環繞MPEG編解碼器之增速及其在TI DSP平台上的實現

Abstract

隨著音響系統的發展，多聲道系統(Multi-channel audio system)已經廣泛的應用在消費性電子產品上，如DVD和數位音訊的廣播服務。環繞MPEG(MPEG Surround)是由ISO/IEC MPEG所制定的一套標準，它能夠在非常低的位元率下壓縮多聲道的音訊訊號。在本篇論文中，對於環繞MPEG編碼器的模組，我們提供了較快速的演算法，並且符合DSP系統的加速。 我們首先分析環繞MPEG編碼器在DSP平台上的執行計算複雜度，發現濾波堆在整個系統中耗費最多的計算量，因此我們利用離散正/離弦轉換和快速傅立葉轉換來實現濾波堆的運算並且減少運算量。在低音聲道(LFE channel)中，我們也避免了不必要的計算。此外針對DSP的架構，我們使用了一些加速的方法像是定點數運算、使用L2快取記憶體、TI DSP的特殊指令群、迴圈的分解與巨集指令。經由以上的加速方法，在TI DSP系統上可對環繞MPEG編碼器加速約500倍。 另外我們也結合了環繞MPEG編碼器和MPEG-4 HEAAC編碼器，將之實現在DSP平台上，並利用downsampled SQMF(synthesis Quadrature Mirror Filter)簡化架構。相較於原本的架構，使用簡化的架構可將整體速度提升至約1.8倍。

With the fast development of the audio systems, multi-channel audio systems are commonly used for consumer electronic products such as DVD audio and digital audio broadcasting services. The MPEG Surround is an efficient audio coding standard defined by the ISO/IEC MPEG (Moving Pictures Experts Groups) committee. It is able to compress the multi-channel audio signals at a very low bit-rate. In this thesis, we propose several methods to speed up the MPEG Surround implemented on a DSP platform. We analyze the complexity of the MPEG Surround encoder and find that among all modules the filterbanks require the most operational cycles on DSP. Hence, we adopt and modify several fast algorithms based on type-IV DCT/DST and FFT for implementing the filterbanks. We also eliminate the unnecessary computation in the LFE (low frequency effect) channel. For the DSP implementation, we use a few DSP code acceleration techniques to speed up such as fixed-point arithmetic, L2 cache, intrinsic function, loop unrolling and DSP macro function. The experimental results show that the modified MPEG Surround encoder is about 500 times faster than the original version. Furthermore, we implement the combination of the accelerated MPEG Surround encoder and the HEAAC encoder together, and simplify the structure by using the downsampled SQMF bank. Comparing to the original structure, the simplified encoder is about 1.8 times faster.