Re-sampling method and apparatus

1. A method of processing samples in a digital sample processor, the method comprising the steps of:

receiving a stream of input samples at respective input sample positions;

specifying output sample positions according to a specified temporal or spatial relationship between input sample positions and output sample positions;

deriving values for new output samples at respective output sample positions from the values of said stream of input samples by summing weighted values of input samples situated within a filter aperture;

wherein the weight applied to the value of each input sample depends on the phase of that input sample within the said filter aperture, and the relationship between the said input sample phase and the said weight is defined by a filter aperture function in which the value of the function is the said weight and the argument of the function is the said phase; and

wherein the width of the said filter aperture is scaled according to either the input sampling frequency or the output sampling frequency so as to obtain the wider of the two possible filter apertures.

2. A method according to claim 1 in which the argument of the aperture function is obtained by taking the numerically smaller value of:

the distance in input sample pitches between the position with respect to the input samples of the contributing input sample and the position with respect to the input samples of the output sample receiving the respective contribution, and

the distance in output sample pitches between the position with respect to the output samples of the contributing input sample and the position with respect to the output samples of the output sample receiving the respective contribution.

3. A method according to claim 2 , in which the said the distance in output sample pitches between the position of the contributing input sample and the position of the output sample receiving the respective contribution is determined as the difference between the output sample position and the required output sample position determined from the position of the contributing input sample according to the said specified temporal or spatial relationship between input sample positions and output sample positions.

4. A method according to claim 2 , in which the said the distance in input sample pitches between the position of the contributing input sample and the position of the output sample receiving the respective contribution is determined as the difference between the input sample position and the required input sample position determined from the position of the output sample receiving the contribution according to the said specified temporal or spatial relationship between input sample positions and output sample positions.

5. A method according to claim 2 , in which data values representing the said specified temporal or spatial relationship between input sample and output sample positions are interpolated so as to obtain intermediate sample positions between specified sample positions.

6. A method according to claim 1 , in which contributions of input sample values to output sample values are accumulated in members of a serially-connected fixed-length chain of accumulators.

7. A method according to claim 6 , in which accumulated values are passed along the chain at the output sample rate.

8. Resampling apparatus comprising:

a terminal configured to receive a stream of input samples at respective input sample positions; and

a resampling filter deriving new output samples at respective output sample positions having a specified temporal or spatial relationship with said input sample positions from the values of a stream of input samples by summing weighted values of input samples situated within a filter aperture such that the weight applied to the value of each input sample depends on the phase of that input sample within the said filter aperture, and the relationship between the said input sample phase and the said weight is defined by a filter aperture function in which the value of the function is the said weight and the argument of the function is the said phase; wherein

the width of the said filter aperture is scaled according to either the input sampling frequency or the output sampling frequency so as to obtain the wider of the two possible filter apertures.

9. Apparatus according to claim 8 , in which the argument of the aperture function is obtained by taking the numerically smaller value of:

the distance in input sample pitches between the position with respect to the input samples of the contributing input sample and the position with respect to the input samples of the output sample receiving the respective contribution; and

the distance in output sample pitches between the position with respect to the output samples of the contributing input sample and the position with respect to the output samples of the output sample receiving the respective contribution.

10. Apparatus according to claim 9 , in which the said the distance in output sample pitches between the position of the contributing input sample and the position of the output sample receiving the respective contribution is determined as the difference between the output sample position and the required output sample position determined from the position of the contributing input sample according to the said specified temporal or spatial relationship between input sample positions and output sample positions.

11. Apparatus according to claim 9 , in which the said the distance in input sample pitches between the position of the contributing input sample and the position of the output sample receiving the respective contribution is determined as the difference between the input sample position and the required input sample position determined from the position of the output sample receiving the contribution according to the said specified temporal or spatial relationship between input sample positions and output sample positions.

12. Apparatus according to claim 9 in which data values representing the said specified temporal or spatial relationship between input sample and output sample positions are interpolated so as to obtain intermediate sample positions between specified sample positions.

13. Apparatus according to claim 8 , in which contributions of input sample values to output sample values are accumulated in members of a serially-connected fixed-length chain of accumulators.

14. Apparatus according to claim 13 in which accumulated values are passed along the chain at the output sample rate.

15. An image processor having resampling apparatus comprising:

a terminal configured to receive a stream of input samples at an input sampling frequency;

a resampling filter adapted to provide a stream of output samples at an output sampling frequency by summing weighted values of input samples situated within a filter aperture; and

an output sampling frequency selector for selecting an output sampling frequency which can be higher or lower than the input sampling frequency;

wherein the weight applied to the value of each input sample in the resampling filter depends on the phase of that input sample within the said filter aperture, and the relationship between the said input sample phase and the said weight is defined by a filter aperture function in which the value of the function is the said weight and the argument of the function is the said phase; and

wherein the width of the said filter aperture is scaled according to either the input sampling frequency or the output sampling frequency so as to obtain the wider of the two possible filter apertures.

16. A processor according to claim 15 , in which the output sampling frequency is selectively variable between a frequency which is higher than the input sampling frequency and a frequency which is lower than the input sampling frequency.

17. A processor according to claim 15 in which a change may be made within an image or within a sequence of images from an output sampling frequency which is higher than the input sampling frequency to an output sampling frequency which is lower than the input sampling frequency.