臺灣博碩士論文加值系統

近年來，MP3音樂格式在網路上快速地風行。以流行歌曲為例，在過去，一張音樂CD最多只能儲存約十幾首歌，但以MP3的壓縮方式，一張CD可以儲存超過一百首的歌曲，我們甚至可以經由網際網路上傳或下載歌曲，卻不再需要像過去得等上幾個小時。然而，有時候MP3的音樂品質實在是讓人不太滿意，尤其是當歌曲中有比較急促的聲音或聲音不是很圓滑的時候，MP3裡的雜訊有時甚至到了不能忍受的地步。我們要如何才能改進呢？
在各種音訊壓縮標準方面，VCD的音訊部份是MPEG-1 Audio Layer 3；DVD的音訊是MPEG-2 Audio（與MPEG-1 Audio相容）或是杜比公司的AC-3；MP3的壓縮規格是MPEG-1 Audio Layer 3。這些壓縮規格在一些特定情下況或多或少都會產生雜訊，這主要是由於壓縮方法的限制，除非我們可以改變壓縮標準或是在每一個壓縮步驟都有人手動調整壓縮用到的參數，否則我們幾乎沒有任何的方法來改進。
MPEG-2 Advanced Audio Coding（簡稱MPEG-2 AAC或AAC）壓縮標準是在1997年12月制定完成的，它捨棄了與MPEG-1 Audio的相容性。沒有了向後相容的限制，AAC的編碼機制在壓縮聲音的信號時，可以維持很高的聲音品質，絕大多數在VCD、DVD和MP3會產生的雜訊，AAC都可以減低到人類聽不出來的地步。
AAC的壓縮技術主要是根據MPEG-1 Audio Layer 3的技術，另外再加上一些修改和加強，使它能儘量維持聲音的品質並同時減低所需要的資料量。這些修改和加強的部份主要包括了：更大的時頻轉換區塊、頻率領域的資料預測、以及時間領域的雜訊塑形、…等等。
在這本論文中，有針對AAC壓縮機制的完整介紹，並且實作一套AAC的即時軟體解碼器。

MP3 music format has come up prevailing in a year or two. One can store more than one hundred pop songs in one CD on which it used to store no more than tens of songs. One can even transmit a pop song at home through the Internet easily without waiting for several hours. However, the quality of MP3 is not always satisfying. Sometimes the noises are even unacceptable for highly pitched or transient sounds when you hear them through PC. How can we improve them?
The audio part of VCD is MPEG-1 Audio Layer 2, that of DVD is MPEG-2 Audio BC (Backward Compatible) or Dolby AC-3, and that of MP3 is MPEG-1 Audio Layer 3. Many of the noises in those media are due to the limitation of the coding standards. Unless we can change the coding standards or perform every encoding steps by human, we almost have no way to improve them.
MPEG-2 Advanced Audio Coding standard is finalized in December 1997. It abandons the backward compatibility with MPEG-1 Audio. Without the backward compatibility limitation, its coding mechanisms can perform higher quality when encoding audio signals. Most of the noises in VCD, DVD, and MP3 can be reduced to a minimum by AAC that most people can not hear them.
In this thesis, a thorough introduction of AAC mechanisms is presented and a real-time software decoder is implemented.

CHAPTER 1 INTRODUCTION3
1.1 MOTIVATION3
1.1.1 Introduction to Audio Coding3
1.1.2 Speech Coding4
1.1.3 General Audio Coding5
1.2 THESIS ORGANIZATION6
CHAPTER 2 PSYCHOACOUSTIC MODEL7
2.1 BIOLOGICAL AND PSYCHOLOGICAL BASES OF SOUND PERCEPTION7
2.1.1 Biological Structure and Functionality of Human Ears7
2.1.2 Sound Encoding12
2.1.3 Masking Effect18
2.2 PSYCHOACOUSTIC MODEL FOR MPEG-2 AAC21
CHAPTER 3 MPEG-2 AAC CODER27
3.1 OVERVIEW27
3.2 FILTER BANK31
3.2.1 Window Shape33
3.2.2 Transform Block Length35
3.2.3 Transform Block Switching36
3.3 SCALEFACTORS AND QUANTIZER39
3.3.1 Quantization39
3.3.2 Noise Shaping40
3.3.3 Iteration Process41
3.4 NOISELESS CODING44
3.4.1 Spectrum Clipping45
3.4.2 Sectioning45
3.4.3 Grouping and Interleaving46
3.4.4 Scalefactors47
3.4.5 Huffman Coding47
3.5 PREDICTION48
3.5.1 Tool Description49
3.5.2 Predictor Processing50
3.5.3 Predictor Control53
3.6 TEMPORAL NOISE SHAPING (TNS)55
3.6.1 Theoretical Basis55
3.6.2 Implementation56
3.6.3 Properties57
3.7 JOINT CODING58
3.7.1 M/S Stereo Coding58
3.7.2 Intensity Stereo Coding59
3.8 GAIN CONTROL59
3.8.1 Polyphase Quadrature Filter (PQF)61
3.8.2 Gain Detector and Gain Modifier61
CHAPTER 4 MPEG-2 AAC DECODER IMPLEMENTATION63
CHAPTER 5 CONCLUSION AND FUTURE WORKS68
5.1 CONCLUSION68
5.2 FUTURE WORKS68
BIBLIOGRAPHY:70
Chapter 1

[1] A. M. Kondoz, Digital Speech — Coding for low bit rate communication systems, John Wiley & Sons Edition, 1994.
[2] ISO/IEC 14496-3: 1999, Information technology — Coding of audio-visual objects — Part 3: Audio.
[3] ITU-T Recommendation G.723.1: March 1996, Dual Rate Speech Coder for Multimedia Communications Transmitting at 5.3 & 6.3 kbit/s.
[4] ITU-T Recommendation G.729: March 1996, Coding of Speech at 8 kbit/s using Conjugate-Structure Algebraic-Code-Excited Linear Prediction (CS-ACELP).
[5] ITU-T Recommendation G.728: September 1992, Coding of Speech at 16 kbit/s Using Low-Delay Code Excited Linear Prediction.
[6] ISO/IEC 11172-3: 1993, Information technology — Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbit/s — Part 3: Audio.
[7] ISO/IEC 13818-3: 1997, Information technology — Generic coding of moving pictures and associated audio information — Part 3: Audio.
[8] Advanced Television Systems Committee, Digital Audio Compression (AC-3) Standard, Doc. A/52, Nov 92.
[9] ISO/IEC 13818-7: 1997, Information technology — Generic coding of moving pictures and associated audio information — Part 7: Advanced Audio Coding (AAC).
[10] Robert Sekuler, and Randolph Blake, Perception, 3rd Edition, McGraw-Hill Editions, 1994.
[11] Bekesy, G. von, and Rosenblith, W. A. 1951, “The mechanical properties of the ear. S. S. Stevens (ed.), Handbook of experimental psychology, New York: John Wiley & Sons, pp. 1075-1115.
[12] Evans, E. F. 1982, “Functional anatomy of the auditory system. In H. B. Barlow and J. D. Mollon (eds.), The senses, Cambridge, England: Cambridge University Press, pp. 251-306.
[1] Marina Bosi, Karlheinz Brandenburg, Schuyler Quackenbush, Louis Fielder, Kenzo Akagiri, Hendrik Fuchs, Martin Dietz, Jűrgen Herre, Grant Davidson, and Yoshiaki Oikawa, “ISO/IEC MPEG-2 Advanced Audio Coding,” J. Audio Eng. Soc., Vol. 45, No. 10, pp. 789-812, October 1997.
[14] John P. Princen, and Alan Bernard Bradley, “Analysis/Synthesis Filter Bank Design Based on Time Domain Aliasing Cancellation,”, IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. 40, No. 4, pp. 1153-1161, December 1994.
[15] Th. Sporer, Kh. Brandenburg, and B. Edler, “The use of multirate filter banks for coding of high quality digital audio,” 6TH European Signal Processing conference (EUSIPCO), Amesterdam, June 1992, Vol. 1, pp. 211-214.
[16] Yu-Chi Chen, Chien-Wu Tsai, and Ja-Ling Wu, “Fast Time-Frequency Transform Algorithms and Their Applications to Real-Time Software Implementation of AC-3 Audio Codec,” IEEE Trans. On Consumer Electronics, Vol. 44, No. 2, MAY 1998.