficients for linear regression with a memory having a size of four lines; the asterisks * indicate the coefficients for a symmetrical FIR filter and finally the dots indicate the coefficients of a conventional PLL design with a fast time constant and infinite impulse response. On the abscissa of the diagram in FIG. 12 the indices of the filter coefficients are plotted. It is noted that the linear regression with and without memory and the PLL filter represent asymmetric filters in contrast to the symmetrical FIR filter.  
         [0082]    Instability in video cannot be printed on paper, however, FIG. 13 gives a rough impression about the achievable improvement. A reference measurement is made with Philips SAA7113H mounted on an original Philips evaluation board. FIG. 13 a  shows the phase-jump response of a synthetically generated VCR search mode and the noise response (SNR=12 dB). The typical instability between two phase jumps seen here as offset between two fields as a comb structure is clearly discernible. The comb structure is marked in FIG. 13 a  by ovals.  
         [0083]    [0083]FIG. 13 b  shows the response of the proposed algorithm using a symmetrical FIR filter. The phase jump response is reduced to 50%. After the phase jump very good stability is achieved and no comb structure due to phase differences in different picture is visible. In addition, also the noise suppression is much better.