Abstract: A method of implementing a dual-mode audio decoder and filter is provided. The inverse modified discrete cosine transform (IMDCT) method and circuit for a dual-mode audio decoder perform the IMDCT with respect to a signal encoded using either the MPEG or Dolby AC-3 standard by utilizing a shared fast Fourier transform (FFT) circuit thereby simplifying the necessary hardware construction. Also, the number of IMDCT outputs used for windowing is reduced by utilizing the properties of the IMDCT outputs of the MPEG bit stream and thus the size of memory necessary for storing the IMDCT outputs is reduced.

Abstract: A method of implementing a dual-mode audio decoder and filter is provided. The inverse modified discrete cosine transform (IMDCT) method and circuit for a dual-mode audio decoder perform the IMDCT with respect to a signal encoded using either the MPEG or Dolby AC-3 standard by utilizing a shared fast Fourier transform (FFT) circuit thereby simplifying the necessary hardware construction. Also, the number of IMDCT outputs used for windowing is reduced by utilizing the properties of the IMDCT outputs of the MPEG bit stream and thus the size of memory necessary for storing the IMDCT outputs is reduced.

Abstract: A method of performing an inverse modified discrete cosine transform (IMDCT) which reduces the amount of calculation, in which IMDCT values y.sub.m (r) expressed in vector form as Y.sub.m =CX.sub.m, where Y.sub.m, C, and X.sub.m represent matrices are obtained by defining values of X.sub.k (m) for ##EQU1## calculating values of y.sub.m (r) for ##EQU2## with respect to an upper half of an upper portion of Y.sub.m by performing calculation with respect to an upper part of an upper portion of a left half of C and with respect to an upper half of X.sub.m ; calculating values of y.sub.m (r) with respect to a lower half of the upper portion of Y.sub.m by inverting the values of y.sub.m (r) with respect to the upper half of the upper portion of Y.sub.m and arranging the inverted values in a reverse manner; calculating the values of y.sub.m (r) for ##EQU3## with respect to an upper half of a lower portion of Y.sub.