The audio coding in the framework of AC-3:transform, coupling, and dithering

• Main Authors: Juang, Shyh-Yan, 莊士賢
• Other Authors: Chi-Min Liu
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/56495932938850908421

碩士 === 國立交通大學 === 資訊工程學系 === 85 === The purpose of audio coding is to reduce the required bit rate, while maintaining the signal quality after decoding. The AC-3 standard is a widely used audio compression standard in many commercial applications. This thesis considers audio coding under the framework of AC3 through three design issues: time-domain-aliasing-cancellation (TDAC) transform, coupling, and dithering. The AC3 is a transform coding and the TDAC plays the role to transform a signal from time-domain into frequency domain. Since that the computing complexity of the TDAC is high, the development of fast computation algorithms for the TDAC has been a major concern for real-time applications. The first issue of this thesis develops a fast algorithm for the TDAC. The fast algorithm is derived through two steps. The first step converts the six formulae, which are for the three forward-inverse transforms used in the TDAC, into a unified formula known as discrete cosine transform through data permutation in the input and the output. For the discrete cosine transform, the second step develops a fast computing algorithm which takes special consideration on the stack size limitation in x86 CPU in addition to achieving the low complexity as traditional fast algorithms. The second issue of the thesis is on the coupling strategies in AC3. Nowadays, the CD quality audio sequence can be compressed into 2x128 kbits per second by independent coding of the left and the right channels. Coupling strategies can provide the design space exploiting the human hearing knowledge which indicates low human sensitivity of the stereophonic in high frequency components. With the use of coupling coding techniques, the compressed audio quality can be improved for the bit rate lower than 2x128 Kbits/s. This thesis presents five algorithms which are applicable to coupling strategies in AC3. We compare the five algorithms through theoretical analysis, objective quality measure and subjective quality measure. Among the five algorithms, three are newly developed. The three algorithms are developed based on the KL transform. Subjective and objective measurements show that the coupling strategy based on the KL transform can provide better stereo audio quality than the others. The third issue of the thesis is on the dithering signals. Dithering can avoid the undesired noise from quantization and provide the means to have better stereo signals. This thesis confirms the above two benefits through experiments and provides the strategies to apply the dithering in the framework of AC3.