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We present a fast software implementation of the MPEG-4 AAC (advanced audio coding) main and low complexity (LC) decoder. The reference implementation is analyzed and selected algorithms are presented to improve the performance of most of its building blocks, i.e., bitstream de-formatter, noiseless decoding, prediction, and filterbank. The code is further optimized by means of assembler procedures for the filterbank and prediction tools to exploit the Intel Pentium streaming SIMD extension (SSE) instruction set. SSE performs operations over four single precision floating-point numbers in a single instruction using 128-bit long registers. Results show that the presented decoder proves to be almost five times faster than the reference implementation, while preserving its full compatibility with the MPEG-4 standard.