Abstract: The quality of a railway track (11) may be assessed with transducers on a track recording vehicle (10). The received data are filtered in a way that introduces phase shifts, the filtration process having an associated transfer function (H). An inverting transfer function H<The is therefore selected which inverts at least the phase differences of the transfer function (H) of the filter. A multiplicity (N) of successive data samples are stored in a memory, each with an indication of the corresponding position or time, and an output data sample is calculated as the integral of the product of the stored data samples with an impulse function (F) centred on the middle stored sample. The impulse function F (T) is related to the inverting transfer function H<The. As each data sample is moved into the memory the oldest such sample is deleted, and on each occasion an output data sample is calculated.