A data processor and method for processing position-related input data from a rotational machine whose angular speed is variable and providing output data at an output data rate. The processor comprises a time-based over-sampler for over-sampling the input data at an over-sampling rate greater than the output data rate, and a down-sampler for extracting samples of over-sampled data from the over-sampler at the output data rate so as to provide the output data. The down-sampler is responsive to an angular timing signal related to an angular position of the machine for selecting the samples of over-sampled data to extract based on the angular position. Application to a rotational machine whose angular speed is variable, in particular to an internal combustion engine to control engine operating parameters as a function of cylinder pressure.