A memory-free modified discrete cosine transform architecture for MPEG-2/4 AAC

Most current audio coding standards use the modified discrete cosine transform (MDCT) to transform an audio sequence from the time domain into the frequency domain. Existing architectures for MDCT use a lookup table to store cosine values in read-only memory (ROM). For MPEG-2/4 AAC, the memory space taken up by the required lookup table makes the circuit implementation inflexible and large because of the long window length. Therefore this study proposes a memory-free architecture for MDCT without a lookup table. The proposed architecture adopts the arithmetic circuit module to calculate the cosine function based on the Taylor and Maclaurin series approximations and makes use of the symmetric and periodic identities of trigonometric functions to reduce circuit complexity. 0.18 m TSMC cell library technology is used to synthesise the proposed architecture, which requires about 9213 gates with a maximum operation frequency of 100 MHz. The proposed architecture, which has a flexible window length to perform MDCT, can be implemented in an area smaller than that of ROM-based architectures.