Method and apparatus for determining a frequency for the sampling of an analog signal

Apparatus for generating digital data from

• analog image data, so as to display an image based on the analog image data on a digital screen,
• and an ideal sampling frequency for re-sampling the analog image data being signalized to the digital screen, the apparatus comprising:
• an A/D converter including a data input for receiving the analog image data, a data output for outputting the digital image data, and a clock input;
• a clock generator including a clock output for outputting a clock signal, and a control input for receiving a clock frequency control signal;
• a phase-shifter including 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 control signal establishing a phase-shift;
• and
• a control 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 output for outputting the signal establishing the phase-shift to the phase-shifter, with the control means being operative in order to select a new sampling frequency on the basis of the ideal sampling frequency signalized to the digital screen, to control the clock generator via the clock frequency control signal to output a clock signal with the new sampling frequency, and to carry out the following steps based on the digital data provided at the input,
• establishing at least two areas succeeding in line direction in the image to be displayed,
• multiple sampling of each of the areas at the new sampling frequency with respective different phases, wherein the sampling frequency is fixed,
• for each of the areas determining the sample phase for the different sample phases, for which a contrast in the area is a maximum or a minimum,
• determining a local course of the sample phases of the areas provided in the line direction based on the sample phase determined for each area,
• deriving a modification value from the local course of the sample phase,
• modifying the ideal sampling frequency by the modification value to obtain a proper sampling frequency for the analog image data, and
• generating the clock frequency control signal corresponding to the proper sampling frequency.