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The paper presents filtering methods for interfacing time-discrete systems with different sampling frequencies. The methods are applicable for sampling rate conversion between any two sampling frequencies; the conversion ratio may even be irrational or slowly time varying. Interpolation by irrational factors requires digital filters with nonperiodically varying coefficients. This is dealt with in two ways. 1) All possible coefficient values are precalculated. This is, in a sense, possible because of the finite resolution needed. Or 2), the coefficients can be updated in real time using either FIR or IIR filters. The first solution requires a huge coefficient memory; the second scheme, on the other hand, is computationally intensive. While discussing both of these solutions, more practical intermediate schemes incorporating both FIR-and IIR-type filters are suggested. The suggested practical implementations are either based on analog reconstruction filters where the derived digital filter coefficients are functions of the distances between current input and output samples or digital interpolators combined with simple analog interpolation schemes for finding the desired values in between the uniform output samples from the digital interpolator.