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Audio. 音訊原理  聲音由聲波構成,由介質傳遞 Human Ears  The human ear is able to detect frequencies in the range between 20 Hz and 20 kHz –Upper limit decreases with.

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Presentation on theme: "Audio. 音訊原理  聲音由聲波構成,由介質傳遞 Human Ears  The human ear is able to detect frequencies in the range between 20 Hz and 20 kHz –Upper limit decreases with."— Presentation transcript:

1 Audio

2 音訊原理  聲音由聲波構成,由介質傳遞

3 Human Ears  The human ear is able to detect frequencies in the range between 20 Hz and 20 kHz –Upper limit decreases with increasing age –e.g.  iobasics/a/test_tones.htm

4 Human Ears  Comparison

5 音訊取樣規格

6 音訊儲存格式  一般音訊格式 : –WAV 檔格式  Microsoft 開發,用於 Windows 系統,副檔 名.wav –AU 檔格式  Sun 公司開發,用於 Unix 系統, 副檔名.au –AIFF 檔格式  APPLE 公司開發,用於 Mac 系統,副檔名.aif

7 音訊儲存格式  MIDI (Musical Instrument Digital Interface): – 音樂樂器數位介面 – 特色 : 有樂譜電腦即能播放  e.g.  MP3 格式 : – 一種音訊的有損壓縮法 – 特色:具有高壓縮頻率, 儲存長度僅有 WAV 格式 1/10 ,音質 僅次 CD Audio 。副檔名.mp3  網路串流格式: –RA 格式 : RealNetwork 公司開發,副檔名.ra –WMV 檔格式 :Microsoft 開發,副檔名.wmv

8 音訊播放  環境檢查與設定 – 硬體設備 : 音效卡、喇叭、音源輸出 – 硬體設備連接檢查 : 在我的電腦按右鍵 → 內 容 → 系統內容 → 硬體 → 裝置管理員檢查 音效卡。 – 檢查與調整聲音有關設定

9 音訊播放  環境檢查與設定

10 Windows Media Player

11 音訊錄製  硬體 : 音效卡  軟體 : Windows 錄音機或其他錄音剪輯 軟體  環境設定

12 Windows 系統的錄音機  Windows 系統的錄音機

13 Windows 系統的錄音機  使用錄音機程式編輯音訊 – 變更音量大小 : 效果 → 增加. 減少音量 – 變更播放速度 : 效果 → 增加. 減少速度 – 反轉音訊 : 相反方向播放音訊。效果 → 反 轉 – 迴音效果 : 效果 → 加入迴音

14 Windows 系統的錄音機  使用錄音機程式編輯音訊

15 Theory

16 Digitizing Sound  Steps Sampling Quantization Processing Recording

17 Digitizing Sound  Sampling –The selection of the sampling rate  If limiting of hearing is 20 kHz, a minimum rate of 40 kHz is required by the Sampling Theorem.  The sampling rate of audio CDs is 44.1 kHz  kHz is commonly used for Internet kHz for speech  DAT (digital audio tape): 48 kHz

18 Digitizing Sound  Quantization –It ’ s usually quantization levels for CD audio  16 bits –Under-sampling a pure sine wave  An analogue signal will be coarsely approximated by samples that jump between just a few quantized values –Dithering  When a small amount of random noise is added to the analogue signal before sampling

19 Digitizing Sound  Quantization

20 Digitizing Sound  Dithering –Dither is an intentionally applied form of noise, used to randomize quantization error noisequantization error

21 Digitizing Sound  Dithering –Concept of adding noise to reduce noise that doesn't make a lot of sense.

22 Digitizing Sound  Dithering –Spectrograph

23 Digitizing Sound  Dither Noise –Spectrograph

24 Digitizing Sound  Adding Dither

25 Digitizing Sound  Dithering –Spectrograph

26 Digitizing Sound  Dithering –Comparision

27 Digitizing Sound  Dithering –Comparision

28 Digitizing Sound  Processing –Tools  Audacity  MatLab  Etc.

29 Audacity  Window

30 Audacity  Recording –Click on the red Record button to begin recording. –Click on the blue Pause button to pause the recording. Press it again to continue. –Click on the yellow Stop button to cease recording. The cursor will return to its previous position, before the recording was started.

31 Audacity  Effect

32 Audacity  Speed

33 Audacity  Amplify

34 Audacity  Pitch

35 Audacity  Tempo

36 Matlab  Lab –Dither  Ref. –http://labrosa.ee.columbia.edu/matlab/ga mmatonegram/http://labrosa.ee.columbia.edu/matlab/ga mmatonegram/ –http://www.soundslogical.com/support/mp acks/documentation/english/documentpar ts/audiodither.htmlhttp://www.soundslogical.com/support/mp acks/documentation/english/documentpar ts/audiodither.html –http://www.users.qwest.net/~volt42/caden zarecording/DitherExplained.pdf

37 Digitizing Sound  Recording and Importing Sound –Sampling rate and sampling size –If level of signal is too low, then resulting recording will be quiet. –If level is too high, clipping will occur.  Fig –Gain control can be used to alter level. –Automatic gain control

38 Compression  Use a bank of filters to split signal into bands of frequencies; 32 bands are commonly used.  The average signal level in each band is calculated, and using these values and a psycho-acoustical model, a masking level for each band is computed.

39 Compression  MPEG Audio  3 layers –Layer 1: 192 kbps for each channel –Layer 2: 128 kbps for each channel –Layer 3: 64 kbps for each channel  MP3 = MPEG-1 Layer 3 compression rate = 10:1

40 Compression  Block Diagram for MP3 –Input audio samples are fed into the encoder. –The mapping creates a filtered and subsampled representation of the input audio stream. –A psychoacoustic model creates a set of data to control the quantiser and coding. sampling_frequencyfrequency specified (kHz) '00'44.1 '01'48 '10'32 '11'reserved

41 Compression –The quantiser and coding block creates a set of coding symbols from the mapped input samples. –The block 'frame packing' assembles the actual bitstream from the output data of the other blocks, and adds other information (e.g. error correction) if necessary.

42 Compression  Block Diagram for MP3 –SMR: signal-to-mask ratio

43 Compression  Block Diagram for MP3

44 Compression  Polyphase Filter

45 Compression  MDCT

46 Compression  SMR –signal-to-mask ratio

47 Compression  Document/Paper –ISO/IEC –REALIZATION OF A PSYCHOACOUSTIC MODEL FOR MPEG 1 USING GAMMACHIRP WAVELET TRANSFORM  MatLab –http://www.petitcolas.net/fabien/software/mp eg/

48 Reference  聯盟教材  /Audacity.ppt#263,6,Before Recording  ml ml   recording/DitherExplained.pdf


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