Key Technology Design of Audio Codecs for MPEG-2/4 AAC and HE AAC

Digital audio coding technology has played an important role in our daily life for entertainment and communication. In this dissertation, key technology designs for the state-of-the-art audio coding, MPEG-2/4 Advanced Audio Coding (AAC) and its extension MPEG-4 High Efficiency Advanced Audio Coding (HE AAC), are proposed. In order to achieve the goal of low complexity applications such as portable devices with audio playback and recording, study on the reduction of the complexity of AAC encoders and HE AAC decoders are discussed. The dissertation is divided into two parts.

The first part presents a low computation, low memory Psycho-acoustic Model (PAM) for MPEG AAC encoding. PAM is the key technology in the MPEG AAC encoder. It has various complicated functions to model the human auditory system. Therefore, the challenge is to reduce the computation and memory while maintaining the sound quality. The main concept of this work is based on the conversion of complicated functions into optimized look-up tables and common functions, and on the replacement of the computation that is unnecessary. Besides, the detection and decision method is modified to improve sound quality. The complexity of the proposed PAM is reduced to 12.2% (by 87.8%), and this design can lead to a real-time MPEG-2/4 Low Complexity profile stereo encoder at 128 kb/s below 20 MOPS with CD quality maintained.

In the second part, fast Quadrature Mirror Filterbank (QMF) in the Low power Spectral Band Replication (SBR) tools for the MPEG HE AAC decoder is derived. QMF is the key technology of the HE AAC decoder. The main concept of this work is to transform the computation-intensive matrix operations in QMF into conventional fast Discrete Cosine Transform (DCT). Therefore, the computational complexity can be reduced up to 2.7% and 7.8% with respect to the original multiplications and additions.

We are convinced that there will be many applications around us with these audio coding standards in the near future. This study can be of great benefit.