EP1512133B1 - Method and device for determining a frequency for sampling an analog signal - Google Patents

Description

The present invention relates to a method and apparatus for determining a frequency for sampling an analog image, and more particularly to a method for determining a frequency for sampling an analog signal provided to a digital screen to image to display the digital screen. Further, the present invention particularly relates to an apparatus for generating digital data from analog image data to display on a digital screen an image based on the generated image data.

Conventional computers or computers include elements, eg. As graphics cards to graphics generated in the computer, for. As images, for display on an external device, eg. As a screen to provide. The conventionally used graphics cards generate corresponding image data suitable for driving a screen, based on the digital signals provided by the computer or its processing unit (CPU). In many applications, the computer-associated display device, the screen, is an analog display having a cathode ray tube. In order to provide the necessary data for this, until just exclusively existing application case, the graphics card comprises a digital / analog converter to convert the image data generated by the graphics card into an analog signal, for example an RGB signal, which then controls the screen allows. In addition to the analogue image data signals (RGB signals), the horizontal and vertical synchronization signals are also output to the screen. which are required for a proper reproduction of the image data on the screen.

Recently, however, so-called digital screens are increasingly used, for example, LCD screens or LCD monitors (LCD = L iquid C rystal D isplay), which, in contrast to screens with cathode ray tubes, require digital control. In this case, it is necessary to digitally process an analog video signal applied to an analog video output of a computer / computer in the screen / monitor. This requires first to re-digitize the analog video signal at a sampling frequency. In order to reconstruct the output data with the most accurate sampling frequency, it is therefore desirable to sample the analog signal with the original frequency and with the correct phase, ie with the frequency and phase, with the from the digital data in the graphics card, the analog video signals at the output of the computer were generated. The phase position here refers to the displacement of the scanning signal relative to the generated scanning signal, wherein the phase angle is generally indicated in degrees, z. 0 degrees, which equals no shift, or 180 degrees, which equates to a shift by half a clock period.

FIG. 1 schematically shows the profile of an analog video signal (see FIG. 1A) at the input of a digital screen. Likewise, FIG. 1B shows an ideal sampling clock for sampling this analog signal. T denotes a period of the sampling clock.

While the generation of images on analog displays using the analog video signals generated by the graphics card is generally straightforward, in particular, does not result in any visible artifacts, resampling of the original on a digital basis Signal-based analog signal is a problem, since it may come here due to the resampling in the digital screen to artifacts in the image displayed, which are visible to a viewer. In order to avoid such artifacts, various approaches are known in the prior art, which will be briefly discussed below.

For example, US Pat. No. 6,268,848 describes a method by which visible errors in an image displayed on a digital monitor are avoided by employing an automatic scanning control system in which the image content of image frames remains substantially the same , a phase of the sampling clock, for resampling the received analog signal as long as it changes until a maximum sample is reached. The phase value achieved at the maximum sampling value then represents the optimum phase shift of the sampling clock for the sampling of this frame.

U.S. Patent 6,147,668 describes a digital display unit that avoids or minimizes display artifacts caused by the aliasing effect of high frequency noise in analog display signals. Similarly as in US Pat. No. 6,268,848, modulation is also applied here to apply different phase delays to successive lines or frames of the sample clock signal so that, due to this modulation, the analog display signal is displayed on the digital display element at different sample points for the same pixel is scanned in different frames.

As can be seen, in the approaches described above, only one sampling phase is varied, whereas the sampling frequency remains unchanged. The approaches described in the above two US patents use sampling clocks based on the one together with the analog video signal provided horizontal and vertical synchronization signals are derived. The synchronization signals represent the reference signal for the digital screen with which a clock generator is locked in the screen to generate a suitable sampling clock based on the reference signal.

Conventionally, the generation of the reference signal for the clock generator is such that based on the received synchronization signals of the analog signal is accessed to a lookup table from which then a suitable for these synchronization signals / ideal reference value is selected, then the clock generator as a reference clock or reference frequency Generation of the sampling clock is provided.

The above approaches work well if it is ensured that the synchronization signals or the reference signal associated with the analog signal actually represents the frequency of the digital signal on the basis of which the analog signal was generated. In this case, the sampling clock generated by the clock generator in the digital screen or in the controller thereof coincides with this frequency. However, this constraint does not apply to all graphics cards, and is usually only true for very sophisticated graphics cards. Other graphics cards, eg. As cheaper graphics cards have tolerances, which cause the frequency used in the graphics card has deviations from the frequency, which is signaled to the digital screen as optimal / ideal sampling frequency. Traditionally, these deviations are in the range of 1% to 5% of the sampling rate actually signaled to the screen.

In such cases, the approaches described above are for sampling analog signals in digital displays to avoid artifacts or interference in the display of the image only limited use, since there is a frequency error in the sampling of the analog signal, which requires further correction.

Starting from this prior art, the present invention therefore has for its object to provide a method and a device which make it possible to generate a sampling frequency which is well adapted to the frequency of a digital signal for the resampling of an analog signal. which was based on the analog signal.

This object is achieved by a method according to claim 1 and by a device according to claim 12.

• (a) Festlegen von zumindest zwei in Zeilenrichtung aufeinanderfolgenden Bereichen in dem anzuzeigenden Bild;
• (b) Bestimmen einer Abtastphase in jedem der festgelegten Bereiche, für die ein Kontrast in dem festgelegten Bereich maximal oder minimal ist;
• (c) Bestimmen eines örtlichen Verlaufs der Abtastphase in Zeilenrichtung, basierend auf den im Schritt (b) bestimmten Abtastphasen in den festgelegten Bereichen; und
• (d) Bestimmen der Abtastfrequenz, basierend auf einem Grundwert und einem Modifikationswert, der aus dem im Schritt (c) bestimmten örtlichen Verlauf der Abtastphase abgeleitet wird.

The present invention provides a method of determining a frequency for sampling an analog video signal by a controller of a digital display to which the analog video signal is provided by a computer to display an image on the digital display, comprising the steps of:

• (a) defining at least two consecutive, row-wise areas in the image to be displayed;
• (b) determining a sampling phase in each of the designated regions for which a contrast in the specified region is maximum or minimum;
• (c) determining a local course of the scanning phase in the line direction, based on the scanning phases determined in step (b) in the specified areas; and
• (d) determining the sampling frequency based on a base value and a modification value selected from the Step (c) is derived specific local course of the sampling phase.

In determining the sampling phase according to the step (b), a sampling phase is determined in each of the designated areas with which the best or worst sampling is achieved, and the contrast in the specified area is thus maximum or minimum

The present invention further provides an apparatus for generating digital data from analog video data for displaying an image based on the analog video data on a digital screen to which the analog video data is provided by a computer

an A / D converter comprising a data input for receiving the analog video data, a data output for outputting the digital image data, and a clock input;

a clock generator having a clock output for outputting a clock signal and a control input for receiving a clock frequency control signal;

a phase shifter comprising a clock input for receiving the clock signal from the clock generator, a clock output for outputting a phase-shifted clock signal to the clock input of the A / D converter, and a control terminal for receiving a phase-shift control signal; and

• zumindest zwei in Zeilenrichtung aufeinanderfolgende Bereiche in dem anzuzeigenden Bild festzulegen,
• in jedem der Bereiche eine Abtastphase zu bestimmen, für die ein Kontrast in dem festgelegten Bereich maximal oder minimal ist,
• einen örtlichen Verlauf der Abtastphase in Zeilenrichtung basierend auf den bestimmten Abtastphasen zu bestimmen,
• die Abtastfrequenz basierend auf einem Grundwert und einem Modifikationswert zu bestimmen, die aus dem örtlichen Verlauf der Abtastphase abgeleitet ist, und
• das Taktfrequenzsteuersignal entsprechend der bestimmten Abtastfrequenz zu erzeugen.

a controller having an input for receiving the digital data from the A / D converter, a first control output for outputting the clock frequency control signal to the clock generator, and a second control input for outputting the phase shift determining signal to the phase shifter, the controller being operative based on the digital data provided at the input

• define at least two consecutive lines in the row direction in the image to be displayed,
• determining in each of the areas a sampling phase for which a contrast in the specified area is maximal or minimal,
• determine a local course of the scanning phase in the line direction based on the determined sampling phases,
• determine the sampling frequency based on a base value and a modification value derived from the local history of the sampling phase, and
• to generate the clock frequency control signal corresponding to the determined sampling frequency.

According to a preferred embodiment of the present invention, the sampling phase having the maximum or minimum contrast in a predetermined range is generated by generating a plurality of reference values at different sampling phases and at the same sampling frequency, the reference value being determined by the sums of the absolute differences is defined by successive intensity values in the specified range. From the thus generated reference values, a maximum or minimum reference value is selected, wherein a maximum or minimum contrast is defined by the maximum or minimum reference value.

According to another preferred embodiment of the present invention, the sampling phase having the maximum or minimum contrast in a predetermined range is generated by performing a first measurement in each of the observed ranges at a predetermined sampling phase and a predetermined sampling frequency by a first Reference value for each of the areas to get. Then, a second measurement is performed in each of the considered areas to obtain a second reference value for each of the areas.

For each of the considered ranges, a difference of the reference values obtained by the first measurement and the second measurement is generated. This measurement is performed at a plurality of different sampling phases / phase values to obtain a plurality of difference values. Finally, for each of the considered ranges, the maximum difference value indicating a minimum contrast or the minimum difference value indicating a maximum contrast is selected from the plurality of obtained difference values. Alternatively, for each of the ranges and for each of the sampling phases, an arbitrary number of measurements may be performed, on the basis of which a plurality of difference values are obtained for each range.

The determination of the local course and the sampling frequency comprises, according to a first preferred embodiment, first determining a straight line passing through the determined best or worst sampling phases. For this straight line then the slope is determined. The modification value is set based on the slope of the line, and the sampling frequency is obtained by adding the base value and the modification value, and a sign of the modification value depends on whether the line is rising or falling, ie the slope has a positive or negative sign. In an alternative embodiment, straight sections and jumps are determined in the course of the sampling phases, and the number of jumps in the course is detected. The modification value then corresponds to the number of hops, and the sampling frequency is obtained again by adding the base value and the modification value. To determine the sign of the modification value, determine whether the straight sections are increasing or decreasing in the local course.

Preferred developments of the invention are defined in the subclaims.

Fig. 1

den Verlauf eines analogen Signals in Fig. 1A am Eingang eines digitalen Bildschirms, und einen zur Abtastung des analogen Eingangssignals idealen Abtasttakt in Fig. 1B;

Fig. 2

ein Blockdiagramm einer Vorrichtung zum Erzeugen einer Abtastfrequenz gemäß einem bevorzugten Ausführungsbeispiel der vorliegenden Erfindung;

Fig. 3

eine Darstellung eines Bildschirms mit aktivem Bild, in dem eine Mehrzahl von Meßbereichen, die zur Frequenzbestimmung gemäß der vorliegenden Erfindung herangezogen werden, dargestellt sind;

Fig. 4

ein Beispiel für die Bestimmung eines schlechten Referenzwerts (Fig. 4A) und eines guten Referenzwerts (Fig. 4B), der zur Bestimmung der Abtastphasen herangezogen wird; und

Fig. 5

den örtlichen Verlauf der besten Abtastphasen für die Mehrzahl von Bereichen in Fig. 3.

Preferred embodiments of the present invention will be explained in more detail below with reference to the accompanying drawings. Show it:

Fig. 1

the course of an analog signal in Fig. 1A at the input of a digital screen, and an ideal sampling clock for sampling the analog input signal in Fig. 1B;

Fig. 2

a block diagram of a device for generating a sampling frequency according to a preferred embodiment of the present invention;

Fig. 3

an illustration of an active image screen in which a plurality of measuring ranges, which are used for frequency determination according to the present invention, are shown;

Fig. 4

an example of the determination of a bad reference value (FIG. 4A) and a good reference value (FIG. 4B) used to determine the sampling phases; and

Fig. 5

the local history of the best sampling phases for the plurality of regions in FIG. 3.

Hereinafter, a preferred embodiment of the device according to the invention will be explained in more detail with reference to FIG. 2. Subsequently, with reference to the block diagram shown in Fig. 2, a detailed description of the preferred embodiments of the method according to the invention.

In Fig. 2, the block diagram of a control device is shown, as used for example in the input stage of a digital screen, such as an LCD screen, use.

The apparatus includes an analog-to-digital converter (ADC) 100 which receives at an input 102 an analog input signal, such as an analog video signal, from a graphics card of a computer. At a clock input 104, the analog-to-digital converter 100 receives a clock signal based on which the analog-to-digital converter performs a sampling of the analog signals received at the input 102. The generated digitized signals are provided by the analog-to-digital converter 100 to its data output 106. The data generated by the analog-to-digital converter 100 is provided at the output 106 thereof to a data line 108. The clock signal applied to the clock input 104 of the analog / digital converter 100 is conducted on a clock line 110. The data line 108 and the clock line 110 continue to extend to the display element of the digital display to provide the data signals and clock signals necessary for display.

Furthermore, the arrangement according to FIG. 2 comprises a clock generator 112, which receives a clock frequency control signal at a control input 114. At an output 116 of the clock generator 112, it outputs a clock signal which is generated as a function of the control signal applied to the control input 114.

A phase shifter 118 is provided which receives at an input 120 the clock signal generated by the clock generator 112. Furthermore, the phase shifter 118 has a control input 122 at which it receives a control signal which defines a phase shift with which the clock signal received by the clock generator 112 is to be applied. The phase-shifted clock signal is then provided at an output 124 of the phase shifter. The output of the phase shifter 124 is connected to the input of the analog-to-digital converter 100 via the clock line 110.

Furthermore, the apparatus includes a controller 126 which receives, at a first input 128 connected to the data line 108, the data signal generated by the analog-to-digital converter. The controller is operative to provide the clock frequency control signal at a first control output 130. Similarly, the controller 126 is operative to provide the phase shift defining signal to the phase shifter 118 at a second control output 132.

The controller 126 operates in accordance with the method of the invention, wherein the control signals for the clock generator and the phase shifter required for carrying out the method according to the invention are carried out, for example, on the basis of sequences / algorithms implemented in the controller 126. Further, the controller 126 includes a signal processing unit to process and evaluate the data signals received at the input 128.

Hereinafter, a preferred embodiment of the method according to the invention will be explained in more detail with reference to the device shown in FIG.

The method according to the invention, as described above, assumes that an ideal sampling frequency signaled to the digital screen for resampling the analog input signal by the analog / digital converter 100 was not the actual frequency of the digital signal underlying the analog signal. Rather, it is to be expected that due to the tolerances of the graphics card used to generate the analog signal deviations from the ideal frequency in the range of a maximum of 1% to 5% exist. This deviation makes it necessary to perform a modification of the ideal sampling frequency to perform resampling / re-digitization of the analog input signal such that an image defined by the analog input data is properly, in particular without visible errors, can be displayed on the digital screen.

In order to determine the frequency required to scan the input data generated by a specific device (graphics card), according to the invention, regions of the analog signal are considered which are repeated. In fact, static frames are used for the method according to the invention, and in this same frame, for example, a single or several screen lines are considered. For the method according to the invention, therefore, preferably the same image / frame is provided for multiple sampling to determine the optimum sampling frequency. Further, the period of the sampling clock provided to the analog-to-digital converter 100 is an integer divisor of the period of the repetitive portion of the analog signal, the horizontal period being a multiple of the pixel period, which is provided by a PLL circuit is produced.

By means of the regulating and measuring loop shown in FIG. 2, the sampling frequency and also the sampling phase from the digital video data on the data line 108 can now be determined.

The inventive method for determining the sampling frequency is based on a method for determining the best / worst sampling phase, but is independent of how this best / worst sampling phase is actually determined. For example, to determine the best or worst sampling phase, reference may be made to US Pat. Nos. 6,268,848 and 6,147,668, respectively, which disclose two approaches for determining the best / worst sampling phase. For the frequency determination, both a method which determines the worst sampling phase, or a Method that determines the best sampling phase can be used.

In the following description of the preferred embodiments, it is assumed that a method is used for determining the frequency, which determines the best sampling phase. A method based on determining the worst sampling phase is analogously applicable.

To carry out the method according to the invention, a "measurement" (sampling) of the analog data of the stationary frame applied to the input 102 of the analog / digital converter 100 is first performed with a freely selected sampling frequency. Based on the received data signals then takes place a calculation of an error indicating the deviation of the selected sampling frequency to the known, ideal sampling frequency (see above). With regard to the freely selected sampling frequency, it should be noted that this principle can be chosen arbitrarily. However, in order to obtain a result in a short time, as after a short calculation period, the freely selectable sampling frequency is chosen to correspond approximately to the expected deviation. Preferably, the arbitrary sampling frequency is chosen to correspond to an expected frequency. If, for example, deviations from the optimum frequency in the range of ± 1% to ± 5% are expected for a graphics card used, then the freely selected sampling frequency is preferably selected in this range around the optimum sampling frequency.

After the repetitive analog signal range M sampling clocks is wide, the sampling frequency may be indicated as M clocks, in the preferred embodiment M being the number of pixels per horizontal line of the digital display.

In order to determine the frequency, that is to say to determine the actual sampling frequency, a plurality of N regions (N ≥ 2) is now selected in the active screen region. In Fig. 3, a screen is shown representing an active image in which a plurality of measurement areas are shown.

Fig. 3 shows schematically the display area 134 of the digital screen, which, as described above, is M pixels wide, ie has M pixels in each horizontal line. Further shown in FIG. 3 is an active image 136 displayed on the screen 134. In the active image 136, a plurality of measurement areas 138 ₀ to 138 _{6 are} shown. These areas 138 ₀ to 138 ₆ are used for frequency determination. In these areas, the best sampling phase is determined, as will be described below. In the embodiment shown in Fig. 3, seven areas 138 ₀ to 138 _{6 are} shown, but the present invention is not limited to this number. In fact, it is sufficient if at least two areas are selected, but the accuracy increases as the number of selected areas increases. The areas 138 ₀ to 138 ₆ are also selected in position depending on the expected frequency error, namely, such that they are a predetermined distance depending on the expected frequency error in the row direction. Two consecutively arranged rows in the row direction should have a distance which is less than or equal to the predetermined distance, which is defined as a rule depending on the assumed error in the sampling in a corresponding number of pixels.

Furthermore, the regions are preferably selected such that image regions are determined in which the best sampling phase is the easiest to determine, which is very easily possible, for example, in regions with high contrast. As can be seen from Fig. 3, it is not mandatory required that all measuring ranges 138 ₀ to 138 _{6 are assigned to} the same line of the image. In fact, these can also be arranged in different lines, as shown in the concrete case of application.

In the regions 138 ₀ to 138 ₆ determined, for example, in FIG. 3, a best sampling phase is now determined according to the invention. The best sampling phase is determined by the method described in more detail below.

A so-called reference value RW is calculated over the specified ranges 138 ₀ to 138 _{6 of} the repetitive range of the digitized input signal. For the same subareas - the analog signal repeats itself - the associated reference values are determined with different sampling phases. In this case, the controller 126 (see FIG. 2) is operative to keep the frequency control signal constant at the output 130 and to provide at the output 132 different phase shift signals for the various computation sections. The best phase setting in one range will then give the maximum or maximum reference value, whereas for the worst phase setting, the minimum / lowest reference value will be set.

The reference value is calculated from the sum of the absolute difference of two consecutive samples of all samples located in one of the measuring ranges. The measuring range may be small up to a measurement of two samples or may extend over several lines of a frame.

RW = Referenzwert,
n = Anzahl der Abtastwerte in dem betrachteten Bereich,
x = Intensitätswert eines abgetasteten Pixels.The reference value is calculated according to the following calculation rule: $$\mathrm{R}\mathrm{W}={\displaystyle \underset{\mathrm{n}}{\Sigma }\left|{\mathrm{X}}_{\mathrm{n}}-{\mathrm{X}}_{\mathrm{n}+1}\right|}$$

RW = reference value,
n = number of samples in the considered area,
x = intensity value of a sampled pixel.

This reference value is thus a value which increases with increasing contrast. The best sampling phase is that sampling phase where the contrast takes a maximum / maximum value. The advantage of the method for reference value calculation just described is that no line or frame memory is required here in order to detect whether the contrast with changed phase becomes better or worse.

To small differences, z. As analog noise, can be provided to sum up only those differences which are greater than a predetermined threshold.

FIG. 4 shows an example of the determination of a good and a bad reference value. In Fig. 4A, a sampling of the analog input signal with a fixed sampling frequency (see period T) is shown, in which the sampling phase is selected so that the sampling results in two adjacent digital values of 0.8 and 0.3, resulting in a reference value of 0.5.

In Fig. 4B, the sampling of the same analog signal at the same frequency (see period T) is shown, but with a sampling phase leading to a digital sample of 1.0 and an adjacent sample of 0.0, so that a large reference value of RW = 1.0, which reflects a high contrast between the two sampled points in the analog signal. Thus, Fig. 4A shows a sample having a bad sampling phase, and Fig. 4B shows the sample having a good sampling phase. Assuming that the reference value achievable in FIG. 4B is the maximum reference value, this is then taken as the basis for the further method for the considered range. In one embodiment In the present invention, where the worst sampling phase was used instead of the best sampling phase, instead of using the reference value determined in Fig. 4B, the reference value determined in Fig. 4A would be used as the minimum reference value.

In an alternative embodiment, different measurements may be taken for the same sub-regions with the same phase setting to obtain a plurality of reference values for each of the regions. In each area the differences of the different reference values are formed. A maximum difference value indicates the worst phase setting in a range, and a minimum difference value indicates the best phase setting in a range. The reason for this is the sampling clock jitter, since the analog signal changes the least in the area of the best sampling, there is also the smallest difference. More specifically, according to the present invention, in this embodiment, first, a first measurement is made in each of the areas under consideration at a predetermined sampling phase and frequency. Subsequently, a second measurement is performed in each of the considered areas. Subsequently, the difference between the measured values obtained by the first and second measurements is generated. The foregoing steps are repeated at different phase settings to obtain a plurality of difference values from which the maximum difference value indicating a minimum contrast or the minimum difference value indicating a maximum contrast value are selected for each area.

Now that the best sampling phase or worst sampling phase has been generated and determined for each of the regions, the frequency determination is now carried out based on the sampling phases thus detected. For this purpose, the measured values obtained are displayed graphically in a coordinate system. For this purpose, as shown in Fig. 5, as x-value (Abscissa) uses the number of the middle sample of the measuring range, and on the y-axis (ordinate), the sampling phase associated with this range is plotted. Thus, for the samples under consideration, the best / worst phase values recorded in Fig. 5 are determined in the manner described above.

The points on the best sampling phases thus plotted along the x-axis are then connected with a straight line, as shown in FIG. 5, and the slope S of the straight lines in degrees per sample is now determined by means of known mathematical methods. For example, the slope is determined according to the following calculation rule: $$\mathrm{S}=\frac{\mathrm{\Delta }\mathrm{°}}{\mathrm{\Delta }\mathrm{x}}$$

However, in calculating S, jumps are to be considered in which the sampling phase values jump between a minimum (0 deg) and a maximum (360 deg) value, as indicated in FIG.

mit:

• M = idealer Abtastwert,
• ΔM = Modifikationswert,
• S = Steigung, und
• Mn = korrigierter Frequenzwert.

After the slope of the line has been determined, the correct sampling frequency can be determined according to the following calculation rule: $$\mathrm{M}\mathrm{n}=\mathrm{M}+\mathrm{\Delta }\mathrm{M}\mathrm{}\mathrm{With}\mathrm{}\mathrm{\Delta }\mathrm{M}=\mathrm{INT}\left(\frac{\mathrm{S}\cdot \mathrm{<figure-callout\; id="360"\; label="M"\; filenames="imgf0005.png"\; state="\{\{state\}\}">M</figure-callout>}}{360\mathrm{°}}+0.5\right)$$

With:

• M = ideal sample,
• ΔM = modification value,
• S = slope, and
• Mn = corrected frequency value.

Alternatively, the corrected or correct sampling frequency can also be determined by determining the number of hops in the course of the sampling phases in the M sampling clocks. This value then corresponds to the absolute value of ΔM. The sign is determined by determining whether the line is increasing or decreasing.

If the sampling frequency is set correctly, the best sampling phase now results for each of the N ranges.

Claims (14)

1. Method for determining a frequency for the sampling of an analog signal provided to a digital screen (132), so as to display an image (136) on the digital screen, comprising the following steps:

   (a) establishing at least two areas (138₀ to 138₆) succeeding in line direction in the image (136) to be displayed;

   (b) determining a sample phase in each of the established areas (138₀ to 138₆), for which a contrast in the established area (138₀ to 138₆) is a maximum or a minimum;

   (c) determining a local course of the sample phase in line direction, based on the sample phases determined in step (b), in the established areas (138₀ to 138₆) ; and

   (d) determining the sampling frequency (Mn) based on a base value (M) and a modification value (ΔM), which is derived from the local course of the sample phase determined in step (c).
2. Method in accordance with claim 1, wherein step (c) comprises the following steps:

   (c.1) determining a straight line on which the sample phases determined in step (b) lie;

   (c.2) determining the slope (S) of the straight line; and

   wherein the step (d) comprises the following steps:

   (d.1) determining the modification value ΔM based on the slope (S) of the straight line; and

   (d.2) determining the sampling frequency (Mn) by adding the base value (M) and the modification value (ΔM).
3. Method in accordance with claim 2, wherein the modification value (ΔM) is determined in accordance with the following calculating rule:

   

   where:

   ΔM = modification value

   S = slope of the straight line, and

   M = base value.
4. Method in accordance with claim 1, wherein step (c) comprises the following steps:

   (c.1) determining straight sections and leaps in the local course of the sample phase; and

   (c.2) determining the number of leaps in the local course of the sample phase, with the course changing at a leap between a maximum value and a minimum value of the sample phase; and

   wherein step (d) comprises the following steps:

   (d.1) determining the modification value (ΔM) based on the number of leaps; and

   (d.2) determining the sampling frequency (Mn) by adding the base value (M) and the modification value (ΔM), with the sign of the modification value (ΔM) being positive or negative, depending on whether the straight sections of the local course of the sample phase are rising or falling.
5. Method in accordance with one of claims 1 to 4, wherein step (b) for each of the established areas (138₀ to 138₆) comprises the following steps:

   (b.1) determining a plurality of reference values (RV) for various sample phases each at the same sampling frequency, with the reference value (RV) being defined by the absolute difference of at least two succeeding intensity values (X_n, X_n-1) in the established areas (138₀ to 138₆), and

   (b.2) selecting a maximum reference value or a minimum reference value from the plurality of established reference values (RV), with a maximum reference value defining a contrast with a maximum value, and with a minimum reference value defining a contrast with a minimum value.
6. Method in accordance with claim 5, wherein the reference value is determined in accordance with the following calculating rule:

   

   RV = reference value,
   n = number of sample values in the area considered, and
   X = intensity value of a sampled pixel.
7. Method in accordance with claim 6, wherein a difference value will only contribute to the reference value (RV), if the difference value exceeds a predetermined threshold.
8. Method in accordance with one of claims 1 to 4, wherein step (b) comprises the following steps:

   (b.1) carrying out a first measurement in each of the considered areas (138₀ to 138₆) at an established sample phase and an established sampling frequency, so as to obtain a first reference value;

   (b.2) carrying out a second measurement in each of the considered areas (138₀ to 138₆) at the established sample phase and the established sampling frequency, so as to obtain a second reference value;

   (b.3) for each of the considered areas, generating a difference of the first and the second reference value;

   (b.4) repeating the steps (b.1) to (b.3) at various phase-settings, so as to obtain a plurality of difference values;

   (b.5) for each of the considered areas (138₀ to 138₆), selecting the maximum difference value indicating a minimum contrast or the minimum difference value indicating a maximum contrast from the plurality of the obtained difference values.
9. Method in accordance with one of claims 1 to 8, wherein step (a) includes establishing a multitude of areas (138₀ to 138₆), with the number of the areas being established in dependence on an accuracy of the resulting sampling frequency (Mn), and wherein the positions of the areas (138₀ to 138₆) comprise a predetermined distance in the line direction, which is established depending on an expected frequency error.
10. Method in accordance with one of claims 1 to 9, wherein the areas established in step (a) are arranged in identical and/or different lines of the image (136).
11. Method in accordance with one of claims 1 to 10, wherein the areas (138₀ to 138₆) established in step (a) are arranged in image areas having a high contrast.
12. Apparatus for generating digital data from analog image data, so as to display an image (136) based on the analog image data on a digital screen (134), comprising:

    an A/D converter (100) including a data input (102) for receiving the analog image data, a data output (106) for outputting the digital image data, and a clock input (104);

    a clock generator (112) including a clock output (116) for outputting a clock signal, and a control input (114) for receiving a clock frequency control signal;

    a phase-shifter (118) including a clock input (120) for receiving the clock signal from the clock generator (112), a clock output (124) for outputting a phase-shifted clock signal to the clock input (104) of the A/D converter (100), and a control terminal (122) for receiving a control signal establishing a phase-shift; and

    a control (126) having an input (128) for receiving the digital data from the A/D converter (100), a first control output (130) for outputting the clock frequency control signal to the clock generator (112), and a second control output (132) for outputting the signal establishing the phase-shift to the phase-shifter (118), with the control means (126) being operative in order to carry out the following steps based on the digital data provided at the input (128),

    - establishing at least two areas (138₀ to 138₆) succeeding in line direction in the image (134) to be displayed,

    - determining in each of the areas a sample phase, for which a contrast in the established area is a maximum or a minimum,

    - determining a local course of the sample phase in the line direction based on the determined sample phases,

    - determining the sampling frequency (Mn) based on a base value (M) and a modification value (ΔM) which is derived from the local course of the sample phase, and

    - generating the clock frequency control signal corresponding to the determined sampling frequency (Mn).
13. Apparatus in accordance with claim 12, wherein the control for determining the sample phase in each of the areas effects a plurality of samples of each area (138₀ to 138₆), so as to obtain a plurality of reference values (RV) for each of the areas, which is defined by the absolute difference of at least two succeeding intensity values,
    with the control, during the plurality of samples, changing the signal indicating the phase-shift at each sample and keeping the clock frequency control signal constant, and
    wherein the control selects a maximum reference value or a minimum reference value for each value (138₀ to 138₆) from the plurality of the obtained reference values (RV).
14. Apparatus in accordance with claim 12, wherein the control for determining the sample phase is operative, so as to

    - carry out a first measurement in each of the considered areas (138₀ to 138₆) at an established sample phase and an established sampling frequency, so as to obtain a first reference value,

    - carry out a second measurement in each of the considered areas (138₀ to 138₆) at an established sample phase and an established sampling frequency, so as to obtain a second reference value,

    - repeat the first measurement and the second measurement at various phase-settings, and

    - select for each of the considered areas (138₀ to 138₆) the maximum difference value indicating a minimum contrast or the minimum difference value indicating a maximum contrast from the plurality of obtained difference values.

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