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Audio Slide 2 Slide 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. http://homerecording.about.com/od/homestud iobasics/a/test_tones.htm Slide 4 Human Ears Comparison Slide 5 Slide 6 : WAV Microsoft Windows .wav AU Sun Unix .au AIFF APPLE Mac .aif Slide 7 MIDI (Musical Instrument Digital Interface): : e.g. http://hk.geocities.com/katherineyau2001/Midi_Download.html MP3 : WAV 1/10 CD Audio .mp3 RA : RealNetwork .ra WMV :Microsoft .wmv Slide 8 : : Slide 9 Slide 10 Windows Media Player Slide 11 : : Windows Slide 12 Windows Windows Slide 13 Windows : . : . : : Slide 14 Windows Slide 15 Theory Slide 16 Digitizing Sound Steps Sampling Quantization Processing Recording Slide 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 22.05 kHz is commonly used for Internet 11.025 kHz for speech DAT (digital audio tape): 48 kHz Slide 18 Digitizing Sound Quantization It s usually 65536 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 Slide 19 Digitizing Sound Quantization Slide 20 Digitizing Sound Dithering Dither is an intentionally applied form of noise, used to randomize quantization error noisequantization error Slide 21 Digitizing Sound Dithering Concept of adding noise to reduce noise that doesn't make a lot of sense. Slide 22 Digitizing Sound Dithering Spectrograph Slide 23 Digitizing Sound Dither Noise Spectrograph Slide 24 Digitizing Sound Adding Dither Slide 25 Digitizing Sound Dithering Spectrograph Slide 26 Digitizing Sound Dithering Comparision Slide 27 Digitizing Sound Dithering Comparision Slide 28 Digitizing Sound Processing Tools Audacity MatLab Etc. Slide 29 Audacity Window Slide 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. Slide 31 Audacity Effect Slide 32 Audacity Speed Slide 33 Audacity Amplify Slide 34 Audacity Pitch Slide 35 Audacity Tempo Slide 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 Slide 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. 12.10 Gain control can be used to alter level. Automatic gain control Slide 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. Slide 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 Slide 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 Slide 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. Slide 42 Compression Block Diagram for MP3 SMR: signal-to-mask ratio Slide 43 Compression Block Diagram for MP3 Slide 44 Compression Polyphase Filter Slide 45 Compression MDCT Slide 46 Compression SMR signal-to-mask ratio Slide 47 Compression Document/Paper ISO/IEC 11172-3 REALIZATION OF A PSYCHOACOUSTIC MODEL FOR MPEG 1 USING GAMMACHIRP WAVELET TRANSFORM MatLab http://www.petitcolas.net/fabien/software/mp eg/ Slide 48 Reference http://www.colquitt.k12.ga.us/cchsmedia/ppt /Audacity.ppt#263,6,Before Recording http://star7.cs.ntust.edu.tw/ckyang/cindex.ht ml http://star7.cs.ntust.edu.tw/ckyang/cindex.ht ml http://www.inote.tw/2006/09/audacity.html http://www.inote.tw/2006/09/audacity.html http://www.users.qwest.net/~volt42/cadenza recording/DitherExplained.pdf