CS 074 The Digital World. Digital Audio

Transcription

1 CS 074 The Digital World Digital Audio 1

2 Digital Audio Waves Hearing Analog Recording of Waves Pulse Code Modulation and Digital Recording CDs, Wave Files, MP3s Editing Wave Files with BinEd 2

3 Waves A wave is a disturbance that propagates through space and time, usually with transference of energy. 3

4 Waves Mechanical waves exist in some medium (e.g., air, water,...). Waves of electromagnetic radiation can travel through a vacuum. 4

5 Properties of Waves 5

6 Frequency : Waves/Second (Measured in Hz) 6

7 Human Hearing 7

8 Human Hearing Human hearing: 20Hz - 20KHz Human speech: 340Hz - 1KHz Amplitude == Loudness Frequency == Pitch 8

9 Recording Sound Analog : the record recapitulates the original signal; Digital : the record is a sequence of numbers representing the amplitudes (pulse code modulation) 9

10 Edison Phonograph Records 10

11 Edison Tin Foil Records 11

12 Edison Gramophones 1880s 12

13 Phonographic Discs Emile Berliner: patents method of recording on flat gramophone discs (1887) 13

14 Trademark: His Master s Voice (HMV) Owned by UK Gramophone Company 14

15 Digital Audio and Pulse Code Modulation - Break time up into discrete intervals; - Record the integer quantifying the amplitude at each interval - Audio represented by the sequence of amplitudes 15

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17 Digital Audio and PCM 17

18 Fidelity.vs. File Size - Variables: Sample Rate and Bit Depth: Fidelity & File Size Increase/Decrease with Sample Rate and Bit Depth 18

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20 Nyquist-Shannon Sampling Theorem If a function f(t) contains no frequencies higher than W cps, it is completely determined by giving its ordinates at a series of points spaced 1/(2W) seconds apart. 20

21 Basic Psychoacoustic Factors Human hearing: 20Hz - 20KHz => sample rate of 40KHz Most experts believe that humans can only resolve between 16 and 18 bits per sample 21

22 Digital Audio Various representation schemes representation protocols and format information packed into headers software parses the header to understand the layout of the data Bit stream may or may not be compressed and/or encrypted (e.g., Digital Rights Management DRM). 22

23 Uncompressed Digital Audio 1982: Sony Compact Discs (CDs) Red Book Sample Rate: 44,100 samples/sec bit depth = 16 bits (= 2 bytes) 44.1K x 2 x 2 x 80 x 60 = 847MB (sample rate x bytes per sample x channels x minutes x seconds) 23

24 CD Anatomy 24

25 CD Spiral Track The Track is ~3.5 miles long if unwound. 25

26 CDs : Land and Pits 26

27 Compact Discs : Land and Pits 27

28 Uncompressed Digital Audio Microsoft WAV Files : uncompressed PCM File format : 3 chunks: 28

29 WAV Files 29

30 WAV Files 30

31 Compressed Digital Audio Lossless and/or lossy compression: Lossless: compression algorithm has an inverse Lossy: compression algorithm doesn t have an inverse but it won t matter 31

32 Lossless Compression Run Length Encoding (RLE) basic idea : replace sequence 40,40,40,40,40,40 with pair (6,40) 32

33 Lossless Compression Huffman Coding : construct efficient variable length prefix codes; huffman codec (more next time) 33

34 Lossy Compression Perceptual coding exploits human psychoacoustic factors: separate frequency bands range masking 34

35 Compressed Digital Audio 35

36 MP3 Files 1990s : Motion Picture Experts Group (MPEG) Audio Layer 3 Lossless compression Lossy compression : perceptual coding An MP3 file that is created using the mid-range bit rate setting of 128 kbit/s will result in a file that is typically about 1/10th the size of the CD file created from the original audio source. 36