MPEG 第二級音訊編碼系統之壓縮, 優化及品質評估研究

Abstract

MPEG 音訊編碼系統第二級(layer 2)壓縮技術是一個針對 CD 音 訊 (音樂)品質而設計的共同性音訊編碼標準。然而，其編碼程序需要大 量的數學計算。在本篇論文中，我們提出一些會損害些許音訊品質的優化 技術來使得編碼程式能更快些。為了能評估出失去品質的量，我們定義了 一個標準來數量化一編碼後音訊流的品質流失率。從該定義中，我們可以 得出由編碼器編碼出有品質流失的資料流浪費的位元(bits)數。在我們的 驗中，我們優化了第二級的壓縮程式，使其可在Pentium Pro 180電腦上 以0.0003%的品質流失率和1.5%的位元浪費率及在Pentium 180電腦上 以0.0009%的品質流失率和4%的位元浪費率來即時編碼。 在這篇 論文中，我們也將提出一些不損耗或僅損耗一點原來音訊品質的方法來降 低第二級壓縮設計的位元率(bitrate)。根據這些方法，我們可以將位元 率由256 kbps降低至接近於第三級位元率128 kbps的160 kbps。 最後我們將介紹一個由我們建構的即時多媒體系統。此系統乃應用由我們 優化後的第二級壓縮程式而成的。 The Layer 2 compression scheme of MPEG audio Coding is a very commonaudio coding standard for Cd quality audio (music). However, its encodingprocess requires a large amount of computation. In this thesis, we propose someoptimization techniques to make the encoding program much faster while allowingto impair the audio quality a little. In order to measure how much quality loss,we define criteria to say that the encoder encoding streams with a quality lossrate equivalently wastes a number of bits, if the encoder that completelyconforms to the layer 2 standard can save the same number of bits while keepingthe same quality loss rate. In our experiments, we optimize the layer 2compression programs such that audio streams can be encoded real time onPentium Pro 180 machine with only 0.0003% quality loss and 1.5% bits wasted,and on Pentium 180 machine with only 0.0009% quality loss and 4% bits wasted. In this thesis, we will also propose some methods to reduce the bitrateof layer 2 compression scheme without impairing the original audio quality orjust impairing a little. Base on these methods, we can reduce bitrate from256 kbps to 160 kbps that is close to layer3's 128 kbps. At last, we will introduce a real-time multimedia system by applyingthe layer 2 compression program optimized by us.