Processing a stream of ordered input data

A data processing system is provided for performing processing operations upon an ordered stream of input data values to form an ordered stream of output data values. A select circuit (18) includes select interval generation circuitry (34) which determines a number (interval number) of input data values between each data value to be selected for output from among the ordered stream of input data values. This interval number varies with position within the ordered stream of input data values. The select circuit (18) can thus perform selection of input data values in accordance with an interval number which may be varied, for example, in accordance with a linear piecewise approximation of an desired curve or, in other embodiments, in a piecewise quadratic variation approximating a desired curve. The processing techniques may be used, for example, in beam forming application, such as 3D beam forming of ultrasonic images.