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A Fast Integer Implementation of MPEG-I Audio Decoder

Hans, Mathieu C.; Bhaskaran, Vasudev

HPL-96-03

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Abstract: We have developed a fast integer implementation of an MPEG-I audio decoder. We obtained a fast implementation using a combination of algorithmic enhancements and by modifying some of the filtering functions. Firstly, we incorporated Chen's fast DCT algorithm to implement the matrixing operation in the subband filtering defined by the MPEG standard. Instead of using the Chen's DCT method as-is, we used a scaled DCT wherein multiplies needed in the final stage of the DCT computations are absorbed in the stage after the DCT. This reduces the multiply count for the DCT form 116 to 84. Secondly, we modified the filtering functions. Based on subjective tests, it was found that a considerable number of subband coefficients can set to zero without degrading the reconstructed audio quality. Setting the subband coefficients to zero reduced the overall multiply count. Furthermore, subjective tests also indicated that setting some of the filter coefficients of the synthesis window to zero did not reduce overall audio quality. Zeroing out the filter coefficients also reduced the multiply and add count. The combined effect of algorithmic enhancements and truncation of subband coefficients and filter coefficients yielded a 13% speed improvement over a naive integer implementation.

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