a a u2062 u2211 k u221e u221e u2062 u2062 x u2061 k u2062 u2147 j2 u03c0 fk f s u2062 u2062 u2147 j2 u03c0ft u2062 u2146 f 5 equation 5 represents a sinc interpolation reconstruction formula in accordance with the invention the integral limits u00b1 a of this function effectively bandlimit the interpolation the interpolation is bandlimited to effect filtering of the data output by the interpolation therefore when f s is less than f u2032 the equation 5 will function as a standard sinc interpolator whereby the data reconstructed by the interpolation will be bandlimited to f s 2 however when fs is greater than f u2032 equation 5 will function as a sinc interpolator whereby the data reconstructed by the interpolation will be bandlimited to f u2032 2 thus the reconstructed data will be bandlimited to f u2032 2 and thereby be filtered from an aliasing effect therefore the prefiltering step to remove an aliasing effect in reconstructed data is no longer required the cutoff frequency u03c9 c is effectively constrained to the minimum of u03c0f s u03c0f u2032 thereby limiting the integral of the reconstruction formula of equation 5 to a sinc interpolation may therefore interpolate and filter the expanded data in a single step fig3 illustrates yet another converter in accordance with the invention converter 30 has a first stage 32 where the sampling rate f s of the input audio data is increased digitally by an integer factor l giving an output y n at l f s y n is also referred to as expanded data the second stage 34 may comprise a simple linear interpolator which interpolates the denser expanded samples of y n at frequency l f s to generate output at required frequency.