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Manipulating Audio

Manipulating Audio. amplitude. time. Why Digital Audio. Analogue electronics are always prone to noise. number. time. The Digital Domain. Based on numbers Digital representation of analogue signal:. Digital Electronics. Less expensive to design Less expensive to manufacture

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Manipulating Audio

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  1. Manipulating Audio

  2. amplitude time Why Digital Audio • Analogue electronics are always prone to noise

  3. number time The Digital Domain • Based on numbers • Digital representation of analogue signal:

  4. Digital Electronics • Less expensive to design • Less expensive to manufacture • Offer high noise immunity • Age immunity • Temperature immunity • Increased reliability

  5. 7 7 6 6 5 5 Analogue signal 4 4 3 3 2 2 1 1 6 7 6 5 3 2 1 2 0 0 s Digital waveform s 6, 7, 6, 5, 3, 2, 1, 2 Digital stream 6.3 7.2 6.4 4.8 3.2 1.9 1.3 2.2 The Digital Domain

  6. Nyquist • The highest frequency which can be accurately represented is one-half of the sampling rate • CD: Sample Rate = 44,100 Hz • Nyquist Frequency = SR/2 = 22,050 Hz

  7. Aliasing

  8. Aliasing • Bach trumpet • SR=22050 - highest frequency in music is about 7000 Hz (no aliasing) • SR=11025 - (some aliasing; adds a little dullness and a metallic quality) • SR=4410 - (lots of aliasing; sounds like bad video game)

  9. Low Pass Filtering • The original signal must be low-pass filtered to remove signals higher than 0.5 of the sampling rate • CD: SR = 44,100Hz thus original signal must not exceed 22,050Hz to be completely sampled • Low-pass filtering avoids frequency fold-over

  10. low-pass filter band-pass filter high-pass filter Filters White noise (unfiltered) Filtered white noise (low pass, band pass, high pass)

  11. Filters

  12. Oversampling

  13. Sampling reality • Critical sampling is not attempted • Lowpass filters can not attenuate the signal precisely at the Nyquist frequency • DVD audio – 96 KHz? • CD – 44.1KHz • Telephone – 8KHz

  14. 7 7 6 6 5 5 Analogue signal 4 4 3 3 2 2 1 1 6 7 6 5 3 2 1 2 0 0 s Digital waveform s 6, 7, 6, 5, 3, 2, 1, 2 Digital stream 6.3 7.2 6.4 4.8 3.2 1.9 1.3 2.2 Quantization

  15. Quantization • The number of quantization levels (N) is dependant on binary word length • N = 2n • n = number of bits • 28 = 256 • 216 = 65536 (CD) • 224 = 16777216 (DVD) • The more bits the better the approximation • Worst error is 0.5 LSB

  16. Quantization Error • Difference between the actual analogue value and the selected quantization interval value

  17. Dithering • A small amount of noise that is uncorrelated with the input signal is added

  18. Dithering

  19. Dithering

  20. Anti-aliasing filter Anti-aliasing filter Sample and Hold Sample and Hold A to D Converter A to D Converter A to D Principal elements Analogue input (L) Processing (error correction) Dither generator Multiplexer Analogue input (R)

  21. Summary • Sampling and Quantization are the two fundamental criteria for a digitisation system • Aliasing occurs when sampling theory is not observed • Sampling is lossless • Quantisation is lossy • Dither can substantially reduce quantization distortion

  22. Fin Fin

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